Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on February 17, 2017, 09:07:39 pm

Title: 180 Watt Super Twin Reverb bias
Post by: TubeGeek on February 17, 2017, 09:07:39 pm

I have a super twin reverb amp I am working on with the intention of selling it.


Ordered a new matched sextet of JJ 6L6's.  Replaced one cracked power tube screen grid resistor.  Powered up tonight, everything seems fine.  Go to turn the volume up and it doesn't have the power I was expecting.  It is powerful but not 180W powerful.


Measure bias on each socket...5-7mA per tube.  Plate voltage 515 VDC.


Are these amps normally biased this way from factory? 


If I want to change the bias, how is it done in this circuit?


Change the value of the 2k7 2W resistor off the balance pot wiper?

Title: Re: Super Twin Reverb bias
Post by: tubeswell on February 17, 2017, 10:56:32 pm

Yeah the biasing is cold in the stock versions of these, but not that cold. Have you done a filter and bias cap change?


The other thing is, some of those old coupling caps might be leaky. I've had three SF amps in the last couple of months that had leaky coupling caps (and bad filter and bias caps - those old white Malorys are notoriously bad)

Title: Re: Super Twin Reverb bias
Post by: Blind Lemon on February 17, 2017, 10:57:12 pm

Yep. But I would get rid of that bias balance and stick in a regular bias circuit. Mod is easy to find.


BL

Title: Re: Super Twin Reverb bias
Post by: TubeGeek on February 18, 2017, 07:08:39 pm

Quote from: tubeswell on February 17, 2017, 10:56:32 pm

Yeah the biasing is cold in the stock versions of these, but not that cold. Have you done a filter and bias cap change?


The other thing is, some of those old coupling caps might be leaky. I've had three SF amps in the last couple of months that had leaky coupling caps (and bad filter and bias caps - those old white Malorys are notoriously bad)

Yes just finished replacing all electrolytics throughout the amp. Made no difference.

Title: Re: Super Twin Reverb bias
Post by: TubeGeek on February 18, 2017, 07:09:28 pm

Quote from: Blind Lemon on February 17, 2017, 10:57:12 pm

Yep. But I would get rid of that bias balance and stick in a regular bias circuit. Mod is easy to find.


BL

Yeah I figured this may be the route to go but it is the super twin reverb which is a little bit wackier than just a twin reverb.  I'm looking into how to convert the bias circuit now.

Title: Re: 180 Watt Super Twin Reverb bias
Post by: TubeGeek on February 18, 2017, 07:44:49 pm

Adjusting the 2.7k 2W balance pot resistor to a lower value is increasing current through the tubes.  I installed a 1.5k 2W and the current is now 17mA per tube with a plate voltage of 500vdc.  I'll go this route first, and see if power feels different.


update: Yes, power is definitely increasing.  Going to shoot for 50% MPD bias point, which is 15W.   

Title: Re: 180 Watt Super Twin Reverb bias
Post by: TubeGeek on February 18, 2017, 08:27:45 pm

900 ohm 5 Watt resistor did the job and it is melt your face off loud.  This is what I expected. My senses were correct.


This thing is squeaky clean with the jbl k120's.