Monday, 17 December 2018

PP Fixed Bias circuit design and calculator

The push-pull design for output stages has persisted since times of antiquity. It was one of the very earliest circuit designs, and has persisted until the present day, with modern solid-state linear amplifiers still overwhelmingly using it.

With tubes, a typical topology is given by the circuit below. (Click to magnify). The anode (plate) voltage on either side comes through the primary of the output transformer.

This design uses a pentode tube, which has a screen grid. This is attached to taps on the output transformer to run in Ultra-Linear mode, increasing efficiency and reducing distortion.

In this design, the cathodes are tied to ground through a very low value shunt resistor. The resistor is simply there to provide a small voltage drop from which the current through the tube can be measured. It plays no other role in the circuit, other than being a fuse if the tube red-plates.

Values of one ohm or ten ohms are typical of this arrangement.


Typical implementation of a Fixed-bias, ultralinear Push-Pull output stage with a pair of pentode tubes (EL84 / 6BQ5)


Because the cathode is at (or very close to) ground potential, this requires the control grid to have a negative DC bias voltage applied to it, to regulate the flow of current through the tube.

If there was no negative bias applied to the tube, it would go into full conduction, the plate would glow red hot, fireworks will happen and that would be Bad, mmkay?

So we need to contrive to feed a constant negative voltage into the grid, along with the signal, to achieve the desired regulation.

The voltage required depends on several factors... as a very rough rule-of-thumb, take the screen voltage and divide it by the tube's mu (gain) to get the maximum negative bias voltage likely to be needed

Looking at the circuit above... An EL84 has a mu of 20 and in this implementation the screen voltage is 350, which gives us 17.5 volts. Multiply by -1 since we're dealing with negative volts. So we're likely to need around -17.5 volts.
In this case, our adjustment range is from -12.5 to -21.5 volts.


How to set up the bias adjustment resistor values

The bias voltage needs to be adjustable. Both tubes need to be drawing the same current, otherwise the net current through the transformer will not be zero, which will lead to magnetisation of the transformer core. This is a most undesirable situation and left unchecked, it will cause quantum fluctuations in the space-time continuum. Well ok maybe not that bad, but the transformer will saturate unevenly and distort the sound.

The usual approach is to use a voltage divider network with a potentiometer, as above.

Couple of points about this design.
  1. The more negative the voltage goes, the lower the current through the tube
  2. In this implementation, if the potentiometer fails, it will fail safe. The most common mode of failure with potentiometers is the wiper lifting off the track. If this happens, effectively the voltage at the grid of the tube will go full negative, reducing the current through the tube to (almost) zero. This is far more desirable than the voltage reaching zero and the tube immediately red-plating.
  3. Expanding on (2) – please don't ever build this circuit with just the pot wiper connected to the grid. When the pot fails (and it will, eventually) it'll likely take the tube with it.
  4. If you're going to build this circuit, it's intuitive to set it up so that clockwise rotation of the pot increases the current through the tube (ie brings the biasing voltage closer to zero)

The next question is - what value resistors will be needed? This is where some trial and error in the calculations is needed. Using Ohms' law, these are the variables:
  • The desired bias voltage adjustment range
  • The input negative voltage from the power supply

From there, you can calculate the values for R34, VR4 and R36, to give you the range you need.

This is where a spreadsheet can be mighty useful.


This is a screenshot from a spreadsheet I made that can help with this calculation. You can download this Excel worksheet here.

Put in the numbers in the red. Experiment with the values for R1 and R3 and the potentiometer, until you get the desired voltage range in the "Output V" column.

The "Build-Out R" represents the load seen by the preceding driver or phase-splitter stage, so watch the maximum "Rg-k" from the tube's datasheet isn't exceeded. In the case of an EL84, that value is 300K. 

The columns of this spreadsheet:
  • Step - the setting on the potentiometer
  • Total R - the total resistance from the bias voltage to ground
  • Output V - the negative voltage as fed to the control grid
  • P(R1) - the amount of power dissipated by R1
  • P(R3) - power dissipated by R3
  • P(Pot) - the power dissipated by the potentiometer
  • Pot pwr % - the power dissipated by the pot expressed as a percentage of the pot's total power rating AND the amount of track being used to conduct. 


About Pot pwr %

In the specifications for the potentiometer, there will be a power rating. However that power rating is across the entire length of the track. If the pot is set to half-position (assuming it's a linear taper which in this design it is) then the power handling drops to half.
So, this Pot pwr % column shows how much power the pot is dissipating as a percentage of its maximum considering the wiper position.


Using the circuit

Putting all this together, it's easy to see how it works. Adjust the potentiometer for maximum negative voltage (wiper closest to the left, in this schematic). Power on the circuit and let it stabilize. Measure the voltage across the cathode resistor. Then adjust the potentiometer until the desired current is flowing through the tube.


