Wednesday, 28 December 2016

Name plate attached

So the courier brought my name plate this morning, I fastened it to the amp and it looks as good as I had hoped:
Looking good with the name plate in place. The image was made by tracing around outlines of photos of the valves in Photoshop. 

Not sure I'm too happy with the placement of the LEDs for the input indicators, but too late to do anything about that now.


While I had the amp off the cabinet to screw on the brass plate, I also decided to take a photo of the rear panel, since for some reason I hadn't done that yet.

The business end. 4-Ohm  and 8-Ohm taps for the output. USB power for the Chromecast. Switched mains output for the separate phono preamp.


Remaining to be done now - just the 3D-printed cover for the power transformer.

The amplifier has been delighting us with its performance and power; despite its tendency to run rather warm, it seems entirely reliable. Its longest runtime so far has been around 6 hours, after which I looked inside carefully to see if there were any signs of thermal distress. Happily, there were none.

Current through the KT88s are stable - I've biased it at 28W idle disspiation per valve.

The amp is driving the big 4-Ohm KEF floorstanding speakers with commanding power and finesse at the same time. Noise level (hiss) is negligible, slight hum is audible with ear glued to speaker but 1m away is inaudible. Clarity and separation of the signal is excellent, bass is strong and authoritative and in no way flabby, nothing dominates at the expense of anything else, and when you need the power, there's ample headroom. We've had the walls rattling with this thing already.

Sadly the mains transformer is physically a bit noisy though, hopefully the cover will muffle that a bit. When the music is playing you don't hear it anyway. 

The transformer slowly gets warmer and warmer as well. Haven't had it too hot to touch, but definitely on the warm side. 

The remote control and input switch and remote power switching all work well.

I am going to call this project a success.

On to designing the next one....

Saturday, 17 December 2016

Important question... Name for the amplifier

So far I haven't discussed the name for the amplifier, I've batted a couple of ideas around but finally I can now reveal the name I have chosen.

The delay was caused by me designing the graphic for the brass nameplate that will adorn the front of the amplifier (the area in the centre within the black screws on the front).

Having completed the artwork, I can now attach this



This is in the Maori language of New Zealand. Titiwai are glow-worms, a natural occurrence in many of the caves around New Zealand. These sites are all sacred to the Maori people. One of the glow worm sites, the Waitomo Caves, are one of New Zealand's premier tourist attractions.

Waru means Eight. So the name for this amplifier translates directly as "Eight Glow-worms"


The design is with the engraver, brass plate pending

Friday, 16 December 2016

Uh-oh... first teething problem

So after finishing the build and moving the amplifier to the living room to use, things were going very well. One reliability issue came up fairly early... a low-level but consistent crackling, ticking through the speakers. At first I thought this was due to power supply interference, but it persisted after I installed a mains filter.

Then I started gently tapping the output valves. Bingo! Second KT88 on the left channel, slight wiggle in its seat and the problem immediately stopped.

I remember during initial power-on that two of the valves were drawing no current whatsoever, which I rectified by replacing the contacts in the valve seats from some donor ones.

This particular valve seat didn't receive donor parts, so it's going to be worth keeping an eye on it in future. 

For now, though, the problem has gone away. But what goes away by itself can come back by itself too. I can see valve seat contact replacement being necessary in the future.


Second problem was a little more dramatic. After the first run, in which I had the amplifier running for a couple of hours, I was happy with the reliability. The next day I ran it for about six hours, and apart from getting rather warm (to use a pleasant euphemism) there were no other issues and the bias voltages remained stable. 

Then I turned it off and left it to cool for a couple of hours, before lifting it off the cabinet and opening the bottom, to take a look at whether anything was showing any signs of thermal distress. No evidence to be found, so yet again I was happy.

Then I plugged it back in again and powered it up. After the 30sec delay the HT came on, to be accompanied by a loud hum from the right speaker and the mains transformer, and a very unhealthy blue glow from one of the output valves on the right side.

Uh-oh!

Having a fairly good idea what caused this, I quickly removed it from service and had the bottom off again, and started probing with the meter, long story short my hunch was correct and the valve in question was for some reason not getting its bias voltage any more.

C'est un Catastrophe!

