Ampeg SVT lll

[Mars Amp Repair]

Hello folks,
Need your help on an Ampeg SVT-3pro amp. Unfortunately I do not have a schematic & no time to procure one.
This amp has a control called 'tube gain'. When turned I is supposed to vary the tube driver effect to the signal....I'm not completely certain. However the result of adjusting the control is the amp hums a bit & the sound goes out temporarily & then slowly fades back in.
From tracing & drawing a parital schemo of the area it appears this control varies the base of a TIP 50 transistor (can't find the board designation) which then adjust the plate voltage to 2 - 12AX7's that appear to be the driver tubes for the solid state output stage. This as many of the smaller B-2 type Ampegs aren't the easiest logistically to work on.
I see the plate votage vary between about 120VDC to 250VDC on the plates, which at least at first blush, appears to be doing what is is designed to....albeit a kinda weird way to go about the effect.
Another curious symptom is that the Fan runs full speed..it will stop if I remove one of the 12AX7 driver tubes I mentioned. I'm guessing that the fan ckt must somehow run off of the current draw from these tubes, but that sounds even weirder than the whole idea of the gain ckt. The fan seems to derive its drive from a couple of TIP31 & 32's mounted on the heatsink. I really haven't had the chance to trace that out.

Has anyone had any experience with this amp?


Any help appreiated. Thanx, glen

[Tom Phillips]

Glen,
I emailed you service information.
Tom

[Mars Amp Repair]

Schematic received

Glen,
I emailed you service information.
Tom

Thanx Tom...that'll help tons. I can see already why the fan won't run withtout those driver tubes in there....how weird is that....I guess that tube gain pot really does vary the plate supply to the driver tubes..now to figure out why it makes the amp behave so strangely.....thanx again, glen

[Mars Amp Repair]

The solution;

Here's the follow up solution.
I don't care how long we work in this field, there just ain't anyone who's human that at times just misses the obvious. I follow up this post for those of us who are also human

If you review my original post, this ampeg amp has a Tube Gain control that through a transistor actually varies the B+ to the driver tubes (I still think that's a weird thing to do).

They in turn then drive the MosFet output stage.

When I turned the Tube Gain control fully CCW (the lowest amount of B+ to the plate load resistors), the amp would 1st cut out & then fade back in quacking & distorting. Turn it up past about 3 & it would clear up but really not have much effect for the rest of the travel.

Long & short of it, it seemed the control was placing the tube in a bad part of its load line. I even tried adding a resistor in series with the pot to move it to a more desireable position just to determine if that would make it act more 'normally', as I couldn't find any bad components in the circuit.

Well here's the part we all have found ourselves in...I OPENED MY EYES & saw on the schematic (granted in very small print) that the final driver tube is supposed to be a 12AU7!!

What the hell you know about that, there was a 12AX7 in there...a 12AU7 is completely fine running its lower gain in that loadline area.

...Hope all can benefit. glen

[Enzo]

Now check all the 12AX7s and see if one has magically transformed into a 12AU7.

[Mars Amp Repair]

Now check all the 12AX7s and see if one has magically transformed into a 12AU7.

Yeah, I knew they were all 12AX7's from the start. Apparently someone replaced them all at one point & didn't make note of what was in there to begin with.
I really have to wonder why manufacturers stopped putting tube charts in amps like this, or at least screening the pcb.

glen

[stokes]

Could be someone thought thy would get more "drive" subbing the AX7 for the AU7,ya know what they say sometimes a little knowledge....

[Amp Kat]

I think it's the Seymour Duncan Convertible that uses the 12AU7's plate impedance as an attenuator. If you have a 12AX7 in it there will be no attenuation but the schematic shows V1 as a 12AX7 but it should be a 12AU7. Will pull your hair out

[Mars Amp Repair]

Yeah, I knew I wasn't alone....we've all been there at one time or another! g

[wizard333]

WOW, I'm looking at an SVT3 with pretty much the same symptoms. I dont have a schematic either, if you could forward one that would be grand. It has all 12AX7s so most likely the same issue...... If I'm looking at the amp from the front, there are 2 tubes on the power amp board, next to each other, one closer to the front once closer to the back. There are 3 on the preamp board, lets call them 1,2,3 from left to right. Which is supposed to be the 12AU7?

Thanks!

[Tom Phillips]

...I dont have a schematic either, if you could forward one that would be grand.

I gave it a try but your profile specifies that you do not wish to receive private messages.

[wizard333]

rcbmusic at sunflower dot com

I'll see if I can figure out how to edit that profile.

Thanks!

[Tom Phillips]

Info sent. Plese let me know via email reply to let me know if all the attachments get through.
Tom

[Mars Amp Repair]

hey Tom,
the 12au7 is the one closest to the front of the unit on that back board....sounds like this is more common that you might think. Someone else forwarded the schemo to me...I don't have it on this computer....g

[Tom Phillips]

Hi glen,
That was me who sent you the service information. I just sent a set to Wizard.
Tom