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Hi, I'd like to replace the 606GC with 6550.
What I need to change? R30 and R31 from 270k to 100k?

You don't need to change R30/31 but you may need to change the bias range resistor R43 so you can set the Fixed bias correctly.
Same with the 500R 5W R34/35 if you plan to run in cathode bias mode. We haven't done this so you will need to CAREFULLY experiment.
I would think it would work with no mods but make sure you do the tests and calculations to determine the safe operating of the amp power tubes or get a tech to make the changes for you if you are concerned.
Keep us posted on your progress.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

its been a year... how did this go?

I tried my new build with KT120's and KT88's. All my voltages are within spec of the build sheet and I was able to have a HUGE bias range with the 22k ground resistor going from the bias pot.

I also am using a solid state diode 5AR4 rectifier. Wanted to get more voltage to the plates. The KT120's can handle 850 VDC. The PT is not putting out quote enough voltage to really take advantage of the higher output wattage of the KT120s but I wanted to test them out.

PT and OT (OT was actually room temp) were still cold enough to grab after a few hours of playing with a loud drummer and through a sealed 2x10" Ampeg bass cabinet.

Hope this helps

There’s this too - viewtopic.php?f=22&t=4250

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps