building an amp: Variac

[3124+]

Hi,

I would like to build an amplifier that is based on an early Tweed circuit. The amp involves two 5Y3 rectifiers powering a pair of cathode biased 6L6 tubes.

I thought it would be cool to incorporate a Variac into the design. With the Variac inserted between the 120vac wall outlet and the mains transformer primary, the operating dc voltages of the amp could be dropped to achieve cranked-up overdrive at lower volumes.

A seperate filament transformer would be fed steady 120VAC to provide constant 6.3V and 5V for the fliaments.

I know this idea will work with a marshall super lead. Will it work with a cathode biased, tube rectified amp? If so, where can I get a transformer with only 6.3vac and 5vac secondaries? Any input would be appreciated.

Thanks!

[Satamax]

Filament tranies can be found at Angela for example, Alied might have some hammond. There's aplenty of supliers. But Imho, you'd better check london power for what you want it's already been done and even can be purchased in kit form.

[3124+]

I don't think Angela or Hammond offers a filament transformer with 5vac and 6.3vac secondaries in a single unit. I will check again though.

I have seen the power scaling stuff.

I still like the variac idea best. Will it work with a cathode biased, tube rectifed amp?

[Satamax]

It will work, up to a certain extent. Since you're dropping voltage on your plates, iirc, your current tend to fall then so is the voltage drop in the resistor.

Here's an exerpt from Aiken's site.

"Do cathode-biased amplifiers need to be biased? The short answer is yes. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. The tube will reach a stable point of equilibrium and stay there. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. For this reason, the bias should always be checked, even with cathode-biased amplifiers. "

Thought i don't know if your voltage difference is real bit if it will keep the bias into specs, as the bias to plate voltage ratio might not be linear.

As for filament transformers with 6.3 and 5V taps, they ough to exist, i'd think angela within the odd ones might have one. And hammond might have one too, if any of thoses two doesn't tough shit to find one!

Best regards.

Max.

[3124+]

Yes, this is what I'm curious about. So the cathode current drops as the variac is turned down?

Hopefully the voltages will drop in correct proportion, so that the bias current will always be proper. Has anyone tried anything like this?

[Steve Conner]

Matamp made an amp with a built-in variac just like what you described. So it's probably worth a try.