guys, I need help w/ an SVT

[stevlech]

Long story short: All original I think ( original 6550's)
Powered up- one tube arced and insane crackling
cheap customer who wouldn't pay to get it serviced
has had issues with the amp for months
has a gig tonight, of course
I have no experience with something this powerful
gut feeling- screen failure?

Any wisdom/guidance/advice/magic spells/etc are greatly appreciated

[UR12]

You are probably right to suspect the screen resistors. You have to pull the board up to get to them if I remember right. Check them all. Also check the power tube sockets and reflow any bad solder connections to the tube socket pins.

[David Root]

I've had exactly the same situation with an SVT. Two of the six screen resistors had croaked. Also possible suspects are the plate resistors. IIRC, the plate voltage should be 695 VDC and the screen 325 VDC. Just keep one hand firmly in your pocket when you have the HT on.

I've overhauled two of these old warhorses, one was the above, the other had been cheaply retubed w/12AX7s where there should be 12DW7s. Check that too, because anyone who sez you can sub a 12AX7 for a 12DW7 in this circuit is full of it.

It's an incredible piece of design work, and we shall not see its likes again. They deserve to be resurrected!

[stevlech]

Thank you oh so kindly. Fortunately, the customer understands that there is a real problem and a sound company is willing to fix it and loan him one of their SVT's for the show. I really appreciate the kindness and helpful nature of this forum. Some others are just a stage for members to spout off opinion after inconsequential opinion, but members here seem genuinely concerned with keeping the world full of tube tone.

Thanks again

[Allynmey]

It's usually the screen resistors. They are more like fuses at that current.