What is the desired current through the tube?

Glad you asked. This depends on the tube itself, and the B+ voltage, and your preference regarding bias.
As a rule of thumb, around 70% is the sweet spot with most types. If you bias low, that's called "cold" biasing. The current through the tube is low, and the sound may take on a thin, glassy, brittle aesthetic. Also the distortion will increase.
Bias too hot and you'll shorten the life of the tubes.
70% is the goldilocks zone.

So. Look at the tube's datasheet. For an EL84 we see the maximum plate dissipation is 12 watts.
70% of 12 watts is 8.4 watts
B+ is 350 volts so using Ohms' law:

we solve for I at 24mA

Across 10 ohms (cathode resistor) our 24mA will give 0.24V

So we want to see 0.24V across the cathode resistor. Adjust the potentiometer until that's the value shown. Then repeat for the other tube in the circuit, then do a final check that they're both the same (or as close as you can get)


Please feel free to use my spreadsheet - I developed it to assist in choosing the resistor network values, and also to ensure the power rating of the potentiometer wasn't being inadvertently exceeded at any setting.


Feedback in the comments please


Wednesday, 14 November 2018

A better time delay startup circuit

The amplifiers I've built so far have all incorporated a delayed turn-on circuit for the high voltage supply. The intention is to allow the 6 volt supply to turn on first and allow the valves to reach operating temperature before turning on the high voltage supply.

This is accomplished with a simple circuit based around a 555 timer IC in monostable mode, set up to a delay of around 25 seconds.

The circuit I've been using, while functioning, had a few problems. Driving a relay directly from the output of a 555 IC resulted in a lot of voltage drop through the IC and the relay coil voltage being low, for a 5V relay it was getting around 3.5 volts, fortunately this is still enough to trigger it, but less than ideal.

My re-design of the circuit was prompted by my addition of a 2-colour LED to the design, to glow red at initial turn-on but change to green when the timer activates and the HT voltage is turned on.

These LEDs are 2-pin, they work by reversing the polarity into them. So they're 2 LEDs in one envelope, and depending on the polarity of the applied voltage, one will be forward biased and glowing, the other reverse biased and dark.

After breadboarding it and measuring carefully, this is the circuit I designed:



Click to enlarge if necessary.

Note in this diagram my symbol library for the MOSFET is wrong... if you're gonna use this same MOSFET be very aware its pinout (viewed from top) is S-G-D instead of G-D-S. So my pin numbers are wrong. Sorry about that.

The 7805 voltage regulator is not strictly necessary but it does result in a nice 4.9V across the relay coil.

The 330K and 68µF cap provide the time constant for the timer IC. The formula in this mode is:

T = 1.1 x R x C

The MOSFET Q1 buffers the output of the IC switching the negative on or off to the relay based on the voltage at the gate, which comes from the output of the IC at pin 3. This starts low until 25sec elapses then goes high and stays high until power down.

The two 330R resistors form a voltage divider, at the mid-point the voltage is 2.5V. When the relay is off, the + voltage will flow through the coil (which is around 62 Ohms) and then into the LED, then to ground through the lower 330R resistor. This results in a voltage drop of around 0.2 volts across the relay coil, not enough to turn it on.

When the IC turns on, the voltage appears at the gate of the MOSFET, switching the transistor on. This effectively shorts the Drain and Source, causing the negative to connect to the relay and the LED. At which point the return path for the LED is through the top 330R resistor, so this reverses the polarity across the LED causing it to change colour.

The reverse-biased diode across the relay is for flyback suppression. 

After breadboarding, I've designed a single-layer PCB layout for this circuit which is 35mm x 35mm utilising a W02 rectifier.

On my board design I've also added a header for a regulated 5V power supply, in case it's needed elsewhere (such as a tone control bypass relay for example)

The current and dissipation is such that no heatsinks are necessary on either the voltage regulator or MOSFET.

Be sure to put the relay on the AC side of the rectifier diodes, relays have a much easier time switching AC than DC and this is reflected in the voltage rating on the datasheet.

Saturday, 28 July 2018

Build completed

The EL84 amp is completed and has been removed from the workbench and is now in the living room where it's been entertaining us the past few days.

This one went fairly well, however there were a few problems.

First, a few pretty pictures


This is the best looking amp I've made so far. Great care was taken with centring and spacing.



The translucent hole to the left of the volume control covers the IR detector

About the name

The amplifier is named "Matariki" which is in the Maori language of New Zealand. Literally translated, it means either "Eyes of God" or alternatively "Little Eyes". 

