Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on November 30, 2011, 09:58:22 am

Title: Is it better to reduce filament voltages with resistors or diodes?
Post by: frank57 on November 30, 2011, 09:58:22 am

What I have is 4 diodes 2 sets on each filament line, and while they're working( bringing it down from 7.7 to 6 or so) they seem to be adding some noise.Not a tremendous amount but something.
Can two resistors be used instead or would that cause further problems?
If so how would one calculate the size or measure the current draw?
What I found was one set of diodes caused an imbalance, so I had to go to 2.

Would simply using an artificial ct lower the voltage anyway?
Although I found it added hiss when I tried it with one set of diodes.

Title: Re: Is it better to reduce filament voltages with resistors or diodes?
Post by: jim on November 30, 2011, 10:13:13 am

Are you checking with tubes in or out?  With tubes in-- it is OK at 6 volts. Jim

Title: Re: Is it better to reduce filament voltages with resistors or diodes?
Post by: frank57 on November 30, 2011, 02:44:24 pm

With 3 12ax7 and 1 12at7 and 2 el 84, with tubes in and no diodes it's 7.7.
And it's balanced I think at 3.8.
With the diodes it's down to 5.8 to 6.

With one set of diodes it's down to 6.8 but the 4 and 5 are 2.9 and 3.8.
An artificial ct corrects this but adds hiss for some reason.

Just wondering if resistors might be better than diodes for reducing the voltage
or impractical because of the size required.