Bell Labs HiFi amp rectifier question

[Randall]

I'm restoring a pair of these Bell Labs amps for a Hi Fi customer. On the schematic, it is drawn with B+ coming off pin 8, but both amps are wired with B+ coming off pin 2. First amp has a slight hum, could this be the reason? I have replaced all the electrolytics and a couple of coupling caps that measured out of spec. as well as rewired the mains with the fuse and switch on the hot side with a grounded line cord.

[nosaj]

I'm restoring a pair of these Bell Labs amps for a Hi Fi customer. On the schematic, it is drawn with B+ coming off pin 8, but both amps are wired with B+ coming off pin 2. First amp has a slight hum, could this be the reason? I have replaced all the electrolytics and a couple of coupling caps that measured out of spec. as well as rewired the mains with the fuse and switch on the hot side with a grounded line cord.

[ATTACH=CONFIG]38726[/ATTACH]

I really doubt 2 or 8 would make a difference. Have you tried pulling v1 or v2 to see if the hum disappears? or swapped the 6v6 s?

Nosaj

[g1]

Pin 2 or 8 is not a concern with a directly heated rectifier like this.
With an indirectly heated cathode like 5AR4, you can get problems taking off pin 2 as you are pulling all current through the heater.

[Richard]

Why is the second half of V1 connected together? Does this reduce noise as opposed to having it disconnected?

[Randall]

I switched it to pin 8, still hums. Still hums with V1 removed, but not V2. I find it stops if I ground either grid in V2. I have swapped out all tubes with no effect. Triple checked all wiring. Getting stumped here.

Another issue is supply voltages. Very high voltages when plugged straight into modern day outlet. When dialed back on a Variac to get 320vdc B+ per drawing, second node is good at 303vdc, but third and fourth nodes are way high at 210vdc and 170vdc respectively, putting my preamp plates in some cases about twice what is called for. I don't understand why are they so high?

I put all four caps in a single can, where they were in two separate cans before, but I can't see what harm that would do. Other than the hum and the high V's it works and sounds pretty good.

[nosaj]

I switched it to pin 8, still hums. Still hums with V1 removed, but not V2. I find it stops if I ground either grid in V2. I have swapped out all tubes with no effect. Triple checked all wiring. Getting stumped here.

Another issue is supply voltages. Very high voltages when plugged straight into modern day outlet. When dialed back on a Variac to get 320vdc B+ per drawing, second node is good at 303vdc, but third and fourth nodes are way high at 210vdc and 170vdc respectively, putting my preamp plates in some cases about twice what is called for. I don't understand why are they so high?

I put all four caps in a single can, where they were in two separate cans before, but I can't see what harm that would do. Other than the hum and the high V's it works and sounds pretty good.

Not sure if this helps but read through all of thisAX84.com - The Cooperative Tube Guitar Amp Project

nosaj

[g1]

Compare your voltages with the good unit. Post any major discrepancies.

[Randall]

This is the good unit.

[g1]

Are the supply nodes in rough agreement between the 2 units?

[52 Bill]

I put all four caps in a single can, where they were in two separate cans before, but I can't see what harm that would do. Other than the hum...

Please describe the original caps, two separate cans, four separate axials?

If what you are saying is that you now have one can with all filters in it, you may have changed the grounding of the power supply and created a ground loop.

[Randall]

Only this amp is functioning at this time.

[Randall]

Oh my, I think I see what I've done. Originally it had two cans, C1 with 30/30 uF nodes 1 and 2 along with 20 uF cathode bias cap grounded at the power supply side at R34. The other can C2 had nodes three and four 20/10uF grounded at the preamp side. I moved the low voltage bias cap to a radial on the tube socket, and put the four power supply nodes in one can, grounded on the PS side. So now I should find a narrow 20/20 uF can and ground it properly, correct?

And could this cause the voltages to be higher than expected on the preamp side?

[g1]

It may affect the hum but I don't think it would cause high supply voltages.
Sorry, I misread and somehow thought one unit was working right.
I can't read the resistor numbers, but can you list your voltages on either side of the supply dropping resistors, 2700R, 47K and 100K ?

[Randall]

See post #5 for voltages.

[g1]

Doh! Missed those too.
Sounds like either V1b or V2b is not conducting properly.
What is voltage at V1 pin 8 and V2 pins 2,5, & 6 ?