Bevois Valley EL84 PP - distortion in one channel - diyAudio
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Old 11th March 2013, 04:58 AM   #1
MrSnrub is offline MrSnrub  Australia
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Default Bevois Valley EL84 PP - distortion in one channel

Hello all,

I've just finished building a Bevois Valley amp from Morgan Jones' book, and have run into an issue with distortion in the right channel.

The DC operating points all checked out OK, after which I put an 8.2ohm 10W dummy load resistor across each channel's output and applied a 2Vp-p 1kHz sine wave on both channels to test them out.

With a scope on the left channel's output, everything looks fine - the sine wave is undistorted and around 12Vp-p. But when I scope the right channel's output, I see a distorted, attenuated sine wave - more like a triangle wave with the peaks clipped.

When I scope the right channel's input at the RCA input jack, I see the same distortion (around 0.4Vp-p, down from 2Vp-p). The unloaded waveform from my signal generator looks fine on the scope.

Does anyone have any ideas how I should fix this? I'm going to try resoldering everything on that channel's input, and see how I go. Possibly I should try swapping the E88CCs in each channel, and see if the fault follows the valve?

Oh, and with inputs shorted to ground and negative feedback hooked up, I see no oscillation on the outputs, so I don't think the distortion is due to me having the phase of the NFB incorrect.

Cheers!
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Old 11th March 2013, 12:20 PM   #2
DF96 is offline DF96  England
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Try swapping the E88CC's, just to eliminate them as a possibility. Re-check wiring around the input stage, and re-check DC conditions there.

Also, what happens if you use a smaller input voltage with NFB disconnected?

Is the grid stopper (330R) right at the grid pin? If you touch the grid or the cathode pin do the DC conditions change? I ask because VHF parasitic oscillation in the input stage might give the symptoms you have. (Be careful not to touch the anode pin!)

As another check, compare DC readings obtained directly with a DMM probe on the relevant point, with DC with a 1M resistor attached to the end of the probe (so in series with the DMM). This should give you 91% (assuming a 10M DMM). Include the first stage grid in this (should be 0V). This check avoids disturbing any oscillation, which normally probes may inhibit.
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Old 11th March 2013, 10:11 PM   #3
MrSnrub is offline MrSnrub  Australia
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Thanks for the suggestions. I found the problem - it was a short in the shielded cable at that channel's input. Too much heat when soldering had melted the insulation on the centre conductor where it met the shield.

I've replaced this, and now I get a clean sine wave on both channels when testing. But now, there's a new problem. With speakers connected and music playing, there's an intermittent clicking noise at approx 2-10Hz. At higher volumes, this breaks out into oscillation.

All my grid stoppers are right against their pins, but I'll try comparing the DC levels on the input grid with and without a 1M resistor in series with the DMM probe as you've suggested.

Could this be a symptom of incorrect gNFB phase? This is my first amp with negative feedback, so I don't know if the oscillation I'm hearing is due to this. I'll try swapping the phase (or disconnecting it altogether), and see what happens.

Or possibly it's motorboating? I have a Maida regulator supplying the 285VDC for the input stage - I might see if this rail is stable when playing music. Maybe an RC section for each channel following the Maida (I have some 10uF 450V caps spare) could be an idea?
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Old 11th March 2013, 11:16 PM   #4
DF96 is offline DF96  England
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2-10Hz probably means motorboating, or possibly a faulty intermittent component. Wrong feedback phase is more likely to create an audio frequency oscillation, so probably not that. Try it anyway, just to eliminate it.

When the oscillation occurs, try to find what parts of the amplifier are taking part. It might be all of it, or just part.
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Old 13th March 2013, 08:39 AM   #5
MrSnrub is offline MrSnrub  Australia
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OK, now I'm noticing some weird hissing and squelchy noises as well as the motorboating. There's also some ringing on the edge transitions of a square wave, so I suspect parasitic HF oscillations.

I've ordered some 10K carbon comp resistors to replace the 330R grid stoppers I'm currently using on the E88CC grids. Will let you know how it goes!
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Old 13th March 2013, 12:18 PM   #6
DF96 is offline DF96  England
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10k might make a difference. Carbon comp is unlikely to make a difference.
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Old 13th March 2013, 01:26 PM   #7
SY is offline SY  United States
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Does the input stage have local bypassing?
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And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 14th March 2013, 11:18 AM   #8
MrSnrub is offline MrSnrub  Australia
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No bypassing at the input stage - both channels are fed directly from the 285VDC output of the Maida regulator. I have some 0.1uF 630V caps that I could use for this, keeping them physically close to their input stage. But what's curious is that I looked at the 285VDC rail with the scope tonight, and it was rock-solid even when I had it pushed into instability again.

I did probe the E88CC grids though, and saw ringing on the square wave edges. I hear a lot of interference through an AM radio when I hold it near the E88CCs - I tried this as well on a SE 2A3 amp I built, and it was much quieter by comparison. So I guess I'll try the 10K grid stoppers. Maybe series RC snubbers between plate and screen grid on the EL84s, too - I'm using Edcor GXPP10-8-8K OPTs.
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Old 15th March 2013, 04:11 PM   #9
tcrane is offline tcrane  United States
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Default oscillatin'

I had a similar problem with my build- it was a while back so I don't remember everything I tried, but what solved it was adjusting the negative feedback. You have to adjust on test because we are using different output transformers than MJ did.

There is a description of how to adjust the feedback network using variable capacitors in this post:

Kofi Annan in: "Oscillatin' in the Bevois Valley'"
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Old 15th March 2013, 05:02 PM   #10
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Try to measure the amount of GNFB you have (the difference of the sensitivity with and without GNFB).
Usually this kind of problem can be solved by reducing the GNFB to max. 10 dB.
This is usefull to try if you have not necessary equipment to analyze the actual reason.
Grid stoppers are not the reason.
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