The Amp Garage

roberto wrote:rockstah,
have you found the problem?

roberto, i decided to check all the ground points. I ended up removing ground lugs and taking a Dremel to the chassis as to make sure the contacts were good and no more hum.

Thanks Jana for the food for thought!

Mark

Yes, by tail I mean the same area that the 4k7 resistor is grounded. The one that goes to the .1 cap, 10K, etc. Even with the volumes off in a plexi, you should still get pops from probing the plate resistors on all except the first stage. "CDE polyester film caps - they would show voltage where th...

A couple more thoughts before my 2nd brain cell gives out. What kind of coupling caps are you using? If they are orange drops, although this can happen with any cap, they are kind of finicky with the leads. Sometimes the lead will detach internally from the foil. It appears like the lead is still c...

welp put it back together without the PPIMV. Something is for sure wrong. i guess the PPIMV was just making whatever is going on more obvious and that makes sense because again i install those masters all the time. the amp makes a slight hum in standby and is real week sounding with ugly highs going...

i hear ya. let me get it back together and ill take it from there... it has be baffled.

didn't see that they are hooked up correctly. if it were hooked up incorrectly the amp would be squealing like a pig if i turned up the PPIMV and they were hooked up wrong. I was referring to the leads going from the PPIMV to each side of the power section, not the output transformer leads (which w...

i just pulled the board out... driving me nuts over here... Have you tried reversing the leads going from the PPIMV out to their respective halves of the power section? Also, have you tested the bias voltage on each side the pot when the volume is at 0 and again at 10? didn't try the reversing beca...

i just pulled the board out... driving me nuts over here...

Why 220K's? There are a couple of possibilities for the situation you are describing. One is that the balance of the differential signals is somehow being compromised which results in the increase in hum. There is hum present at this stage of the amp but it gets nulled out because of the opposite p...

Are you certain that you have it wired correctly? Is it possible that the wires going to the pot are connected to the wrong lugs? With it on 10 it wouldn't matter but at anything less than 10 it would matter. If the wiring checks out okay, what happens if you jumper each of the 2.2M with a 220K? po...

hums when its on zero and is quiet on 10....

Firestorm wrote:Does the hum diminish gradually from 0 to 10? Or is there a sudden drop when the wiper hits any resistance?

gradually as the volumes goes up to 10 where it goes away completely.

Jana wrote:Just out of curiosity, measure the bias voltage going to the power tubes at various settings of the pot. Measure at the grids of the power tubes to get the actual reading and not at the bias feed portion of the circuit.

did that. -41.89 dcv at all settings of the pot on all tube grids.

Jana wrote:I would suspect a bad pot that isn't making good contact with the trace and it's "ground" tab.

i swapped out the pot and checked all connections.

I thought you invented it :wink: Nice! This master was developed over at Metroamp. When i was trying it inside a marshall, while looking for a better solution, (i didn't like the way the rich mod sounded) I came across it pouring through ken fisher data. It was Larry that came along and added the s...