Buzzing noise in 1966 Fender Pro Reverb

[fushifushi]

Hi everyone,

A buzzing noise started a couple weeks ago in my 1966 Pro Reverb. Here's the typical sequence of events:

- Turn on my amp and take it off standby.
- After about 20 seconds you can hear the amp come on.
- Another 20-30 seconds later a loud buzzing sound starts.
- After another 2-3 minutes the buzzing sound fades away, but I can still hear it faintly in the background.
- The noise occurs regardless whether a guitar is plugged into either channel. It also occurs when the volume on both channels is zero.

Here's a video where you can hear the buzzing noise as it comes on about 40 seconds after I turn on the amp. [FYI, the amp was rehoused in a 1970s Fender Showman Reverb head at some point.]

Earlier this year, before I bought the amp, a tech had serviced it. He only replaced the first two filter caps and the bias cap. Everything else was original in the amp except the tubes had been changed out at some point probably in the 1980s. So today I decided to replace the other three filter caps with F&Ts. The new filter caps dramatically reduced the noise floor of the amp, which is awesome. But the buzzing noise still occurs.

Based on my description and the video, can you suggest any parts of the circuit that might be causing this noise?

Thanks!
Jason

 

[AntonyB]

As Jack Darr suggests in his service/troubleshooting procedure (thank you Skip Simmons), start by the speaker and back track from there all the way to the input.
I am assuming the noise is coming out of the speaker.
Before swapping things though, I'd check to see if nothing is loose, like tubes of their socket etc.
- can you try another speaker?
- swap your power tubes (if noise still there with a different set, put the original ones back, only changing one parameter at a time, same procedure for subsequent tube stage)
- swap your phase inverter (V6 12AT7?)
- swap you 2nd gain stage (V4 7025/12AX7)
- then swap your v2 since you are on the virbato channel.

That sounds sounds like a bad contact honestly.
The steps above should allow you to isolate the location... the noise is amplified by one of those tubes, so isolating where it disappears should help you.

Please report back, am curious now

 

[jackleg]

input jack grounding issue?

 

[SoK66]

I can never understand why techs don't replace filter caps as a set. That amp is almost 60 years old, the original electrolytic caps are way past their expected life span. YMMV, but......

 

[Paul G.]

I'd check/clean input jack ground first. Make sure they are tight to the chassis. Make sure the star washers are present. Check that all of them have hot grounded when no jack is installed. If not, give them a clean with contact cleaner and then by slipping some rough paper between the contacts and letting the contacts close then pulling the paper out.

 

[Wally]

The amp needs full service by a tech.

 

[schmee]

Rectifier tube. Try another one and see if it goes away. I have one or two old rectifiers that do this.

 

[fushifushi]

Thanks for all your input!

Rectifier tube. Try another one and see if it goes away. I have one or two old rectifiers that do this.

I don't have a spare GZ34, but I'll try to borrow one from a friend.

- swap your power tubes

I did have a spare 6L6, so I swapped out both tubes. The noise mostly went away when I swapped one of the original ones out. I kept turning the amp off, letting it cool down, and turning it back on again to listen for the noise. Only one time has the noise kind of come back but much quieter.

That sounds sounds like a bad contact honestly.

A friend also suggested it's a bad contact in the power section. Interestingly, the noise is affected by tapping the power and rectifier tubes and poking the sockets and chassis with a chopstick. Sometimes a tap will reduce the noise, and then the next tap will increase the noise again. Almost like switching the noise on and off by tapping.

The amp needs full service by a tech.

This is probably true, but it's too bad that it was already given a "full service" by a tech in the past few months.

 

[Wally]

This is probably true, but it's too bad that it was already given a "full service" by a tech in the past few months.

Earlier this year, before I bought the amp, a tech had serviced it. He only replaced the first two filter caps and the bias cap.

I do not consider that to be “full service”. Ommv.
Your tap testing is of interest as is the power tube substitution…..but the results of the tests are not definitive. You could have a dirty tube socket or loose sockets that need retensioning. A solder joint might need to be reflowed. AI be might be bad. Did the replacement of that one power tube eliminate the noise when tapping or moving the rectifier? Tapping the board? Tapping the power tubes?
Are all of the nuts and screws tight?

 

[fushifushi]

I do not consider that to be “full service”. Ommv.

I agree. That's why I put "full service" in quotes. I was being snide about it. I think he did the minimum amount of work to get the amp back out the door.

I did clean the power tube sockets with isopropyl alcohol and re-tensioned the sockets a few weeks ago, but I could try that again. I could try reflowing the solder joints, too.

I'll do more swapping and tapping later today and report back any new findings. Thanks!

