1980 Hiwatt: Variac Recommended?

[BrewGuitar]

I recently purchased a 1980 or 1981 Hiwatt DR504. I have heard that older Marshalls and Hiwatts prefer to see 110Volts as opposed the nowadays common 115-120V. Does a 1981 model prefer to see lower voltage or is okay at the higher voltages. What changes to tone would I recognize?

[stratman_el84]

The only safe way to determine what AC line voltage is optimum would be to take it to a tech experienced with tube amps, especially if you're not absolutely certain of the amps' manufacturing date.

Using a variac without checking out the operating parameters of the power supply (both high-voltage and heater), the bias supply voltages, and the operating condition changes to the power tubes of the amp in question (idle current, screen voltage, heater voltage, etc) before and while the AC line voltage is altered is asking for disaster. It's a very bad idea unless you're very technically experienced with tube amps.

Cheers!

Strat

[Tripower455]

It was desigend around 115VAC.

The voltages inside will be running higher than designed, but it will most likely work fine at 120-125VAC.

That said, I run my '79 DR504 with a variac set at 115 and it gives me 470 on the B+ and 6.3 on the heaters. Using wall voltage (which is typically about 125 at my home), I am running over 500 B+ and 7 VAC on the heaters.

Running higher than desigend voltage definitely affects the tone of my amp in a negative way. Almost all of my NOS preamp tubes get microphonic with the voltage in the 125 range.

YMMV, but the Hiwatts were pretty consistent and I'll bet that yours will be perfectly happy at 115.

[Lammy]

mmmm. hiwatt. yes.

[callaway_1]

Tripower455 is right on the money with his post.

Almost certainly will your amp sound better and the tubes will definitely last longer if you are supplying it with 115v. as opposed to 120+v.