In more common usage, it is the name given to the Pleiades star cluster, when it becomes visible (which is mid-year, mid winter here) and has traditionally become associated with renewal, the Europeans decided to call it the "Maori New Year"

There was also a rare southern right whale which made an unusual appearance in Wellington Harbour recently, during Matariki, and the whale was thus informally named Matariki.
While all this was happening, I was designing this amplifier.
Hence the name





The case is aluminium, sourced from AliExpress, of the type I usually use. The front panel is 8mm thick, brushed aluminium. It required pockets being milled on the CNC from the back to accommodate the controls mounted through it.

The lights are 3mm LEDs but I decided I don't like the bulging appearance they give when pushed through the front panel, so we laser-cut some 2mm clear acrylic into 3mm circles, so the lights on the front could be flat and flush. They press-fitted perfectly and the look was 100% what I was wanting.

The STBY LED is red, and the PWR led is dual-colour, it starts red at power-up and then when the HT switches on after 30 sec, it turns green.

The power switch and input selector are a rotary encoder: push to toggle power, rotate to cycle through the inputs. 


Inside the case


Pic taken with phone under workshop fluorescent lights, sorry for quality!

Inside the chassis there's the amplifier mainboard, which contains 6 tubes and is the phono, tone, gain and phase splitter. To the left of that is the base of the output valves, the long thin board contains the bias and cathode shunt resistors, test points, and on the track side, the four trimmers for adjusting the bias voltage.

The green boards are bought-in components: input selector, mains switch/standby, remote volume, and microcontroller.

The power supply contains the usual array of resistors and capacitors needed to provide the various voltages, as well as the usual 30 sec startup delay timer relay circuit I always use.

The DC voltages provided by the power supply are:

+350V
+300V
+270V
+250V
+6.0V (DC heaters for Phono stage, rectified from the 5vac secondary with Schottky diodes)
-27V for fixed bias

In addition there's the standby transformer which provides 9vac at around 200mA to power the microcontroller and standby board.


Things that went wrong in this build

1. Phase splitter grid current

I have a continuous improvement philosophy in that each build needs to be better than the last. I don't profess perfection in any of these projects, but provided each shows improvement, I am happy.

In this one, I built it essentially to the same circuit as the previous, but owing to finding a supplier of NOS Soviet valves, I changed the circuit replacing the 12AX7 with 6N2 and the 12AU7 with 6N1.

This is where the problems began. The different characteristics of these valves meant that my original choices of operating points and voltages (I like to DC-couple the cathodyne phase-splitter) were wrong. I thought I knew how to work this stuff out, turns out I didn't, and the results on the scope were disappointing.


This was the kind of nonsense I was seeing on the output from the cathodyne


This was the output from the gain stage (yellow) with input inverted, scaled and superimposed (blue)

The initial gain stage was running into clipping quite badly as it went positive. This was disappointing; it meant that I was inducing grid current into the phase splitter and the gain stage couldn't drive it. If I pulled the phase splitter tube the waveform immediately reverted to an undistorted sinewave. Back to the drawing board to figure out what I'd got wrong.

This is how you learn. After some head scratching and calculating, I arrived at a set of voltages and operating points that - while not eliminating this problem - shifted it beyond the range of signal levels that I would need in this amp. So now at 28V p-p it looks like this


Which no doubt purists will jump all over me for, but I can live with it. We're driving EL84s here, so we don't need huge levels.


2. Gain Deficit

So subbing in the 6N2 in place of the 12AX7 resulted in lower overall gain, despite the two having on paper the same µ

The result of this was that when applying 10dB NFB, it was taking something like 1.0 vrms (2.8v p-p) to drive it to full output. On quieter passages of music this meant that even at full volume, it was too quiet.

In the end, I had to reduce the NFB to around 7dB, a figure which according to some is worthless, the advice I had was to try for 20dB but realistically in this amp it would mean adding another gain stage.

This issue is unresolved in that I've left the NFB at 7dB but even at that level, you have to crank the volume control over more than expected. It makes me think twice about building this design again

On the plus side, it sounds absolutely wonderful, and it leaves me asking myself why I really need to add more NFB. Is it to satisfy some purist urge to get to the holy grail 20dB or what? I need to understand more about NFB - in this amp the bass is solid and tight, not flabby at all, and the sound is the purest of any amp I've built so far (to my uneducated 47 year old ears at least)


3. Low voltage timer delay and LED colour reverser circuit

In each of my amps I use the same 555-based startup delay circuit. It's based off a rectified 6.3 volt using a W02 rectifier which then feeds the 555

The output to the power relay is taken from pin 3 of the chip and also this feeds another small DPDT relay which reverses the polarity of the dual-colour LED. Except that this smaller relay when switched on, buzzes like a bumblebee for the first 4 or 5 sec, and I measured its coil voltage, it's only getting 3V which is weird because the 555 is being powered off the rectified 6.3 volt... somewhere there's a ton of voltage drop happening. 
Measured the positive pin of the rectifier, only 5VDC. That was surprising, I'd have expected 7.5 volt, even allowing for the 1.4volt forward voltage drop. Then there's the voltage drop through the 555.
So somehow I need to re-design this circuit for my next amp so that the relay coils are getting 5 volts. This is currently an unresolved, work-in-progress - I'll leave it like this for this one but re-design the circuit for the next one.




