Monday, 17 July 2017

Fine tuning the big amplifier

So I've lived with the big KT88 amplifier for the last few months and have been mostly happy with it, though there were some niggles that I resolved to get around to. 

Specifically, the amp ran hot, and needed more ventilation holes drilled. 

Also, there wasn't enough Negative Feedback (NFB). I wasn't too worried about this until I built the EL84 amp featured elsewhere on this blog, which had more NFB. On hearing the difference, I resolved to correct the situation in the big amplifier, but this would need some equipment I didn't have.

So, first up was a shopping trip online to get some power metal film resistors for a dummy load, it's very important to have a non-inductive load for tuning NFB. These were duly mounted to a large heatsink.

Next I needed a pair of old-style variable capacitors, the kind you'll find in an old valve radio. eBay to the rescue, and these eventually came all the way from Bulgaria.

The method I intended to use for fine tuning the NFB was from Morgan Jones "Building Valve Amplifiers" p.290-291. 

Today, I managed to get the amp back onto the workbench. Pulled out all the valves and attacked it with the drill, to make some new ventilation holes. Problem 1 fixed, and it remains to leave the amp on for several hours to determine its effectiveness.

Problem 2 was also resolved, though this was a good deal more time-consuming. Utilising Morgan Jones' method, and armed with a healthy stock of film capacitors of various values, I started making the necessary modifications to the circuit, first with potentiometers and variable capacitors, before subbing in fixed components.

First order of business was to reduce the NFB resistor from the (fairly useless) 100K to something lower. After experimenting with the input sensitivity, I dropped this to 33K.

This got the amplifier's gain to where it needed to be, and eliminated the problem of the very touchy volume control.
It did however introduce another very serious problem: high-frequency ringing. The Williamson design is prone to this, and adding NFB in any quantity will exacerbate it. 

This was the result (red trace) at the speaker terminals of dropping a 10kHz square wave into the amp, after reducing the NFB resistor from 100K to 33K

Yeah :(

Don't know about you but I don't want to listen to that amplifier like that. Apart from anything else, it'll set all the dogs in the neighbourhood howling. And things will be getting mighty hot with all that high frequency energy to dissipate.

So clearly some compensation needed.

So watching the trace on the scope, using Morgan Jones' method, I arrived at these changes to the circuit:

Added compensating capacitor and resistor parallel to the anode resistor in the first gain stage

Dropped feedback resistor from 100K to 33K
Added compensating capacitor and resistor parallel to feedback resistor

This was the result:

Speaker trace in red

Yeah, I forgot to clip the CH1 probe back on to the input. No matter; it's the same signal at the same amplitude.

So far so good, this is all textbook from Morgan Jones. However in the course of my experiments I discovered something else that Morgan Jones apparently neglected to mention which I pass on here in the hopes that it may help someone.

Specifically. Jones' method calls for the feedback resistor to be bypassed by a variable capacitor and resistor, which I did, and I noted that the resultant waveform at the speakers looked pretty much like the above already. 

Thinking I wouldn't end up needing anything bypassing the anode resistor, I acted on Jones' recommendation and put a 220nF capacitor across the speaker terminals as a test, and watched the output go absolutely crazy. It looked much worse than the amp with no correction at all, and in fact was on the very edge of falling over into uncontrolled oscillation.

I then decided to bypass the anode resistor in the manner suggested, this resulted in some fine tuning of values as these are all inter-dependent. Eventually I got it to approximately the same level of cleanliness on the output as I'd seen with just the feedback resistor bypassed,

Then I tried the capacitor-across-the-output trick again.

Result: The amplifier barely even noticed the capacitor. A complete fix of the problem :)

Conclusion: the anode load resistor bypass doesn't do much to alter the oscillation into a resistive load, but it makes the amplifier much more stable into a capacitive load.

Morgan Jones did not mention this anywhere I could find.

So for reference this is the circuit diagram of the amplifier now (click to see full size)

The power supply implements the timer circuit which is not shown here for clarity, refer the circuit diagram of the EL84 amplifier on this blog for details on that.

This is the last modification or fine tuning I expect to make to this amplifier.


Morgan Jones "Building Valve Amplifiers", Newnes press, 4th. Edition. pp. 290-291

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