So a bit of probing with the multimeter quickly revealed the problem. Three days ago, when I replaced the resistors in the bias section on the power supply board, I'd made a poor solder joint. The repeated thermal expansion and contraction had stressed it and it had let go.

So I quickly removed the board, took it to the workbench, re-soldered it and tested, then replaced it - the work of five minutes. Re-assembled and tested and working happily once more.

Hopefully the one and only fireworks incident!

Thursday, 15 December 2016

(Almost) the end of the road

With the arrival in the mail today of the Emergency replacement KT88 output valve to replace the one with the internal short, we now have a milestone: a working amplifier

The amp has been sealed up and moved from the workbench to the living room, and connected to the big floorstanding KEF speakers, wired up and biased up, and some listening tests have been successfully conducted.

The moment I've been looking forward to for months, since I started this project.

What did the listening tests reveal??

First, this amp has plenty of power. The volume control hasn't been advanced beyond about 20% and at that level I was anticipating howls of protest from the Teenager upstairs when the parents' ancient music invaded through her headphones. Such howls were not forthcoming however, but I did keep the high-volume tests to a fairly brief period in the interests of domestic relations.

Second. There's a bit of hum, as completely expected with valves. If you put your ear up to the speaker you can hear it. If you stand about a metre away you can't. I am happy enough with that.

Third. The mains transformer makes a bit of physical hum... I had already noticed this when I loaded it up on the test bench. This is by far more more noticeable than the hum from the speakers.

Fourth. The sound quality is exactly what I was hoping for. In an hour of burn-in listening we were delighted with its imaging and separation and clarity.

Fifth. These KT88s get HOT! They're biased up to 28W idle disspiation and they will readily scorch unsuspecting fingers. The rest of the chassis gets warm, but nowhere near too hot to touch.


The build process is not quite complete however. The brass nameplate is the next item on the agenda, that will be in the centre under the row of indicator lights. Also I might make a few aesthetic changes yet, and the 3D-printed cover for the butt-ugly mains transformer still needs to be designed and made.

But for now, we have an extremely powerful yet easy-on-the-ear home designed and hand-assembled amp to listen to. Win!

A few photos of the usable (I won't say finished yet, until the aesthetics are completed) product:

The unmistakable warm glow of valves 

The mains transformer is the rudest part of this amplifier. It looks ugly and it hums.

Looking pretty :) The orange glow reflected on the turntable is in fact coming from the phono preamp, also valve, on the other side of the turntable

Wednesday, 14 December 2016

Final photos of the internals

Progress since last posting
  • Tidy-up of the internals
  • Final placement of LEDs and IR Remote sensor
  • Removed PSU board and replaced 1K8 bias resistors with 2K7 after testing to determine optimum value

Testing
  • Power into Dummy load reliably produces 80W at 1% THD on the 4Ohm tap (where I'll be using it)
  • Freq. response -3dB point at 46kHz

My new KT88 is in the mail to replace the dead one. Soon as that's arrived, it's being installed, final test before sealing up the case, photo shoot, then moving it to the living room where it will be connected to the speakers, biased up, and some listening can start.

These are the final photos of the inside with building complete. Click to see larger 

After a bit of tidying

The front-end circuitry: Initial gain, inverter, driver

Bias adjustment system (and using some space on the tagstrip for the feedback resistor too, spanning each end. That's the one with the heatshrink on its leads)

Side view of internals

Other side


Power supply board. At top left is the delay circuit consisting of a rectifier off the heater supply, 5V regulator, 555 IC with 30 sec delay, and the two relays which switch either side of the AC secondary to the HT rectifiers.

Still waiting to be designed: the brass nameplate. My next task...

Monday, 12 December 2016

Circuit as built and tested

As the build phase of the amp is completed, this seems a good opportunity to publish revision 13 of the circuit schematic. 13 seems to be the lucky number in this case as this is the "as-built" version, complete with reference voltages as tested.
Initial testing (with a loudspeaker as well as an oscilloscope) shows this working well.

The resistors in the bias section may perhaps need changing later, if I can't achieve 50mA quiescent through the KT88s with the resistors I've got. This would be a case of increasing the value of R26 and R27.

Other thing this schematic does not show is the delay turn on for the HT with the 555 timer IC. I'll get around to drawing that later.

