Ampeg SVT VR New Tubes Bias Control / Balance Issue

[dw1003]

Hi Everyone!

Home from tour and finally got around to putting new tubes in SVT VR
I got 6 Sovtek 6550WE Tubes (from tubedepot.com matched sextet no 24 burn in), swapped them out and I've got a few issues...

1. Bias Control 1 (Green LED) is not lighting when it's taken out of standby. I've adjusted it and cannot get it to light.

2. There is a small (but present) hum when the amp is taken off of standby. As I adjust Bias Control 2 (the one that does light) The hum gets a bit louder when it gets to bright green.

3. Should the balance light be glowing yellow when taken out of standby? I believe this balances the driver tubes 2 12au7's...

Both Bias Control 1 and 2 glow green and the balance LED glows Yellow when the amp is IN STANDBY. When taken OUT of STANDBY I can only get Bias Control 2 to glow bright green when adjusted.

For background... I blew (cracked) 2 tubes on the last tour, one from each side of 2 power tube clusters (top right of triangle one, bottom left of triangle two... I believe this is how they are clustered)

Thanks! - Daniel

 

[JimmyM]

Swap the power tubes to the other side and see if the problem follows.

 

[dw1003]

OK will do!

thanks

 

[dw1003]

fyi currently all six tubes glow normal and look good when powered on

 

[dw1003]

same problem after switching the tubes around

 

[rodl2005]

Does the OTHER light now not glow? Did the problem follow the swapped tubes?

 

[dw1003]

no the problem did not follow the tubes. Bias Control 1 still won't light

 

[beans-on-toast]

When your tubes blew, they make have taken their screen resistors with them. If these are open, the amp will not bias properly. They need to be checked to see if they are open.

There is one screen resistor for each power tube. Unfortunately they are located deep in the amp on the power tube board so some disassemble is required to inspect and replace them. They are 220 ohm, 1/2W, 5%, flameproof components labeled R2, R7, R8, R9, R10, and R12 on the power tube board. The resistors are intended to act as fuses and blow to protect the transformer. So it is best to replace them, if necessary, with a part that meets the required specs. Some people install larger wattage resistors to prevent them from blowing when there's a problem with the tube. This isn't good.

This is the part that I use, NFR25H0002200JR500, available here: Invalid Link Removed.

 

[dw1003]

Ok great thank you! Sounds like a trip to the tech... Can anyone recommend an affordable ampeg guru in LA area? Thanks!

 

[beans-on-toast]

Ok great thank you! Sounds like a trip to the tech... Can anyone recommend an affordable ampeg guru in LA area? Thanks!

If Thousand Oaks is convienient: JohnK_10. There is a list of techs in the FAQ at the top.

 

[dw1003]

Update... fried some tube resister screens. My friend who is a great amp tech was in town and was nice enough to service the amp for me. replacing all the fried resisters and hopefully be up and running tomorrow!

 

[beans-on-toast]

Update... fried some tube resister screens. My friend who is a great amp tech was in town and was nice enough to service the amp for me. replacing all the fried resisters and hopefully be up and running tomorrow!

Yes those are the ones. Better the resistors than your output transformer.

 

[beans-on-toast]

Looks like scorch marks on other resistors as well. Maybe superficial collateral damage but they should all be checked out.

 

[dw1003]

yes, definitely everything will get checked at this point. While the amp is all taken apart, it's a good opportunity to clean and test everything...

 

[dw1003]

Resistors replaced, tubes tested, amp is biased and balanced... Sounds great one again!