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Old 8th June 2011, 07:33 AM   #1
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Default KT120, PSE

Hello everybody,
Like many other peoples I am addicted to tubes receivers and amplifiers. Until now I have built tube preamplifiers, final stages and as well restored a few old radios. Always I have used proven schematics and I was very happy with the results I got, nevertheless I am far away being an expert. Since the KT120 came out, I couldn’t stop thinking about it and finally I decided building an amplifier with this powerful device. Was a long search – as the tube is still young, there are not so many schematics. Finally I found “Le KT120” from Leven – looks good and is a proven schematic – thus I considered the power is a little low. Not because I would need lots of W but because the speakers I use are not so sensitive. As well I was not so happy with the driver stage from Leven – for me Aikido with 6SL7 + 6SN7 sound and look better together with KT’s.
Now the Leven’s schematic loose the driver stage and receive an Aikido instead. Beside, the final tubes would be in PSE, UL 50 % connected to an OTP with 1,5 K primary and 4 / 8 Ohm secondary, rated at 70W RMS. There should be no feedback at all. Power supply would be overrated. The power transformer I have available offers:
315 V at 1150 mA with a tap at 215V – final stage (enough for 440 V B+)
280 V at 80 mA - driver
60 V at 100 mA - bias
6,3 V at 14 A – heating final
6,3 V at 3 A - heating driver

Now I need the experts advice. Is there any issue regarding aikido driver stage for PSE configuration of KT120 ? Would this work properly together?
Thank you all and best regards from Germany,
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Old 8th June 2011, 09:35 AM   #2
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Quote:
Is there any issue regarding aikido driver stage for PSE configuration of KT120
One of the problems with parallel output tubes is the Miller capacitance. Your aikido driver should not have a single problem with that. As it is apparently a very good driver for difficult loads.
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Old 8th June 2011, 04:13 PM   #3
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....Würde ich so nicht machen...
Well, the Aikido certainly does have a low output resistance since its output stage is a cathode follower. On the other hand, the amplification factor is limited and is about µ/2. So a 6SL7 in Aikido gives an amplification factor of about 35 which is way too low. IMHO there are much better ways for a driver than a the SRPP configuration in the aikido like high gm Pentodes with anode chokes.
No Feedback in such an amp is a fail if you ask me. Especially when not connected as triode. I would even use feedback with pseudotriodes since their sound differs clearly from direct heated Triodes with the typical flat box anode. Even the cheapest SS amp sounds better than a Pentode without feedback.
With two KT120 you're getting lots of power but with no feedback this gives a strange combination. Just imagine the cone of the speaker. With more power, the distortion in the speakers rises dramatically, this is a fact. But with no damping the amplifier has no ability to control the counter movement of the cones. No feedback does only make sense at Power levels about 5W and suited power triodes (low µ)
What Operating Point did you choose for the KTs ?
I don't want to mock your design but I'd go for PP Class A and use the TFK V69 design. Means Phase split transformer and feedback from KT120 Plate directly to the driver tubes cathode.
Think about it

Gruß aus München
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Old 8th June 2011, 10:41 PM   #4
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Default ....Würde ich so nicht machen...

Hallo Manta,
Some years ago I build the "red hot pp" and my father in low still use it almost daily. PSE with this new monster would be more challenging than a PP configuration… Beside there is already SE or PSE designs made out of aikido driver and 300B or 2A3 or KT88 finals – and most comments are extremely positive. . One side you might be right is in respect of amplification factor, but I think it deserve a chance. Regarding operation point of the final tubes I am thinking to set at 440 B+ and 120 mA each KT. As the power supply is able delivering up to 1150 mA continuously I would be able rising as I wish and try different values. I think this would be a reasonable start
Best regards to all ...
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