6SN7/6SL7 and DC Heaters

[rp]

Here's an extra tidbit from the brethren at the pub across the street. It regards the very large cathode bypass cap you see on on V1 in old amps.

http://el34world.com/Forum/index.php?topic=17617.0 -> HBP ->

Short Answer: This was very likely used to eliminate hum in the input stage due to heater-cathode leakage. This is a tube-specific problem, because with time and a big-enough pool of tubes you can select those with the least leakage and hum. However, it is also most-problematic for the first stage because even the smallest hum here gets amplified by the rest of the circuit.

One take away here is if you ever wondered if you really need to put that 330mf cap in your Marshall...

Things I noticed: the more preamp octals you have the more hum, if you will have 1-2 you'll likely be fine. This concerns 100/120htz hum but upping the filtering and/or adding a stage to make a more modern power string helps too on the really ancient stuff. Tough choice on a vintage clone but we are less accommodating of noise than they were in 1952. I was born in 1960 but I remember that if something buzzed, hissed, and hummed it meant it was working.

[bal704]

Interesting read. I'm adding notes to my build notes for tricks to try if I have a hum, and this one will go on the list.

[kwijabo]

i built a 5B3 last year and used AC heaters. no more or less hum than any other amp i've built, so i'll add +1 to try with AC.