This schematic is for the amp proper,

Note the power supply the diodes are paired in series, this is because PSUD2 warns the PIR voltage across them would be exceeded - albeit only slightly. So I decided to play safe and pair them, with balancing capacitors.

Note also the main electrolytics for the power supply are connected in series, not a usual configuration. This is because these caps are rated at 450V and my B+ is 560V.

So they are series-connected with balancing resistors. Voltmeter tests show the balancing resistors all have the same voltage across them: 280V plus or minus a couple of volts. So they are doing their job.

So this is a fairly conventional Williamson design with KT88 finals in ultralinear configuration. Gain and inverter are provided by a 12AX7, driver stage is a 12AU7. I have not used a CCS.

Click as necessary to see full size

As a passing comment, I was derided for choosing an excessively ambitious design as my first valve amp design and build. I took that as a challenge, rather than the discouragement it was intended as. I'd even considered naming this amp the "Defiance" in reference to it, but instead I have called it "Waru Titiwai" which is a Maori phrase (Maori are the indigenous people of New Zealand in case you didn't know) which means "Eight glow-worms"

New Zealand is full of glow worm caves, some of which are tourist attractions, others more out-of-the-way. All are sacred places to Maori.

Apart from possibly changing the values of the resistors in the bias section, I don't anticipate any further circuit changes.

[EDIT] This schematic is out of date, as I've subsequently fine-tuned the NFB which resulted in some changes, there's a current schematic here

Sunday, 11 December 2016

Milestone: Working Amplifier (kind of)

Today I have finished the build phase of the amplifier, and it's working... kind of.

There's a problem however – one of my KT88 output valves is no good. It has a short-circuit between the control grid and the cathode, which only manifests when the valve is heated.

So this means I have three functioning output valves. So until I can source a replacement (and these things should really be bought as a set) then I can not use this amplifier other than one-sided.

This wasn't the only problem I hit during the construction of the output stages either. It turns out my valve sockets for the output valves were a bit less than ideal... when powering up, two of them were drawing no current at all. Some of the pins were not connecting properly. This necessitated procuring and installing replacements, with all the attendant re-soldering and swearing that entailed.

However, each channel now works perfectly with a functioning pair of valves. Watch as I power the amp up, feed some signal through it and listen to it through a tiny little monitor speaker. Then stick around for a tour of the internals


After this successful test I re-connected the dummy load and connected the output to the oscilloscope and ran the signal generator into the input. Then I increased the volume until THD reached 1%

The result is that this amplifier will deliver 80 watts into a 4 ohm dummy load at 1% THD. I am very happy with this result.

A few photos of the completed internals

Insides all completed now. This has taken a long while to get this far!

Preamp stage all tidied up and with the negative feedback in place

Interior looking tidy(ish)

Close-up of the final stage and bias adjustment circuit

Worth mentioning one item was left to experimentation - the negative feedback. By luck I got the phase correct the first time around, and I used a potentiometer to determine the optimum level of NFB, while watching on the oscilloscope. The correct point became very clear in short order: too low resistance and the feedback was visible. Too high and noise waveforms were visible. Just right and neither were present. That Goldilocks point came at 100K, so that's the value of the NFB resistors I used.

There's still some work to complete before the amp is completely finished... specifically, the input selection LEDs and remote sensor need to be mounted properly, the way they're set up at the moment is a bit rude.

Also, there's going to be a brass nameplate on the front, that needs to be designed first. Once these two parts are done, the amp can go the right way up and migrate to the living room

But first, before any of these steps, replacement output valves are needed!





Wednesday, 7 December 2016

Construction, Part 3

A good amount of progress has been made since the last update:

  • The output transformers have been mounted
  • Output transformers secondaries wired up to speaker terminals
  • The volume / input selector / remote power-on switch has been wired up to the standby transformer
  • The USB power supply has been wired up to the standby transformer
  • The Bias pots have been mounted to the chassis
  • The driver valves (2 x 12AU7) have been wired up and tested


The books I've been reading (mainly by Morgan Jones) advocate a big-bang approach to first switch-on - complete with an unintentionally hilarious guide to the smells you may encounter if something goes wrong:



Contrary to this advice, I've been powering up the circuit in stages, as each is built, I'll add dummy loads to sub out the following unbuilt stages, then apply power and check voltages and signal levels on the oscilloscope.

