62 vibrolux….this is a weird one

[pintail78]

My main gigging amp, turned it on today and it was humming pretty bad, and it sounded like it had a octave pedal going through it. It seems like something to do with the vibrato circuit. I cleaned and swapped the preamp tubes, checked the bias on the power tubes and its modulating up and down with the vibrato, even some when its off. Chop sticked everything I could in the chassis, resoldered all the components (check for open resistors, checked the pre amp voltages from the schematic too) going into V3, which I think is the vibrato channel, I even looked at the filter caps-which are new blue sprague atoms. Plate voltage was fine so rectifier seems ok.

Im at a loss and think I checked all the main culprits. Any ideas???? The strangest thing is the bizarre octave tone coming out of the amp…..never heard that before.

[Leo_Gnardo]

checked the bias on the power tubes and its modulating up and down with the vibrato, even some when its off.

#1 suspect is the bias filter cap. If it hasn't yet been replaced, now's the time. And it is normal for bias to drift up & down - just a little - in these amps.

Not far behind in the suspect lineup - hi voltage filter caps. A failure in either bias or hi voltage filters will give you a whole lotta hum and strangely modulated guitar tones.

[Chuck H]

What he said ^^^^^^^^^^^

Also... Since the filter caps have been replaced it's reasonable to suspect the connections for the filters. Especially grounds.

[Leo_Gnardo]

Since the filter caps have been replaced.

Woops I missed that, but they're still suspect and so are their connections. Even "premium" Spragoo Atoms have been known to go bad, or have mis marked + and - polarity.

[Chuck H]

Woops I missed that, but they're still suspect and so are their connections. Even "premium" Spragoo Atoms have been known to go bad, or have mis marked + and - polarity.

I stopped using them because I had some new builds with symptoms of bad filters I suspected a failure on the sellers part. Sprague specs a shelf life and stock rotation regimen. This applies to any electrolytic cap but Sprague actually use to specify it. I don't think this is always done by distributors. Changing brands fixed the problem There were some suspect runs of those caps at that time and I haven't read about any recent problems. I'm still using the brand I switched to though. Partly because the price of the Atoms is silly high anymore.

[nosaj]

I stopped using them because I had some new builds with symptoms of bad filters I suspected a failure on the sellers part. Sprague specs a shelf life and stock rotation regimen. This applies to any electrolytic cap but Sprague actually use to specify it. I don't think this is always done by distributors. Changing brands fixed the problem There were some suspect runs of those caps at that time and I haven't read about any recent problems. I'm still using the brand I switched to though. Partly because the price of the Atoms is silly high anymore.

Do tell.. .which brand?

nosaj

[Alex R]

from memory the vibrolux trem works by wobbling the bias up and down. No reason yet to assume the trem is the problem. As a first try pull out the PI tube - that'll tell you whether it's a pre or power amp problem. Pulling out other preamp tubes can also give a rough guide to the whereabouts of the issue.

[Leo_Gnardo]

Do tell.. .which brand?

nosaj

Lots of brands including "eminently trustworthy" Sprague Atoms. Goes to show ya, anything can be counterfeited. Our friends at The Amp Garage had a long running thread about 5 years ago, with photos. Should you have a peek, there's been a problem last week or so, claiming that TAG's a malware distribution site. Not so, just another website folly, they'll get it fixed one of these days. Link:

The Amp Garage :: View topic - Inexpensive Electrolytic Capacitors

[Chuck H]

Do tell.. .which brand?

nosaj

I build from scratch and don't do many repairs. I switched to radial leads because of the greater selection/availability, better specs and lower price. My choice won't help anyone that needs axial leads. That said... I'm using Nichicon PW series. I haven't had a need for axials in awhile, but the last time I did I used the standard F&T caps that all the amp parts distributors sell and didn't have any problems.

[Chuck H]

Lots of brands including "eminently trustworthy" Sprague Atoms. Goes to show ya, anything can be counterfeited. Our friends at The Amp Garage had a long running thread about 5 years ago, with photos.

That was actually later than when I had problems. But I read at the time that there were counterfeit Atoms around AND EVEN BEING SOLD BY MAJOR DISTRIBUTORS! It's probably happened a few times. How the counterfeits get into rotation with major distributors I can't imagine, but that was the word on line. I had always bought them from Mouser. Hard to imagine they were selling "street junk" and not buying direct from Vishay/Sprague.

I'm not bashing the Atoms anymore (I use to though). If I got counterfeits that shouldn't reflect on Sprague. And if I got old stock that wasn't properly treated that shouldn't reflect on Sprague either. But since I don't NEED axials I CHOOSE not to pay $18 US for the Atom 450V/100u cap rated for 1000 hours when I can get the Nichicon PW 450V/100u with lower ESR and rated for 5000 hours for $4.50 and a much lower possibility of getting a bad, new part!!! On my own builds that were made with axials I've even taken to shrink tube modifications for the radial Nichicons to retrofit them.

[guitician]

It could be the output transformer going out. Can you swap in a sub to check it?

[pintail78]

Hey everyone, it was the bias cap…..which ironically turned out to be a sprague atom! Working fine now thanks for the advice. JT.

[Chuck H]

And the winna is..... Leo Gnardo

BING! BING! BING! BING! BING! BING! BING! BING!

One and done

[Leo_Gnardo]

Those darn Atoms, ya just can't trust 'em. Clean, safe, too cheap to meter, woops that's the other kind (thanks Harry Shearer for reminding me every week.)

Atom smashers, they're so 20th century, now you can be an Atoms Basher.

[gui_tarzan]

I took the opportunity to measure my electrolytics last night after reading this thread, just to see where mine fall in. All of my high volt caps were within spec except one of my 100/100 F&T bias caps. It read in the mid 50s which really surprised me. Some of my F&T 80s were in the mid 90s but I think that's acceptable. Brands didn't matter, most everything was really close but I'm glad I took the time to check them.