Tube Questions: 12AU7 in SVT-3PRO - What does it do exactly?

Groover · #1 05-20-2010, 09:41 AM

What is the function of the 12AU7 in the SVT-3PRO amp? Says driver tube. Is the 12AX7 shown in specs is the phase inverter, or is the 12AU7 the phase inverter? - maybe there isn't a phase inverter? Sorry, confused here I'm comparing to SVT tube functions in my head, and trying to make analogy.

Specs:

3 x 12AX7 Preamp Tubes
1 x 12AX7 Plus 1 x 12AU7 Driver Tubes

If anyone could clarify that would be the awesome!

MAMMOTHvolume · #2 05-20-2010, 02:04 PM

driver and phase invertor refer to the same thing.

Groover · #3 05-20-2010, 02:51 PM

My SVT-HD amp has a Preamp Section (5 tubes), a Phase Inverter (1 tube), then driver tubes (2 tubes), then off to the power section (6 tubes).

Just analogy here, the SVT-3PRO has the Preamp Section as (3 tubes as shown on Ampeg Site), a Phase Inverter (maybe the single 12ax7?) then Driver tube (maybe the 12au7 as it's draws more current perhaps?), then off to the Solid State Power amp section.

I found the schematics here, but I'm not electronically knowledgeable enough to determine which one is the phase inverter

http://www.mif.pg.gda.pl/homepages/tom/schematics.htm

Thanks again

BassmanPaul · #4 05-20-2010, 03:01 PM

Quote:

Originally Posted by MAMMOTHvolume

driver and phase invertor refer to the same thing.

No not necessarily. While the phase splitter/inverter usually drives the output tubes directly, many many amplifier topologies insert a tube section between. In the former case the PS is the driver tube in the second it is not.

Paul

BassmanPaul · #5 05-20-2010, 03:08 PM

Looking at the schematic, the amp does not have a phase slitter as such. There is a driver tube feeding a complementary output section. Thus while one set is being pushed positive the other set is pushed negative by the same feed.

Paul

Groover · #6 05-20-2010, 03:12 PM

Cool, thanks! So why wouldn't they make the two driver tubes the same? (the 12AX7 and the 12AU7) - Those are the two that you are referring to, right (as in one being positive and one negative push)

Thanks again

rickdog · #7 05-20-2010, 10:13 PM

Quote:

Originally Posted by Groover

Cool, thanks! So why wouldn't they make the two driver tubes the same? (the 12AX7 and the 12AU7) - Those are the two that you are referring to, right (as in one being positive and one negative push)

Actually, the 12AU7 is connected as a push-pull driver. The key to understanding that is C4, which connects the plate of the upper section to the grid of the lower.

Think about what happens as the input to the grid of the upper section goes more positive. That section will conduct more, increasing the voltage drop across R8, so the voltage at the plate decreases. C4 couples this to the grid of the lower section, and the more negative voltage decreases the current through the lower section. The opposite happens when the input to the grid of the upper section goes more negative: the upper section conducts less, the lower section conducts more.

This gives the driver stage the capability to both source and sink a lot of current, so it can drive the capacitance of all those MOSFETs in the output stage.

The 12AX7 just provides some voltage gain (the first section) and current gain (the second section) to drive the 12AU7.

Why is that last tube a 12AU7? Well, my tube books are out of reach, and I don't want to disturb the cat in my lap

but I think it's because the 12AU7 has lower transconductance but can drive a little more current. If you want to experiment with something in between, you could try a 12AT7 there, but I haven't tried it in my 3PRO, so it's up to you.

Groover · #8 05-21-2010, 06:13 AM

Nice breakdown rickdog! I found some tube datasheets linked below. In a nutshell and to my understanding, the 12au7 has twice the transconductance, ten times the current, and about 20% of the amplication factor compared to a 12ax7.

So whatever that means. It's starting to make sense. I'll take a look at a 12at7 if I do end up trying different tube types.

www.drtube.com

and specifically good comparable 12ax7 and 12au7 ones linked here

http://www.drtube.com/datasheets/12ax7-rca1962.pdf

http://www.drtube.com/datasheets/12au7a-rca1961.pdf

Thanks again for the great replies.

Groover · #9 05-21-2010, 06:57 AM

I also just found this. According to this review, the 12AU7 in the SVT-3PRO is an "Overdrive" tube.

http://www.retrosynth.com/~analogued...guitars/ampeg/

MAMMOTHvolume · #10 05-21-2010, 07:28 AM

Quote:

Originally Posted by BassmanPaul

No not necessarily. While the phase splitter/inverter usually drives the output tubes directly, many many amplifier topologies insert a tube section between. In the former case the PS is the driver tube in the second it is not.

Paul

in most amps they do, SVT's are generally in AB2 and the second valve provides more current gain / drive to drive all the 6550's up.

Also i don't know a whole load of amps, that use a similar set up to an svt, theres just not a whole load of point in smaller valve amps.

BassmanPaul · #11 05-21-2010, 09:31 AM

I would be concerned with subbing a 12AT7 for a 12AU7. The difference in plate dissipation may be too much for an 12AT7. As Rick says, that gate capacitance of the twin banks of MOSFETs requires a low impedance, high current drive. The 12AU7 is ideal for this, the 12AT7 less so.

Paul

BassmanPaul · #12 05-21-2010, 09:37 AM

Quote:

Originally Posted by MAMMOTHvolume

in most amps they do, SVT's are generally in AB2 and the second valve provides more current gain / drive to drive all the 6550's up.

Also i don't know a whole load of amps, that use a similar set up to an svt, theres just not a whole load of point in smaller valve amps.

As a matter of interest, are you talking MI amps here? There are many HiFi amps out there that use a driver tube after the PS. A lot of bass amps do too.

Paul

Groover · #13 05-21-2010, 09:40 AM

Thanks again Paul. I'll most likely stick with the 12AU7. I'm just going to try a few variants, as in brands, plate types etc.

Jerrold Tiers · #14 05-21-2010, 09:46 PM

The 12AU7 is used substantially because of the gate drive issue........ and the bias current.

The earlier statement

Quote:

My SVT-HD amp has a Preamp Section (5 tubes), a Phase Inverter (1 tube), then driver tubes (2 tubes), then off to the power section (6 tubes).

Just analogy here, the SVT-3PRO has the Preamp Section as (3 tubes as shown on Ampeg Site), a Phase Inverter (maybe the single 12ax7?) then Driver tube (maybe the 12au7 as it's draws more current perhaps?), then off to the Solid State Power amp section.

isn't entirely accurate......

in both cases the driver tube is PART OF the power amp, not some sort of pre-amp...... from which the signal goes "off to the power amp"

The SVT-HD has the splitter drive the actual driver tubes, and those drive the power tubes..... the separate drivers are needed because the SVT power tubes draw grid current (not a lot, a few mA) at full power. The splitter could not supply that.

In the case of the 3PRO..... the mosfets are the only SS parts that actually do any work in the power amp section.... and all they do is act as "followers" with no voltage gain, to supply the large current for the speaker. they basically replace the power tubes and transformer.