I recently came across some information on eurotubes.com about a modification to the Blackheart BH100H hothead that allows it to run 6550 power tubes instead of EL34's. From their description, it's not so much a modification as it is a simple bias adjustment. Something about taking the cathode bias up to 50ma?
Their info left me wondering several things. The hothead is fixed bias in class AB mode and cathode bias in Class A mode. Did they have to change an actual resistor to get the class AB fixed bias into an acceptable range for the 6550's? There's a bias pot for class A cathode inside the amp on the PCB looks like you can get a tiny screwdriver into. I'm not sure what the stock fixed and cathode bias is from the factory, but it's worth noting that it's dialed in for JJ EL34 (not the EL34L's).
Here is a pic of the 6550's running inside of the hothead, boy they sure do look awfully hot!
...and a link to the page on eurotubes site
I'd like to do this to my BH100H amp. Any input on whether this is a safe thing to do? If so, what bias settings would you recommend? Thanks!