ampeg svt keeps blowing tubes

daankill · #1 09-13-2011, 04:49 AM

i have an 2001 ampeg svt av. bought from guitar center 1 year ago. when i first bought it i got it home and a tube blew guitar center replaced them all with new electro harmonix. i have since then blown 4 more tubes each time i replace the set of 2. my tech said that there have been problems with e.h. tubes. anyone else heard this. i have 2 of these amps and am only having trouble with one . the other still has original sovtecs. any help would be awsome.$$

Webtroll · #2 09-13-2011, 06:54 AM

you'll need a tech or a warranty return to mfg to fix this i think, i can't imagine anything a user without amp repair experience would be able to do to fix this

jgroh · #3 09-13-2011, 07:06 AM

Maybe it had a step father with boundary issues...

Jerrold Tiers · #4 09-13-2011, 07:11 AM

I'm not sure what "blowing" tubes means here......

Can you describe what happens?

Tubes normally fail slowly, and gradually, or just quit working. Generally there is nothing going on that one would describe as "blowing" the tube..... So I am puzzled by the description, unless you mean the tube just fails.

Replacing 2 at a time is a decent idea, IF they are matched to each otehr and to the tubes still in there...

But even if of the same grouping match-wise as the old, new tubes will be different than old ones, and won't match as well. Any tubes that require MORE negative bias will tend to "hog current" and probably will fail faster.

Then also, if the amp itself has a problem, the tubes might fail faster. If for instance, one or more of the cathode current sensing resistors is open, the other tubes will work harder, dissipate more power, and may fail faster.

problems with the sockets or bias supply can cause one or more tubes to glow red..... that is not necessarily a tube problem, but more usually an internal amp problem.

Finally, if the manufacturer of the tubes didn't do their job right, the SVT has rather high voltage, and the tubes may arc. We had a shipment of a different tube type that were not properly "flashed off" at high voltage, and a large number of them failed during amp testing. They arced internally, failing in a spectacular manner with showers of "white fire" inside the tube.

We built a high voltage power supply and "flashed them off" at low current to get rid of the bits of crud that cause arcing (they are vaporized and caught in the "getter" material). The manufacturer should have done that.

So how do they "blow", and how do you know that they are bad?

beans-on-toast · #5 09-13-2011, 07:14 AM

I'm not aware of any problems with EH 6550's installed in an SVT.

It is possible that they are installing bad tubes. Dealers buy cases of tubes and weed out the ones that are not up to spec. The bad ones aren't always disposed of but are sold off as a group to some other dealer at a low price. They usually know what they are getting. These tubes eventually find their way into amps. This is why it is important to buy your tubes from a reputable dealer. Sometimes you pay a little more but you are getting burnt in and tested sets of tubes.

It is possible that your amp has something wrong with it other than bad tubes. For example, your power transformer might be overwound and is putting out a higher voltage than it should be. Your tech should be checking out the amp with the new tubes installed to verify that the plate and screen voltages on the power tubes are correct. If everything is in order, have them install a set of tubes from a different manufacturer. Their stock of EH tubes might be bad. If you want to take the risk, have the tech swap in the Sovtek tubes from your other amp and see what happens.

kosmicwizard · #6 09-13-2011, 10:02 AM

This "the plate and screen voltages on the power tubes are correct" is the reason you can't just put new tubes in (even a matching sextet) and not blow tubes!

RickenBoogie · #7 09-13-2011, 11:13 AM

You are in need of an amp tech. Take it in, unless you still have warranty, then take it there.

daankill · #8 09-13-2011, 05:21 PM

i really appreciate the help thanks. by blowing tubes i mean the first two times the tubes just stop glowing s bright as the others and the bias lights were very dim. the last time when i turn the amp on after letting it warm up in stand by there is a flash of lightning inside the tube and sometimes a "pop" sound
since it's been over 30 days guitar center says it's mine to deal with. the tech i talked to says that sometimes you just get bad tubes and it's a weeding out process. but this is getting expensive.

daankill · #9 09-13-2011, 05:23 PM

by the way i have been buying tubes from tubedepot.com
they are a couple bucks cheaper.

