Vintage Ampeg SVT troubles

[Joe H.]

I'm a newbie to this site, so bear with me. I have a 1977 Ampeg SVT head which I bought brand new back in 1977. It is of Magnavox - Elkhart, vintage, serial # 117377 with split black line face. The nomenclature on the back says V9 - SVT!! It is all original except for one pre-amp tube and some diodes. It has several of these discontinued 12DW7 tubes in it, as well as the original Magnavox 6550 tubes. I use it with a 4 ohm 2 x 10 cabinet, and a 4 ohm 1 X 15 cabinet. I play in a Texas dancehall band that tours around the south Texas dancehall circuit. Last Saturday night during sound check in Beeville old Ampzilla died, just turned itself off. Luckily, I carry two GK 800RBs with me at all times for backup. I checked the fuse and it was blown. The fuse is a Littelfuse 3AG, 10A, 32V glass type. It is Ampeg factory original as far as I know. It also has a three digit series # on, but only the first number, 3, is visible. We thought we smelled something when the amp died, but we weren't sure.
Questions; Is it critical to use exactly the same type fuse in this amp, i.e. 10A, 32V, 3AG, or can I try a 10A, 3AG, 250V? I don't know if it is a slo-blow or fast-blow. Does it make difference? This is a high current, high voltage amp and I don't want to try to bring back up with the wrong fuse and do more damage. Also, is possible that since the fuse is 33 years old that it just failed due to age and fatigue? When you turn this amp on, the local substation sends out an alert to the power company. Class 2 wiring, yeah just barely!!! Anybody got any ideas? Thanks.

[Albert Kreuzer]

Questions; Is it critical to use exactly the same type fuse in this amp, i.e. 10A, 32V, 3AG, or can I try a 10A, 3AG, 250V?

10A / 250V will be fine.

I don't know if it is a slo-blow or fast-blow.

Slow blow.

Does it make difference?

Yes. A fast one will not stand the inrush current at power-up.

This is a high current, high voltage amp and I don't want to try to bring back up with the wrong fuse and do more damage.

If the new fuse blows again, take the amp to a tech. Chances are that you may have a bad tube.

Also, is possible that since the fuse is 33 years old that it just failed due to age and fatigue?

Yes.

Cheers,
Albert

[Amp Kat]

One of a kind and heavy as hell amp. If you can replace the tube that failed and caused the fuse blow and it sounds good still then let it go but as Albert stated that yes the amp is old and along with it are some of it's components like the Electrolytic filter caps which I believe in that amp are the can type black paper that are rivoted to the chassis as are the ones inside. If the amp still runs good with those in there which is a compliment to how well things were made back then you may want to keep them in. Total maintenance on it to bring it back to standard could be costly but well worth every penny as well as you know.

[52 Bill]

As for the fuse dying from metal fatigue:
The question is what does it look like? If there is no visible damage to the fusible link and the the fuse just doesn't have continuity, then maybe.

If the glass is blackened or the link is vaporized, there is something wrong with the amp.

[Joe H.]

Thank you for the SVT help

Thank you all for your insight on my old Ampzilla SVT. To Amp Cat, yes this amp has the caps that are the black cardboard type that are reveted to the chassis. I don't see any leaks coming from them. If the PT died then that's going to be expensive to replace I would imagine. But like you said, this SVT is a keeper. For bass guitar nothing blows as hard as an SVT!! Everything else just sounds cheap. I originally had the eight 10 bottom that went with it, but I just got tired of lugging around that refrigerator. To 52Bill, the fuse did vaporize. It isn't the type that has what looks like a piece of guitar string inside, instead it has (was) a strip of metal about 1/8'' wide, so I guess that it is a slo-blo type. I'm going out to pickup some slow blow 10A fuses this afternoon and try powering up the amp again. I thought I could see a crack in the glass of one of the 12DW7 pre-amp tubes, but I'm afraid to start screwing around inside this monster with all the residual charge that can be present. I'll just take it to our local repair tech for some TLC.

[Amp Kat]

Good choice and as far as the tube goes if it's cracked it will lose it's vacumn and turn chalky white inside and you'll know it's a gonner. I wouldn't suspect the 12DW's as fault for blowing a fuse but I'd have you're tech test the 6550's / 7027 because that is the biggest % of failure for blowing fuses by far.

[Joe H.]

Good info to learn

Amp Kat: I dig this website because I'm learning things about amps and tubes that would have been so useful to me through the years! The 12DW7 must be OK because I don't see any white caulk buildup inside. I should mention that a clown that I was playing in a local band with a couple of months ago was helping me unload my SVT head out of my van one night. For some reason (probably drugs) he dropped it out the side door onto a cement curb. Now this amp weighs about 85 pounds, so you know it took a hard hit, right onto one of the metal corners. Needless to say, I don't play in a band with this joker anymore. When I turned it on in the club it was kind of crackling, but after a while it sort of "cleared its throat". I've used it on a couple other gigs since then with no problem or change in sound until it died last Saturday night. Question; If it turns out that a 6550 is bad or damaged, what is the consequence of changing out just one of the power tubes? Isn't the sextet of 6550s supposed to be matched? I've yet to find a source for a matched set of six 6550s. I've considered replacing all the 6550s with a matched quartet plus a matched pair of ElectroHarmonix 6550EH from Parts Express. That's a causal $203 plus shipping. Any thoughts? Many thanks.