Things that went right in this build

This was the first all-on-one-board amp I'd made and it was successful. Everything worked exactly as expected on the first power-up. All the components fitted on the board, the board itself was a success (first project with the new temperature-controlled PCB etching tank) and the board looks fine (although there's no soldermask or silkscreen on it, it really is just single-sided naked copper tracks on FR4)
Likewise for the power supply.

The level of tidiness inside the case is better than anything I've achieved before, although I don't think I'll ever get to the level I am looking for... which is OK, because when you shoot for the moon you're not gonna hit it, but you will end up in the treetops, which is a whole lot better than being on the ground.

The level of aesthetic appeal on this one is better than any of my previous projects as well. I am completely happy with that aspect.

From a technical standpoint, on this build I'd designed the board to allow phase compensation into the NFB loop. This is because NFB produces high-frequency ringing which you can see on the oscilloscope if you put a 10KHz squarewave into the input. At the output you get something like this:


Nasty ringing through the NFB

The prescribed method to resolve this is to phase-compensate the NFB with resistors and capacitors, the values of which are determined by experimentation. After doing this, the 10KHz squarewave output now looks like this


NFB after phase compensation added

Which would again probably prompt some scorn and ridicule from purists, but it represents around a 99.7% improvement, which I am happy with.

Finally, one aspect I am well pleased with is the listening test. Subjectively this is the cleanest sounding amp I have built to date.


Remaining to do

The last steps before I can call this one finished is just to complete the performance measurements. Input sensitivity, Freq response, noise, and THD all need to be measured.

Thereafter, the remaining to-dos are:
  • Fix the Low-voltage 555-based circuit so the relay coils get 5v not 3v (or else just use 3v relays if they exist, that'd be a quick and ugly hack!)
  • Track down and understand the source of the remaining hum in the RIAA stage and modify the next design accordingly
  • Increase the gain so the NFB can be increased
When performance measurements are completed I'll post up the final project page.


Schematic as built

Click to enlarge. Might need to download / Save-As, to be able to read it






Tuesday, 17 July 2018

Advice comes at a cost

This post is a bit of a rant, and also a warning to those embarking on this craft and seeking the advice of experienced or expert designers and builders.

No pictures in this one sorry.

I've debated whether to post this for a while, but recent events have compelled me to.

When I started this blog, I was completely new to designing and building amplifiers and valve gear in general. I was delighted to see all of the resources available on the internet, and I joined one or two of the more popular forums. After sitting and watching for a while, and reading as much as I could, I started posting up a few questions, and a couple of schematics I'd designed, to get some input and opinion from the wise and experienced folks.

The input and opinion I got was not quite what I was expecting or hoping for. In my mind I'd imagined that the experienced folks would be tolerant of – or even welcoming – to the newbie, and take time to give explanations or point to resources to further my understanding.

Instead I was the recipient of sarcasm, scorn and ridicule. Both on the boards, and in private messages. It became quickly apparent to me that the prevailing attitude seemed to be that unless you know all of the common topologies by heart, you have no business even picking up a soldering iron. 

My particular approach has been that I don't want to just find a schematic and build it, I want to understand how it works. I'll only build something I can describe the working of to another person. So I'm gonna ask questions... that's how you learn.

Besides the condescending remarks, another thing I had to contend with was opinion stated as fact. Some examples:

  • "Hammond Sucks. Edcor all the way"
  • "No audio circuit has any business using the 12AU7, it's so non-linear."

So one of the first skills I had to pick up was the ability to discern fact from strongly-held and expressed beliefs.

The next problem I encountered was a peculiar way of offering recommendations. The most recent example was concerning the use of a Constant-Current Source for preamp tubes. This particular recommendation was given to me in an email by another old-timer in a way that implied that any amplifier without a CCS is some kind of useless toy. When I questioned this, my question was taken as a challenge, and I received an insulting and profanity-laden email in return.

Here's the thing, though. If someone tells me I need a CCS - or any other such recommendation - they should expect me to ask why. This is not to challenge or disagree – but rather because I want to know the reasoning. I need to know if this is another opinion-stated-as-fact, or whether there is some basis for the recommendation. I want to know:

  • Why would I need a CCS?
  • What problem does it solve?
  • How bad is that problem?

