Swede52 wrote:Hi all! I have recently upgraded the power transformer to one that makes it possible to use KT88's in "The Pride of my fleet". I use Shannons newest revision of the ST-70 upgrade board. The secondary VAC is exactly 360-0-360 and the voltages after the rectifier tube are only SLIGHTLY lower that the voltage chart (within 5%) and symmetrical. My problem is I can't get BIAS low enough! Even with the 10K pots at the extreme left I get more than 0.62V in the right channel and only slightly lower in the right channel. The amp act and sound VERY good but biasing problem won't give me any relief. Balancing is no problem but the levels remain too high. I read Bob Latino thinks KT88 tubes in this environment need 42-43 mA (corresponding to ABOUT 0.42-0.43V voltage drop over a 10 ohm resistor). How do I get that low? Please help me!!!
With a STOCK ST-70 PA-060 power transformer you should never use KT88 or 6550 tubes. They draw maybe 30% more current and you can smoke an original PA-060 power transformer.
In an ST-70, if you do have a beefier transformer from Triode or Dynakitparts, IMHO you CAN use 6550 or KT88 tubes in an ST-70. You have to bias 6550 or KT88 tubes a little higher than the 42-43 Ma that you mention above. About 50 ma is right when in an ST-70 which is .50 volts over a 10 ohm resistor on each tube.
KT88's or 6550 tubes will last longer in an ST-70 than EL34 tubes. KT88's and 6550's are designed to "look at" 475 - 500 volts DC on pin 3 (plate) and pin (4) the ultralinear screen tap. If KT88 or 6550 tubes are used in and ST-70 they only look at 410 - 430 volts. IOW they will be operating at mayb 15% less voltage than they were designed and as such will last longer. The two down sides to their use is that they cost more and at times can cause biasing problems with the bias paramaters on an ST-70 which are set up for EL34 tubes. On a stock bias system (as has been mentioned by others on this thread) you will have to alter the two 10K resistors on the 7 lug terminal strip. On an VTA boarded ST-70 the fix is simple - just pull the 10K R39 and replace it with a resistor of a lower value like 7.5 K or 5 K. If you are ONLY going to run KT88's then you can just pull the 10K R39 and replace it with a jumper wire (no resistance). The only caveat with the jumper wire is that now EL34's won't bias.
Bob Latino