Issue With Bassman 5F6A Build

[Guitarnut]

Instead of going through all the bias test point install this morning, I went with Martin's instructions on testing the OT primaries. I tested with JJ 6L6GC

If I did this correctly, here are the numbers.

Powered Up - Bias adjust pot at mid point

V4
Red to Brown 50Ω (powered down)
Red to Brown 1.4v
28mA

V5
Red to Blue 44Ω (powered down)
Red to Blue 1.5v
34mA

Or,
JJ 6L6GC max output in push/pull is 25W or 17.5W @ 70%.

My V4 plate is 431V. 17.5/431 = .040mA
.040*44Ω = 1.75v

My V5 plate is 435V. 17.5/435 = .040mA
.040*50Ω = 2.00v

Peace,
Mark

[martin manning]

So you are a bit cold right now, and the output tubes do not match exactly (no surprise). In this case I would dial V5 up to 2V across its side of the primary and check the voltage and current balance again.

6L6GC are 30W tubes, so you will still be only about 58% of max plate dissipation if the B+ doesn't drop (and it will).

Here is another layout for the 1R current sense resistors. I agree with Tonemerc that external test jacks are not much advantage when you can remove the top panel and get full access to the inside of the chassis, and you have to remove it to get to your bias trim pot anyway. The resistor lead bridge between the pins 1 and 8, and the extra ground lug are convenient places to clip your meter probes. If you wire it this way, you can still add the test jacks later if you want.

[martin manning]

Did you figure out what is up with V2 yet?

[Guitarnut]

I found that the 25K mid pot was grounding out. I removed it, took it apart and there was some solder dribbled inside shorting it out. Cleaned it up, meters fine, reinstalled it.

Plugged in and it actually played!!! Very low levels but I was getting signal through. I unplugged to check voltages and a few of them moved.

V1 pin 1 up to 181, pin 6 up to 196

V2 pins 1 & 6 down to 288, Pins 7 & 8 down to 244.

V3 pin 1 down to 233. Pin 6 down to 220. Pins 5 & 8 up to 46.

Plugged back in and no sound again...gotta be something loose in V1a or V1b.

[Guitarnut]

I get a fairly loud pop on both switches when I power down.

[martin manning]

Are you sure that you have the 100k resistor installed on the V2 socket correctly? Pins 1 and 7 are tied together and should have identical voltage.

[Guitarnut]

Crap! Reversed them when I rewired the sockets. R is going from 1 to seven and 6 is jumped to 1.

Gotta go fix that.

[martin manning]

I'm going to open a beer, because I think it's about to get loud.

[Reeltarded]

Hooorrrraaaaay!

Excellent help provided by the best guys in the World, and way to stick with it there, Mark.

[jelle]

I'm going to open a beer, because I think it's about to get loud.

Good job!!

[Guitarnut]

We are so close! It gets plenty loud when I strum but as soon as I put any umph into my playing it sort of pops and goes silent. Touching any one of the pots or cable brings it back but same thing happens again.

I poked around with a probe while powered up to see if I could find anything loose but nothing shows itself.

It's almost like it's voltage related.

Any ideas?

[martin manning]

What do the voltages look like now? Get a complete set including the power supply nodes, and the idle current for the power tubes. I'm wondering if the V2 tube got damaged, and is the cause of the signal loss.

[Reeltarded]

Turn the lights off and watch it for sparks too. Watch the tubes and the guts...

[Guitarnut]

What do the voltages look like now? Get a complete set including the power supply nodes, and the idle current for the power tubes. I'm wondering if the V2 tube got damaged, and is the cause of the signal loss.

PT
Yellow 5.33
Red 352
Red 355
Recto DC 446

V1
1 232
3 1.89
6 216
8 1.89

V2
1 189
3 1.25
6 340
7 189
8 190

V3
1 262
3 50.8
6 246
8 50.8

V4
3 446
4 445
5 -53.4
6 445

V5
3 445
4 445
5 -53.4
6 444

Power Tube Currents. Bias pot at mid point.
Red to Brown 46R (powered down)
Red to Brown 1.06
V/R 23mA

Red to Blue 50R (powered down)
Red to Blue 1.27
V/R 25mA

[Guitarnut]

Turn the lights off and watch it for sparks too. Watch the tubes and the guts...

Didn't see anything but even with the lights out it's still fairly bright...so, to be more accurate, I didn't see anything obvious.

Peace,
Mark