variac for tube amplifier

[Robalo]

These devices are useful for controlling the AC line voltage, e.g, bringing up AC slowly when servicing vintage Vacuum tube Guitar and HI-FI Vacuum tube preamplifier/power amplifiers and tube type radios.
Also useful for adjusting filament voltage for preamp and power output tubes.
****now can you explain me how can I used this device on radio amps on repairs. PLEASE Iam working on tl-922a right now and before replace the new 3-500z bulb need to check all circuit working ok,and also how many amps do you recommend for safety 10,20 or 30 amps. thank you and 73's

 

[nomadradio]

I tend to use a VVT only to check the amplifier behaves itself on standby. Current draw for that is the fan, lights, bleeder resistor current and filament power only. Two or three Amps should cover that unless it has more than two tubes. The amplifier gets plugged into a full-voltage outlet when it's time to key the thing.

And if there is some hidden overload fault in the HV, the 1-Amp breaker in line will minimize the risk of damage.

We only have 120-Volt VVTs on hand. A socket adapter lets you turn a 240-Volt amplifier only halfway up. But there is a workaround for that.

We use a "suicide cord", a line cord with a 3-prong plug on one end, gator clips on the other. Hooking the clips up to one of the two 120-Volt primaries on the HV transformer allows you to bring the HV secondary side slowly up to full standby voltage, more or less. An amp with a single transformer will draw more current. It has to light the filament(s). A two-transformer setup will draw only the power to run the HV bleeder resistors and transformer losses. Under one Amp in some cases.

Any amplifier with a soft start will usually smoke the start resistor if you turn it up slowly with a VVT. Clipping directly onto a transformer's primary winding bypasses this pitfall.

And creates a hazard level so high I should regret even suggesting it.

I have been promising myself I would gang two 120-Volt variacs on a single shaft and make a proper 240 Volt variable voltage AC supply.

For years.

We use the VVT only for the first safety check after an amplifier's power supply has been disturbed or repaired. Mostly we don't operate the amplifier full bore that way.

73

 

[Dr_DX]

"Any amplifier with a soft start will usually smoke the start resistor if you turn it up slowly with a VVT."

Absolutely True...

 

[linearone]

Any amplifier with a soft start will usually smoke the start resistor if you turn it up slowly with a VVT. Clipping directly onto a transformer's primary winding bypasses this pitfall.

This...I watch bbi's videos and he does this on every damn amp and most of his boxes he tests like ameritrons and Kenwood's etc all have soft starts which I am positive he's destroying. But it's "the right way"...so many people think that.

 

[nomadradio]

If what you're doing is to test for faults after working on a power supply, bypassing any startup gadgets is just part of the picture. Testing for work errors without poofing the new parts you just installed can be immensely valuable.

And an amplifier that's a simpler design will be simpler to work on.

73