Variac'd Superlead type amps; SL68, Helios, et al

SpiderWars

Well-known member
I have a homebrew Superbass that I variac down and run into a PS2, add some delay/reverb/post-EQ, and run into a couple of 4x12s.

It sounds great. I am wondering if any of these "SL-with-built-in-variac" type amps will get the same tone but simplify that setup. It seems that in order to have the delay/reverb/TC1140 in the path that I'll still need the PS2 anyway.

I also would prefer to not have my Superbass biased for 90vac supply because then I can't plug into regular wall voltage without rebiasing.

Maybe I'm just looking for an excuse to buy one, as if I needed it (the amp or the excuse). First world problems.
 

Rick Lee

Well-known member
I have a '69 1959SLP clone I built (to stock spec) and later had my tech install a Variac pot. I don't remember how far down it takes the voltage, but it definitely lowers volume and increases gain. I usually run it with the Variac totally off, but recently forgot to bring my Power Brake to band practice and had to use the Variac pot to not blow the house down. Sounded totally awesome.
 

SpiderWars

Well-known member
I built an amp using London Power power scaling and ime it's a whole nuther level of complexity. It requires a small transformer for the filaments (so they don't change) and a not so basic circuit such that the bias voltage adjusts to the new lower plate voltage. You also have to add another power supply node for the 'upstream' side of all that circuitry.

I don't know what the techs did that you guys are talking about but having tried it in another build...I think I'd just rather buy (or even build) something else. Thanks for the suggestion tho, I will ponder that.
 

Rick Lee

Well-known member
The Metro build instructions for a '68 12000 series include a three-way power switch for the Variac engagement. I built one of those too and it rocked.
 

fusedbrain

Well-known member
The SL68 has a dual voltage secondary and a modified bias circuit with 2 bias trim pots.
I believe the dual secondary switching is done with a 3PDT power switch, with the 3rd pole switching the bias trimmers.
If a quality dual secondary power transformer could be sourced, a similar amp could be built for fraction of the cost of a new SL68.
In the SL68 gut shot, note the 2 recommended bias settings in sharpie under the PT.
Edited to say the 3PDT switch is available from Valvestorm.
SL68 guts.png
 

glpg80

Well-known member
You have to be careful with relays on the primary side - with AC circuits you’ll get inductive kickback breaking or making connections or with relay bounce which could translate into extreme high voltages on the secondaries. It can be done with single phase snubber circuits but you have to know what you’re doing and what types of components to use. Definitely use a beefy SP3T on the secondary to kick in and out resistors to drop the DC voltage, filament voltage, and switch to a different bias trim pot of the same divider. If the relay fails it fails in one state or the other - it won’t be intermittent.
 

SpiderWars

Well-known member
That's way more doable fusedbrain, thanks. But does it sound as good as a variac'd Marshall (or equivalent)?

For example, for many years in the 70s the 100W amps had much higher voltages than the 50W. The 50W often just under 400vdc. But to me a 50W doesn't sound the same as a variac'd 100W. It just sounds like lower voltages. Variac'd amps seem to have something else going on that makes it sound and feel better. Might be my imagination.
 

fusedbrain

Well-known member
TBH, if the SL68 is any indication, I'd just stick with your Variac set-up.
The Voltage selector on the SL68 should have been labeled "Tight" and "Mush"
 

harddriver

Well-known member
I think the virtual variac in metro was actually a dual voltage tap transformer one set of taps was the normal 480-490 DCV and the lower taps was 400DCV around where a plexi amp plate voltage would be variaced down to 90ACV, this allowed the heater voltages to remain unchanged.

I have on it my 68 metro plexi and I don't think it has the exact same feel and tone as when you actually variac the amp down that also lowers the heater voltage which affects how the preamp tube distort as well.

If you enjoy the true 100% variac'd tone I don't think any gimmicks cam accurately reproduce it IMHO. I doubt the SL68 while it sound great would be worth the coin. My 69 SL was a great sounding amp and it did not need variacing and it came real close to my variaced 68 but only close.

Maybe the Soldano HR25 might fit the bill, Mike built them around lowered plate voltages as it would be variaced. I thought those were very plexi like.
 

harddriver

Well-known member
That's way more doable fusedbrain, thanks. But does it sound as good as a variac'd Marshall (or equivalent)?

For example, for many years in the 70s the 100W amps had much higher voltages than the 50W. The 50W often just under 400vdc. But to me a 50W doesn't sound the same as a variac'd 100W. It just sounds like lower voltages. Variac'd amps seem to have something else going on that makes it sound and feel better. Might be my imagination.
No, you aren't imagining things. Even with two power tube pulled in my 68 variaced it still punches harder and feels different from my standard 125ACV 1972 Marshall 50 watter.
 

fusedbrain

Well-known member
Dave Friedman has talked about Variac's and Eddies amp a couple times on Tonetalk.
I could have sworn he said the plate voltage was around 350V when the variac was set to 90V.
In any case, if ya want Variac tone, use a variac. :thumbsup:
 

Meanjoegreene

New member
Anyone know if the voltage switch on the Friedman Phil X acts like a true variac? I use it from time to time but it’s pretty subtle. Seems to add some compression but also seems to tame an overly bright guitar a bit. Amp is still tight and responsive, just less in the lower voltage setting.
 

harddriver

Well-known member
To follow up on the point that 100 watt amps even with two tubes pulled running at 50 watts still punches harder than a 50 watter equates to the larger transformers laminations, core windings, inductance and such technical attributes.

Mojaves amps addressed this by building the 50 watt Scorpion amp which was the 50 watt version of the 100 watt Peacemaker plexi amp with 100 watt power and output transformers and then wired the 8 ohm tap as 16 and 4 ohm tap as the 8ohm taps respectively just as you cut your impedance in half if you pull two tubes out of your 100 watter to run it at 50 watts.

And now you have a 50 watt amp that punches like a 100 watter, quite brilliant really...:2thumbsup:

I read an interview where he explained some of the technical aspects of why and how it works. Here's the article:
 
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mixn4him

Active member
Anyone know if the voltage switch on the Friedman Phil X acts like a true variac? I use it from time to time but it’s pretty subtle. Seems to add some compression but also seems to tame an overly bright guitar a bit. Amp is still tight and responsive, just less in the lower voltage setting.
No it just bypasses one of the voltage dropping resistors for the PI and PreAmp tubes..
 

Nigel

Active member
To follow up on the point that 100 watt amps even with two tubes pulled running at 50 watts still punches harder than a 50 watter equates to the larger transformers laminations, core windings, inductance and such technical attributes.

Mojaves amps addressed this by building the 50 watt Scorpion amp which was the 50 watt version of the 100 watt Peacemaker plexi amp with 100 watt power and output transformers and then wired the 8 ohm tap as 16 and 4 ohm tap as the 8ohm taps respectively just as you cut your impedance in half if you pull two tubes out of your 100 watter to run it at 50 watts.

And now you have a 50 watt amp that punches like a 100 watter, quite brilliant really...:2thumbsup:

I read an interview where he explained some of the technical aspects of why and how it works. Here's the article:
My Zinky is like that. Got a 100 watt PT, and a 5k6 OT that sounds best running the 16 ohm tap into an 8 ohm cab, depending on the room. Punches hard for two KT77’s, and the tubes are cathode biased at about 30 watts each idling.
 
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