So far there has been no drama - for which I credit the prep work I did in building the test rig. When I powered up the circuit today to check the 12AU7s, everything just worked... voltages, signal levels - all exactly as per my test rig.

The only real difference, other than there being twice as many valves (two channels... this little thing called STEREO...) is that I'm trying – as best I can – to make the wiring tidier than the haphazard mess that was the test rig.

So - today's photos:

Preamp section is complete, the coupling capacitors can be easily seen

Note component leads bent at right-angles wherever possible, and heatshrink to avoid accidental shorting

Next step will be to build the output stages - wire up the bias supply and the KT88 sockets, then power it up and see what sort of current we're pulling through them. That will be an exciting day because – unless the amp goes into crazy oscillation (always a risk with the Williamson topology) that will be the day I could theoretically put a signal through it and connect it to a pair of speakers.

I've decided I'll get a set of cheap disposable speakers from the auction site for the first actual test, I don't want to risk this thing going unstable and pummelling my beloved KEFs with the best efforts of an angry pair of KT88s driven at 560V plate voltage.

Stay tuned... 

Saturday, 3 December 2016

Construction, Part 2

Owing to other commitments that can't be avoided, at the moment I can only snatch 15 minutes here and there to work on the amplifier. Frustrating but that's life. Each 15-minute stretch is divided up into two halves. The first half is spend pondering where to put things and how to make everything fit and look as elegant as possible.

The second half is spent trying to make that happen. A Master Craftsman I am not, so it often happens that something is 2mm too long or too short, necessitating rework.

However, despite the time constraints, there's some progress. A couple of phone pics because I didn't have time to drag the SLR out and do it properly, but since the phone has a Leica camera on it, I think phone pics are sufficient

Trying to be as tidy with the wiring as possible. Using solid (single) core since this stuff is bendable and stays where you put it


Trying to be as tidy as possible with the component leads. My needle-nose pliers make the right-angle bends in the resistor leads easy


As construction progresses, it will become apparent whether my chassis design is too cramped, or well thought out. So far I haven't hit any major issues, putting all the power supply components onto a PCB has certainly been convenient. I am really glad I went through the learning curve of how to do that.

There's a good reason I want this amp to be as tidy as possible: apart from being somewhat of a perfectionist, this has a practical aspect: It's likely this will not be the last amp I build. In fast I already have another that I've been commissioned to build after this one, so this one has to sell my amp-building abilities. 

Friday, 2 December 2016

Construction , part 1

No intelligent thoughts or ramblings or decisions in this post, just a few photos of the construction process

Internals. Heaters all wired up, taking care to ensure the same phase of each is connected to the same terminals on the output valves, thus cancelling any heater-induced hum. Tip from Morgan Jones. Power Supply board is my own creation, see earlier post. Small mains transformer is for the standby circuit (remote control) plus it'll power the USB supply for the Chromecast on the back panel 

Cathode earthing for the output valves. These will be running Fixed bias, so the 10Ohm resistor is for measuring anode current


The top of the chassis. Featuring the world's ugliest mains transformer (eagerly awaiting a decorative 3D-printed cover). Note bias test points behind the Octal sockets


Looking pretty with all the glassware installed and glowing

These KT88s have a fairly discreet glow, no great exposed heater piping here

Easily the most beautiful electronic thing I have ever constructed

My horrible small messy cramped workbench. The computer doubles as my oscilloscope. Note amplifier is doing a headstand, the mains transformer is so heavy that it's quite stable sitting like this.

Monday, 28 November 2016

Home stretch

Today's entry marks the beginning of the home stretch: the chassis is finished and the assembly work has begun.

I estimate I'm going to need two to three full days to complete the assembly, likely including a few trips to the electronics store to collect various fasteners and such like, before the job is complete and testing can begin.

Just to satisfy myself that I'd done something, I began assembling a few components onto the chassis, pleased to say everything seems to fit; the holes all line up and are the right size (so far!)

This amplifier will contain TWO mains transformers: the large main transformer, and a smaller one to provide 9v AC which will be the standby power. This transformer will power the input selector, remote volume control and standby control, and the chromecast USB power supply.

