Help with an SVT

[Buddha's Guitar Tech]

Have an old SVT...
So far, there was a 6X4 where there should have been a 6C4, and i found a fried resistor. Replaced some electrolytics, so now it sounds fine idling.

But it's way too quiet.

This is the first SVT i've worked on - 22mvac on channel two's first-stage grid gets me 300+mvac going into the power enclosure.

That looks like a solid jump to me, but i dunno if it's right.

Answering this might help me narrow down where i should be looking.
Thanks all.

[selloutrr]

What year is the SVT?

Make sure u have a snug connection between the preamp and power amp chassis.

Are you sure the tubes are good?

[Buddha's Guitar Tech]

Thanks for the reply.

Unsure of year, but it uses 12WD7s and 6C4.
The caps that came out were pretty old, so i'm thinking 1970s...?

Fairly sure i was getting good continuity between preamp and power stage.

Bought two NOS 6C4s and both yield same result.

i have two new JJ ECC832s for the 12DW7s; not enough for a full-on replacement.

12AX7 no problem, but I don't have any 12BH7s here.

i think all the tubes in the preamp section are ok.

At the moment, i'm not getting any crackle suggestive of a dying tube.

my next mission is to look for the 10R plate resistors on the 6550s and measure voltages accross them to make sure they are conducting.

[David Root]

I worked on two of them a few years ago, one of them my own '75. I think you're on the right track. The 22 ohm screen resistors, the 10 ohm 5W plate resistors especially should be checked.

I would also check the 8K2 15W dropping resistor right after the plate node in the PS. Mine had gone up substantially and had to be replaced.

This circuit dumps gain all the way thru it so all those gain dumping resistors are worth a look too.

[Buddha's Guitar Tech]

Shall give it a whirl tomorrow, thanks a bunch.

[Buddha's Guitar Tech]

Something semi-interesting...

The 8K2 resistor you speak of (R49) is a 4K7 - i think someone's done a little 'work' before i got here. Might explain the light-show i'm seeing with preamp plate resistors.

The 10R 6550 plate resistors all measure 5ohms, which is fine for my purposes.
But the drop along them suggests that four tubes are pulling around 15mA each and two conducting @ 26mA.

[David Root]

Glad I could help. Sounds like some nitwit wanted 800V on the 6550 plates! I would check everything now, a knucklehead that would do that is capable of anything.

Set up right the 6550s are supposed to draw 12mA each. At 695 Va. 26 mA is too much.

I was thinking about that 6X4 where the 6C4 should be, that must have screwed something up too.

[Buddha's Guitar Tech]

"a knucklehead that would do that is capable of anything. "

Yes, this evil genius will destroy us all!

Actually, you've struck a chord there - the two with the high currents are the fat-envelope style (simply marked KT88/6550) and the other four are old magnavoxes. Same grid voltage across all (-47).

Another couple of burned resistors later, this things is back to square one. Sounds awful mind you... preamp section terribly noisy. Wonder if the grounds/shielding need looked at.

[selloutrr]

make sure all bolts and screws are tight, transformers , chassis, pots. should help with the noise. If it's still running stock caps or anything over 15+ years it most likely needs a cap job.

[Buddha's Guitar Tech]

Already did the electrolytics, apart from the cans.

The flimsy ground connection between power enclosure and pre came undone at the molex and i got a very rude awakening earlier... 400 volts, yeah.

[UR12]

Something semi-interesting...

The 8K2 resistor you speak of (R49) is a 4K7 - i think someone's done a little 'work' before i got here. Might explain the light-show i'm seeing with preamp plate resistors.

The 10R 6550 plate resistors all measure 5ohms, which is fine for my purposes.
But the drop along them suggests that four tubes are pulling around 15mA each and two conducting @ 26mA.

Some of the older SVTs from the early seventies used a 4.7k 15 watt instead of the 8.2k. The 4.7k is probably proper for this amp unless someone was looking at the wrong schematic.

[Buddha's Guitar Tech]

With that 4k7, the preamps were getting way too much voltage. 400V original cap being inundated with 500v.
Preamp plate resistors burning up. Even the poor 6C4 cathode resistor (god bless its poor soul) giving me the death sparks.

Funeral held, phoenix rising... Beer anyone?

[UR12]

With that 4k7, the preamps were getting way too much voltage. 400V original cap being inundated with 500v.
Preamp plate resistors burning up. Even the poor 6C4 cathode resistor (god bless its poor soul) giving me the death sparks.

Funeral held, phoenix rising... Beer anyone?

There is also a 15k resistor that sets up a voltage divider. R50 IIRC. Might want to check it also. If it is way out of tolerance it might throw your voltages off. Just a thought...................

[UR12]

Double post

[Buddha's Guitar Tech]

R50 was one of my burn-outs, but on this model it's a voltage drop resistor before the 12DW7 plates in the poweramp. Maybe we're talking about the same thing but giving it a different name.