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Original Message

Jeff Korneff e-mail to me ...............

Posted by Cut-Throat on May 17, 2021 at 12:29:58:

Jeff Korneff also sent me the following e-mail. He also told me to share this and his schematic as well.


That 45 amp was a take off on Eric Barbour's Single Ended Glory for under $100. I attached that schematic. You can download and enlarge. I never bothered with a written schematic of my own, the changes I made were all in my head. You could build the amp exactly as shown, PP transformers and all, and it will sound great. My first build was a mono version very similar to this one, even using the Sansui/Hashimoto outputs, just to see what might be there. I was totally taken in. Don't forget Eric was using junk box parts to build a great sounding amp. Genius.

The changes I would make as a final build to get the full 2 watts, not in any particular order:

use a Hammond 372FX power transformer (a nice transformer, better than 200 series)
use a 5u4 (octal) or 5Z3 (4 pin) rectifier with 20uf/500V electrolytic input cap and 20K 12W load resistor at B+ take off
use separate filament transformers for the 45's with 100 ohm 2W hum balance pots
use 1500 ohm -5W or better- 45 cathode resistors (if separated, above) with say 30uf-100uf / 100V bypass caps
use a pair of true SE 5K outputs from Electraprint- or your maker
use a pair of 50-50 uf/500V dual section electrolytics after the choke pairs
the Heath W series 46-12 (#?) chokes look nice refinished, seems to me they were ~10H, 150 ohm?
use min 2W 6SN7 plate load resistors
suggest using coupling caps to be 0.22uf (not 2.0), 400V or better, try Mundorf silver Foil in Oil
I would personally skip the mylar bypasses across the electrolytics
seems to me I used a 50 or 100uf/16V electrolytic for the 6SN7 bypass cap
It all fits nicely on a 10 x 17 chassis- photo attached. Aluminum.

You're looking for 275 plate volts for the 45 (plate to cathode), 56 volts dropping across the 45 cathode resistor, about 160 plate volts at 6SN7. You will need approx a 335V supply off the rectifier to get to these voltages. Using any other rectifier choices than those I mentioned may/will necessitate the use of a different power transformer rating.

Other notes: I did tidy up the 6SN7 and 45 filament supply voltages with 5W cement dropping resistors to put them "on the money". The amp was buss grounded to a power transformer mounting bolt. Also, the amp was dual switched to allow for preheating the 45's before B+ voltage could be applied- the B+ switch will do nothing without the 45 preheat switch being on. Don't be tempted to use high capacitance values.

The amp was built to run USA old stock 45's. If using overseas versions you would want to consider their specifications.

It's a power amp and will want a preamp ahead no matter that it could be driven directly from some sources. Seems to me the input sensitivity was around 2.6vac .. not sure now. But more than 2.5 and less than 3.

Be safe. High voltages can be lethal.

Hope this helps, feel free to share, Eric did!