[Amp Kat]

Tubes are very subjective and as far as the 6550's go (current production) I really like the Ruby 6550 STR but the JJ's are very nice too and I'm sure Bob at Eurotubes would match you up a sixtet if you asked him or even Doug at Dougs tubes. Not sure how good the EH would be matched or if you would encounter an out of box failure because these scenario's are when it seems to happen. See if maybe you're tech has a tube offer because then he will be able to test them and see how they bias up and burn in after about 20 minutes or so. I've oftern had customers bring me tubes to install and finding one out of the bunch either bad or mismatched so consider that. Then again you may get a smooth trip and be just fine. Thing is you won't get the lastibility that you did with the ones the amp came with as those were NOS which I'm sure you can imagine what the cost of a sixtet of those would be.

[vistacruiser]

Got my SVT working

Joe,

I have a similar '77 SVT, ser# 133012. Same fuse problem too. I took it out on a cold night a week ago, after I set it up, it worked for about 10 mins. and then blew the 10 amp fuse. Replaced the fuse and it blew out again as I took it off standby.
After reading your thread and some others, I went at it to see what the trouble was.
I pulled all the 6550a's and replaced the fuse, with 4 ohm cab plugged in, I switched it on without the power tubes, did not blow when on or off standby. Checked pin voltage between tube sockets and chassis front grounding point, all good. There is alot of voltage here so be careful, pin 3=695vdc., pin 4=350vdc. Anyway, that told me the power transformer was ok. I shut it down and replaced the 6550's one at a time until I found one that arced and flashed inside. Shut it down quick! They make crackly noises too!
On inspection the bad tube had several cracks at the base, just above the metal clamping ring.
Don't know if the cold temps caused it, I don't have a road case, just a vinyl cover. I replaced all the 6550a's in 1988 with Westinghouse, so maybe it was time.
Anyway, I'm glad it was just a tube and I want to thank every one who gave advice that helped me get to the fix.

Thanks!
Brad

[Joe H.]

Again, thanks for the SVT support

You guys are the best! To Amp Kat; So what you are saying is it would be best to have a complete matched set of 6550s. Parts Express also sells Ruby Tubes 6550s, but again no sextet, only quartets, and for the same price as the EH. I will have to wait until my local tech isolates the problem, and if it's one of the 6550s, I'll order up a new set and him install and bias. EuroTubes has a interesting site and I'll probably talk to them about a matched sextet of 6550s. I'd hate to replace the original Magnavox 6550s, they've lasted 33 years. Right up to just before my SVT died, our FOH was telling me I was too loud, eventhough my volume setting usually is set about at 9:30, about where the tone on this head actually starts to kick in. I usually go direct into our sound system (miked), which runs at 14,000 watts. Sometimes I don't mike up my amp at all, and it's still plenty loud in the mix. To Vistacruiser; Congratulations on being part of the SVT brotherhood. To own and use a 1977 SVT is to know the karma. I will try what you did, pull the tubes and check each one in series. I didn't know you could do that but it makes sense. According to the Tube Amp Book by Aspen Pittman, the SVT was designed with a fail safe system so if a 6550 shorted out there is a 5 watt fuseable link that suppose to drop out the tube and let the amp keep running at reduced power. I don't know why that didn't happen with your amp and probably mine too. Any ideas? Thanks.

[vistacruiser]

Joe,

Yes these are incredible amps, I bought mine used in '82. I love your pet name "ampzilla". BTW, in Wisconsin, when I say "a cold night" I'm talking around 10 degrees.
About those 5 watt links, it's true they did not blow this time... but, several years ago when I wasn't playing much, the amp stayed at a friends house. I don't know what happened exactly, but I suspect it was turned on with little or no speaker load. Three of the Six, 5 watt resisters did blow. They connect pin 3 of the 6550's to the 695v supply. Also R-50, which supplies pins 1 and 6 of V1 in the pre was open. I don't know how that one relates, I'm not an amp tech, just a DIYer. But at that time the tubes and fuse were not effected. I mean, they were still good.

Getting your SVT running again, after it was dead, is like winning the lottery. Feels great! Just be careful around all that power. I fried the tip off one of my probes when I touched some where I should not have.

Thanks again all,
Brad

[soundmasterg]

Joe,

It sounds like all the electrolytic caps in that amp are original, in which case they probably should be replaced. Notice I said electrolytic and not ALL caps....just the electrolytics need replacement over time unless the others go bad. You won't often see physical signs of leakage (though sometimes you do or the positive end of the cap is bubbling) because there is another failure condition where the cap will leak DC. Caps are only supposed to pass AC but as they age the electrolyte inside them dries out and the cap does not work how it should. Replacing the electrolytics in a vintage SVT can be a challenge because most are can caps that are not made anymore so you need a good tech to get creative about how to replace them without butchering the amp, but it will benefit you with less hum and crackling, better and tighter bass, and an overall tone that is richer. More to the point however, a bad cap or two could have caused the problem you have in addition to being a bad tube, and if that is the case, replacing the caps will allow the amp to work properly again. Replacing electrolytic caps in vintage amps is sort of like replacing the tires on your car....you don't run around with 30 year tires on your car....shouldn't run around with 30 year old electrolytic caps either.

That said, it could very well be bad tubes. I've got a 1981 MTI-made SVT. These were made in Japan and imported. They use mostly Japanese components, though the early ones of these used surplus American transformers, and mine is one of these. The early ones also used the same exact circuit with the same tube complement. So it is essentially the same amp as yours except for the cosmetics. I retubed mine about 10 years ago and stuck in a sextet of Winged C 6550C's and they are very nice and work well. I use JJ KT88's in my Sunns and like those too, but for the SVT I used the Winged C's and couldn't be happier. You can get a sextet of them from Antique Electronics for $204 plus shipping. Antique Electronic Supply . They also sell other types. Any KT88 type will work in that amp also and might give a little more power. JJ makes a brand new 12DW7 that is fine and cheap.....you might want to stock up to have some spares.

Greg