DC driver heater question

[Paul Joppa]

A customer asked me this question the other day via PM:

"I'd like to know what do you think about the possibility to power filaments of input tube of the Paramount with DC. I've read that AC powered filaments may compromise the bass reproduction! As my pair will only drive woofers......."

I thought it was a question others might have as well, so I'm posting it here along with my response:

For indirectly heated tubes, hum and noise on the heater supply are isolated from the signal elements (primarily the cathode) by a layer of insulation. Nevertheless, some of these noises do get into the signal at a low level. The amount that gets through varies widely with different tubes and among different samples of the same tube type. For some tubes, I believe it would make an audible improvement.

The risk is whether the rectifier noise would communicate to other windings within the transformer. To minimize this risk with the Paramount power transformer, the 6.3v winding center tap should continue to be grounded, and Schottky diodes should be used.

We did not implement DC heaters for the Paramount driver, but I did design the power transformer with enough capability to do so - the maximum current in the 6.3v winding is 1.2 amps RMS.