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[gazzgary]

I have built a GSG with great results and i wanted to pass around an idea i had and see what people think.

I was wondering if the poseidon board is capable of driving a pair of the new kt120 tubes at full power or close to it or if it would all become an expensive melted paperweight.

my idea is to use an antek AN-4Tk400 transformer with the sds labs cap board from triode electronics using solid state rectifer (i realise the bias tapping voltage is wrong on this antek transformer) im not sure but i dont think that the P-782 transformer would be up to the task.

as for output i was thinking maybe the CXPP100-MS-4.2K from edcor.

I have built the GSG and i have a ST35 on the way but i dont think i have enough experience to be changing resistor values etc for a project like this just yet.

for the poseidon board to drive KT120,KT100,KT90 tubes to full output or close to, would it be a matter of just having a bigger power transformer and adjusting the bias to suit the larger plate dissapation of these tubes or would resistor values etc have to be changed or is it possible at all.

for the antek transformers 70v bias tap some resistors would need to be changed to start with i would imagine.

I think this would be a very worthwhile project and im sure that there would be interest in a 100w + tube amp.
Kind Regards
Gareth

[dcgillespie]

Gareth --

The new KT-120 is basically a heavy duty KT88. It has a greater plate dissipation rating, and a higher plate voltage rating, which allows it the capability to produce more power, but it's bias and drive characteristics are very similar to the KT88, so the Poseidon board will drive it just fine.

Notice I said "capability". Simply dropping KT-120s where KT88s once were will produce little power increase since the power supply and output transformer impedance basically set the power output a given transformer set is capable of producing.

I would define your goals clearly. Real KT88s can produce about 90 watts RMS in UL with low distortion, but they are pushed near maximum ratings to do so. If your goal is 100 watts RMS without running the output tubes to their extremes, or you simply want to use the KT-120 tubes, then that determines the tube of choice for you.

To build an amplifier that will output an honest 100 watts RMS into a real world load with KT-120s, and assuming you're using your 4200 ohm OPT transformer with no greater than 40% UL taps, you will need a B+ of 575 vdc under full power output conditions. As is so typical, this makes the design of the power supply very unforgiving. It must have minimal resistance to prevent the B+ from soaring excessively under no signal conditions, and plenty of current capability to handle full power output current draw with little sag -- in other words, good regulation. A high current SS voltage doubler supply is almost a must here. To handle the full 20 - 20 kHz power bandwidth with little distortion, the OPT must be able to handle this bandwidth at 150 watts to account for losses. If your current choice is a 100 watt transformer, power bandwidth will be compromised at the frequency extremes under full power conditions.

Given the above stipulations, the Poseidon board will drive the tubes rather easily with a B+ to the board of at least 450 vdc, and possibly a current adjustment to the phase inverter CC circuit. And, the resulting amplifier should produce very little distortion as well.

It would be an ambitious project for sure, but certainly doable. Keep us posted on your thoughts, ideas, and design goals.

Dave

[battradio]

If you would be using an Antek power transformer , i would use the Antek AT 100 . http://www.antekinc.com/pdf/AT-100L.pdf .

.
This is a baldwin that i modified to use four 6550s in PPP and will produce 100 watts .



This is the dual mono monster i'm working on right now .
I realy like the sound of the antek outputs .

Not that i don't like the sound of the Edcors


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Mark