This helps me build understanding and further my knowledge. I did not profess to be an expert in this area - it remains a hobby which I fit around a career and a family. I do strive to learn something from each project, and make each one better than the last.

To that effect, I have made a decision which I should have made back in 2016 and this is the reason for this longwinded post. From now on, I am receiving my knowledge from books, or the small number of personal sources I trust, and I recommend anyone else starting out do likewise. 

Either that or develop a thick skin against the attitude you're likely to encounter.

For my part, if anyone asks me for my knowledge, I'll happily share it without condescension, such as it is.





Saturday, 14 July 2018

PCB party

The demo amp is taking shape... the chassis is back from laser engraving and milling, the back panel is assembled, and the PCBs have been made.

Some photos for now.


The amp main board.

 About 10 hours design work went into this at the PC in several sessions. Then another hour on the exposure and etching (hooray for the new Etching Tank!) then about two hours on the drill press (I do this all manually and this one has about 346 holes) then about four hours stuffing and soldering.
  • The right-hand third of this board is the phono/RIAA stage. 
  • Bottom half of the left two-thirds is the Active tone controls
  • Top half of the left two-thirds is the gain and phase splitter stages



Flip side

The tubes are at 50mm spacing. As you can see it's a single-sided board. The unfilled holes at the bottom left (in this view) are for the phase correction in the NFB loop. These component values will be determined by experimentation



The total size of this board is 160 X 100mm (or 4" X 6.25" if you insist)


I'm using Soviet military NOS surplus tubes which I found a supply of. So this is configured for 6N1 / 6N2 tubes.


We also have a power supply. This one is a bit of a squeeze because I had a 100 X 100mm cutoff bit of PCB which I thought of using. Ideally it should be bigger but I didn't feel like cutting anything.

The supply contains my usual start-up delay (that's the IC and relay you can see) where the AC power to the rectifier diodes is switched on after about 25 sec, to give the tubes warm-up time. I've also got my usual 2-colour LED driver (it reverses the polarity to the LED when the B+ power comes on, turning the LED from Red to Green. I designed this in the last amp and I liked it, so I bought a few 2-colour LEDs and this can now be a permanent feature in my designs)



Touch the capacitor with blue writing and you'll jump across the room. These are 2 caps of 470µF in series (since they're only rated 250V and my B+ is 330). They have Balancing Resistors.



The cap lying down is too high to fit into the case upright. It's 47000µF at 10V, it's for the DC heater supply for the phono stage. The top right semiconductors are the schottky rectifiers for it (since I'm using the 5V secondary for this, low voltage drop diodes will keep my filament supply within voltage spec)


Loving how clean the tracks are using the new etching tank




Finally there's the chassis. The drilling and punching was done by hand on the top panel, since this is a one-off the setup time on the CNC would have been not worth it. If I am gonna make 5 of these amps then I'll CNC it.

The front panel is adhesive vinyl, laser-cut then peeled, and then there's three coats of clear lacquer to protect the vinyl letters.


Not all of the controls are installed yet.


This one will have a motor-driven remote controlled volume and the input selector is a Rotary Encoder which will cycle through the inputs, with indicator lights to show the selection. The knob will also push to turn the main power on. This is also accessible through the remote control, the perspex window for which is to the left of the volume control.


Around the back

The back panel, nothing especially amazing here. The lettering is laser-etched into the aluminium.

The transformers will be Hammond. 370FX for power and 1650E for OPTs.

More photos as the build progresses.



Thursday, 5 July 2018

Measure twice, cut once

It's been too long without a project on the workbench, and I've got a few leftover parts from previous projects. Plus, I happened across some NOS Soviet military-spec 6N1 and 6N2 tubes. It would have been a grave sin of omission not to do something with them.

So, the idea of building a new amplifier took shape. This one doesn't have a new owner waiting for it, but rather I'm making it as a demo unit. Idea being to use it to hopefully drum up a few orders and to test the market to see if I can sell it at a price that recovers the parts cost and makes a profit.

Topology-wise this will be a tried-and-true amp, I'm not breaking any new ground electronically with this one, but I am refining the construction as far as my skills will allow, and hopefully the results at the end will be worth the effort.

So, we're looking at (yet another) EL84 push-pull amp in ultralinear with fixed bias, a split-load phase splitter, preceded by a gain stage, the same active tone control as I've built twice before, and a Phono (RIAA) stage, again the same one as I made before.

This time, however, I've spent a bit of time on the board design. My photosensitive board blanks are 160mm X 100mm, so I decided to see if I could fit the RIAA stage, tone controls, gain and phase splitter stages, all on that board.

