Vox AC30 reissue ( Marshall made ) noise reported

[Arbutt]

Ok Hope we had a nice weekend. Only had about 2 hrs time to work on it this morning before i had to go out
I have fitted the humdinger and yes an improvement
Fitted a 470R LOG Pot across the heaters with the wiper of pot going to ground. Disconnected the TX CT.

when i turn on with all volumes turned down and adjust the 470R pot. i can adjust for quite, once i turn up the volumes noise is there. If i then adjust the 470R i can remove the noise but then it will be noisy on another channel

Too cut a long story short if i adjust the 470R for every channel i can get it quite
I have connected the wiper of the pot to various different grounds inc to the EL84 no difference

At this point i'm thing of separating some of the grounds to different points as all are going to the same Star gnd

idea's
many thanks
Arbutt

[pdf64]

I wonder if this isn’t all just due to some of the ECC83 having poor heater-cathode insulation. Have you tried some known good ones in there, especially V8, V10 and V6?

[Helmholtz]

Fitted a 470R LOG Pot across the heaters with the wiper of pot going to ground.

At this point i'm thing of separating some of the grounds to different points as all are going to the same Star gnd

Why a log pot? A linear one should give better control.

The fact that the humdinger allows to completely eliminate the noise at some setting proves that it's actually heater-cathode hum.
Other hum sources would produce different waveshapes that could not be completely compensated by the pot.
Different channels reacting diffferently might be due to the wiring to out-of-phase inputs of the PI.


Nothing wrong with proper star grounding. It doesn't allow for ground loops.

[Mick Bailey]

At this point i'm thing of separating some of the grounds to different points as all are going to the same Star gnd

I advise against that - the job here is to fix the fault, not re-design the amp (to Paraphrase Jack Darr). If there was a problem with the grounding scheme, all of these amps would hum from new and always have a problem, and they don't. There's either a component or connection fault that needs to be thoroughly investigated and fixed.

[Arbutt]

i fitted the log pot as that was all i had in stock. I have put it all back together and tweaked it up. all seems to be ok. ish

I think im trying for perfection. which i ain't going to get

thanks too you all i have learned stuff in the last few days,

Arbutt

[Helmholtz]

BTW, you should check the time base calibration of your scope. Actual horizontal deflection coefficient in your picture must have been around 2ms/div as the repetition rate of the spikes is 50Hz not 100Hz. That caused some confusion with HT ripple.