My guess is still grounding. Also no grid stoppers, and the attenuator is rather too close to the PSU. Not easy to see where the rectifiers are, but they may be on the end of the same tag strips as the audio circuitry.
Shorting out the output cap would reduce the anode voltage by imposing a DC load. This would reduce gain. If it kills hum completely, then grid stoppers may be the answer as parasitic oscillation might be killed off by the reduced gain too.
Shorting out the output cap would reduce the anode voltage by imposing a DC load. This would reduce gain. If it kills hum completely, then grid stoppers may be the answer as parasitic oscillation might be killed off by the reduced gain too.
I thought that possibly using the stepped attenuator could be introducing noise due to the larger surface area of the exposed wires + proximity of the PSU.
Grid stoppers may be a good idea. How are these installed?
A grid stopper is a resistor attached to the grid connection, right at the valveholder with a short lead on the valve side (but as long as you like on the other side). Value not critical, but typically 1-10K for output valves or a bit higher for small signal valves. The idea is that the resistor forms a CR low pass filter with the internal capacitance of the valve so it damps any tendency to RF oscillation. An oscillating valve can produce strange results, including hum and distortion, but the problems sometimes stop as soon as an oscilloscope or meter probe enters the room.
It is good practice to include grid stoppers as a matter of routine, as they do no harm unless too high in value and retrofitting them later on can be fiddly.
It is good practice to include grid stoppers as a matter of routine, as they do no harm unless too high in value and retrofitting them later on can be fiddly.
Really?--That high (1-10K)?
Ive always used a few hundred ohms generally on signal valves and as low as 100 ohm on outputs....
--I normally use carbon-compos for 'em...
No need to use carbon composition. A normal film resistor is OK up to VHF; above that it becomes a lossy inductor which will still damp resonances.
A few hundred ohms is usually enough to kill off VHF/UHF oscillation, but it won't have much effect at HF frequencies. You need a few K for that. You might use 47-100 ohms as a stopper for a video amp or a low HF amp.
A few hundred ohms is usually enough to kill off VHF/UHF oscillation, but it won't have much effect at HF frequencies. You need a few K for that. You might use 47-100 ohms as a stopper for a video amp or a low HF amp.
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