Several hours of editing on the PC later, and I had a design which has passed 3 stringent eyeball checks. I am happy to build it and see what happens.

Circuit-wise it's the same as the previous one I made but those were all on separate boards. Also in the Gain stage I've incorporated phase compensation in the NFB both on the cathode and the load resistor.

I'm even using the exact same chassis as the last one. So, the first job was to work out the component placement. 

So, I printed out my PCBs onto paper at 100% size and placed them in the chassis. Then I added the PCBs for the remote control volume, standby, and input selector (thanks Aliexpress!) Finally, the connectors and other things that go inside the case to complete the job. It's all a big jigsaw puzzle, and I find this the easiest way to visualise what the inside of the case will look like, and whether there's anything that'll need re-arranging.



Luckily there's enough room and I don't need to stand anything on its edge. This case only has 50mm height so this is good news.

So the printed board at top left is the RIAA / Amp / Tone Control board. That has 6 tubes on it in two rows of three, with 50mm spacing.
The sockets for the EL84s are next, proceeding clockwise, and the long thin printed board is the bias board. Same design as I've used previously each time.
then we have the volume control which will be mounted to the front panel. Continuing clockwise, this is a cardboard cut-out of the 9V transformer which will supply standby power for the remote control board, giving us the ability to turn the amp on remotely. Then there's the mains relay. 

The 100 X 100mm printed board is the power supply incorporating all the resistors and capacitors and usual power supply things. It also incorporates my usual 555-based startup delay with the driver for the 2-colour LED, like in the previous project. (It turns on red to begin with but then changes to green when the high voltage switches on)

The remaining two boards are the input selector and the driver board for the remote control receiver.

My next job is to score up the case and cut the holes needed, then make up the three boards.

I got tired of using a dish for etching boards, it takes too long and is a bit hit-and-miss. So I bought an etching tank with a heater:



The heater keeps the etchant at the correct temperature and should improve the process. When I get to making these boards, I'll do a video of it to publish here.

More entries as this build progresses...

Wednesday, 30 May 2018

Hammond Power Transformer temperature problem

The last EL84 amplifier I built (with the phono stage, tone control and headphone stage) has a Hammond 370FX power transformer. It was noticed this transformer gets uncomfortably hot to touch after about one hour's use of the amplifier.

Being unfamiliar with how-hot-is-too-hot, I've adopted a cautious approach and ordered a higher spec transformer to replace the current unit. However this is a few weeks away (coming from Canada) so in the meantime I decided to measure the temperature rise to gain a deeper insight into the problem.

First thing I tested, before doing any measurements, was to pull the tubes from the headphone stage, thereby relieving the power supply of 44mA of B+ and 1600mA of 6.3volt. As expected, this resulted in a much slower heating up of the transformer.

With all tubes plugged in, the quiescent current on the B+ is 140mA
This is a centre-tapped transformer, so conventional wisdom is that in this mode of usage, the secondary should be rated at 1.2 times the desired DC current, which in this case would be 168mA

In the case of the 370FX the secondary is rated at 173mA

Assessment: OK


On the Low voltage side. the 6.3volt is rated at 5A and the total draw on it is 5.2A so a little over (by 5%)

Assessment: Not ideal
(The replacement unit ordered has an extra 1000mA there).


So. Down to the measurements. I ordered a digital pyrometer (infrared surface temperature measurement gun) and when that arrived, I ran the amplifier for 3 hours, measuring the surface temperature on the top of the transformer, every 5 minutes.


(Posed photo. Te measurement target shown was not the actual measurement location due to radiant heat from the output tubes).


Over the three hours, this was the result:



The measurements were made each time at the same spot on the top of the transformer, from the same distance.

After around 45-50 mins the transformer became uncomfortably hot to touch if resting the hand on it. This corresponded to a temp of mid-to-high 40s. Once the temperature was in the low to mid 50s it became uncomfortable even to a fingertip.

Conclusions

1) This is an unscientific test with a cheap uncalibrated instrument from AliExpress. I have some confidence in it because the baseline temperature reported (22ºC) at zero minutes was exactly the ambient temperature in the room reported by multiple other thermometers.

2) Electronic components are rated at 105ºC. I do not know what the temperature difference between the windings and and the outside of the transformer would be, so I am going to make a totally wild and uninformed guess of 20°C. Therefore the windings are at around 80-85°

3) From reviewing others' experiences with Hammond power transformers, it seems a commonly reported phenomenon that they run hot. Therefore this transformer is behaving as expected, although it is causing considerable unease in doing so.

4) Because of my wild assumption in (2) above, I have no confidence that this transformer will be safe or indeed what detrimental impact sustained running at high temperature will have on it.

5) When the new transformer arrives and is installed, I will repeat the experiment.

