RM50 bias issue (post bias mod)

[Mattfig]

Hey guys...So I did the mod where you replace the resistor in R35 to a 3.3k ohms to get EL34s to hold a bias properly...At first, no issues...Then, I check my bias and realize it's very low...I go to turn up and and can't get it past 25 or 26ma....WTF? So I changed tubes and tried again...Nothing....Same issue...The only ones I could get to bias up were 6550s and the were VERY jumpy around 35ma wanting to jump up to 50 or higher with a breath in the wrong direction...
Should I change R35 to something else???

Both tubes are showing a match so it's not one particular spot...Replaced the PI just to see....Tried many tubes and even 6L6s won't get hot enough....Very weird...

Thanks! All help is appreciated...

 

[wesarvin]

Bad pot?

 

[Mattfig]

Bad pot?

I don't know...Could be...It isnt sticky and moves consistently up and down smoothly- just not within the right range...Could that still be it?

 

[kc2eeb]

Check the value of R35 you just installed and make sure it actually reads
3.3k. Check the pot also. Then, take the output tubes out and actually measure the bias voltage to see what it is actually putting out.

 

[Mattfig]

Check the value of R35 you just installed and make sure it actually reads
3.3k. Check the pot also. Then, take the output tubes out and actually measure the bias voltage to see what it is actually putting out.

My R35 3.3k ohm resistor is 1/4 watt...Should it be 1/2? I will check the actual bias output tonight and get back to you...

 

[kc2eeb]

1/4 watt is no problem. The bias supply can produce, I think around -85 volts or so. What you want to check is that you can get -30 volts or less so you have enough sweep to set the bias, typically -38 volts or so.

 

[Mattfig]

1/4 watt is no problem. The bias supply can produce, I think around -85 volts or so. What you want to check is that you can get -30 volts or less so you have enough sweep to set the bias, typically -38 volts or so.

Thank you! Perhaps I'm just getting odd readings from the bias points in the back? I will check asap...

 

[Mattfig]

Bad freaking multimeter....My trusted Autozone multimeter was giving poop readings....All amps were way OVER adjusted....I tested the RM50 inline and the bias was just fine...

I tested with another multimeter (Like I should have done in the first place) and confirmed that I am, in fact, almost retarded...

Thanks for the help!

 

[Totenkampf]

are the bias points on the randall gear usually pretty spot on? the folks at eurotubes helped me find a problem with my peavey xxx where the bias points were garbage.

 

[Mattfig]

are the bias points on the randall gear usually pretty spot on? the folks at eurotubes helped me find a problem with my peavey xxx where the bias points were garbage.

Accurate...Not the same reading as my inline Weber but it was a cheap multimeter and the readings were close...

Edited to add: I can't believe I was using a bad multimeter on all my amp bias...Had I not encountered this I would have burned through many tubes and hours wondering where my issues were....Everything was consistent on the bad one, the readings were just not the same...It was great for years...??? I'll miss that 10 dollar tube eating ***** of technology... 8)

 

[kc2eeb]

Great! Glad I was helpful.

 

[Mattfig]

Great! Glad I was helpful.

Dude, you have been MOST helpful in many ways...Just reading your posts has educated me...So hat's off to you!

 

[kc2eeb]

Thank you very much. I really appreciate it.

 

[warlok]

At least you didn't bias them to OHMS!! That was when I just accepted that I am almost retarded! :?