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Driver Tubes (SVT)

Groover

Groover

Jun 28, 2005
1,756
19
Ohio, USA
#1
The SVT-HD (maybe others as well) has 3 Driver tubes. (1) 12AX7 and (2) 12BH7.

Does one 12BH7 drive three of power tubes, and the other 12BH7 power the other three power tubes, or do they all work toghether?

The reason I'm asking is that I'm thinking of using some older RCA tubes, but one is a Black Plate, and the other is a Gray plate.

Keep them the same, or is it OK to mix?

Any advice on these particular tubes would be great.
 
#2
The driver tubes should be matched for best performance. For the price of a matched set i think it would be preferable to replace them as a matched set. It is interesting that it uses 12bh7 tubes instead of 12au7's like the classic. I know that they can sometimes be interchangable but i thought the 12bh7 had a higher current draw? Would also be interesting to hear from someone with more experience with these 12bh7 tubes than myself.
 
#3
The original driver tubes were 12BH7's, as in when the amp was first designed and built. Interesting that they fallowed the original design thast closely! Maybe there is hope for LOUD Tech?? Are they also using the 12DW7 and 6C4? I would hope they used a single section or paralelled the two triodes of a 12AU7 instead of the 6C4, that was always the weak link. I was using a high temp O ring under the 6C4's to dampen noise as most I could find were microphonic.
To the OP's question I don't have that schematic here at work, but maybe Johnk_10 will stop by.
 
#6
Yes, each 12BH7 is driving three power tubes. The 12AX7 is not actually a driver. It's a phase inverter.

On paper, the driver tubes should be at least resonably matched for best performance.

In reality, tubes' specs drift during their lifetime and it's pretty rare to find truly matched drivers or power tubes in musical instrument amplifiers. It's much more important in Hi-Fi applications.

You can always try your old RCA tubes and see how they sound. You won't hurt anything, and if you like the results you don't then have to shell out the dough for a matched set of tubes. If you don't like the result, you haven't lost anything by trying.
 
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#10
The 12BH7 has a higher current capability than the 12AU7, but in the SVT circuit, you can't USE that.... so it doesn't matter....

There is a 47k ohm resistor in series with each grid...... so the worst case load is a total of about 15k... well within the 12AU7 capability, and the 12AU7 is a LOT more available. And they plug in the same socket

The noise I recall from JAN 6C4 was always hiss noise... no O-ring fixes that. We had a lot of trouble.
 
#11
The trouble I ran into was microphonics not hiss with the 6C4's, most didn't even give hiss a thought? I guess with tweeters and better drivers today, hiss now becomes an issue. There are still two amps out there that have a 12AU7 adapter in the 6C4 socket that I made. They were a pain to make but worked nicely.
 
#14
JimmyM said:
why is it that 6c4's are always microphonic? something inherent in the design?
Click to expand...

The tube was orig. designed for use in small radio receiver circuits not high gain or Hi Fi circuits so shortcuts were taken in the structure of the tube.

derridiandrift said:
The OP is talking about an SVT-HD. You referenced LOUD... are you confusing the OP's 80s head with the current production SVT-VR? The VR uses a 12AX7 and two 12AU7s for driver tubes.
Click to expand...

Yes I did brain fart, I for some reason was thinking of the new US built offering from LOUD.
 
#15
I just picked up one of these.... it's probably going to hum like crazy... Doesn't look very solid...

!BnlpZPw!Wk~$(KGrHqEH-D8EuDDnkEgwBLkDhRpnRw~~_12.jpg
 
#18
None of them are very solid. It's a... er... feature of the design. Yeah. Let's call it a feature.

The O-ring can go around the tube itself, like a belt or around the pins before you insert the tube (which works better). Around the tube, it helps dampen vibrations in the bottle. Around the pins, it creates a cusioned "seat" for the tube to sit on which helps isolate it from vibrations.

It's not foolproof, but it does help.

Old Gibson amps using 6C4s and 7199s used to be built with tiny O-rings around the mounting rivets for the tube sockets; one above the socket and one beneath it. It was the same idea, but it isolated the socket from the chassis instead of isolating the tube from the socket. It also worked pretty well - to a degree.
 
#19
Place the O ring around the tube pins before inserting the tube is how I did it. AC O rings or hydraulic O rings are usually rated for the heat from preamp tubes. O rings for R134A are a higher temp O ring than the old R12. I never tried to place the O ring around the tube glass, they do sell dampners but I was always concerned with pin spread due to the added wieght?
 

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