Help me understand this Ampeg SVT- 3 Pro Output Circuit

[defaced]

I'm helping a friend of mine who has the SVT-3 Pro bass head. He's looking to get more gain out of his setup. I don't have the preamp schematic for this amp, but the power amp seems a little interesting, at least not something I've never seen. I'm curious if the 12AU7 can be swapped for an AT7 or AX7. My guess is "no", but here's where I'm at...

I'm trying to understand the function of the last two triodes, V1 and V2. It looks like V1 is just a standard gain stage with a DC coupled cathode follower behind it. The following tube is a 12AU7, it looks like it's serving the same purpose as a phase inverter, I guess, but it's not like any PI I've ever seen.

So if I'm looking at this right (V2), the upper anode and lower cathode are held at very close to B+ and ground, respectively. This allows the center junction between the two to swing +/- whatever DC point that junction floats to. I'm going to guess that this tube doesn't offer any gain to the setup, so changing it to a 12AX7 is going to do nothing.

So the 10 cent question is, does this tube have to be a 12AU7? If I had to guess, it's a 12AU7 so it can drive all of the parallel inputs of the output MOSFETs.

[Ronsonic]

In that circuit V2 is providing no gain. Look at the top half as a cathode follower and the bottom as an active load, a virtual near infinite cathode resistor. You'll get no gain from any tube in that circuit. The signal at the plate of V2a is inverted from the input at the grid so when it is fed to the grid of the lower half, it "fights" the current flow, looking like a huge resistor, on the other half of the wave it conducts freely looking much smaller. There are similar circuits, like the SRPP which can deliver gain, but won't have the low output Z of this circuit. Here's more about that particular circuit. I expect it is the other components in the output section that require the low Z and not the IRFP devices which should show a very high gate Z.

You might increase the Plate resistor on tube V1a, but then you'll need to change the cathode R on V1b and you won't have much to show for your efforts.

IMO, get your gain before this stage and leave this alone to do its drive and react to the compression effect of that plate voltage regulator.

[teemuk]

Google search: white follower

[defaced]

Thank you both. I'll add this one to the tube configuration log in my wee little brain.

[Enzo]

Leave that power amp alone. If you need more gain, either fix the preamp or add something in the loop jacks.

Here. SVT3Pro preamp schematic.

[defaced]

Leave that power amp alone.

I chuckled when I read that. Something tells me there's a story or two behind that comment.

Just read the notes on that schem, dirty ground. Either someone has a sense of humor, or I missed a term in ground school

[Steve Conner]

Dirty ground is a standard term, it just means a ground rail that you don't want to ground sensitive audio circuits to, because it's likely to have noise on it.

The circuit is a White cathode follower as was pointed out. Originally invented for driving heavy capacitive loads at high frequencies in TV transmitters, and the TubeCad journal has a whole series on it: http://www.tubecad.com/october99/page4.html

Yes, it has to be a 12AU7, or similar tube like a 12BH7, 6SN7 etc. A 12AX7 can't pump out enough current to drive the MOSFETs properly. A 12AU7 barely can.

[Enzo]

And I would report that that 12AU7 is the most common part to replace on all the SVTs in my experience.

The story behind the comment is not so interesting. I was just objecting to the common reaction to problems being "let's modify the circuit."

[defaced]

Thank you for the report on the common replacement of the AU7s. I will pass that along.

I was just objecting to the common reaction to problems being "let's modify the circuit."

Actually, the guy wanted to wholesale replace all the tubes with 12AX7 because they had more gain and I told him to wait until I found out what the AU7 circuit was doing, which my guesses were pretty much right. My suggestion to him was dirt or EQ out front and/or different pickups to give him a more percussive sound. I prefer not to mod commercial circuits because it's a big pain in the ass with a sizable risk of screwing up.

[Ronsonic]

Sounds like what the guy needs is a real SVT, the heavy kind with the big tubes and the roar.

[mac1amps]

Just this week I serviced a newer SVT "classic" that had failed [EH] 12AU7's. I replaced them with a [new] pair of 12BH7's (no further alterations needed). I've found these to be a worthwhile "upgrade" for the more recent model SVT's (copying the design of the nice older models), whereas I've seen a bunch of these (12AU7's) fail in this application.

[Jazz P Bass]

When you installed the 12BH7, did the output stage idle current (bias) change?
I was under the impression that the idle current should be checked when replacing the 12AU7 driver.