Having two transformers allows the provision of the full remote control, including power switch.

So the top plate of the chassis will need to support the mains transformers and the output transformers. It's made of 4mm aluminium and it seems equal to the task without bending, based on some benchtop tests. Time will tell if it starts to sag in use

More pictures and progress reports as assembly continues. Some quick phone snapshots for now.

Power Supply, USB supply, rear terminals, volume control and rotary encoder in place


The back panel. Note switched power output – this will run to the separate (valve) phono preamp.
Outputs will be 4 and 8 Ohm.
All these separate panels are gonna need some heavy-duty earthing!

Tuesday, 22 November 2016

Lots of holes

Other than the occasional messing around with bias resistors and measuring signal levels on the test rig, not much progress has been made at the workbench. This is because I've instead been designing the chassis, which needs to be completed before any further construction can proceed.

So - after a bit of research I decided on LibreCAD as my design software of choice. It has one (and only one) redeeming feature – its price: free. (As in speech).

Apart from that, this software is a nightmare. Everything works to about 60% and then you have to work around its shortcomings. For a complete CAD virgin such as myself, this proved somewhat challenging. A few times I came close to abandoning the process and instead cutting out bits of paper and re-arranging them on a sheet to simulate the layout.

However, I persevered, and eventually I had some artwork that I was able to send away to be laser cut. This was an exercise in spatial thinking since I was effectively working on the top and bottom side of the top panel simultaneously. A jigsaw puzzle which took some consideration before everything worked.

After the artwork was sent to the laser cutter, a prototype chassis was cut into MDF. Today I have collected this:






Already I have around 10 changes I want to make before the final chassis is cut into 4mm aluminium. This prototype has admirably served its purpose.

Next steps will be actual construction, when the final chassis is completed.

Monday, 31 October 2016

Since we're building power supplies...

Following the success of the main power supply for the amp, I decided to improve the USB power supply I'd previously built on a piece of stripboard. The idea of the USB supply is to power the Audio Chromecast which will be feeding into this amplifier. Thus saving the need for a wall-wart.

What's needed is an isolated regulated smooth 5V DC power supply – so a simple circuit powered by the heater AC, containing a rectifier, some capacitors, and a voltage regulator, is all that's needed.

So utilising my newly-acquired PCB design skills, I decided to make my own PCB for this and build the circuit improving on the previous effort. This is the result (the SD card is just a scale prop, since the photos of the previous supply didn't have one I thought it might be a good idea this time)

A bit smaller than the last one.

Tested and working quite happily, it powers the Chromecast and the DC is very smooth.

This is the PCB for it:

Actual size 45 x 35 mm


And changing the subject completely, I thought it might be a good idea to publish a new circuit diagram, since the previous one on here is well out of date by now.
You'll need to click this to see full-size

The circuit diagram however does not show the delay circuit I built with the 555 timer and the relays, so the power supply here is "conceptual" rather than as-built. The relays switch the AC secondaries going into the rectifiers.

Now, the desk-bound work continues - designing the layout of the top, front and back panels, for the CNC machine to cut out.

More later...

Wednesday, 26 October 2016

Testing the power supply

So I put the power supply in a test chassis and wired it up, this video shows the first switch-on.

Did it work? Was there smoke or fireworks? Watch and see...


Power Supply board, Part 2

After the previous less-than-completely-successful circuit board fabrication effort, we now have a board that is 100% correct. I am using the "press 'n peel" method of printing the layout onto a plastic sheet then using a hot iron to transfer the design onto the copper side of the PCB, before then putting the PCB into the Ammonium Persulphate etchant to dissolve away all the copper not covered by the resist (=transferred printer toner)

The transfer process was time-consuming and not particularly effective, it required multiple passes with the iron and then careful peeling off to see how much of the resist was transferred before then carefully re-ironing.... rinse and repeat several times until it seemed no more resist would be transferred. Then it was a case of filling in the missing parts with a sharpie, fortunately not much was needed.

From there it was into the chemical bath for 10 mins being constantly agitated until all the remaining copper was dissolved off.

Then... drilling, cleaning, then stuffing and soldering.

