Modifying Widow makers with an isolation transformer

[Daven]

Hello All,

I've decided to install an isolation transformer in my vintage Kustom Kraft 600 tube amp. Its a 'widowmaker' 3 tube amp with a 12au6, 35w4, and 50c5. The chassis is isolated from the line but the heaters are not. If a heater to cathode short would happen then the line would be connected to the chassis and therefore the guitar strings. I really don't wan't to chance it.

I sized the transformer by measuring the line current with a full volume signal going into an 8 ohm dummy load. If I remember correctly the current was the same regardless of the volume. I guess this makes sense right, since the amp is class A. I think the line current was something like 350 mA. So 350 mA x 115 volts = 40 VA. I have a 50 VA on order from newark electronics for $18. My question is this, can anyone help me with how I would go about calculating the power requirements of an amplifier if I were to attempt to design one in the future.

Thanks,

Dave

[J M Fahey]

Hello All,

I've decided to install an isolation transformer in my vintage Kustom Kraft 600 tube amp. Its a 'widowmaker' 3 tube amp with a 12au6, 35w4, and 50c5. The chassis is isolated from the line but the heaters are not. If a heater to cathode short would happen then the line would be connected to the chassis and therefore the guitar strings. I really don't wan't to chance it.

Excellent decision, way to go

I sized the transformer by measuring the line current with a full volume signal going into an 8 ohm dummy load. If I remember correctly the current was the same regardless of the volume. I guess this makes sense right, since the amp is class A. I think the line current was something like 350 mA. So 350 mA x 115 volts = 40 VA. I have a 50 VA on order from newark electronics for $18.

Sounds very reasonable.

My question is this, can anyone help me with how I would go about calculating the power requirements of an amplifier if I were to attempt to design one in the future.

Thanks,

Dave

You can add filament consumption to tube plate current plus preamp consumption and add a reasonable safety factor, say 50% exra ... or you can plain measure it as you did successfully

Never used it, but maybe Power Supply Designer includes such a case.