Variac sound without a Variac ;) ???

Ruben · Post by **Ruben** » Thu Jun 15, 2006 8:42 pm

Is it possible to run my Plexi '67 Superbass on 240/250 while the current draw here in Europe is 220? I just change the 'plug-style-pill' from the the 220 option to 240/250....Simple ain't it

The voltage of my amp lowers from 462 to 420....Is that a safe way to run the amp?

master keeper · Post by **master keeper** » Fri Jun 16, 2006 3:15 am

I'm doing it to both my 12XXX amps, that's how I got them down to 440 B+. That's where the TONE is!

Ruben · Post by **Ruben** » Fri Jun 16, 2006 4:21 am

Anyone else doing this trick?

What happens to the overall sound? Won't it hurt the amp?

mightymike · Post by **mightymike** » Fri Jun 16, 2006 8:11 am

440 B+. That's where the TONE is!

This is true

Bainzy · Post by **Bainzy** » Fri Jun 16, 2006 9:34 am

Ruben wrote:Won't it hurt the amp?

It won't hurt the amp (directly anyway) but it kills the power tubes, shortening their life expectancy to about 30 hours. This is because of cathode stripping inside the tubes. Larry posted a graph of it recently:

http://www.metroamp.com/forum/viewtopic ... ght=#60630

Looking at Power Scaling it appears that lowering the plate voltage going to the power tubes doesn't change the tone at all, so theoretically the "variac sound" must be the effect it has on the PI and preamp. If that's the case, couldn't you get that same sound just by changing the 10k/8.2k voltage dropping resistor combo? That seems a lot more sensible to me than stripping the power tubes' cathodes and prematurely killing them. It might not give the slight (and pointless) volume reduction, but it should give the exact same tone as lower plate voltage would have.

5150loveeddie · Post by **5150loveeddie** » Fri Jun 16, 2006 11:39 am

Bainzy wrote:
Ruben wrote:Won't it hurt the amp?
It won't hurt the amp (directly anyway) but it kills the power tubes, shortening their life expectancy to about 30 hours. This is because of cathode stripping inside the tubes. Larry posted a graph of it recently:

http://www.metroamp.com/forum/viewtopic ... ght=#60630

Looking at Power Scaling it appears that lowering the plate voltage going to the power tubes doesn't change the tone at all, so theoretically the "variac sound" must be the effect it has on the PI and preamp. If that's the case, couldn't you get that same sound just by changing the 10k/8.2k voltage dropping resistor combo? That seems a lot more sensible to me than stripping the power tubes' cathodes and prematurely killing them. It might not give the slight (and pointless) volume reduction, but it should give the exact same tone as lower plate voltage would have.

OH yes.......and,

you have to maintain 6.3VAC on the heaters...........

Namar · Post by **Namar** » Fri Jun 16, 2006 1:17 pm

Hey, cool.

My reissue p/t runs at exactly 440, sweet! I'm already there!

Heh.

Maybe that'd explain why whenever I've tried using a variac with it in the past, it just makes it sound really bad.

mightymike · Post by **mightymike** » Fri Jun 16, 2006 1:26 pm

440v stock is better than a sore dick, you just can't beat it.

The Super Ultimate Attenuator has a 90v Output that you can plug your amp into, but you'd still get cathode striping I reckon. That's why I like the compensated PT.

45auto · Post by **45auto** » Fri Jun 16, 2006 5:11 pm

this is what i keep asking...are these tones attributable to plate voltages? i would think the preamp tubes would be in there too. what if you adjusted these voltages & maintained the filament voltages as they should be? what do i know?

Bainzy · Post by **Bainzy** » Fri Jun 16, 2006 6:24 pm

45auto wrote:what if you adjusted these voltages & maintained the filament voltages as they should be? what do i know?

That's what the "virtual variac" mod on the Metroamp 12000 series '68 Plexi does. It sounds just the same as using a variac, because filament voltage has no effect on tone.

To be fair though, you could get exactly the same tone as using a variac on 90v by increasing the resistance of the voltage dropping resistors by a few k ohms.

45auto · Post by **45auto** » Fri Jun 16, 2006 10:47 pm

is this what the dual "lower" taps are for?

mightymike · Post by **mightymike** » Sat Jun 17, 2006 5:26 am

master keeper · Post by **master keeper** » Sat Jun 17, 2006 2:22 pm

Bainzy wrote:
Ruben wrote:Won't it hurt the amp?
It won't hurt the amp (directly anyway) but it kills the power tubes, shortening their life expectancy to about 30 hours. This is because of cathode stripping inside the tubes. Larry posted a graph of it recently:

Hey... my heater voltage is close to 6.3 at 440 B+ 0n all my 3 amps... they all exceeded 6.3 before we adjusted them... I've run 12267 since Jan 2004 on the same tubes no problem, my RI I've run fro years with the same TESLAs in the power section. A few days ago I got another original 12xxx amp and it clocks in at 441 volts at the 240 tap and sounds KILLER. As long as you dont drop heater voltages to much below 6 you're fine...

Bainzy · Post by **Bainzy** » Sat Jun 17, 2006 3:04 pm

That's sorted then - if you can keep the heater voltages constant, and lower the B+, you've got the perfect setup.

mightymike · Post by **mightymike** » Sat Jun 17, 2006 3:45 pm

In a nuts hell yes.