Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on October 23, 2017, 08:05:14 pm
-
I just repaired a 5150. These don't have a choke. They use the 400ohm flame proof resistor instead. Since I don't have one of these, I used four 5 watt 100ohm wire wound resistors in series just to make sure that's all it needs. It fired up and played fine. I have a couple questions.
- Should I replace with a flame proof, or would the 4 wire wound be fine? Also, this amp was modded. They made it adjustable fixed bias with a pot hot glued to the board. Would biasing the amp too hot blow this resistor?
-
Just measure the voltage drop across it during peak load, which should be pretty small, then calculate the current based off the resistor's value. After that calculate power. using the calculated current and voltage drop. From there you can use whatever you want,
but yes you're fine with rated above 3 watts in that spot most likely.
Any wire-wound resistor you get isn't going to have enough henries to make a difference. You'll have to find a spot to place an actual 5h choke to see a benefit, which might be a good idea that it's potentially more aggressively biased.
Edit: The schem I'm seeing lists a 10W resistor in that spot. Measure and see what it's really using for power.
-
That resistor burned because a tube shorted screen to cathode.
It will happen again.
You can't use a 400 WATT resistor to avoid burn-out. Therefore I would use a Flame Proof resistor so it can pooof without starting a fire.
-
I should re tube and order a flame proof resistor than. Thanks PRR
-
That resistor burned because a tube shorted screen to cathode.
It will happen again.
This is most likely with the output tubes rather than the preamp tubes correct?
EDIT: You said SCREEN to cathode short. Obviously in output tube. Not preamp. These are the things I miss when reading. :dontknow: