Dual Pultec Build Ground Noise

[Janalex]

Only positive and negative hooked up between rectifier board and bypass boards. No reference ground to chassis on either. There was a yellow wire heading up from the negative on the rectifier board to chassis ground but it was adding tremendous noise.

 

[RoadrunnerOZ]

Seems like there’s some noise generated by the bypass/LED network - I’ve had faulty LEDs causing dreadful noise in equipment before. Does the noise change with the bypass on or off (although there may be an LED associated with this as well)? I’d also disconnect the LEDs one at a time to see if one of them is a culprit.

 

[Janalex]

I could try. It’s a true bypass board so when it’s engaged there is zero noise. Also I can’t find the schematic for it but let me double check that the board is not being grounded by way of pin 1 via the chassis. Then the addition of the rectifier board ground wire would cause a ground loop, no?

 

[RoadrunnerOZ]

I could try. It’s a true bypass board so when it’s engaged there is zero noise. Also I can’t find the schematic for it but let me double check that the board is not being grounded by way of pin 1 via the chassis. Then the addition of the rectifier board ground wire would cause a ground loop, no?

Yes it could by connecting two different supply grounds in multiple places.
Is there noise on the rectifier board DC when not grounded? If so I’d be checking those LEDs. Taking them out of circuit 1 by 1 and seeing if the noise goes.
I had a MemoryMoog that had a problem with erratic behaviour caused by an LED - when it switched on the whole board would go crazy - the LED appeared normal - it turned on as it should but when on it caused so much noise in the 5V DC rail anything on that same rail would do random stuff. The LED metered and measured as normal so initially it led me to look at the circuitry enabled by the mode the LED represented! When I put a scope on the 5V rail the DC would continually drop and rise rapidly caused by the LED failure and recovery at a high frequency.

 

[Janalex]

I did check the bypass boards before reading your message and now I’m away from the unit. Looks as though the bypass board ground nodes are already grounded so has to be via pin 1. Seems like I don’t need another ground wire. I’ll do the final led checks but did hook it up to the rig and I’m not hearing any noise at reasonable and high reasonable volumes. I should figure out how to put a number on the noise level but all my testing before was done direct into a powered focal twin 6be speaker at full blast since it didn’t have a volume adjustment. Now in the rig with monitor control I’m very happy but will double check one more time.

 

[RoadrunnerOZ]

All good then - just leave out the extra ground wire. Probably just a hum loop from the addition of another wire to ground.

 

[Janalex]

How about possible noise from the tubes? There are amperex tubes in there now. Would a low noise 6922 be more helpful? Not that I’m really concerned but could easily replace tubes if worth it

 

[RoadrunnerOZ]

You’re only likely to get hiss from noisy tubes but they can be susceptible to radiated noise. Some good 6922 tubes wouldn’t go far astray but in saying that the Amperex are really well built tubes - which ones are in yours?

 

[Janalex]

I’ll have to go in look again but yellow label made in holland ones is all I can recall

 

[Janalex]

Well I used the eq in a session yesterday and was incredibly pleased with its performance on kick and snare and no noticeable noise to speak of. Gonna stop analyzing and get back to making music! Thanks all for your consistent help and feedback!