Musicman RP-100, where to install a 1 Ohm resistor for bias ?

[Colas LeGrippa]

Hi !

I am working on a Music Man RP100 and wondering how to do the bias : I just don't know where to solder a 1ohm resistor since the cathode is tied to a transistor. Also,the main problem was a screen resistor totally burnt on 1 tube socket which I replaced: is it possible that a defective tube draws that much screen current ?

Thanx

Colas

[Colas LeGrippa]

sorry, I should have said : ....each cathode is tied to a transistor ( 2 tubes ).

[sluckey]

I just don't know where to solder a 1ohm resistor

Don't bother. Just measure the voltage across R77 for V1 or R78 for V2. Divide that voltage by 3.9 to get the cathode current. Close enough.

But if you must use a 1Ω resistor, put it between the tube cathode and the transistor's collector. Measure voltage across the resistor, not from cathode to ground.

is it possible that a defective tube draws that much screen current ?

Yes, that's a fairly common failure when using 470Ω/1W resistors on a 6L6. Some people like to replace those with 1K/5W resistors.

[Willabe]

Be careful in there Alvin. That amp has around 700vdc on the output tubes plates with enough current to fry a large horse.     

I had an RD50 years ago, not a bad sounding amp. It was the only amp I could actualy use the bridge PUP on by itself. Never heard the RP-100. Let us know how it sounds when you get her fixed.


             Brad     

[Colas LeGrippa]

Be careful in there Alvin. That amp has around 700vdc on the output tubes plates with enough current to fry a large horse.

Yeah, I have noticed. with the standby switch on the Hi side.  ouch !

But if you must use a 1Ω resistor, put it between the tube cathode and the transistor's collector

I should have thought of that, I had ''ground'' in my head and couldn't figure it out, in fact, the 1 OHM right between the cathode and whatever comes after

Yes, that's a fairly common failure when using 470Ω/1W resistors on a 6L6. Some people like to replace those with 1K/5W resistors

That's what  I did right away, every amp that comes in here deserves that small operation, costless, almost.

Thanks guys!

Colas