ampeg SVT noise issue

[pontiacpete]

I have a 70's SVT that's making a severe breaking up noise. This amp doesn't get used a great deal for a month or 2, but this passed month it was used for a week straight on gigs. Started out fine and toward the end of the week it started making with this horrible noise upon some heavy attack.
I noticed it has some paper filter caps in the power supply and wondered if they should be replaced? Along with the ones in the cans? There's a 30uf/600vdc cap that I think might be OK to leave in.
Also the coupling caps used in this amp are these yellow, square radial types, are these paper and suspect to fail?
I've checked all the tubes and they all appear to be good.
Any suggestions?

[Amp Kat]

The first thing to be concerned with in this amp is the tubes as I've found 1 or sometimes 2 of the 6 -6550's will be bad and cause the symptoms you listed. Have also seen a bad 12DW7 in V3 cause that sympton. I would suggest a cap change in that amp and the paper cap is a multisection cap as well as the cans. You can find a close replacement at Tubesandmore .com and I used a few Axials in the supply. Keep an eye on the increasing voltage from a cap job and you may have to tweak that dropping resistor that goes from the 700 volts to the lower preamp tube voltage as it can change with the cap job but yes it (caps)could also be the problem. Try running just 2 or even 1 6550 and see if you still get that noise. I think those coupling caps are probably ok but just depends so it wouldn't hurt to check them for DC leakage.

[pontiacpete]

Thanks AmpKat
What about the electrolytic in the preamp?
Also, what do you suggest as a technique to check for DC leakage?

pete

[Amp Kat]

The more e-caps you change the better you'll be as those are 40 years old and most likely all dried out just depends on the climate and usage. I found one the other day about 40 that was still full of electrolyte. Check the coupling caps for DC on the opposite side of the high voltage with the black lead on chassis ground and if there is over a few volts of DC on them I'd change them. I'd be more worried about the e-caps though than the signal caps..

[pontiacpete]

Thanks, AmpKat!

[Euthymia]

It makes a crackling noise when you play a note?

An old SVT can have any number of problems that lead to that symptom.

1st, make sure all your tube sockets are tensioned and clean of oxide.

After that, one or more preamp/driver/splitter tubes may be making noise.

Pound on the top of the cabinet with your fist and see if you can induce the crackle. That'll point to a bad connection and/or microphonic tube.

Electrolytic caps are way down the list of things I'd check given that symptom, but it is true that it won't hurt to replace them anyway.

[pontiacpete]

Not quite a crackling noise, it's actually more like a loud distorted fart jumps out which on stage was noticed by the band leader and audience.

thanks for the suggestions, will give it a try.

pete

[Amp Kat]

Erik is correct on all of his analysis and could be a number of things. I can only tell you what changing the caps in some of the ones I repaired did and it was 10 fold better sound. Farting out could be several things including tubes,connections ,low bias, bad sockets,open resistors,bad coupling caps and yes E-caps. It's not an easy amp to work on, it's heavy as crap and frustrating. You have to drill the paper caps out because they are rivoted in and the replacements aren't the exact caps. You'll like it though when you finish it and there is no other amp like it in the world as far as quality tone in Bass Amps. Did I mention that I heard you the last time you played it .

[Metro]

I third what Euthymia and Amp kat suggested. Generally the first thing I would check is the preamp and phase tubes, then follow on with the other checks as above. A good way is with the volume lowish and plugged into the cab tap each tube with a chopstick or pen. If there are any microphonic tubes they will usually pop or fart when you find the culprit. Good luck

[pontiacpete]

Thanks to everyone who has replied.
I'm working on replacing the e-caps. I went got some sprague 100uf/450 's and 40uf/450s. Of course, these are huge. I didn't know this when I placed the order. Probably should have looked up the dimensions. I could use another brand and save on space, also could restuff the cans....but I'm inclined to go with the spagues.
Is it okay to use either of the 2 cavities next to the PT and OPT?
Does anyone have any suggestions from their experience with tacking them in place that they'd like to share?
thanks very much, again

regards
pete

[Amp Kat]

I used the multisection replacements from Tubesandmore that had close to the same values as the original and used screws to tighted the can holder instead of rivots but if you have a rivot gun that will work also. I layed axials exactly where the others were laid down and silicon to hold them in and tie wraps. You could use that area by the tranny's but the existing can holes are the way to go. You could also mount terminal boards in the holes where the rivots were and put the across the terminal strips if you had to.

[pontiacpete]

thanks ampkat

Went ahead replaced all the e-caps, the cans as well(there I put in the F&T 50/50uf @500vdc and the 100/100uf@500vdc). It's getting higher voltages alright - at the top (+)of C23 there's 400vdc - a 100v higher than what the schematic calls for. Said something about a dropping resistor to we aware of, are you talking about R49?
At this point, I'm getting a square wave. Sounds like it's oscillating or something, no sound.

pete