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

[Silvertone Jockey]

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.

High ripple = hum. A`shorted transformer allows current to flow which if the transformer is only partially shorted means that it will produce sound in the speakers with no tubes installed. So possibly a two fold problem where the shorted output iron has`caused the filter caps to start to fail as well. That's my theory

[Tom Phillips]

High ripple = hum. A`shorted transformer allows current to flow which if the transformer is only partially shorted means that it will produce sound in the speakers with no tubes installed. So possibly a two fold problem where the shorted output iron has`caused the filter caps to start to fail as well. That's my theory

Could be. I asked marcumh to ohm out the OT back in post #7 but have not seen a report on the results. When there is more than one thing wrong, by coincidence or because one caused the other, troubleshooting becomes more difficult.

[The Dude]

High ripple = hum. A`shorted transformer allows current to flow which if the transformer is only partially shorted means that it will produce sound in the speakers with no tubes installed. So possibly a two fold problem where the shorted output iron has`caused the filter caps to start to fail as well. That's my theory

If the OT is defective, and we have yet to determine that, it would have little effect on the filter caps. DC is actually lower on the caps and the current path becomes the near shorted transformer. The AC present would be from excessive loading rather than failing caps.

[Tom Phillips]

...The AC present would be from excessive loading rather than failing caps.

One of my concerns is that the ripple is too high when the standby switch is open. In that unloaded condition the ripple should be very much less than the 6 Vac reported.

[The Dude]

Possibly correct. It's been a while since I had need to scope B+ on one of these amps, so I could be way off here. Don't kill me if I'm wrong. That said, I do know that these amps depend on the push pull effect of the output to cancel much of the ripple allowing for a "less filtered" supply. I'm not entirely certain that 6VAC of ripple is uncommon for this amp in standby. I wish I had something similar opened up in the shop right now to verify my "theory".

[Tom Phillips]

...I'm not entirely certain that 6VAC of ripple is uncommon for this amp in standby...

I used Duncan's power supply calculator and all indications are that the ripple in standby at the first filter node should be less than 0.3Vpp = 0.12 Vrms. My recollections of actual measurements of similar circuits agree.

[g1]

Nothing more frustrating than defective replacement parts. New does not necessarily equal good. Do you have some other caps to try for the first stage filters?

[überfuzz]

(I just skim read random posts)

If I got this right, you did a cap-job and ended up with a bad filter. Can you lift one leg of each cap check them, measure etc just to make sure that they're hunky dory?

Edit, forgot to write, even if it's beneath you, triple check everything you wired.

[Tom Phillips]

By coincidence I just benched a 1964 Fender Delux that has never had any repairs other than tube chafes. The original filter caps were bad and the fist filter node voltage readings were 325Vdc (low) and 109Vac (very high). That is about 30% ripple and of course results in a high hum level. As an experiment I pulled the power tubes and listened. The hum stopped and there was no buzz from the speaker. None of this is surprizing.

To summarize the facts related to marcumh's repair issue there seems to be at least two problems.
1) The first node filtering problem evidenced by the excessive ripple when the amp is in standby mode.
2) The buzz signal getting through to the speakers when the power tubes are pulled out. I think the Silvertone Jockey's previous posts in this thead point to the OT as the suspect cause.

Tom

[The Dude]

Just a quick thought after skim reading the thread again. Have you subbed a known good rectifier tube?

[marcumh]

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.

Here are a couple of scope shots. #1 is with only recto tube installed, #2 is with 6l6's and PI installed.

I'm pretty much just a sine wave guy,....use of the scope for seeing anything other than clipping eludes me; my apologies


I checked res from CT to secondary. I'm a little unsure of what I should see here; I get 9ish M or -2ish M depending on the orientation of my probes.

[g1]

Where is the scope connected to for those pics? What are the voltage settings per square?

[überfuzz]

1. What voltage swing do you see at the secondaries on the PT?
2. Are you sure the rectifier tube is healthy? You don't happen to have a spare one, or maybe a ss rectifier you could try just to rule that out.
3. Check every capacitor in the filter, if it's not 1 or 2 you probably have a bad filter. (quite beefy caps I might say.)

[Tom Phillips]

If you still have the high ripple on the B+ as reported in post #24 then I say that you need to repair the power supply before proceeding.

[The Dude]

Is the first picture in or out of standby? That will tell us if the noise we see is from the OT or the power supply. Repeat the test and supply scope shots in standby (no voltage to OT) and out of standby (voltage to OT)- only the recto installed and measuring the B+ line on the "hot" side of the standby switch.

Edit: As has been suggested, try a known good recto tube.
And, another note, the OT should be wide open primary to secondary. You may have been measuring through your fingers. You can not touch the probes and get accurate readings on higher scales.

Edit (again....sorry). As g-one suggested, voltage and frequency settings would also be helpful when posting scope shots.