Peavey Alphabass round two-troubleshooting

[catalin gramada]

Do you suggest to move R52 /22K from v2 local ground to v3,v4 local ground ?

[Enzo]

It appears you jumpered the cable from send to return, to eliminate the jack board. But my suggestion was to shunt signal path to GROUND at the connector. Idea is to divide the circuit to determine where the noise comes from.

[Enzo]

Do you suggest to move R52 /22K from v2 local ground to v3,v4 local ground ?

I don't recommend moving anything until you find out WHERE the hum is coming from.

[catalin gramada]

Are we talking about pre out / PI in, right ? The circuit was broked between v2b out and v3a in and v3a pi input was shunted to ground at its local point. The hum is gone. The conector is between pre out and pi in. and not between fx send fx return. In the moment you pull off the conector the PI input is in air, if shunt the PI input to ground the hum is gone. I tried a jumper to conector just to do a link which not run through DI board ( which have its grounds g points to chassis) and the local ground was running to chassis by separate wire.

[Enzo]

Your hand drawing shows a jumper wire between the green and white wires, which you label as pre out and power in. I called those send and return. I did not intend to mean the jacks.

[catalin gramada]

This is basically how those part of circuit looks like

[catalin gramada]

Your hand drawing shows a jumper wire between the green and white wires, which you label as pre out and power in. I called those send and return. I did not intend to mean the jacks.

The jumper was used for a test to avoid to run the signal through DI board. At same time the jumper was not used and the PI inputt was shunted to ground from the same point. Shunting PI input to its local ground stoped the hum. Otherwise with the jumper used keeping the v2b output conected to pi input but without any tube in v2 socket the hum was still present

[catalin gramada]

I.m not pretty familiar with the actual grounding structure in my amp. Please let me understand...the local supply grounds are defined by the patches on the board separate in string by those 47 ohm resistors whilst the local signal grounds are related one by the other by ground points to chassis please ? Do that mean any "dirty" earthing may introduce unwanted noise in signal chain ? Lifting the earth cable from chassis may show if is a problem ? Thanks.

late: actually the mains I use did not have a separate earthing. As much as I know the earthing in my building (protection null) is linked with the working null in the distribution pannel- sortta old economical mains wiring...don't ask.

[Enzo]

Earthing is different from "grounds" at least inside the amp. The 47 ohm resistors are there to ensure a complete circuit, but the various ground sections are connected via the chassis, mostly at jacks. In other words, with all the jacks tight to the chassis, if you try to measure the 47 ohm resistor, you will see zero ohms or close to it. If you do see 47 ohms than one of hte ground connections to chassis is open.

[catalin gramada]

Earthing is different from "grounds" at least inside the amp. The 47 ohm resistors are there to ensure a complete circuit, but the various ground sections are connected via the chassis, mostly at jacks. In other words, with all the jacks tight to the chassis, if you try to measure the 47 ohm resistor, you will see zero ohms or close to it. If you do see 47 ohms than one of hte ground connections to chassis is open.

This I.m talking about. They used 47 ohm to separate the supply returns by signal returns.Then all signal ground will run through chassis. Then a noisy earthing may induce noise in the signal ground ?

[catalin gramada]

I asked just because it is a AC potential diference between mains plug and chassis. I recorded 132Vac to one pin in and 72Vac to the other...As time as the signal is running through chassis but also ac mains current components should be pretty normal to interfere. The only solution I see here is to completly isolate the chassis, possible just lifting the earthing connection...think

[Enzo]

Your earth connection to the chassis does not necessarily form a circuit with other ends of the chassis.

[catalin gramada]

...

[catalin gramada]

...I did a different grounding test of V2b input (fx return) not through the return jack but grounding C48 2.2u in different point of chassis. I shorted with a wire c48 input to the chassis jack ground point and then few inch away also to chassis. The noise was reduced. What does this mean please ?

[catalin gramada]

After checked and recovered all ground traces, swapping lot of tubes...etc. Finally found it: I leaved the power transformer aside and hum is gone. It was the last thing I imagine to think about, but yes: the layout is noisy due to PT position. Unfortunately is not room inside to do some so I have to live with it as it is...I will try to isolate from chassis later and ground it directly to PS but don't think it will help...

late: grounded/ungrounded PT didn't do any difference...Turning PT 90 degree and noise is almost none, but unfortunately no room inside ☹