Ampeg SVT VR tube is dead

D Bopp · #1 08-28-2010, 01:40 AM

I received an SVT-VR this week, and finally got to play it live last night at my gig. The sound was incredible for about 45 minutes. Then it started crackling and popping. I looked in the back and saw one 6550 tube was not lit up. All the others were blue while the amp was off standby.
Do I need to replace the set of tubes, or just the dead one?
This might be a newb question, but I just don't research this stuff too much, so I'm not sure.

JimmyM · #2 08-28-2010, 01:48 AM

you migh be able to replace the dead one if you can find a match for the other two in the set, but if not, the most you should have to replace is 3. any tech who says you need to replace the entire set is either incompetent or a thief. svt power tubes work as triplet sets, and each triplet set has its own bias.

DLEE · #3 08-28-2010, 05:26 AM

Replace the one tube,I've done it on my old SVT's and never had a problem. You'll readjust the bias when you do it so it will be within range. Back in the 70's I blew a power tube and just popped a new one in with no rebias. I played it for over 2 years before I did a tube change. Never had one bit of problem with it. Probably not recommended but it worked.

silvertones · #4 08-28-2010, 05:49 AM

I agree with replacing 3 however if it was me, and I know it's a fair amount on $$$, If you bought the amp used ,as i suspect, I'd do them all and save the others for spares just in case.

beans-on-toast · #5 08-28-2010, 06:22 AM

The blue in the tubes is not a problem. As has been said, the power section has two sets of three tubes.

The SVT is biased conservatively by design. If one of the three tubes goes, the other two will not be pushed beyond their operating limit. This means that the other two will not blow because the third one died. It doesn't mean that you can run the amp with three tubes on one side and two on the other. It means that you have enough time to shut the amp down while minimizing any damage.

In a pinch you can pop in a spare and carry on. The bias won't be optimal but it will work fine.

You can get the working tubes tested and if the others are still good, depending on your finances, either replace the single tube with one that matches the other two, replace three, or replace all six. With either option, the bias and balance settings will need to be checked.

For new production tubes, I like SV6550C SED "Winged C" tubes in the VR. They are based on the original GE 6550 which is the holy grail for these amps.

Bassmec · #6 08-28-2010, 07:00 AM

There might be markings on labels on the valve bases reading
something like: M/A 44 and M/AV 5
These Ancient hieroglyphics are for tube matching purposes if indeed these where a matched set in the first place.
Order a tube by the same manufacturer within similar trans-conductance /bias range and having checked the bias
you are good to go assuming the grid resistors are fine.
Name and shame the tube manufacturer for us though will ya!

D Bopp · #7 08-28-2010, 09:03 AM

Quote:

Originally Posted by Bassmec

There might be markings on labels on the valve bases reading
something like: M/A 44 and M/AV 5
These Ancient hieroglyphics are for tube matching purposes if indeed these where a matched set in the first place.
Order a tube by the same manufacturer within similar trans-conductance /bias range and having checked the bias
you are good to go assuming the grid resistors are fine.
Name and shame the tube manufacturer for us though will ya!

The tubes in there now are Sovteks. But I just had the head shipped to me from San Francisco to Atlanta. The shippers did a good job of removing and wrapping each tube in bubble wrap.
When I played the head last night at a gig for the first time, it was fantastic.
It started making a popping and cracking sound. Do you think that is from the tube being out, or might I have another issue with the head? It happened a few times while I wasn't playing at all, but mostly happened when I played loud a few times.

Tim Skaggs · #8 08-28-2010, 09:14 AM

I would definitely try a tube first, but you may find it's something other than the tube that's affecting that tube socket.

Not much way to tell via text exchanges; popping and cracking covers dozens of possibilities. A dark tube is at least a starting point.

BassmanPaul · #9 08-28-2010, 09:38 AM

By not lit do you mean that this one tube was not glowing blue or that the heater was out?? I too would try one tube. You should watch very carefully to see if its plate gets any hint of red. if so don't use the amp until you get a re-bias done.

Paul

D Bopp · #10 08-28-2010, 09:45 AM

Quote:

Originally Posted by BassmanPaul

By not lit do you mean that this one tube was not glowing blue or that the heater was out?? I too would try one tube. You should watch very carefully to see if its plate gets any hint of red. if so don't use the amp until you get a re-bias done.

Paul

I don't think anything is coming out of the tube. Even when the head is on standby, it's not glowing orange at the bottom like the other tubes.

BassmanPaul · #11 08-28-2010, 10:20 AM

Then the tubes heater is open and the tube needs to be replaced.

Paul

D Bopp · #12 08-28-2010, 10:29 AM

Quote:

Originally Posted by BassmanPaul

Then the tubes heater is open and the tube needs to be replaced.

Paul

Ok. I will change the tubes and then let you guys know my results. Thanks! I'll probably go with the Winged C's as mentioned before.