Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ListerHayes on February 19, 2020, 01:45:48 pm

Title: Pro Junior Mods Continued
Post by: ListerHayes on February 19, 2020, 01:45:48 pm

So, I did an octal socket conversion and plugged in a pair of JJ 6V6s tubes.  Everything works, but she's running a bit hot.  Aside from switching to 470k/3w resistors, any suggestions for cooling her down?

Title: Re: Pro Junior Mods Continued
Post by: sluckey on February 19, 2020, 02:06:51 pm

Quote

470k/3w resistors

Where are you putting those?

Title: Re: Pro Junior Mods Continued
Post by: EL34 on February 19, 2020, 02:24:37 pm

Quote from: ListerHayes on February 19, 2020, 01:45:48 pm

So, I did an octal socket conversion and plugged in a pair of JJ 6V6s tubes.  Everything works, but she's running a bit hot.  Aside from switching to 470k/3w resistors, any suggestions for cooling her down?

Did you check the power tube bias current?

Title: Re: Pro Junior Mods Continued
Post by: sluckey on February 19, 2020, 03:10:35 pm

6V6s will need a lot more than -10.4v for bias. I doubt you can even get enough negative voltage from that circuit to run 6V6s. I see a couple of easy solutions...

1. Put the EL84s back in. Easiest.
2. Keep the 6V6s and convert to cathode bias. Very easy.
3. Modify the bias circuit to provide -25v to -40v. Not so easy. May not be possible with that PT. Using a suitable PT would likely require overhauling the B+ rectifier/filters as well as the bias circuit. Not worth it IMO.

Title: Re: Pro Junior Mods Continued
Post by: ListerHayes on February 19, 2020, 05:04:40 pm

Cathode convert it is.  Thanks for the help!

Title: Re: Pro Junior Mods Continued
Post by: 2deaf on February 19, 2020, 05:36:07 pm

Raise the value of R29.  Could also make R29 variable for a bias level control.  Maybe 47K in series with a 100K pot so that the R29 slot is adjustable from 47K to 147K.