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RE: Consertinasplitter for KT88 in PP

Posted by Thorsten (M) on April 22, 2014 at 07:27:08

In Reply to: Consertinasplitter for KT88 in PP posted by Stewen on April 20, 2014 at 10:36:51:

Hi,

First you likely should drop the screen voltages low to make sure you can produce enough swing (lower screen voltage = less bias). Crank up the current (more current = less bias).

Secondly, use a high perveance tube as splitter, run with loads of current. Make sure you can drive the tube close to 0V across it (read low Vak(min)) remaining linear.

The cathode voltage should be (Va - Vak(min))/4. This will maximise the swing of the splitter. Unless your input tube can swing close to 0V while remaining linear, or at least far enough to drive the output tube driven from concertina cathode into cut-off, you need to AC couple concertina and gain stage.

Sometimes you have to raise the gain-stage anode voltage and thus the concertina cathode voltage, reducing available swing in the concertina, if you want to direct couple the two. Going SRPP/Mu-Follower for the gain-stage may help to allow more swing with limited anode voltage than a plain common cathode gain stage.

If you can work out the conflicting requirements to something that works, I find Gainstage -> DC -> Concertina -> Coupling Caps -> Output Tubes one of the best PP Amp's I can make.

A way out would be to have fixed bias available and make the concertina tube also a cathode follower driving the output tube grid. Anode signal drives a second cathode follower via cap, make sure it is in effect an "output-less concertina", manipulating the resistors values somewhat can adjust the harmonic profile...

Yes, I know it is a crazy proposal, but it actually makes a surprisingly capable self-phase-splitting Amp with the potential to get much more swing and drive the output tube grids positive. Use a single 5687WB as driver and a single 6072A Mu-Follower as gain stage, or maybe a 6SL7 Mu-Follower etc.

Output stage - my recommendation would be ultra-linear with cathode feedback from the secondary (needs 0-4-8-16R secondary) near max. allowed plate dissipation (70-80%), this can run open loop without needing loop feedback.

Not the most powerful amp under the sun, but definitly something else.

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

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