The stuffing and soldering took around 2 hours, this included testing the delay circuit to make sure the timer circuit (my design) was working. Happily it was.

The timer circuit takes the 6.3v AC heater supply, feeds it through a bridge and then into a 5V regulator and then to a 555 Timer IC with suitable component values to give around a 30sec trigger delay before closing the relays and applying the high-voltage AC to the rectifier diodes. This is to give the valves a chance to heat up and be ready to draw current before any HT is applied.

This is the completed supply:





On this board we have:
  • Timer delay circuit including relays
  • Rectifier (Diodes in series because of peak inverse voltage exceedance) plus balancing capacitors
  • Main capacitors (4x, in series-parallel configuration, with balancing resistors)
  • Separate supply for input valves (x2, 300V)
  • Separate supply for Driver valves (x2, 360V)
  • Separate supply for Negative bias voltage for output valves (X4, -80V)

Next step: testing

Monday, 24 October 2016

Power Supply board, part 1

Following on from my previous post about the power supply board, I made a number of improvements to the layout since the initial design. Also I completed it by adding the bias supply components.

Confident I had something I was ready to produce, I went through the process and learning curve of making a PCB. This resulted in some difficulty transferring the design successfully from the plastic film it was printed on, to the board ready to etch.

However a few trials and errors and finally I had this ready to start stuffing, after carefully etching and drilling:

Ain't it beautiful?

The first usable PCB I ever made, or at least, almost usable... 

On this board is a galactically stupid n00b mistake which renders it completely useless: I'd neglected to mirror-image the pinout of the 555 IC I'm using as a start-delay. So in order to get this working I'd need to mount the IC to the underside (ie. solder-side) of the board.

Not happy with that at all. 

So I've corrected the design and it's back to the electronics shop, can't go tomorrow as it's a public holiday, so now need to curse my naivety until Tuesday when I can get another board and transfer the design onto it one more time...

THEN I will have a power supply board that works...

However this experience has taught me a few useful things. Firstly, the heat transfer method of getting an image from a laser printer onto a board is a bit hit-and-miss, but I found polishing the board with Brasso first then roughing it up with steel wool helped.

Secondly, despite this board having 110 holes (or thereabouts) the drilling process did not take long, the drill stand makes it easy.

Thirdly, sharpies are magic for filling little holes in the resist prior to etching the board.

Tuesday, 18 October 2016

The volume control that comes with unintended consequences

So I finally got my hands on a decent-ish signal generator. A few online reviews of this device haven't been particularly kind to it, but the shortcomings discovered are well outside the range that I'll be using it at anyway. For my purposes it's 5-stars excellent. 

This is the device, an AliExpress-special, it's called a "MHS-5200A":


It hits all the high notes the previous two signal generators I was using couldn't... so today I was able to test my response out to 100kHz.

Which revealed something extremely interesting... my volume control, which is simply an attenuator on the input, is having an effect on frequency response.

Not entirely surprising I suppose, given that the combined effect of the volume control and the capacitance of the valve would form an RC network.

So to recap, this is the relevant section of the circuit:


With a sig generator that will easily give 100kHz (and more) and at a nice 200mV RMS (which the previous couldn't) it means I can test the frequency response with the attenuator at maximum (equivalent to maximum volume on a finished amplifier) as well as with the signal being attenuated.

The test was taken from the output of the driver stage - in other words, the signal went in the initial gain stage, through the concertina, and then through the driver, before going into a X100 probe and then to the oscilloscope.

The result was interesting... and confirmed my suspicions. Volume controls come with hidden extra features. Check freq response graph below... I tested at 5kHz intervals from 1kHz up to 100kHz, so this graph doesn't contain any extrapolations.


So the green trace is the freq response out to 100kHz with the volume control set to maximum and a 200mV RMS input. I had a probe on the input as well and my spreadsheet normalized for slight variation in signal voltage on the input (verified as being the same whether connected to the amp or running open-circuit)

The red trace shows the response with the volume control at -10dB... so the signal is going through some carbon track.

Probably not enough to make an audible difference, but interesting and informative.

I'm also quite rather pleased that my naive circuit built using valves that are everyday quality rather than audiophile spec, managed to turn in such a good response with zero attenuation dialled in.

