Can't bias tubes on amplifier

I presume that this is the first time you tried KT88's and had not previously (before the first failure to bias) tried them. If that is the case, it is possible that the KT88 tubes you tried were just 'hot' tubes and that selection of selected/matched tubes with a lower 'bias point' might work. Might. I've had 6550's too hot for one of my amps, but by careful selection I was able to use different 6550's without problems.

More likely the amp had been optomized to run on the smaller tubes by Paul in the first place leaving the option to change the internal settings to operate on KT88's if desired. Don't know, but it's a reasonable guess. Very few amps can handle both the low power and high power tubes within the range provided without some type of switch or adjustment to accomodate the differences.

I'd be looking for a techie with a good tube design background/understanding of the problem. Don't know one off hand, but you might ask the Tube Forum on Audio Asylum where a lot of knowledgable tub-o-philes check in.

I agree with Newbee. It's possible that the window of bias adjust,i.e. negative voltage, is insufficient to properly bias the KT-88's you received. Therefore the tubes are running away. A tech needs to measure the maximum bias voltage at the tube socket to see if it's sufficient. If it isn't, a different resistor could be installed to 'open' the bias window to allow for a wider, higher, bias voltage adjustment provided the minus bias voltage was high enough to begin with from the power supply. The example would be, with the KT-88's you received and based on the plate voltage your amp runs the output tubes at, you need a bias voltage of -55Vdc. Your adjustment maxes out at -48Vdc, so the tube runs away.

Thanks for the advice, Newbee and Hifigeek. I have been able to bias KT-88s in the past with this amp at he 500-550 mV settings, but have never been able to bias tubes 1 and 3 at 200 mV to achieve a single ended class A operation as Paul describes. Now I can't get any of the KT-88s to stay below 600 mV.

I can stay with the EL-34 tubes, which sound very good, and I can achieve the single ended bias with them. But one of the things I like about this amp is that you can use a number of different types of tubes and get a different sound.

Have you tried 6550's??

Haven't tried 6550's although they are approved for the amp. I'm reluctant to put more money into tubes if I am having the bias problem with KT's.