Elevating heater supply

[debk]

I have a SRPP circuit using 6n6p Russian tubes.  I am using 6.3VDC for the heater.  With a B+ of 300V it puts the upper triode heater to cathode voltage about 143V about 50% above the max 100V difference.

How would you suggest I elevate the DC heater supply?  If I do nothing how fast would I expect to damage the tubes.

Thanks

Debra

[Paul Joppa]

Elevate the heater supply to about +75v, with a voltage divider across the 300v supply. That brings both cathodes well within the usual +/-100v limits. A 220K and 68K resistor is close enough.

Bypass the 75v point to ground with a capacitor. I'd use an electrolytic of at least 10uF, bypassed with a ceramic of 0.01uF or greater. The large cap is to remove any common-mode hum from the filament winding, the small one is to take out radio frequency noise (the electrolytic has a relatively large parasitic inductance and looses effectiveness somewhere around 10kHz-100kHz).

To be extra-anal about it, center-tap the filament supply wit a pair of small resistors, 25 to 100 ohms each - or use the CT if the transformer winding has one.

Historical note - the first version of the Paramour had no bias on the 12AT7 heaters, which were in SRPP. I was counting on the leakage between cathode and heater to settle at an intermediate voltage. However, this contributed to a bit of a hum problem. Now I always ground the heater power, with a capacitor if it is biased and directly otherwise.

[debk]

Paul
Thanks for the info.
One other question somewhat off topic.
I assume using LED's to bias the lower triode is no problem, but is using a CCS instead of the resistor between the the upper and lower triodes of the SRPP of any advantage?

Thanks again

Debra