Need help choosing the rite variac for my tube amp.

[xMichaelx]

I have a SC au33 signet 30watt tube amp. I want to ad a variac to this amp but I'm not familiar with variacs. Tubes are, 7025 12AX7, 12AX7/ECC83, 6U8 7687, 5U4GB, and two 6L6GC/7027, all tubes are Stromberg Carlson. I want to get that deep purple eddie van halen sound, I use the Pro Co Rat distortion thats the only pedal I've used. I've read about and seen videos of the power soak, variac and I know I'm playing with fire, horrible things can happen if I'm not careful using the variac. Thanks

[Jazz P Bass]

A variac is a very useful "tool" when troubleshooting electronic equipment.
Using a variac to alter the voltages on a tube amplifier, in my opinion, is not the correct application.
First & foremost you will be lowering your heater voltages.
That in itself is a good reason to "not" use the variac.
Try a powersoak at the speaker output.

[xMichaelx]

THD Hot Plate

A variac is a very useful "tool" when troubleshooting electronic equipment.
Using a variac to alter the voltages on a tube amplifier, in my opinion, is not the correct application.
First & foremost you will be lowering your heater voltages.
That in itself is a good reason to "not" use the variac.
Try a powersoak at the speaker output.

How about the THD hot plate 8 ohms? my amp speakers are 8 ohms.

[Jazz P Bass]

The amp will certainly be quieter.
Never used one so I cannot comment.

[Gtr_tech]

Don't buy all the BS that EVH has spouted over the years. He said alotta stuff that was deliberately wrong just to throw ppl off his trail who were try'n to cop his sound. Its been exposed that his "magic" plexi head that he always said had a zillion mods and ran off a variac was nothing more than a *stock* 68 plexi.

Players who think they have a unique sound always hate it when others try to cop it. They've been known to give lots of incorrect info just to keep ppl wondering. EVH and Blackmore's tone were *very* different BTW....

[Jazz P Bass]

PLUS, Eddie & Richie both can play the s**t out of a guitar.
Sidenote.
There was a story circulating about a player that had a real crap amp on stage.
He is up there fighting the thing & cursing the amp.
He looks over & there in the wings is none other than his friend, Eric Clapton.
So, the story goes, EC is on the sidelines giving the "eyes" like he would love to play a couple of songs.
The player is sh**tting bricks, thinking " oh man, not with this crap amp."
So Eric comes on stage, plays a couple of chords, winces, turns a couple of knobs & starts PLAYING.
The sound from the amp was EC. No mistaking it.

[ThermionicScott]

PLUS, Eddie & Richie both can play the s**t out of a guitar.
Sidenote.
There was a story circulating about a player that had a real crap amp on stage.
He is up there fighting the thing & cursing the amp.
He looks over & there in the wings is none other than his friend, Eric Clapton.
So, the story goes, EC is on the sidelines giving the "eyes" like he would love to play a couple of songs.
The player is sh**tting bricks, thinking " oh man, not with this crap amp."
So Eric comes on stage, plays a couple of chords, winces, turns a couple of knobs & starts PLAYING.
The sound from the amp was EC. No mistaking it.

I think I read that story in one of Ritchie Fliegler's books. The best part was EC at first saying that he wasn't going to go on, so the stage guys didn't bother trying to find a decent amp for him until it was too late.

- Scott

[Gtr_tech]

This is true. Although Claptons tone is pretty ratty since he got into bed with Fender.

As far as the OP's Stromberg Carlson amp goes....I can't find a schematic that matches that version, but since it uses 12ax's it should be very moddable to be more guitar friendly. I would go that approach before torturing teh output section with some tranny toaster....

[xMichaelx]

Don't buy all the BS that EVH has spouted over the years. He said alotta stuff that was deliberately wrong just to throw ppl off his trail who were try'n to cop his sound. Its been exposed that his "magic" plexi head that he always said had a zillion mods and ran off a variac was nothing more than a *stock* 68 plexi.

Players who think they have a unique sound always hate it when others try to cop it. They've been known to give lots of incorrect info just to keep ppl wondering. EVH and Blackmore's tone were *very* different BTW....

I'm just trying to get that tube distortion sound without having to turn my amp volume all the way up, so heres what I've been doing. I turn my amp volume to max,and I use the volume on the pro co rat distortion instead. my amp has one volume control. heres a schematic for my amp if it will help.

[Gtr_tech]

That uses grid leak bias on the input stages. Typical mic preamp for that time. Don't sound so hot for guitar tho. You've got all the available real estate there to make a decent 2 channel amp. I'd start with a JCM800 type front end, but change the coupling caps to .01 instead of the normal .022. And change the bias on the second stage to 2.2k bypassed with a 1u or .68. That'll get you into classic rawk territory with a good amount of overdrive, but will also clean up with single coils by rolling the gtr vol down. For the #2 input, you can use Fender BF values there and resistively couple that to the 3rd stage grid of the OD section. I'd think that would be enough gain to drive the output section to full output while staying relatively clean. But....season to taste.

You'll need to diddle with the dropping resistor values for the input stage b+ supply rail (r31,r32). Somewhere around 275-300 with 100k plate loads is what I shoot for. You can get by with less on the clean side, but more supply V will give better clean headroom.

Looks like a fun project. Give me an hour with that on my bench and I'd have a real screamer....

[xMichaelx]

That uses grid leak bias on the input stages. Typical mic preamp for that time. Don't sound so hot for guitar tho. You've got all the available real estate there to make a decent 2 channel amp. I'd start with a JCM800 type front end, but change the coupling caps to .01 instead of the normal .022. And change the bias on the second stage to 2.2k bypassed with a 1u or .68. That'll get you into classic rawk territory with a good amount of overdrive, but will also clean up with single coils by rolling the gtr vol down. For the #2 input, you can use Fender BF values there and resistively couple that to the 3rd stage grid of the OD section. I'd think that would be enough gain to drive the output section to full output while staying relatively clean. But....season to taste.

You'll need to diddle with the dropping resistor values for the input stage b+ supply rail (r31,r32). Somewhere around 275-300 with 100k plate loads is what I shoot for. You can get by with less on the clean side, but more supply V will give better clean headroom.

Looks like a fun project. Give me an hour with that on my bench and I'd have a real screamer....

Thanks for your help Gtr_tech. I'll get the amp moded with the info you gave me, hopefully this wont cost me a fortune lol. thanks again for your help.

[Greenie Fleetwood]

I thought that the Powersoak was a no-no. I used one for a while in the 80's and was told by the tech I used at the time that it was the cause of my melting down a Plexi one night. That amp was never the same...

[g1]

I think JPB was using the term "powersoak" generically for speaker emulators. What you heard about in the 80's was probably the actual Scholz Powersoak, which seems to have a bad reputation for being hard on output transformers.

[ThermionicScott]

The way I understand it, those devices aren't harmful in and of themselves; it's the fact that you can run your amp flat-out for hours on end that overstresses the OT.

[xMichaelx]

Stromberg Carlson

I took the S/C amp to Dale Laslie at djl vintage audio. The amp was rebuilt based on the Sovtek mig50 design. The amp sounds amazing.