1963 Fender Concert developed buzz.

[Helmholtz]

Because I'm having trouble with the Concert and don't trust what I would be reading. Should I?

Sure, why not?
A current is a current.

[g1]

OK bias probe says both tubes 3.5mv. I can crank them up to about 6.5mv, but for now I'll leave it where it was.

So at 6.5mV you are at the end of the pot travel? What DC bias voltage is there when set for 6.5mV?
You said you added the bias pot, how did you do it (schematic)?

It's odd that plate voltages are low yet tubes are almost cut off. Can you remove the power tubes and see what the B+ goes to?

[rockman627]

I just realized I never measured cathode current as requested by pdf64. As there are no resistors on the cathodes, how do I do this?

[pdf64]

It was ‘anode or cathode’ current; cathode current will be a milliamp or 2 higher than at the anode. As it also includes screen grid current.
It may be worth rechecking the anode voltage and current with the valve in V6 the phase splitter removed. In case oscillation is being induced as part of taking the measurements (note the 12AX7 LTP and absence of control grid stoppers).

[Helmholtz]

OK bias probe says both tubes 3.5mv. I can crank them up to about 6.5mv, but for now I'll leave it where it was.

So the 2 methods give similar results.

You didn't report if anything changed with the different tubes performance-wise.

Your bias probe might actually be measuring cathode current.
But it's always a good idea to add 1R cathode resistors.

What is the bias voltage across the 56k resistor?

[rockman627]

I'll get to all these tonight when I get home. Thanks for the input.

[rockman627]

Sorry for the delay, it took me 2 1/2 hrs to get drive home, I took the measurements but was too tired to post. But here they are (with matched tubes):

AC mains 120VAC
rectifier in 776VAC
rect out 422Vdc
CT 418Vdc
V8 voltage drop .495v
V7 drop .423v
V8 6 422v
4 422v
3 422v
V7 6 422v
4 422v
3 421v
V6 6 291v
3 27v
1 308v
V5 6 309v
3 3.3v
1 315v
V4 8 62v
6 288v
V3 6 331v
3 1.7v
1 108-215v
V2 8 0v
6 405v
3 0v
1 405v
V1 8 0v
6 403v
3 0v
1 403v

Filter pack:
red 421v
yellow 420v
brown 408v
blue (upper) 349v
blue (lower) 349v
white 328v

No change with V6 removed
Output tubes removed: V8 p3 12v
V7 p3 15V

Bias readings V8 4.6mv
V7 3.5mv
across 56K resistor 51v
at 220k-220k junction: 58v; with bias at 6.6: 50v

I'll post my bias pot drawing soon.

[rockman627]

I'm a little worried: I previously read 3.5 mv bias on both tubes, now I get 4.6 and 3.5. Should I put in some other tubes and not risk un-matching my good ones?

[Helmholtz]

I'm a little worried: I previously read 3.5 mv bias on both tubes, now I get 4.6 and 3.5. Should I put in some other tubes and not risk un-matching my good ones?

Always let the amp warm up and stabilize before measuring voltages.
Are you sure your meter (battery) is good?
No risk to harm the tubes especially at such low currents.

Please let me know if you still have your original problem (buzz, low output).

[rockman627]

Always let the amp warm up and stabilize before measuring voltages.
Are you sure your meter (battery) is good?
No risk to harm the tubes especially at such low currents.

Please let me know if you still have your original problem (buzz, low output).

I usually do a 15 minute warmup.
If I take my meter battery out, I can't test it (just joking)
Volume on both channels is good, Any previous mention of volume mismatch between normal and vibrato channels is no longer an issue. Supposedly other brown face Concert owners had the same issue and said it's the way it is designed. I accept that and the lower volume works in my favor at home.
The buzz seems a little lower, but still very prominent.

My shipment of caps and resistors should be here today. I gotta figure out how to re-wire the two 20uf/500v parallel filter caps to two 100uf/350v caps in series.

[rockman627]

Whoa! I found something: I was checking the heater voltages on all the tubes, and when I stuck the probe on V2 p9 the wire came out. Apparently it was touching but not in the socket. I fixed that and the voltages on V1 and V2 changed significantly. I'm going to a 15 minute amp warmup, check the battery on my meter and do another full voltage chart. PS still buzzing.

[rockman627]

New readings, after fixing V2p8 connection, in BOLD

AC mains 120VAC 119
rectifier in 776VAC 763
rect out 422Vdc 400
CT 418Vdc 400
V8 voltage drop .495v .5
V7 drop .423v .47
V8 6 422v 394
4 422v 393
3 422v 394
V7 6 422v 394
4 422v 394
3 421v 394
V6 6 291v 261
3 27v 24.4
1 308v 275
V5 6 309v 290
3 3.3v 3.14
1 315v 295
V4 8 62v 57
6 288v 270
V3 6 331v 308
3 1.7v 1.47
1 108-215v 131-204
V2 8 0v 1.4
6 405v 174
3 0v 1.36
1 405v 177
V1 8 0v 1.41
6 403v 186
3 0v 1.28
1 403v 179

Filter pack:
red 421v 400
yellow 420v 393
brown 408v 364
blue (upper) 349v 326
blue (lower) 349v 326
white 328v 308
to pre-amps 269v

[rockman627]

OK, it took me a while to figure some things out, but I figured it out and it seems to be working. First thing is I wanted to change the 2 parallel 20uf 500v caps to 100uf 350v in series for 50uf/700v performance. But when I looked at Psionic Audio's video, it looked like his balancing resistors were color coded as 7.2 meg ohms, but they were the standard 220k ohms. Then I had to figure out how to do the series wiring because the wiring for the series caps in Psionics video were different and my red wire couldn't stretch that far. Fortunately, EL34's website had a layout I could follow. Working on power supplies is a new experience for me so I went over everything dozens of times in my head until I understood what was happening and not just aping others. So I finally did the work today, plugged into my current limiter and everything seems good. There was a tiny bit of motorboating, but it went away after 10 minutes. I'm letting it warm up right now. Tomorrow I'll close up the cap can and take new voltage readings.

[Enzo]

I have never seen nor heard of a 72M resistor. Plus it would not be a standard value either.

[rockman627]

I have never seen nor heard of a 72M resistor. Plus it would not be a standard value either.

Sorry that should have been 7.2 M .Yeah, I had to read up on balancing resistors and somebody mentioned once you get to about 1M, you wouldn't get balancing done. But in the video, it looks like a 5 band resistor and I see violet-red-black-yellow with grey(?) for tolerance. I suppose you shouldn't count on a video to get a correct color code.