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Old 15th May 2013, 02:51 AM   #11
Enzo is offline Enzo  United States
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OK, groud the scope probe to the amp, and probe the speaker output or the plate of the output tube, or for that matter, any plate in there. AC coupled of course. Since this seems to happen even in the absence of signal, you would expect the signal path to remain a flat line at zero. If you have what appears to be a wide lit band across your scope, turn up the sweep speed and see if it resolves into a very high freq waveform.

At sweep speeds suited to audio, the RF will just look like a band of light instead of a line.
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Old 15th May 2013, 03:37 AM   #12
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If you have X10 or X100 probes you can set the scope input to AC and look at the 6V6 anode directly. Chech for oscillation at ultrasonic frequencies.
If you are "sus" about your or the scopes safety doing that then you should be able to see any ultrasonic oscillation (greatly reduced) on the secondary of the output transformer.
If you really want to see what is happening then lift the 0V side of the 6V6 cathode bias resistor and bypass cap and put a 10 Ohm resistor in there and look across that resistor with the scope.
As a first step, I would just put the grid stop resitor in and see if your problem goes away.
Cheers,
Ian
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Old 15th May 2013, 05:04 AM   #13
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Post up a picture...in case it's layout-related...???
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Old 15th May 2013, 11:27 AM   #14
DF96 is offline DF96  England
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Most amps benefit from grid stoppers. Guitar amps, with their peculiar layout and long wires, definitely need them and often use stoppers which are so high in value that they act as (fixed) tone controls. The exact value is not too critical, but something around 5k for the output and 10-20k for the earlier stages - wired at the grid tag.

Put the scope on the output stage grid. If there is a signal here when there should not be, what is its frequency? (Roughly - subsonic, 60Hz, 50kHz?) If possible, use a 10:1 probe as that will add less capacitance. At the same time continue monitoring the output cathode voltage. If it changes when you connect the scope probe then this is a classic symptom of oscillation.
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Old 15th May 2013, 04:19 PM   #15
cancon is offline cancon  Canada
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Thanks everyone. I'll have a go at the grid-stoppers & scope and post the results.

How does a grid-stopper work? Would it be worthwhile to add a grid leak resistor? I can see one is already in place at the input to the first triode.
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Old 15th May 2013, 04:25 PM   #16
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Quote:
Originally Posted by cancon View Post
Would it be worthwhile to add a grid leak resistor? I can see one is already in place at the input to the first triode.
All three stages already have them.
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Old 15th May 2013, 04:57 PM   #17
DF96 is offline DF96  England
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A grid stopper does two things:
1. It, in conjunction with Miller capacitance, rolls off the HF gain of the stage so stray capacitance can't create unwanted feedback loops (typically over several stages).
2. It adds losses to the grid circuit, so anything which looks like a VHF/UHF resonator is heavily damped so the stage (on its own) can't turn into an oscillator.

In order for the second thing to work the resistor needs to be right at the grid tag.
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Old 15th May 2013, 05:25 PM   #18
cancon is offline cancon  Canada
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Thanks for explaining, very helpful!

I found this very short (yet concise) link about common gain stages and what you typically see immediately surrounding a tube. Check it out here;

Questions keep flooding in...
How does Fender end up with 1.5K as a common grid stopper value?
What are your thoughts about the low plate voltage for the 12AU7s (40-80V)?

Last edited by cancon; 15th May 2013 at 05:32 PM.
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Old 15th May 2013, 06:21 PM   #19
Enzo is offline Enzo  United States
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Have you tried more than one 12AU7? A bad tube is always top of the list.

SOlve your oscillation problem and those voltages may fall into place.

Or pull the 6V6 and see what the 12AU7 voltages are on their own.
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