Wrong Transformer in Vibro Champ?

[highoctane]

I have a 1971 Fender Vibro Champ silver face. I have been going over it really well and trying to get it ship shape. It is making a weird buzzing sound when abruptly overdriven. This sound appears "on top of" the nice warm tube distortion one would expect. I have been trying to troubleshoot this. On an oscilloscope it appears as an abrupt spike which then ramps back down. I know this can't be a normal waveform or sound. Could it be a capacitor discharging all of a sudden?

The amp sounds good at low volumes, nice and clear.

One thing I notice is that whoever "serviced" this thing last did not put the stock audio output transformer in it. Instead, they put a 125A20B (an 8 ohm, 3.5 watt) transformer in it. The factory spec called for a 125A35A (a 3.5 ohm, 5 watt secondary). Could this wrong transformer be what is causing that buzzing / farting sound? Or would this be due to a bad output tube?

Any ideas or experience you could share would be much appreciated.

[MWJB]

A mismatched OT (either half or double the rated speaker load) won't normally cause unwanted noise, you may experience slight drop in power compared to a matched load.

More likely culprits are the tubes & power supply caps. You say that the amp has been serviced, are the power supply caps original '71 (metal can soldered to the chassis)? If so, they could do with changing.

[highoctane]

The caps have been changed

The OEM schematic on this amp shows qty 3, 20 microfarad capacitors. Someone has bypassed the original Mallory can caps and used electrolytic instead. The weird thing is, they used qty 2 20 microfarad, and qty 1, 47 microfarad caps. I don't know why. I have tried swapping the preamp tubes, with no change. I guess I need to find out if my power tube is bad. Is there any way to test a tube without having another one to test against?

[MWJB]

47uf is fine for the filter cap that connects to pin 8 of rectifier...that's what I would use, or even 100uf.

"I guess I need to find out if my power tube is bad. Is there any way to test a tube without having another one to test against?" Best way is to play the amp with the tube in it, unless you find something that is obviously wrong elsewhere that cures the problem, you're pretty much wasting your time if you don't eliminate the tube first.

[captntasty]

Hi all - I'm quite interested in this post as I believe I am having a similar problem. Mine is a modified 5F1 clone which I know is different from a vibro. Does that extra distortion sound kind of like a blown speaker? Sort of a farting noise on top of the nice overdrive? I've checked and of course it is not the speaker or any of the tubes. I've tried moving the OT around and shortened all leads as much as possible - I've heard these things can cause such distortion.... I also had removed negative feedback on this amp so I put it back switched. The NF mellows it out some but the distortion still remains. Anyhoo, just interested. Hope you get it figured out...

[highoctane]

Problem solved (mine, at least). It turns out I had a bad output tube. And, yes, captntasty, I would describe my symptom like you did, like a blown speaker or a farting / buzzing noise on top of the overdrive. It looks like we might have a different problem though, since the tubes didn't fix it for you. New output tube fixed it for me.

[MWJB]

Captntasty, any pics of your build?

[BloozeGit]

I'm looking in on this thread as well as my 5E3 clone with a vintage OT is having the same issues. Changed tubes, undid mods, changed out carbon comp resistors, redone signal wiring, tried different speaker etc but still the same issue.

I've tried testing the filter caps according to http://www.repairfaq.org/sam/captest.htm#cttes and they seem ok, but I'm not sure if there are any issues under load.

Interestingly, this phenomena occurs when the tone knob is above a certain level. Some weird resonance/feedback of some sort?