Saturday, 24 March 2018

EL84 Amp II: Progress Post 6 – Listening Tests

With electronic construction complete the amplifier has been moved from the workbench to the listening room to get some initial listening impressions. 

The speakers it's running into are floorstanding KEF C95s from around 1990. Efficiency is quoted at 90dB. These are sealed bandpass.

The amp has 3 line inputs and one phono. So the source used for listening tests was the Chromecast Audio and my turntable with its Audio-Technica AT11 MM cartridge.

First up - the line input. An easy test. Passed with flying colours. The measurements on the workbench were showing around 15 watts power into the 8Ohm dummy load before any sign of clipping, and the THD was reading less than 1% rising as the power level approached maximum. So expected behaviour, in other words.

The EL84s are biased to about 8 watts dissipation, which at the B+ of 365V means 22mA. The cathode shunt I am using is 10Ohm so that's a reading of 0.22V at each cathode test point.


EL84s with bias adjusters

The topology of this amp is that the first thing the line-level input signals encounter is the Cathode Follower which forms the first stage of the tone control. This means the input impedance is around half a meg. So a very easy load to drive. Most amplifiers route the input signal to the top of the volume control pot, so the input impedance is effectively the track resistance of that pot, usually 50 or 100K.



Hum levels are acceptable. There is a barely detectable hum if pressing the ear to the front of the speaker cabinet when no music is playing. However since I don't know anyone who would use the amplifier in this manner, I'm not going to allow myself to get too worried about it.

Sound quality assessed subjectively is clean, pleasantly detailed, no trace of any distortion or harshness in the treble, and with plenty of power to the bass. I would be happy having this sound quality as my daily driver.

The tone control behaved exactly as the previous build of this circuit, this is the second time I've built this one.

The phono stage however was not such a stellar performer.

The only problem with this one is an unacceptable level of hum. This is normal with phono stages and simply indicates a signal grounding problem somewhere.

Other than the hum, the RIAA stage sounded well. So I am confident some simple experimentation with the earthing will resolve the hum.

I tested the RIAA compliance on the workbench and found it to be within 1dB from 45Hz to 15KHz. This is acceptable.



One aspect of this build I am less happy with is the power transformer.

This is a Hammond 370FX and it's loaded to its rate current. Around 140mA DC load on the B+ and around 5A load on the 6.3v

However this transformer runs hot. After 30 mins run time it's noticeably warm and after 60 mins you can touch it but if you wrap your hand around it, you'll want to remove it after 3 or 4 seconds.

I found a few discussions on DIYAudio commenting about the heat output of these transformers, and to be honest it makes me reluctant to use them again, either that or I'll be sure to apply a generous de-rating margin next time.

Cosmetically I still have some work to do





The person who I am building the amplifier for has indicated his dismay at the power switch. Something with a chrome toggle is preferred. Finding something that will a) fit without fouling the adjacent EL84 socket and b) fit the existing 20mm hole, is going to be a challenge.

Secondly, on the right hand side, the hole in the front is for the remote control sensor. This needs to be permanently fixed in place internally and a piece of translucent white acrylic press-fitted into the hole.

Electronically I need also to arrange a mute circuit to avoid switching spikes going through to the speakers when toggling the speakers/headphones switch, and to avoid the open-circuit hum going through to the speakers when the switch is set to headphone (thus disconnecting the input to the preamp)

First order of business though: cure the ground-loop hum on the phono stage...

Saturday, 3 March 2018

EL84 Amp II: Progress Post 5 – Confessions!

It's been a little while since I posted an update on this project, and there's been a lot of progress, as well as one or two hiccups.

Thought I'd put up a few photos today since I've been taking plenty.

First up - I've had a few requests from people (offline, as well as on) for some photos of the PCB fabrication process. Since I took photos during the etching process of the power supply PCB, I submit for the admiration of sev'ral viewers [anyone get the obscure reference?] the PCB etching process in stages...


Etching is just starting



Copper exposed to the UV light is being slowly etched away


A few more minutes and it's nearly done


Finished! Ready for washing and drilling


The completed Power Supply board was already shown in the previous post, so these photos are a trip back in time *by popular request*

I did make up another little board though, by necessity. The motorised remote control volume control board I bought from AliExpress didn't work, so I had to buy another one, only this was a different type, and of course it needed a different voltage, that I didn't have to hand. 

I needed 9V DC for that board, which I also needed for the signal relay which switches the signal to the headphone stage. So necessity being the mother of invention, this was the result


5vac in - 9V DC out. 

A voltage doubler and regulator board. A couple of diodes, some capacitors, and a 7809 regulator. Plus a switch (on the other side) and on a board that's smaller than a SD card. 

So – on with the amplifier. 

