Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AmberB on March 04, 2024, 07:42:33 pm

Title: Simple question, Minimum wattage for cathode resistor on PP 6V6?
Post by: AmberB on March 04, 2024, 07:42:33 pm

Hey there people!
Just a simple question because I couldn't find the answer in a search of the forum.  What would be the minimum recommended wattage for the cathode bias resistor  for a push pull 6V6 amp?  The amp will be like a Princeton, but cathode biased.  The power transformer doesn't have a bias tap and I don't want to build a neg bias circuit due to space constraints in the chassis.

Title: Re: Simple question, Minimum wattage for cathode resistor on PP 6V6?
Post by: sluckey on March 04, 2024, 07:59:24 pm

5 watts

Title: Re: Simple question, Minimum wattage for cathode resistor on PP 6V6?
Post by: AmberB on March 04, 2024, 08:27:38 pm

Thank you!

Title: Re: Simple question, Minimum wattage for cathode resistor on PP 6V6?
Post by: tubeswell on March 05, 2024, 05:09:32 am

Or, at least 2x the maximum power that is expected to be dissipated through the resistor. From Ohms Law, we know that:
Power = volts x amps
Amps = volts/ohms
A Single ended cathode bias 6V6 output tube idling at Pmax with a 360V B+ might get somewhere between 36mA cathode current, rising to 44mA. Assuming a 470R cathode resistance you might get 20.7V on the cathode. So 20.7V x 0.044A = 0.9W


So you could get away with 2W or 3W rated resistor for a single 6V6. But you need to change up the numbers for a shared cathode resistor for a pair of 6V6s.

Title: Re: Simple question, Minimum wattage for cathode resistor on PP 6V6?
Post by: sluckey on March 05, 2024, 08:15:58 am

Fender used a 250Ω/5W in the very popular 5E3.

Title: Re: Simple question, Minimum wattage for cathode resistor on PP 6V6?
Post by: PRR on March 05, 2024, 01:13:40 pm

The voltage on this resistor is typically less than 1/10th the voltage on the plate. Two 6V6 will dissipate 24 Watts. So the cathode resistor should not feel more than 2.4 Watts. Resistor watts are CHEEP. If you offer a warranty (or maintain it yerself), always double-up. 4.8 Watts minimum.