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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!!!

KT88 and 6550 tubes bias differently than EL34 types, 60 to 70ma per tube is about normal.

Your 5AR4 rectifier will NOT last very long, I would suggest solid state rectifiers or a Weber WZ-68 replacement.

You may want to check out the other kt88 discussions here.
Make sure you are not overloading the choke (if you are
using the stock PSU with a single C354 type choke).

http://www.diytube.com/phpBB2/viewtopic ... highlight=

Assuming you kept the stock bias supply which has the bias pots between 10K fixed resistors what you need to do is change the 10k fixed resistor closest to ground to a 5k and change the other 10K resistor to 15K.

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Actually I misspoke. In running some PSUII simulations its the 10k resistor BETWEEN the two filter caps that needs to be lowered.
Burnedfingers has the right idea. Wire a 30K or a 10K across the 10K resistor BETWEEN the two filter caps NOT the 10K closest to ground.
The 30K wired across the 10K will give a resistance of 7.5K.
The 10K wired across the 10K will give a resistance of 5k.

Quote:
Assuming you kept the stock bias supply which has the bias pots between 10K fixed resistors what you need to do is change the 10k fixed resistor closest to ground to a 5k and change the other 10K resistor to 15K.

What would happen if you changed the value of the resistor 10K (the first resistor before the pot) to a value around 7.5K and left the one connected to ground at 10K?

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

I have a stereo 70 that I installed a switch in which to parallel a resistor across the 10K resistor so that I can use 6BG6GA's and have them bias correctly.
One direction of the switch allows the usage of EL34's and the other direction 6BG6GA's.

I might try a 20k pot instead of upper 10k resistor and 10k pot, maybe that will give enough range without being too twitchy. Yellow_Light_Colorz_PDT_09

I bias my KT88's at 50mA each in my Quicksilver mono block amps. I found no difference in distortion figures between running 50mA each and running 60-65mA each.