Fender BF 1965 Super Reverb Buzz (with no tubes installed)

[Tom Phillips]

Also verify that you have proper screen voltage at the power tubes on pin #4.

If the screen voltages missing or very low then the tubes won't conduct. Although hat will not explain your buzzing it would explain your low current readings.

[marcumh]

Screens look good (roughly equal to the plates). Neg bias v @ -52,...I even checked the heaters lol... Every thing there seems to be "normal".

[marcumh]

I'm still plugging away on this thing and trying to think outside my box (however small it may be).

So forgive me before I ask this,.....Should I see AC voltage on 6L6 plates & at OT CT?

More specifically; 126Vac with no input signal.

[Tom Phillips]

...Should I see AC voltage on 6L6 plates & at OT CT? More specifically; 126Vac with no input signal.

You should not see 126Vac with no input signal. Normally, with no input signal, you would see a small AC ripple voltage riding on the DC voltage. In a properly functioning SR the ripple will typically be 3% to 4% of the DC voltage value. Your high AC reading may be due to faulty first stage power supply filtering. That could be the cap, a bad ground connection, mis-wiring etc.

Can you post gut shot photos of this amp?

[marcumh]

Here are some shots. I have done filter caps, cath byp caps and 2-3 prong ac conversion. This problem was present before this work and still remains.

[Tom Phillips]

I suggest the following diagnostics with the amp turned on and the rectifier tube installed :
1) Put the standby switch in the standby position. Does the buzz stop.
2) Measure the voltage at the OT center tap. It should be ~0 Vdc and ~0 Vac.
3) Measure the voltage at the +end of the first stage filter capacitor stack. It should be higher than normal Vdc (probably over 500V) with very low AC ripple because there is no load connected other than the balancing resistors. Note that some meters don't give an accurate AC reading for this test because they cannot accurately measure a small AC voltage in the presence of a high DC voltage.

Please report the readings you get.
It's OK to leave the other tubes installed during these tests. The results of the measurements should be the same either way.

Based on your answers we can then decide what to do next.

[The Dude]

512VDC B+, yet 126VAC is odd. If filtering were poor, I'd expect lower B+, yet it is high. I'm thinking something is wired incorrectly. Did you perhaps disconnect something and put it back in the wrong place?

[Tom Phillips]

512VDC B+, yet 126VAC is odd. If filtering were poor, I'd expect lower B+, yet it is high...

Good point. It would be best to scope the B+ and be able to see the actual waveform.

[marcumh]

1) The buzz does stop if the amp is in standby
2) ~0Vdc & ~0Vac (standby on)
3) I was a little unsure on this one so I did it a couple of ways.

Measured at the standby switch: (?)
~390Vdc / ~90Vac (standby on)
~510Vdc / ~6Vac (standby off).

Only rectifier tube installed.

Thanks for your help and have a happy Thanksgiving

[The Dude]

This is likely unrelated, but I was scanning your pictures and noticed that there is cold solder on one of your screen resistors. It's the joint on the far left screen resistor end. Visible in your second picture.

[Tom Phillips]

1) The buzz does stop if the amp is in standby
2) ~0Vdc & ~0Vac (standby on)...

OK. That tells us that the standby switch is functioning as it should and that the source of the buzz is through the B+ feed.

3) I was a little unsure on this one so I did it a couple of ways.
Measured at the standby switch: (?)
~390Vdc / ~90Vac (standby on)
~510Vdc / ~6Vac (standby off).

Only rectifier tube installed...

Interesting that the Vac readings are so much different than previously measured at the plates of the power tubes.
The ~390Vdc / ~90Vac reading is more in agreement with what the Dude said he would expect. (Both readings are abnormal but they are in more correct proportions) Both of the AC readings you just posted are too high. These readings further support that the first stage filtering, made up of the two 100uF / 350V caps in series, is faulty. I suggest that you check all the connections and the caps in that circuit as your next step. With the amp powered down and all voltage drained from the caps check the resistance from the negative lead of the leftmost cap in your posted photo to chassis ground. Check from the lead wire of the cap. Not the eyelet or hookup wire. It should be ~0 Ohms.

Edit: Also verify the connection between the two 100uF filter caps in the first stage stack. That connection runs under the eyelet board.

[Silvertone Jockey]

My 1484 did that exact same thing, traced it down to short between the windings of the output transformer

[Tom Phillips]

My 1484 did that exact same thing, traced it down to short between the windings of the output transformer

My thinking is that, although your bad OT may have caused a similar buzz, the high ripple voltage that marcumh is observing with the amp ion standby mode must be caused by another fault. I hope he reports news soon.

[The Dude]

That thought had occurred to me- a near shorted OT loading the power supply. I initially threw out that idea because B+ remained high. New measurements indicate that B+ is indeed pulling low out of standby. The buzz could be transformer arcing. It would sure be nice to hear it or see a picture of it on a scope. I've replaced a few 1484 OT's, not so many older Fenders. They are built a little heftier than the Silvertones. Certainly still a possibility. The OP could check the resistance primary to secondary on the OT.

[Silvertone Jockey]

Thats what I observed. I was able to get a reading from primary to secondary. Not a dead short but far from the infinite reading it should have had. In my case the amp still had the death cap (since removed) so that was possibly another source of hum contributing to the horrible sound it made. I replaced the xformer and all was good