Ampeg R12-R Reverberocket power transformer replacement

[beatnik]

hi guys

just wanted to update you with the situation about this amp

i have got the transformer re wound by a specialist company here in the uk. great service and affordable as well

i have replaced all electrolytic capacitors and a couple of resistors and put in a fresh set of tubes. i used jj 7591s for the power amp

the amp works nicely and all the voltages seem to be in the ballpark, but I am now struggling with a hum problem. the hum is present with nothing on the input, and regardless of the potentiometers settings

without the preamp tubes the hum disappears, but if I put in the phase inverter it comes back. i have tried three different 6sl7 tubes, so I doubt it's a tube issue. also tried another 5y3 rectifier just in case, but no difference

i am wondering if this can actually be a ground problem. right now i have all the 0V points tied together and they go to chassis at a single point, on the input jack sleeve

i think that originally the amp had the 0V signal split in half and one side went to chassis at the power supply filter can capacitor and the other half grounded at the input jack sleeve

i didn't think that my modification to the grounding would cause any problem. what is your opinion on this matter ?

[Ted]

An amp of this era will probably never be totally hum free. But it shouldn't be obtrusive. Some older ampegs didn't have switching jacks on the inputs and hum like bastards until you actually plug something in. Even if they do have switching jacks corrosion sometimes measn they don't work properly - a bit of switch cleaner on a trip of paper dragged between the contacts may fix it.

But the grounding arrangement you describe is almost certainly not the original one.

Try grounding the cathode resistor of V1 (and the power electrolytic for that stage if its not part of the multicap) to the input jack and all others to the tab on the multicap (if there is one)

hi guys

just wanted to update you with the situation about this amp

i have got the transformer re wound by a specialist company here in the uk. great service and affordable as well

i have replaced all electrolytic capacitors and a couple of resistors and put in a fresh set of tubes. i used jj 7591s for the power amp

the amp works nicely and all the voltages seem to be in the ballpark, but I am now struggling with a hum problem. the hum is present with nothing on the input, and regardless of the potentiometers settings

without the preamp tubes the hum disappears, but if I put in the phase inverter it comes back. i have tried three different 6sl7 tubes, so I doubt it's a tube issue. also tried another 5y3 rectifier just in case, but no difference

i am wondering if this can actually be a ground problem. right now i have all the 0V points tied together and they go to chassis at a single point, on the input jack sleeve

i think that originally the amp had the 0V signal split in half and one side went to chassis at the power supply filter can capacitor and the other half grounded at the input jack sleeve

i didn't think that my modification to the grounding would cause any problem. what is your opinion on this matter ?

[beatnik]

what you are pointing out is actually true. the input jacks are switching type but they are not meant to ground the input when no cable is plugged in. the hum is also present when the guitar is plugged in though.

the amp had been serviced before (quite poorly) so I am not sure what was the original ground arrangement. When I got the amp part of the circuit was grounded at the capacitor can, and the section closer to the preamp grounded at the input jack sleeve. Since the new capacitor had the ground tab isolated, I decided to join everything to the input jack sleeve.

the modification you suggested requires quite a lot of changes to the wiring, but it makes total sense to have the more sensitive input circuit grounded separately. do you suggest to separate just the first triode cathode resistor or the second triode as well ? (they are part of the same V1 tube, although the second half seems to be part of the tremolo circuit)

[g1]

Where is the center-tap of the HV winding grounded?

[beatnik]

the ht and filament center taps are joined on the multisection filter cap, and from here they are tied to chassis

i have moved v1 filter capacitor to this point as well (before it was grounded at the input jack sleeve) - this lowered the hum a little bit although I am not entirely happy and would like to improve things

right now there are 5 different points where the circuit is grounded to chassis :

1) the portion that is grounded to chassis at the input jack sleeve is shared between V1 first and second triodes - should i try only the first triode on this point ? (the second triode cathode is grounded at point 5 described below)

2) tone capacitor grounds on the potentiometer shell - this is definitely stock wiring, but maybe would be more correct moving this at the input ground ?

3) tremolo speed control grounds at the potentiometer shell

4) tremolo intensity control grounds at the potentiometer shell

5) the rest of the circuit ground signals are joined together and tied to chassis at the multi section filter capacitor negative terminal, this includes the h.t. and filament centre taps

do you think this layout could be improved ?

I forgot to mention that instead of 60uF for the first filter capacitor I have used a 20uF, since that's used on the previous revisions of this amp, with 6V6 power tubes, I assumed it would have been fine

[Ted]

Is the hum 50Hz or 100Hz? (100Hz is approximately Ab on the low E string on a guitar)

If it's 100Hz it will be power supply related, if its 50Hz it's almost definitely grounding or screening.

The grounding arrangement you describe sounds sensible to me - those extra tremolo grounds probably won't make a difference.

[beatnik]

i have modified the ground scheme by grounding at the input jack sleeve only the first triode grid resistor and cathode resistor + bypass capacitor. the rest of the 0v signals are grounded at the power supply side

the hum dropped noticeably and right now let's say the amp is usable as it is, but i would like to make some further improvement. maybe would be a good idea grounding also the tone cap and the volume potentiometer at the input jack sleeve ?

another thing to be mentioned, the heater wires are not twisted. would probably be a good idea replacing the wiring with a tightly twisted pair ?

[Ted]

Twisting the heater wiring might give you some improvement. You may be looking at a bunch of small, incremental improvements. You can try changing the tone cap and volume control grounding. Might work. might not.

It's easy to get obsessed with getting rid of every tiny bit of hum, to the extent that you're hearing things that are imperceptible to anyone else. I've done it a few times...

i have modified the ground scheme by grounding at the input jack sleeve only the first triode grid resistor and cathode resistor + bypass capacitor. the rest of the 0v signals are grounded at the power supply side

the hum dropped noticeably and right now let's say the amp is usable as it is, but i would like to make some further improvement. maybe would be a good idea grounding also the tone cap and the volume potentiometer at the input jack sleeve ?

another thing to be mentioned, the heater wires are not twisted. would probably be a good idea replacing the wiring with a tightly twisted pair ?