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With the long weekend I decided to open up an old can-o-worms that I never could figure out. It's a Marshall 2204 style circuit except the preamp gain control is after the second gain stage instead of the first and there's no switching business at the input jack (just one input jack). The amp also has a post-PI MV in addition to the regular pre-PI MV. The amp hums excessively. I've tried swapping tubes...it's not faulty tubes. Here's what I've found so far:
With the post-PI MV full-up and the pre-PI MV fully off...dead quiet.
With the post-PI MV full up and the preamp gain fully off, increasing the pre-PI MV causes a light but still unacceptable amount of hum with some 'crackle' to come through and it increases with increasing pre-PI MV settings.
With the post-PI MV full up and the pre-PI MV full up (so you're already hearing the light-ish hum/crackle), as you increase (lol, barely crack) the preamp gain an over-the-top amount of hum comes through and increases with increasing preamp gain. This hum doesn't have the 'crackle' that the previous 'light-ish' hum did and it quickly swamps the light-ish hum.
Pulling V1 gives the same result as having the preamp gain fully off (remember, preamp gain control after second stage), i.e. the light-ish hum/crackle increases with increasing pre-PI MV settings.
Pulling V2...dead quiet with all volumes full-up.
Voltages seem fine including heaters. Heaters are balanced AC-wise and when off/unplugged each heater side measures 0.6 ohms to ground. I've tried a ton of different things with grounds but no effect at all. These are new filter caps and I've tried a known good cap in every one's place. Using my multi-meter it settles down to the same ~10-20mvac 'ripple' on every power supply node. I've tried poking moving wires...zero effect. I've measured for strange dc voltages on the 'other' side of blocking caps...zero volts every time.
I'm stumped. Any other 'tests' I can do?
Oh yea - measured from the three prongs of the power cord to inside the amp including ground prong directly to chassis, all good (i.e. 0.5 ohms on my meter). Other amps work fine using the same cord/outlet/guitar/speaker cab/etc. I've also measured from the ground tab on each filter cap can to chassis...all good.
Another thing just remembered - I tried directly grounding the grid of the first (input) stage...zero effect.
With the post-PI MV full-up and the pre-PI MV fully off...dead quiet.
With the post-PI MV full up and the preamp gain fully off, increasing the pre-PI MV causes a light but still unacceptable amount of hum with some 'crackle' to come through and it increases with increasing pre-PI MV settings.
With the post-PI MV full up and the pre-PI MV full up (so you're already hearing the light-ish hum/crackle), as you increase (lol, barely crack) the preamp gain an over-the-top amount of hum comes through and increases with increasing preamp gain. This hum doesn't have the 'crackle' that the previous 'light-ish' hum did and it quickly swamps the light-ish hum.
Pulling V1 gives the same result as having the preamp gain fully off (remember, preamp gain control after second stage), i.e. the light-ish hum/crackle increases with increasing pre-PI MV settings.
Pulling V2...dead quiet with all volumes full-up.
Voltages seem fine including heaters. Heaters are balanced AC-wise and when off/unplugged each heater side measures 0.6 ohms to ground. I've tried a ton of different things with grounds but no effect at all. These are new filter caps and I've tried a known good cap in every one's place. Using my multi-meter it settles down to the same ~10-20mvac 'ripple' on every power supply node. I've tried poking moving wires...zero effect. I've measured for strange dc voltages on the 'other' side of blocking caps...zero volts every time.
I'm stumped. Any other 'tests' I can do?
Oh yea - measured from the three prongs of the power cord to inside the amp including ground prong directly to chassis, all good (i.e. 0.5 ohms on my meter). Other amps work fine using the same cord/outlet/guitar/speaker cab/etc. I've also measured from the ground tab on each filter cap can to chassis...all good.
Another thing just remembered - I tried directly grounding the grid of the first (input) stage...zero effect.
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