Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on December 07, 2012, 09:49:54 am

Title: Musicman RP-100, where to install a 1 Ohm resistor for bias ?
Post by: Colas LeGrippa on December 07, 2012, 09:49:54 am

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

Title: Re: Musicman RP-100, where to install a 1 Ohm resistor for bias ?
Post by: Colas LeGrippa on December 07, 2012, 09:51:31 am

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

Title: Re: Musicman RP-100, where to install a 1 Ohm resistor for bias ?
Post by: sluckey on December 07, 2012, 10:23:11 am

Quote

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.

Quote

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.

Title: Re: Musicman RP-100, where to install a 1 Ohm resistor for bias ?
Post by: Willabe on December 07, 2012, 11:16:00 am

Be careful in there Alvin. That amp has around 700vdc on the output tubes plates with enough current to fry a large horse.     :w2:

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      :icon_biggrin:

Title: Re: Musicman RP-100, where to install a 1 Ohm resistor for bias ?
Post by: Colas LeGrippa on December 07, 2012, 02:54:53 pm

Quote

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 !

Quote

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 :think1:

Quote

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