Long story short when I got the amp back, he told me that he put in a new pair of matched JJ EL84s and adjusted the bias to run them healthier. My tech looked at it and replied that the tubes were running way too hot. The label damn near burned off the EL84s. My initial thought was premature power tube failure and/or transformer issues. I turned it off right away and took it to my amp tech as soon as I could. A few months ago my amp just cut off volume on me. I tried it wioth a bypass cap but found it sounded much the same as fixed bias so left off the cap to get a distinct sonic difference between fixed and cathode bias settings.I have a specific question about my Marshall 2061X.

It was basically a London Power Standard with full power scaling.įor cathode bias I used a common 470 Ohm resistor in the cathodes of a 6V6 pair without a bypass cap. Perhaps not surprisingly - one of the settings he uses a lot is with one pair in fixed bias pentode mode and the other pair in cathode bias triode mode. Each Output pair was switchable between fixed and cathode bias and between Triode and Pentode Mode. I've actually built an amp for a friend using 4 off 6V6 into a 3400 Ohm Marshall JT45 Output Transformer. Grab any 1900 Ohm or 2200 Ohm Raa Output Transformer (Marshall or Fender 100W) and drive it with 8 off 6V6 (4 per push pull side).

One more 'off the wall' suggestion for a big 6V6 sound. In fixed bias this is 100K which is a sever load for the phase splitter/driver. Individual Cathode Bias resistors of 525 Ohms each tube (try 2 off 270 Ohms or a 220 Ohm + 270 Ohm in series)Ĭathode bias gives you one benefit - the maximum value for the KT88 grid leak resistor is 220K.

Primary Impedance for Output Tranny of 4 KOhms You can also run them in triode mode for just on 30W out. B+ is 500V, I got 122 watts jus beloow clipping and 135 Watts in heavy overdrive. I've only ever run KT88 in fixed bias, I have a prototype HiFI Amp running a quad of KT88 in UIltralinear into a Hammond 1650T Output Tranny. Individual Cathode Bias resistors of 460 Ohms (use 470 Ohms) Primary Impedance of Output Tranny Raa = 9K Have a look at the linked data sheets for other possibilities but the application data gives one possible operating condition.

Cathode bias guitar amp