PSE KT88 using Mikael's basic schematic.....

[jmillerdoc]

Hi,
Having built the Mikael KT88se amp found on many threads here I was very pleased with the result. I was curious if it would be feasible to use the same driver stage (6n1p) to drive a pair of KT88's into a 2.5k or 3.5k OPT. Anyone with any input regarding this idea? I was going to keep everything else the same...B+, cathode resistance on the KT88, etc....just drive a pair into a lower OPT impedance.
Jeff

 

[Tubelab_com]

I have done this with an SSE which is a similar circuit. You need to either use half the resistance in the cathode resistor if the two tubes are wired directly in parallel (matched tubes required) or use a seperate cathode resistor for each tube (recommended and what I did). In fact there are seperate cathode resistors, cathode bypass caps, grid and screen stoppers for each tube since I just wired the two output tubes in parallel at the coupling cap, screen and plate connections to the OPT on an SSE PC board. The second coupling cap was removed so 1/2 a 12AT7 drove both KT88's. This works fine as long as you don't try to run A2. I ran it this way in triode and UL mode. Did not try cathode feedback, but it should work.

 

[jmillerdoc]

Excellent! This is my next project then.....thanks George.

 

[Pano]

Cool. Can't wait to see what you come up with. A couple of us are kicking around the same idea. Could be fun.

 

[jmillerdoc]

Went back and did a little research on the 6n1p tube....it should be able to handle driving both tubes without much difficulty. I have a pair of James OPTs that have 2.5k and 3.5k primaries so I will probably switch between the two to see which does best.

I have a nice chassis ready to go and I think a power transformer sitting on the shelf. Ot shouldn't take much to get this operational.

Once I start I will start a dedicated build thread.

Thanks for the input.
Jeff