Output Transformer Impedance Mismatch 6L6 to KT88

[leadfootdriver]

Probably another Topic cover many times, but lets keep it alive!

My next project is building a Deliverance (60) clone with a JCA50 Chassis and Transformers.. I've really flip flopped on the Power Tubes I'll use given the original iron was meant for 6L6's.. A big part of that sound is the authority a KT88 can deliver. I figured a KT66 would be close and a good match for my OT.

However, a respected member on another Forum stated that the lower Impedance of a KT88 would not be a problem and that I should bias 70% for a 30 watt Tube, thereby limiting the current demand on the OT. This sounds reasonable to me. He also recommended using 100K Grid Resistors for Bias vs 220K. Even the JJ Data Sheet recommends this. https://frank.pocnet.net/sheets/163/k/KT88.pdf

I can supplement the Heater Current with this little guy feeding the 5-Preamps and use the stock PT for the Power Tubes. https://www.mouser.com/ProductDetail/Ha ... eTYw%3D%3D

All knowledge and experience welcome!

[leadfootdriver]

I found a great post on the subject from Wilder Amplification back in 2009. https://www.marshallforum.com/threads/v ... ost-184531

"To find the "rough minimum" plate-plate impedance I square the B+ voltage, then divide that by the plate dissipation rating of one valve multiplied by the number of power valves that will be used -

(B+^2) / (Plate Dissipation Rating x Number Of Power Valves Used)

So for EL34s running at 425 Volts -

(425 x 425) / (25 Watts x 2 Valves) = 3612.5

A 3600 ohm (3.6K) plate-plate load would be the closest "standard" impedance to this. There is some leeway here since impedance is never constant...Marshalls with 2 x EL34s typically use 3.2K - 3.4K so something roughly in that range would work. You just don't wanna use something DRASTICALLY lower than the calculated value...such as trying to run 2 x EL34s @ 425V on a 1.7K plate-plate load or something like that."

With this formula, I'm not far off at all.

Discuss!