Voltage Problem [resolved]

[Bob]

Currently building a pair of Paramount 300b. Completed 1st mono block, all tests well, sound check ok. Completed 2nd mono block, resistance reading within range, added power to check voltages --- on startup, the 5670 tube glowed as you would expect, but the 300b did not, turned off. Went back and checked connections, validated visual all wiring looked good, exchanged 300b tube with know good tube. Same Result. The 300b would not glow --- I rechecked the wiring for A1, A4, check all wiring back from there to FC, back to Power supply board, power supply. All looked good.

I waited about 30 minutes and turned amp back on  --- I went ahead and checked voltage at OA - right on at 300.1, i set the plate voltage on the 5670 tube and followed the sequence for the balance of the voltage checks.

All voltages check except the following were wrong ---

Power Supply Board -
6v-Red should be 3.1v, reading 0v
6v-Blk  should be 3.1v, reading 0v

Four Pin Socket -
A1 should be 74.1v, reading 0v
A2 should be 440v, reading 545v
A3 should be ~0v, reading 0v
A4 should be 69.2, reading 0v

Clearly there is an issue with the 6v Blk/Red power connection to the Power Supply Board - I have visually checked the power supply, I have tried a known good 5670 tube, both glow as expected (both sides), i have checked wiring, check Diode string, all test good.

Any guidance would be much appreciated !

Thanks

Bob

[Paul Birkeland]

What is your DC voltage between pins A1 and A4?  Do you have your 4 pin socket oriented properly?

The Red/Black wires are AC voltages, my guess is that you had your meter set for DC.  (If those were truly 0V, your 5670 wouldn't glow)

[Bob]

Paul,

Thanks for the quick reply.

I do have the 4 pin oriented properly --- A1, A4 closest to the on/off switch (large pins of 300b tube).

The red/black --- thank you -- I was reading those as DC, they now red as 3.0 AC -- properly!

DC voltage is -0- on A1, A4 and between A1, A4.

Thoughts ?

Thanks Again

Bob S

[Paul Birkeland]

On the power supply PC board, what is the DC voltage between +F and -5?  (If you still get 0V, unplug the 300B and retest)

This is certainly why the rest of your voltages are funky.

-PB

[Bob]

Paul,

Again thanks for the quick reply. I checked the voltage between +f and -5 --- It read 5.5v DC !!!    At this point I traced all wires again from power supply thru FC-1 to A1, A4 --- I discovered a small cut in the red wire from FC-1 to A1 --- I am not sure how I missed it - but when voltage was present between +f and -5 - i figured there must be a wire or solder joint somewhere needing attention !!!

Thanks again --- the voltages all check now --- I will go through the amp again and make sure I do not have any other issues with nick'd or cut wires !!!

Appreciate all you do !

Regards

Bob S

[Paul Birkeland]

Again thanks for the quick reply. I checked the voltage between +f and -5 --- It read 5.5v DC !!!    At this point I traced all wires again from power supply thru FC-1 to A1, A4 --- I discovered a small cut in the red wire from FC-1 to A1 --- I am not sure how I missed it - but when voltage was present between +f and -5 - i figured there must be a wire or solder joint somewhere needing attention !!!

Hello Bob,

This is a job well done!  Enjoy the tunes.

-PB