• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.


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Joined 2004
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The kismet design is a good choice with the 45 and 2A3, but does not have enough gain or headroom to properly drive a 300B.

The 5687 is configured as an SRPP which may not be obvious as there appears to an error in the drawing in the PFO article.

Variations using 6SN7/7N7 (and their 12V cousins) also work quite well. Arguably they work slightly better with slightly higher voltages of 300V or so.. (1K cathode resistors, 8mA ~150V across each section)

You can also use 45 if you need less power or 6B4G with 6.3v filament which must be dc heated to avoid excessive hum. 45/2A3 may be ac heated without significant hum problems due to the low filament voltage.

I designed a somewhat similar amplifier in the mid 1990's using the 45 and 6SL7. I am a big fan of fixed bias because it eliminates an expensive high quality cathode bypass cap on the output stage and also permits easy fine tuning of the operating point to suit your reflected load impedance and the tube you are using.
Choice of OPT


I bought a pair of Lundahl LL1620 30mA SE OPT for my 45 tube amp replacing the good old Hammond 1628SE.

The result is extremely good. The Lundahl OPT out performs many OPT's I have heard so far.

I am building another 45 tube amp for my friend which will employs the Amorphous LL1620AM 30mA SE OPT's. I will report once it is done. Hopefully it will be done during the weekend.

Surely, the LL1620 can be configured for use the 300B. Give it a try. Highly recommended.

Kismet 2A3

Hello fox! Thanks for answers!
I've made a mistake, my mind was thinkink to 2A3 but my hand was writing 300B, mea culpa!

Yes, I would like to build this amplifier but I never build a valve amplifier. I've made many amplifiers with cipamp and transistors for me and my friends but never a valve one. I asked you becouse I want to start with a simple circuit that work.

Any sugestion is welcomed!

Thanks again!
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