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amirmk 25th September 2005 12:09 PM

Design Questions
I've built several tube amps but this one is the first one i'm designing.
I want it to be SE Class A KT90 triode with a Parallel 5687 driver and input stage.
My KT90 will be using a 3K OPT and a 400V B+ to get about 140mA of current. The cathode resistor is set for about 680Ohm for a -40V grid voltage.
My questions are two:
Are there any major faults of the design i've written about?

If the grid voltage is -40v does it mean I need about 40 volt swing to drive it to full power? it's a gain of 20 which i can easily achieve with the 5687.

Thank you. sorry for my English.

amperex 25th September 2005 01:39 PM

Per my DIY experience.......
I prefer a direct coupled input & the best I heard are a Tung-Sol 6P5GT or 76 to a Sylvania 6J5GT. (Do not bypass the cathode resistor with a capacitor). If one does not prefer crystal clear sonics, use another design. It is well known some audiophiles like some slight smearing in the audio. The DC 6P5GT to 6J5GT design hides nothing. Slight smearing can make a poor recording sound better. I prefer Sylvania 5R4GYB or the Mullard family of rectifiers including the EZ80, EZ81 & GZ34. A rectifier tube can effect sonics to a fairly large degree & I can not explain why this is true.

BTW- The KT90 is supposted to be excellent in triode configuration. Good choice.

Audio transformer:
I noticed unloading (increasing) a tube audio transformer primary impedance produces a few percent less watts out, but the sonics are superior. An example is my DIY PPP triode connected 6V6. At 4K the tubes sounded just so-so. At 6K primary the difference was just jaw dropping. Of course, the speaker load plays a large role here. My existing Pro Ac 2.5s are well known to be 'tube friendly' speakers.

Power Supply:
The use of motor run oil caps are superior to any electrolytic type. Also, inexpensive Solen polyproplyene are excellent. If using a LCLC PS filter, the first cap (L-first C-L-second C), an electrolytic at the first C position makes no difference.

Audio Coupling Caps:
Paper in oil or Russian Teflon are well known by many audiophiles to be high performance coupling caps. The most inexpensive couplinjg cap that performs well are AuriCap. These require about 20 hours to sound just ok & about a whopping 200 hours to settle into smooth sonics. Use of a Sprague 716P as a coupling cap will smear that midrange compared to PIO or AuriCap. It is simply the sonics one wants. Some audiophiles like the sound of a Sprague 716P 'orange drop'.

My 2 cents.

amirmk 26th September 2005 09:40 AM

Thank you very much.
One more thing, how can i know how much voltage swing is needed to swing it to full power?

jlsem 26th September 2005 01:21 PM


One more thing, how can i know how much voltage swing is needed to swing it to full power?
I would use a step-up input transformer (1:2) and bias the 5687 at -8 volts. This will give you enough voltage swing (80v p-p) to drive the amp to full power from a cd player.


amperex 26th September 2005 01:23 PM

I never looked at the KT90 triode connected plate curves, but -40 volts is easy to drive. The design I mentioned with 275 volts on the 6J5GT plate & 75 volts at the 6P5GT plate (direct coupled to the 6J5GT) produces about 65 volts peak into a 100K grid resistor with approximately 1 volt input.

amirmk 26th September 2005 02:17 PM

Thank you both, jlsem, can you please explain a bit more about what you're suggeting?

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