Help trouble shooting an old SVT

[Leif_Bodnarchuk]

Idling it sounds ok, no pops or undue hums.

Cranked, it's like a 5-watt amp on '1'.

As this is the first SVT i've got into, i'm not sure whether the signal is low in the preamp stage or the power section.

On channel two, 22mvac on the first-stage grid gives me 24mvac on the 6C4 output.

(Channel one not working yet.)

i don't want to blindly replace the whole set of valves if i'm missing something obvious.

Anyone any ideas?

Thanks

[edit]: whoops, output not 24mvac, but 300+ ... my bad

that looks like a solid jump to me, but an outside opinion would be most helpful.

[soundguruman]

the B+ has crapped out or the output transformer is bad? the speakers are bad?
I would not start randomly buying and installing parts until a tech tells you what is really wrong with it. This is an expensive mistake to make.

An OLD SVT is a REAL SVT. There are no other real ones.
There are imitation SVTs, but they do not live up to the name.
Since it is a real one, it is worth fixing RIGHT.

[Leif_Bodnarchuk]

i am the tech.

Got most of it sorted. The last guy in there replaced an 8k2 HT drop resistor with a 4k7, causing too much voltage to get into the preamps. Too much current and subsequent burning of plate and cathode resistors.

And there was a burnt resistor in the power section - 15k drop resistor supplying the HT for the 12DW7 plates.

It now sounds like an amp... but not great.

i'm thinking maybe a ground loop or bad shielding between the pre and power enclosures, as it is quite noisy.

[soundguruman]

In the SVT*, there is a kind of sloppy AC heater layout on the circuit boards.
The AC heater lines are located too close to the signal path. This inevitably leads to some induced AC hum in the signal path.
A good way to deal with it is to change the preamp heaters over to DC.
There is also one particular tube which is used as the cathode follower in the final stage of the preamp. The 6C4 is particularly microphonic and overly sensitive. It is very difficult to get a high quality low noise 6C4.
(tap on that one with your chopstick, and you will see what I'm talking about)
In some of these, I have converted the 6C4 to a 12AT7 instead.
Then, if you remove the front grill, you will find the balance and bias controls, hum balance, which may be helpful in lowering the noise.

But old filter capacitors, out of balance power tubes, ceramic disk capacitors in the preamp stages, lack of shielded audio path wires, will all contribute 60 cycle hum in the output. And yes you are right, there is a couple ground loops too.
In other words, to make one work just right, and minimize the noise, it may take several hundred dollars in parts, tubes, and many hours of work. This is not an easy amp to work on. OK I admit I like KT88s in the output stage.
BUT it's worth it, to have a real working SVT, the finest and heaviest bass amp ever made. (85 pounds, dry)
At least the Stones and the Faces think so.
There has never been a bass amp made that was as great. And there probably never will be again.
And there is your motivation for tweaking it out just right!

*the real SVT, not the imitation ones. The real ones were made by Magnavox.

[Leif_Bodnarchuk]

Biggest change i made in the end, apart from changing some electrolytics, and plate / cathode resistors was a separate audio cable from the preamp to the poweramp. Got the original signal carrier off the umbilical cord and provides a more solid earth connection.


You make some good suggestions there. i don't think the owner wants to pay a for a zillion changes though. i've suggested all new 6550s, as they are poorly matched, but hey, i ain't spending my money on it, so there you go. Sounds a lot better, so we're good there.

[Ted B]

I have a '70s SVT rig that arrived seriously down on power and with issues.

- New filter caps
- New tubes (including JJ KT88s)
- Replaced several resistors in h.v. locations whose value had drifted considerably.

The amp sounds like new.

Was it cheap to service? No, but that's the nature of the beast. They don't need service often, but when they do, it costs.

[Leif_Bodnarchuk]

Awesome, can i quote you on that?

[Ted B]

Awesome, can i quote you on that?

I think you'd better!

[soundguruman]

Out of match power tubes will not help the hum situation. This causes hum.
But you do have a balance control in the front of the chassis.

[Leif_Bodnarchuk]

if i may sharpen my pencil - out of balance valves don't exactly cause hum, they just don't have the combined capacity to cancel the inherent hum that the reservoir caps don't quite smooth out.

Well, that's my take on it.