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In Reply to: Supply voltage timing on DHTs posted by mikeyb on January 25, 2007 at 12:57:21:
I have an 01A DHT driver to a 45 DHT. I only AC heat the 45 at 30 KHz but I have tried to do both. There's a problem when you do both. 30 KHz leakage is amplified quite a bit from the 01A heater (or 26 in your case) and mixes in the output stage and there's ultrasonic signals everywhere now. This is bad. So what's the only alternative now? I DC heat the 01A (you want to DC heat the 26 in your case) with a SLA battery that's floating. The 01A sounds really good even with DC heating, better than any IDHT driver I tried. I also tried the 26 and prefer the 01A by a large margin, although those are rarer and more costly. YMMV, but AC heating a DHT driver is prone to problems, at any frequency.
> 30 KHz leakage is amplified quite a bit from the 01A heater
How did you drive the 01A heater? I'm thinking it would need a
center-tapped transformer, and maybe also a balance pot.
I drove it from a center tapped pot just like you mentioned. But it doesn't cancel out completely and a lot gets through a 26 and a 01A even still.
I don't typically use DHTs in early stages, but I've considered whether ultrasonics would be the appropriate methodology in such cases. You know, if the frequency was higher, say 100 kHz or more, you could add a tuned trap to ground at the anode of the preamp. That might notch the RF by 25-35 dB and wouldn't affect audio signals. Just a thought.
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