Ampeg vt40 combo old enough to smoke

[Rays]

On standby amp is quiet all filaments normal. When I disable stby switch amp begins to buzz very loudly. To a point where I need to hit standby quickly before I blow the speakers. Also some smoke and arching from the preamp solder side section around r10. I have a hunch what is going on but I'll look to you folks to confirm my suspicions.yes I pulled the plug..

I dont know the value of this amp and wonder if is worth the effort and cost. The amp is in very good physical condition. nice pair of 7027a groove tubes as well..........

[Rob Mercure]

Rays,

The VT-40 is a loud, clean well built amp and a working one in good conditions seems to be worth around $350-500 depending on the market, etc., but most I've seen go for the low side of that estimate. Generally, for roughly the same wattage and features Ampegs are undervalued as compared to Fender which is sorta the standard,

Funny place for around R10 as my schematics show this as a resistor in series with a switchable cathode bypass resistor with only around 22 volts there - if this area were shorted you'd have lotsa distortion but no smoke. Instead something that's drawing lotsa current is shorted. I don't know what your level of experience is sot I'm not sure how much to suggest but first off lemme tell ya that under no circumstances should you operate the map with the speakers disconnected - you stand a good possibilty of frying the output transformer if you do. But you might try pulling the 7027s and see if the buzzing stops - if so there's something funky in the output stage (tubes, bias, screen resistors, etc,.). If not then there may be a glob of solder or something shorting the heater, B+, or primary circuits to ground.

What's your hunch? You've got the amp in front of you so your hunch may be the most valuable diagnosis. Oh, by the way, you noted a "pair" of Gravy tubes 7027As - this amp should have four output tubes, are there there two other output tubes? If they are severly mismatched this could be your problem as the amp doesn't have individual output tube bias so you have to use fairly well matched quads.

Rob

[Rays]

Hi Rob,

yep a duet not a quad. the internal tube chart shows two power tubes. model says vt4o!!I pull the power tubes no buzz

I put the amp away about two years ago as it sounded somewhat distorted but plenty of power. No symptoms like the present! The amp was never touched.

My hunch bad caps in the power supply maybe, causing funky AC ripple for those circuits that feed on dc causing the smoke...

Maybe you can add more based on this info.

PS I do have a dummy load.

Thanks

[Rob Mercure]

Rays,

Well, if you're cruising on the original filter caps all sorts of trolls, gremlin, and ogres can come out of the amp so replacing them first is a great idea. Might not solve your problem but your amp sounds like it's in good enough conditon to recap it and them continue diagnosis.

Funny about the model numbers but Ampeg is known to do this and I've got 4 X 7027 schemos but the circuit is probably identical, or pretty dang close. Now believe it or don't tubes can fail in storage (lost a great pair of Sylvania STRs in a B-15 in storage - tubes just lost all emission with no damage) so it's possible that your tubes have simply failed. JJ makes a good 7027A that's fairly inexpensive (identical to their 6L6GC with the extra base pin connections) but any good quality ("winged C") 6L6GC will work - you need a matched pair - and might solve your problem. There' nothing special about "gravy tubes" 'cept the price and most of their tubes are simply selected tubes that someone else, like the companies I've mentioned and others, have manufactured.

Oh, and the smoke. Can't understand why you'd have it from the R10 area but when you recap it you should replace that cathode bypass capacitor so you'll be able to inspect the area well and find out what's shorted (probably a fried ogre <grin>).



Have fun with this beast, if you get it running it'll outlive you.

Rob

[telemoxy]

This is a great amp and it is definitely worth fixing IMHO.

I just did some extensive work on mine and it really paid off. Here is a link to my thread " Ampeg VT40 Tube Replacement Blues" on the subject. To get the goods go directly to the last post "I did it". If you can read the other posts do so. Our compatriots on this board gave me some very valuable advice and interesting info.

http://music-electronics-forum.com/s...ght=Ampeg+Vt40

The VT22 combo had 4 7027As. The VT40 has 2.

I found 7027As to be more money for no real gain. Ampeg wired the VT40 for 6L6GCs. That is good advice on the Winged C SED 6L6GC. This is what I settled on and they are outstanding. It is very important to have the amp biased when you change power tubes in my experience.

Once I replaced the filter caps I was amazed at how quiet and powerful the amp is.

Good luck and let us know how it works out.

[Mark Black]

I've seen versions of the VT-40 schematic that were almost identical except the part designators were different, hence R10 on some schematics (and some circuit boards) is not R10 on others.

Any way to scan & post the schematic that is on the amp cover? They are usually correct for the amp they are on...

[Rays]

Ampeg vt40 combo old enough to smoke

Hi Rob,Guys,

BTW the amp is a combo with 4-10's. 68 pounds of beef too.

Yes, I tried known good jj6l6's for a quick spin, same problem. I mean this buzz is loud!

I know PS cap kits go for about 60+ bucks. need to wait on funds.

I looked into the pre amp and it seems that the arcing is between the pc lands!

no burnt components to the eye. still have to check for shorts though.I still think the issue is power related.

I'll keep you posted, may be some time though.

Guys,
Thanks for your input!!