 

[Rowdyman]

Definitely not the speaker.
I'd be trying a rectifier tube swap and looking for loose tubes/sockets, cold solder joints, etc.
Very possible to be any tube in there.
It should really be properly serviced by an experienced tech. Anyway.

Good luck! Great amp, I had two of em, back in the day,,,, RM

 

[fushifushi]

Here's what I can report after a few days of swapping and tinkering.

- I swapped out the GZ34 rectifier with a new JJ that works, and it didn't eliminate the buzzing.

- Swapping out the old Silvertone 6L6 for an old Realistic 6L6 I have on hand seems to be the most effective way to get rid of the buzzing. It might be as simple as that, but I'll keep an eye on it for a while to make the sure the buzzing doesn't come back. I might also re-tension the sockets again and reflow the solder joints to be safe. The old 470 ohm carbon comp screen resistors in the sockets have drifted higher than tolerance, so I'm wondering if replacing them might be wise given their tendency to overheat and drift. I've got 2w metal film replacements for them on hand.

- Since replacing the old filter capacitors made such a difference in the noise floor, I went ahead and replaced the rest of the original electrolytic caps in the preamp (i.e. the cathode bypass caps). The difference wasn't as dramatic, but the noise floor did get a little quieter. An interesting side effect of replacing the bypass caps is that the gain must have increased in the tremolo tube, which is to be expected. There had been tremolo ticking before, but now the ticking is much stronger due to the increased gain. So that's the next issue to iron out.

Thanks everyone for your input,
Jason

 

[Wally]

Is this a buzz or a hum?

 

[fushifushi]

Is this a buzz or a hum?

I'm not totally sure how to classify it, though I've been calling it a buzz. You can decide for yourself in the video I linked to in my original post.

 

[Wally]

I'm not totally sure how to classify it, though I've been calling it a buzz. You can decide for yourself in the video I linked to in my original post.

That video will not load for me. I Remember trying it when this thread opened…..it doesn’t load today, either.
How would I described the difference between a hum and a buzz?? Generally, a buzz has a ratcheting aspect to it…..maybe like the difference between humming a song and using a piece of paper wrapped around a comb through which to hum. With that paper and comb, the paper vibrates against the comb and creates a buzzy effect. Sometimes in amp amp, a buzz has an electrical arcing sound.

 

[fushifushi]

Thanks for describing the difference. Even with your description, I'm afraid I still am not sure how to classify the sound. To me, it sounds more grinding and aggressive than what I would consider a hum. So I'll still say it's a buzz. Maybe someone else who is able to view the video can give their opinion.

Yesterday, as I was reflowing the socket joints I decided to replace the 1.5k ohm grid stopper resistors and 470 ohm screen resistors in the power tubes since the originals had drifted higher than tolerance. Not sure if any of this had anything to do with the buzzing (since that now seems to have been an issue with the failing 6L6), but the amp seems to be the quietest it has been yet. The noise floor is has gotten low enough that I almost can't tell when the power tubes kick in when I've turned the amp on.

I've been dealing with some cloudy static at times though which might have to do with the PI. That'll be for a separate thread though.

Thanks,
Jason

 

[pippoman]

Hi everyone,

A buzzing noise started a couple weeks ago in my 1966 Pro Reverb. Here's the typical sequence of events:

- Turn on my amp and take it off standby.
- After about 20 seconds you can hear the amp come on.
- Another 20-30 seconds later a loud buzzing sound starts.
- After another 2-3 minutes the buzzing sound fades away, but I can still hear it faintly in the background.
- The noise occurs regardless whether a guitar is plugged into either channel. It also occurs when the volume on both channels is zero.

Here's a video where you can hear the buzzing noise as it comes on about 40 seconds after I turn on the amp. [FYI, the amp was rehoused in a 1970s Fender Showman Reverb head at some point.]

Earlier this year, before I bought the amp, a tech had serviced it. He only replaced the first two filter caps and the bias cap. Everything else was original in the amp except the tubes had been changed out at some point probably in the 1980s. So today I decided to replace the other three filter caps with F&Ts. The new filter caps dramatically reduced the noise floor of the amp, which is awesome. But the buzzing noise still occurs.

Based on my description and the video, can you suggest any parts of the circuit that might be causing this noise?

Thanks!
Jason

If you have to pull the chassis, set it on a couple of short 2x4s and plug the speaker up. There’s a ground wire that runs from the front of the chassis to the breadboard, just a bare single strand wire, no shield. Even though it looks like it’s still soldered in place, try moving it with a wooden dowel to make sue it’s actually soldered; sometimes the solder joint breaks and it barely makes contact. If not, replace it with a piece of multistranded shielded wire. That happened on both a 66 Vibrolux and a 65 DR a few years back. I don’t know why Fender used basically a paper clip to ground that chassis.