Newly completed GSG, LOW bias voltage ...

[Toddm]

LOng-time lurker, etc

I just finished my GSG using an Edcor power transformer and Transcendar outputs, fired it up and the one channel that makes sounds, sounds great - of course, you can see where I'm going with this, the other channel has no out, the 6b4g doesn't even light up ... so, naturally, I ask for your help, Forum.

Hoping I can get some general direction as to where to look, but here are a few observations/measurements:

--switching out put tubes, and putting in another 6ax5gt does nothing to change the situation, and both swaps make no difference in the measurements or symptoms - both the outputs always light up and make sound in the good side, and neither ever does in the bad side
-- the most glaring "wrong thing" is that the bias measures ~3.5VAC on either side when connected, but 6.3VAC when disconnected (i.e., measuring between the windings directly); on the bad side of the amp, there is less than 1VDC present with the windings connected top the terminals, on the other, there's 54VDC, and the good side does sound very good, to be sure
-- other measurements seem off, esp around the 6sl7
-- I've completely eliminated the switch, inrush limiter, etc., and have the primaries connected directly to a 2-wire AC input (using the switch on the power strip for on-off)

I used a supposedly-NOS 6sl7 and 6ax5, and I've already got replacements on the way

I've looked over the board to make sure there's no stray bit of wire, etc., but this is a totally new build, very clean, and I'm stumped (I actually had a eye- and wallet-opening dumbass moment this past summer when I rebuilt a fisher 400 and couldn't find a stray wire under a transformer mounting bolt, so I've been extra careful in checking for mismounts, stray wires, etc., though I'm certainly open to the chance that lightning has indeed struck again)

Yay, Happy New Year!

Thanks,

Todd

[20to20]

Your ohm meter will probably solve this one. Use a systematic comparison between the good channel and the bad one with your ohm meter. It's easy to misread resistor color bands or even socket pin numbers, so use the meter and compare like points on each channel to find the bad circuit. Then trace the bad circuit backward until you find the point of failure or miswiring.

HTH

20

[Shannon Parks]

Todd, could you write down all your test voltages and post them here? Use the same format from the test voltage chart in the manual. Secondly, a couple nice, huge close up shots would be very helpful, too.

Shannon

[Toddm]

Some are close, some are wacked - for the first time ever, I hope it's a bad tube

I'll upload pics as soon as I can get them off my camera (my PC is very old and slow with the iPhone downloads) - it's a rat's nest in there, nice colored wire and better sockets will be here tomorrow, this was a first pass, I-can't-wait-to-fire-it-up kind of thing

V1
1 0
2 163
3 0
4 0
5 168
6 2
7 55
8 55

V2
1 0
2 .2
3 340VAC
4 0
5 340VAC
6 0
7 3.5VAC
8 371

V3 (dead side - no sound, no light)
1 0
2 -3.7
3 363
4 0
5 4.1
6 0
7 3.7
8 0

V4
1 0
2 50
3 342
4 0
5 .3
6 0
7 57
8 0

Thanks again for the help! Like I said earlier, I'm totally prepared to find that I butterfingered something, but hoping it's really not me ...

Todd

[Toddm]

Here are some big pics of the wiring, board, etc - they're in PNG format, so they should be manageable - again, this is all first pass, proof of concept, literally rough edges and all

[TomMcNally]

Reverse the wires on J-3 3 and 4 ... you have filament and grid reversed.

[Shannon Parks]

Reverse the wires on J-3 3 and 4 ... you have filament and grid reversed.

Yup - we have a winner! Pictures make it so much easier to diagnose these things.

Shannon

[Toddm]

And it works! Yay to Tom McNally for catching the airhead mistake

Now to go back and add the all the "luxuries" - a switch, a real power cord - heck, even a fuse!

Actually, I do have another question - the AC from the transformer secondaries are fine (i.e., 6.3 VAC) when disconnected from pins 1 & 2 on J3 & J4, but measure ~3.7 VAC at each pin when connected; all the other voltages around the circuit look to be within reason, so I'm tempted to clean it up and button it up

ETA: The questions is, why could it be that voltage difference is happening?