Framus cobra 6550 questions

[jd267]

I have been running 6550 power tubes in my cobra for a few days and LOVE the way it sounds . Framus said the transformer is rated at 100 watts to 160 watts max for a short period of time . So I replaced my screen grids with 1k 5 watt resistors which is what it had stock but I moved them away from the PCB like jerry said in a post.

What I want to no is should I install a better power transformer?

Also what resistor do I change in bias circuit to get the bias a little lower ? Right now they are running 50 mv, my plate voltage is 465 so they are running at almost 70% right now with bias pot all the way down. If I change the resistor I will have more swing on the pot to run the 6550 tubes lower than 70% and could swap to el34 again .

Let me know fellas any help would be great . Amp techs only. I have done bias mods to a few 5150 amps and resistor swaps in jcm 800's . Also built a few pedals . I also bias my own amps and repairs but I'm not a amp guru .

 

[jd267]

http://www.warwickbass.com/warwick/data ... bra_V5.pdf

 

[jd267]

http://www.warwickbass.com/warwick/data/Warwick.de/Technical%20PDF/Framus%20Amps/Cobra/Cobra_V5.pdf

Looking at the layout is the resistor R83?

 

[jd267]

Figured it out. Had to remove power tube board to get to dam resistor between the two caps. Removed the 22k and installed a 15k . Low side with 6550 tubes is now in the mid 20's. Can also bias el34 tubes past 70% . Amp sounds better with the 6550 tubes at about 65% they fill in. Know wonder jerry charges what he does to work on the framus cobra and dragon. They suck to work on lol.

 

[RJF]

I like KT77's in my Cobra.