First order of business was to get the top panel of the chassis ready. This involved a lot of measuring and drilling - the mounting holes for the boards and transformers, then the chassis punch for the valve sockets. A lot of swarf ended up on the floor during this process.



After getting the top panel ready, it needed to go to the laser etching workshop before I could do anything with it. This is to get the identifiers for the valves etched on - this design uses four different types, so it's important to know which type goes where!

Once that was back, it was time to begin assembly. Mounting up the transformers and sockets to the top, and circuit boards underneath. A delightful jigsaw puzzle, but everything fit together nicely and it was not necessary to utter any curses.


Transformers and output valve sockets in place



Starting to assemble the business end



All boards in place, ready for wiring up


Next the back panel needed drilling - this design will have four sets of inputs – three line-level and one phono, with the necessary separate earthing point. Also the speaker terminals will expose 4Ohm and 8Ohm outputs, and of course the standard IEC Mains connector.

Everything was mounted onto the back panel, just to make sure it all looked OK and didn't foul anything inside the case when in place (it didn't, so again, no cursing necessary!)

Then it was a case of removing all the terminals and sending the naked panel off to the laser engraver to get the descriptions and other vital pieces of information added to it.


The back panel, before laser engraving


Then, just because it would be remiss not to, it was time for a photo session


Front panel legend - this is a valid design technique and don't let anyone tell you otherwise!






Showing the bias adjusters and test points for the output valves (same design as the last EL84 amp I made)


So at the top of this post I mentioned one or two hiccups. This firmly comes under the category of "learning from mistakes". Those who are more experienced at this may choose to laugh at my misery if they are of a vindictive nature, or sympathise if they are more empathetic... but I screwed up the low-voltage side of this amplifier rather badly and it's going to need a rather ugly (and obvious) rescue.

I will disclose my thinking and why it didn't work here, in the hopes it might help someone.

Warning: There are no more pictures, and it gets a bit technical from here on.

So going by the previous pictures you will see there is a lot of glass here – 13 valves to be exact. This is because of the configuration - a RIAA stage, Tone control, headphone stage, and push-pull output.

Long story short, the amount of 6.3 volt needed exceeded the rating of the transformer. Plus, in a RIAA stage, it is preferable to run the heaters on DC. Handily, the transformer I am using (Hammond 370FX) has a 5v winding. So I thought I could run the RIAA stage off an arrangement like this - the capacitor is 47000µF













Then, to relieve the load on the 6.3v I thought it might be possible to run 3 more filaments from this arrangement, for a total of 5 12A*7 tubes running on DC heaters.

Sadly this arrangement was not suitable – the DC voltage dropped to 5.5V which is too low to run filaments on, plus that diode was running rather warm, to make an understatement.

If I removed all load from the DC except the RIAA stage, the DC voltage was closer to 6.1 which is tolerable. But this leaves me with the following problem:












The transformer is rated at 5A on the 6.3V winding. So, my original plan was to use the DC to take about 900mA of that load. Alas this plan did not work so now I need to find around 1000mA of 6.3v ac from somewhere.

Worse, I also discovered that the tracks on the PCB I'd set up for the 6.3vac were not thick enough for the 5A load. They were dropping around 0.44v which at 5A equates to 2.2 watts of heat.

Having PCB tracks dissipating 2.2 watts of heat is a Very Bad Thing.

Clearly some thought and remedial action required!

The initial idea - swiftly dismissed - was to re-make the PCB with larger tracks. However in the end I opted to rework the connection to the EL84s so that they don't go through the PCB. That will save the board from burning up.

Only problem is that it will look ugly. It will work fine but I am not pleased.

Second problem. Where's that extra 1000mA of 6.3v going to come from?

Only one possible solution. A second, smaller, filament transformer.

As luck will have it, a transformer rated at 6.3v 1000mA was sitting in my box of spare parts. And with a small amount of re-shuffling, will fit inside the chassis.

However. It will add weight, and it will forever bear testimony to the error made in the calculation. So I am doubly not pleased.

However, the happy ending to this awful debacle is that it doesn't set the schedule back too far, and the amplifier will work completely as intended, and I do not need to re-make the board. So not a total disaster.

But definitely some lessons learned for the next project.

Here endeth the confession. 

Ending on a good note

Before the Great Filament Supply Disaster of 2018, I had the output stage and the amp board running and the B+ and other HT voltages were exactly where I planned them to be (so my high voltage design is fine, just the low voltage stuff I messed up!) and the amp was running with a signal source (=old iPod) connected, through a small pair of speakers. It sounded great, and the quick measurements through the oscilloscope with the function generator showed the response and power exactly where they were supposed to be. And there was no hum!

More later when the panels are back and more building is completed...