JimmyM · #10 09-13-2011, 06:32 PM

Quote:

Originally Posted by daankill

i really appreciate the help thanks. by blowing tubes i mean the first two times the tubes just stop glowing s bright as the others and the bias lights were very dim. the last time when i turn the amp on after letting it warm up in stand by there is a flash of lightning inside the tube and sometimes a "pop" sound
since it's been over 30 days guitar center says it's mine to deal with. the tech i talked to says that sometimes you just get bad tubes and it's a weeding out process. but this is getting expensive.

That's one nice thing about a tech...a tech can weed out the bad ones easier and more efficiently than you can, and then you just replace the ones that are busted. It's a little expensive but not too bad, and once you get a set that works, they should last a good while.

**B-string** · #11 09-13-2011, 07:05 PM

I actually have had trouble with EH 6550s holding up to high plate and screen voltages. The screen grid have flashed to the plate.

The EH KT88s hold up better (if you stay with EH).

Jerrold Tiers · #12 09-13-2011, 08:27 PM

Quote:

Originally Posted by daankill

i really appreciate the help thanks. by blowing tubes i mean the first two times the tubes just stop glowing s bright as the others and the bias lights were very dim. the last time when i turn the amp on after letting it warm up in stand by there is a flash of lightning inside the tube and sometimes a "pop" sound

OK, the flash of lightning deal is an indication of bad tubes.....

Actually, not necessarily "bad", but ones that a crucial manufacturing step was left out of..... the part about "flashing off" the tubes. That, or maybe they DID do it, but at too low a voltage, maybe they used the 6L6 voltage for 6550s..

EVERY tube has bits of crud in it.....stuck here and there where they shouldn't be. Generally, they tend to lower the 'flashover" or "arcing" voltage of the tube.

So, manufacturers are supposed to hit the tube with high voltage at limited current. Enough to zap the crud, but not enough to cause the "lightning". This step avoids problems with high voltage, and it really cannot be left out.

When it is done, the voltage is left on until no more arcing is found in teh tubes being processed. They are then good for voltages up to 80 or 90% of that voltage, peak.

The voltage used needs to be higher than the peak voltage the tube is expected to see, per the specs.

An amp with 450V B+ produces peaks of 900V or more on the tube plates. The SVT, with 700V or so B+, can produce peaks of 1400V or more. The difference can be significant.

Arcing can damage the tube and cause it to have low gain, or even basically stop functioning, depending on where it arcs, and/or for how long or how often.

it can also damage components around the tube, depending on what the arc is to inside the tube.

If that is going to happen with the EH tubes (which I HAVE heard bad things about) then DO NOT USE THEM.

Have the amp checked for problems caused by the arcing, and get a set of decent tubes in it. Should end the problem.

MusicBear · #13 09-17-2011, 06:28 PM

Take it to an authorized Ampeg service center. There was a mod issued for the SVT Classic to improve tube life. Your amp may qualify. There were also several other engineering changes issued for them as well addressing other issues. Specify that you want the amp checked to be sure all engineering changes/upgrades are in place.

Jerrold Tiers · #14 09-17-2011, 07:57 PM

Quote:

Originally Posted by MusicBear

Take it to an authorized Ampeg service center. There was a mod issued for the SVT Classic to improve tube life. Your amp may qualify. There were also several other engineering changes issued for them as well addressing other issues. Specify that you want the amp checked to be sure all engineering changes/upgrades are in place.

Those CSBs and the circuit changes are from 1997, and he says his is 2001. They would all be present already as normal production.

Rockmusician · #15 09-17-2011, 08:04 PM

Most Ampeg heads have a 5 year warranty on electronics and 90 to 180 days on tubes. If the tubes keep blowing, there must be a defect in the electronics. It should be covered under the warranty.

**B-string** · #16 09-17-2011, 09:20 PM

Quote:

Originally Posted by Rockmusician

Most Ampeg heads have a 5 year warranty on electronics and 90 to 180 days on tubes. If the tubes keep blowing, there must be a defect in the electronics. It should be covered under the warranty.

If it is a 2001 the warranty ran out 5 years ago?