1963 Fender Concert developed buzz.

[rockman627]

A little history: a friend gave me this 1963 Fender Concert when he quit playing. I had previously fixed a few things for him and the amp had no sound when I got it. I had the feeling I would be changing a cap or resistor every month, so I decided I was going to build a new circuit with a turret board and over spec'd components. Amazingly I got it right and everything worked. The vibrato channel was very low compared to the normal channel so I made some mods and re-routed the normal channel to the vibrato circuit and the vibrato channel straight to the output. I also installed a bias pot. I've gigged with the amp like this for 5 years since. Recently, I felt the vibrato channel was too dull so I replaced the .1uf cap in the vibrato tone stack with a .02uf. That worked out OK. And though I changed the filter caps and bias circuit, I never replaced the old rectifier diodes so I did that too.
Everything was okay for 2 or 3 days then the amp developed a loud 120hz buzz.I re-checked my rectifier and am fairly sure the diodes and soldering are good.But I failed to notice the schematic calls for 600v filter caps and I used 500v. So, I am going to follow Psionic Audio's method and put two 100uf 350v caps in series for 50uf 700v performance. In his video, https://www.youtube.com/watch?v=P1b4...wyAkUS&index=2 PA seems to use 7.2M resistors for balancing, and most people use 220k. I think I will go with 220k. But my voltages mostly went lower and really high in some spots.
original reading (before diode change), then new reading
V8: p6 487v 428
p3 484v 429
V7 p6 487v 429
p3 486v 429
V6 p6 255v 234
p3 47v 44
p1 263v 241
V5 p6 366v 321
p3 3.4v 3.05
p1 368v 325
V4 p6 333v 291
p8 73v 64
V3 p6 383v 337
p3 1.6v 1.6
p1 around 170v, never stayed still (vibrato?)
V2 p8 1.7v 0
p6 211v 402
p3 1.6v 0
p1 212v 403
V1 p8 1.7v 0
p6 212v 401
p3 1.6v 0
p1 222v 403

492v off the standby switch 429
447v brown wire off filter pack 408
490v yellow wire off filter pack 427
408v blue wire off filter pack to 2- 100ks 358
407v blue wire off filter pack to 56k 357
-57.8v bias -57
330v above V1 and V2 408

I originally thought something in the circuit was drawing too much current and reducing voltages but now I think the new diodes supplied more voltage and overwhelmed the filter caps. While I'm waiting for my new caps, you guys got any ideas?

[pdf64]

Maybe a bad heater wiring connection between V3 and V2?
What anode or cathode currents are the output valves drawing at idle?
When measuring voltages inside an amp, it’s good practice to measure the mains AC voltage being fed to it at least once during the process, preferably several times if it’s not typically very stable.
And also check the heater voltage, eg across the pilot lamp. Looking for a reading of about 6.3VAC, not 3.15V to the chassis.
Power supply voltages inside an amp are fundamentally just a ratio of the mains voltage; as that goes higher or lower than nominal, the voltages in the amp will track it.

[Helmholtz]

Neither the 500V caps nor the new diodes should lower your B+.
7.2M balancing resistors would be nonsense.
Carefully check your wiring and caps' and diodes' polarities.

Also not all new caps are good caps. An overstored new cap may perform worse than an old used cap.
Some ecaps develop substantial leakage when not used for more than 2 years since production.

Does any get warm?

[rockman627]

Sorry for the delay, busy day at work yesterday.
So, measuring he 2 red wires going into the rectifier: 750VAC. Coming out of the rectifier: 415VDC.
I re-checked the diodes and re-soldered them. They seem OK
I'm also reading 6.6VAC across the pilot lamp.
I have no experience measuring current, but I'm going to rely on Ohm's Law. Some questions first:

If I measure resistance across the 2 output tube plates, that is the same as the output transformer primary resistance? So, the resistance from 1 plate to the center tap is the resistance I calculate with? Then I measure voltage from each plate to ground and the center tap to ground and I have my voltage drop?

[Enzo]

Why do extra math? You measure resistance from CT to plate, now you know resistance. When it is running, just measure voltage CT to plate. No need to make extra readings to ground to calculate the same thing. That gives you resistance and voltage, so Ohm's Law gets you current.

[rockman627]

I'm getting older and my hands aren't 100% steady anymore.It's better for me to clip the black probe to the chassis, and stick the red probe here and there. But I wasn't really sure about measuring the OT and you verified that for me. Thanks. Now to take some measurements.

[rockman627]

OK, today's readings:
118VAC mains voltage
755 VAC into rectifier, 425VDC out .
V8 p3 to CT : 101 ohms, .04VDC voltage drop
V7 p3 to CT : 113 ohms, .38VDC voltage drop

If you look at the layout, I am using where the right side of the standby switch connects as the center tap.
ps: the voltage drop was so low I used both hands and carefully measured from plate to CT to verify.

[g1]

I'm getting older and my hands aren't 100% steady anymore.It's better for me to clip the black probe to the chassis, and stick the red probe here and there.

Mine weren't steady ever, so I'm in the same boat. You can clip the black to the standby switch instead, then you only need to use one hand.

[g1]

V8 p3 to CT : 101 ohms, .04VDC voltage drop
V7 p3 to CT : 113 ohms, .38VDC voltage drop

Any chance the decimal place is off? 4V and 3.8V seem more likely.

[rockman627]

Every other range on my meter gave me 0 volts; when I used the 2v range, I got those readings. That means .04/101= 3.96 amps? And .38/113=.0033 amps?

[Helmholtz]

OK, today's readings:
118VAC mains voltage
755 VAC into rectifier, 425VDC out .
V8 p3 to CT : 101 ohms, .04VDC voltage drop
V7 p3 to CT : 113 ohms, .38VDC voltage drop

Looks like power tubes are unbalanced and close to cutoff, i.e. hardly conducting. I'd expect something like 2.5V.
Try a good new pair and clean and retension tube sockets.

[g1]

Also report DC volts for V7 and V8 pin4, not pin6.

[rockman627]

OK I put my dual bias probe on my working Hot Rod Deville and went through my 6L6's. Geez most of my previously matched tubes are way off now. I think I'm going to build some single ended 6L6 amps some day. Anyway, my TAD tubes were within 2 mv of each other so in the Concert they went. So here are today's readings:

mains 116 vac
rect in 746 vac
rect out 415 vdc
v8p3 413 vdc
v8p4 412 vdc
v7p3 413 vdc
v7p4 412 vdc
v8 voltage drop .297 vdc
v7 voltage drop .348 vdc

[Helmholtz]

So here are today's readings:
v8 voltage drop .297 vdc
v7 voltage drop .348 vdc

Any difference in amp's performance now?

From your "voltage drop" readings I'd expect less hum but distorted output.
An OT primary voltage drop of 0.3V corresponds to roughly 3mA idle current. That's about a factor of 10 too low.

As you have a bias probe, why don't you use it with the Concert?

[rockman627]

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