Metropoulos Forum

coldengray wrote:One thing to check out - jumper leading from the plate resistors. I swapped a 100 watt resistor during my troubleshooting and it could have caused the under board connection to partially come out.

With what you describe that is most likely.

I'll have to rig up a way to chopstick under the board tonight...using chopsticks!

Also, I think I'm going to separate the Presence and pot grounds...I think I will drill a new ground point. I will run a lead from the pot bus to the chassis ground connection I drill, keeping the Presence on it's own. It's pretty quiet when cranked, for a Marshall...but it could be quieter.

coldengray wrote:I'll have to rig up a way to chopstick under the board tonight...using chopsticks!

Also, I think I'm going to separate the Presence and pot grounds...I think I will drill a new ground point. I will run a lead from the pot bus to the chassis ground connection I drill, keeping the Presence on it's own.

I would cut bus wire before the bass pot and run dedicated black wire from presence pot to the same ground as the PI can.

That is where the entire bus is grounded now - PI can ground. I will separate and keep the Presence grounded at the PI can ground and drill a dedicated input/pot ground.

Thanks Neil!

Ok...I believe my problem was two-fold:

1.) bad mixer bypass cap, probably fried after 3 mixer value changes.

2.) dying V1, replaced last night (need to keep an eye on this, make sure the amp isn't eating V1 tubes)

The noise seems to manifest after about 30 minutes of playing, as the amp heats up. I'll see if I'm good tonight. If not I think I have a bad volume pot on channel 1 or loose jumper off the plate resistors.

Still didn't get a chance to modify the ground scheme, I'll do that soon.

As always I am open to thoughts, ideas and suggestions from my fellow builders!

Played a while tonight, no issues. Scumback S75-PVC and H75-PVC is new for my 2x12 and sounds awesome.

coldengray wrote:Played a while tonight, no issues. Scumback S75-PVC and H75-PVC is new for my 2x12 and sounds awesome.

Excellent, but what was the fix?

Sorry for the delayed reply, been busy. The fix was on the underside of the board, the input resistor connection for the High Treble channel. I fixed it up and no issues now. I did add a dedicated ground for pots and inputs, and put the Presence pot on the cap can ground...it actually sounds more noisy now so I may put it back. While I had it open I changed the NFB resistor and moved the connection to the 8 ohm tap - don't like it, going to move that back but it was worth experimenting once.

I've made many changes to the amp and always go back to stock.

coldengray wrote:Sorry for the delayed reply, been busy. The fix was on the underside of the board, the input resistor connection for the High Treble channel.

Cheers to you, Neil! You ID'd the problem, nice work.

I'm not thrilled with the grounding experiment, I will probably change that back.