Saturday, 15 October 2016

Additional measurements, power supply design

In the last couple of weeks I've spent a lot of time agonizing over how on Earth I am going to make a chassis for this project. I have zero sheet metal skills and very little equipment. I vacillate between buying a ready made chassis from AliExpress and somehow cutting all the holes I need in it, to ambitiously planning a magnificent construction with corner braces and a polished aluminium top plate.

Fair to say this is not resolved yet and will be the main factor holding me back now.

However on a happier note, I've decided that the power supply circuitry will be going onto a PCB. This is necessitated designing a layout, which I have been doing completely the old way - manually. Measuring component footprints and putting the pads on manually.

Those who are more seasoned at such things will observe my technique and doubtless recoil in horror in much the same way you might when you see Grandma transcribing an email because she doesn't know about copy-and-paste. However with nobody to get me started or show me how to do this, I've resorted to the electronic version of essentially manual sketching.

So on this power supply board - which is 150mm X 150mm (and once again seasoned professionals at this type of work could probably fit all this onto a board quarter the size!) will be the rectifiers and main capacitors (4 of) plus the voltage droppers and capacitors for the 4 input valves (one each) plus the output valves bias supply. As well, there is a delay circuit consisting of a 555 timer IC and the necessary discrete electronics, to switch on the HT supply (on the AC side) after approx. 30sec warm-up time for the valves.

This circuit will be fed with a rectified and regulated 5V DC sourced from the heater supply. An LM2940 5V regulator will be used, solid state is allowed in this amplifier, just not in the signal path!

The PCB layout isn't quite finished yet as I still need to add the bias supply. Those of a professional disposition may wish to avert the eyes at this point. This is what the layout looks like so far:


As you may deduce, this is the first PCB I have ever designed

If anyone's got a genuine "Warning - if you build that, then XYZ will happen!" to contribute, please let me know. But if you just want to criticize how crap my design is, kindly please don't. As long as it works, I will be happy, and I am not mass-producing these things. 

I will post photos of this board from both sides once I've stuffed it.

Yes the main caps are in series, this is intended, yes I know that series connecting electrolytic caps is weird. There will be balancing resistors across them... it's just that the caps are 450V rated and my B+ is 560V.

Moving on, I have also sourced a signal generator that goes to 65 kHz, the previous one I was using had a max frequency of 20kHz. So I have made some more measurements of the test rig on the bench - this is the input, splitter and driver stage. The -3dB point measures at around 50kHz. With the cheerful assistance of my daughter as a lab assistant entering numbers into the spreadsheet that I called out as I made measurements, this is the response I am getting:


Which I am also reasonably happy with.

In other news
 - the KT88 output valves have arrived
 - I am still waiting for the remote volume/input switcher from AliExpress
 - and I still haven't figured out what to do about the chassis!



Friday, 30 September 2016

Substituting the output stage load

So far I've built the gain, splitter and driver stages on my test rig. While I am well pleased with the results, this has been with a higher B+ voltage than intended. As a consequence, I've had to use higher resistors than planned to get my HT+ voltages right for the valves.

Part of the reason for my high voltage is that I don't as yet have the output stages - each of the output valves will be biased to a quiescent current of around 50mA... representing a 200mA load on the B+ which I would expect to drop the voltage by some amount. 

Question is.... will the extent of that voltage drop require me to change the values of my dropping resistors?

Without the output stage present, my thoughts turned to constructing a dummy load, something that I could sink around 200mA at 550V DC into.

A few calculations later and I concluded that three 60W lightbulbs wired in Series would do the trick. So construction of the world's ugliest dummy load was commenced... this is the result:

We have tamed the B+ to 565V which is right where I want it.


In place and power applied. Some nice voltages flying around on my bench today

Just to make sure my calculations weren't completely wrong, I wired the bulbs in series with a 10ohm resistor. Which is dropping 2.25V across it, meaning that the lights are drawing 225mA

So about 25mA more than I'd planned... I can live with that. Valve electronics is not always an exact science.

So what did this do to my voltage? It took around 15-20V out of the supply. Perfect.

I changed my supply resistor to the 12AX7 from 270K to 220K and I will change the 12AU7 supply resistor to 27K from 30K as soon as I have a 27K 5W resistor... yep... back to RS for more parts!