Variac, how to ?

Nikon1975 · 12th October 2010, 02:42 PM

Dear All,

I have some question on the use of a Variac. When is attached between the main and the power transformer I can decrease the AC HV. But doing this I will decrease all the filament voltages. Do I have to power all the filaments with separate psu ? If I have to do this, can I power the filament of the rectifier with a normal transformer ? I understand that in DH rectifiers the filament gets all the B+, I am not sure if the insulation is up to this.

In general, I am fascinated by the process of "voicing" an amp, ans starting with the voltages is kind of easy. Any other tip ?

Thanks,

Davide

Doz · 12th October 2010, 02:48 PM

Hi,

Perhaps the easiest way to do this, is to use the variac to supply a separate HT transformer, and use that to feed the exsisting rectifier, you can leave the heaters and the rectifier heater on the old TX then. Just beware if the transformer has a bias winding.

Also be aware that a variac does not isolate you from the line. I've built MI amps like this, before I used a series regulator to adjust the HT.

I must say you can have more interesting effects by limiting the HT's current than lowering it's voltage.

Happy playing!

12th October 2010, 02:42 PM	#1
Nikon1975 diyAudio Member Join Date: Nov 2009 Location: Tokyo	Variac, how to ? Dear All, I have some question on the use of a Variac. When is attached between the main and the power transformer I can decrease the AC HV. But doing this I will decrease all the filament voltages. Do I have to power all the filaments with separate psu ? If I have to do this, can I power the filament of the rectifier with a normal transformer ? I understand that in DH rectifiers the filament gets all the B+, I am not sure if the insulation is up to this. In general, I am fascinated by the process of "voicing" an amp, ans starting with the voltages is kind of easy. Any other tip ? Thanks, Davide

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12th October 2010, 02:48 PM	#2
Doz diyAudio Member Join Date: Jul 2010 Location: Sat Down	Hi, Perhaps the easiest way to do this, is to use the variac to supply a separate HT transformer, and use that to feed the exsisting rectifier, you can leave the heaters and the rectifier heater on the old TX then. Just beware if the transformer has a bias winding. Also be aware that a variac does not isolate you from the line. I've built MI amps like this, before I used a series regulator to adjust the HT. I must say you can have more interesting effects by limiting the HT's current than lowering it's voltage. Happy playing!

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