An 813 tubes amplifier (500W to 1KW)

I know, QRP is nice and also, one should be cure the antenna system to minimize losses and increase radiated power, but there are times when one may want a bit more "kick", now, I just stumbled upon this amplifier design and found it to be very clever, so I thought to post it here in case someone may be interested

The original site hosting the infos doesn't seem to be online anymore, luckily the page can still be found at the web archive

http://web.archive.org/web/20071213043718/http://www.raibeam.com/zl1axb/813_amp.html

which also carries the schematic for the 2x813 version of the same amplifier

http://web.archive.org/web/20071214021540/http://www.raibeam.com/zl1axb/813_p2.html

I believe the design of the input matching circuit is really clever and worth a look, on the other hand, I don't really like the idea of placing the HV transformer and circuitry in a separate box, in my humble opinion a larger box hosting the whole circuitry would be nicer and allow more airflow; as for the HV transformer, while looking for schematics, I found another design for a 2x813 amplifier built using "scrap" parts

https://people.ohio.edu/postr/bapix/813amp.htm

that design uses another smart idea, that is the author uses a pair of recycled microwave ovens transformers to obtain the HV to feed the anode of the tubes, the idea is quite interesting and may be applied to other amplifiers too (not just 813 based ones), for further informations about the power supply, see here

http://web.archive.org/web/20030627162157/http://www.vaxxine.com/phil/QEX_HV_Power_Supply.pdf

as for the 813 tubes, the UR4LL http://www.ur4ll.net/ web site sells them at $35 each

http://www.ur4ll.net/#tetrodes

a honest price, I think, and on the same website it's also possible to find air or vacuum variable capacitors which may fit

That's all, nothing special, but if someone is considering to build an HF amplifier I hope the above links may give some helful ideas