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In Reply to: RE: TAL - Jeff Korneff 45 Clone Mono Amplifers Review ....... posted by Cut-Throat on May 11, 2021 at 17:04:22
It seems that the Korneff SE amplifiers are held in high regard by many.
In terms of the schematic : a tube rectified cap input power supply with a 10H smoothing choke and a two-stage cap coupled signal circuit. The circuit has been lightly modified by the builder.
It looks like a standard 2-stage SE 45 circuit , perhaps there is more to it than the schematic reveals.
Is it the Korneff circuit or the selection of components that makes it a legendary SE 45 amplifier?
DT 667
Follow Ups:
I'll speculate that the choice of output transformer plays a large role. That UBT-2 (apparently no longer available) has huge magnetic headroom when used with a 45.
My Actual Build uses Hashimoto output transformers. Which was a substantial upgrade.
Interesting. Hashimoto is certainly a good name - do you have a model number or link? I'm always interested in good-sounding transformers and what characteristics might be associated with the sound.
The circuit of the amp is an excellent example of classic simplicity - nothing exotic, just everything done right.
Closeup Pic.
Thanks!
Hashimoto H-20-3.5U; details easily found on the web. I was very interested to see that a 3.5K primary is being used, as opposed to the 4.8K of the UBT-2. The best 45 amp I've heard happened to use a 3K primary.
In any case, the rating of 80mA DC provides pretty hefty headroom for a 45!
"The best 45 amp I've heard happened to use a 3K primary."
what 45 amp would that be Paul ?... parafeed ?... I have a pair of tfa-2004 to use.. thanks
Neat primary arrangement; offers the ability to run different g2 ratio if applied to a pentode final... :)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
This is a Schematic that was drawn by Craigtone Audio on DIY Audio. It was under the Tutelage of Jeff Korneff Himself. So this is what he was building.Jeff was inspired by an Article called 'Old Glory' that appeared in a Magazine called 'Glass Audio' by Eric Barbour, who put the Amp together using sub Par 'Spare Parts'.... Jeff may have tweaked that original schematic, but the one you linked to was the Schematic of the 'Jeff Korneff' Amplifier, as certified by Jeff.
Edits: 05/17/21
Could someone post the recommended values for B+ and B2+, please. (I would like a rule against using "B+", without defining its value on a schematic). It would be very helpful if someone with a working amp could measure and add some voltages on the schematic too, with expected currents through the tubes.
Edits: 06/10/21 06/10/21 06/10/21 06/10/21
There I circled it for you....
Great! Thanks very much!
"Korneff 45". Jeff Korneff adapted the circuit from a Glass Audio design using old Sansui PP transformers. That design was likely taken from the RCA Tube Manual.
A quote from another audio website regarding the "Korneff" SE 45 design.
Not sure about the RCA tube manual reference. I'd need to look in my copy to see if there is a similar circuit in that old text book.
DT 667
And here was that Article that Jeff Himself sent me. He also sent me a list of the Modifications that he had done to this Design.
Schematic shows a cathode resistor which is common to both 45's ..
I have Monoblocks, so I would have separate ones of course.
as posted above.
Korneff does not seem to favor the cathode bypass multi-cap arrangement in his schematic.
The austerity of a single cathode bypass capacitor does have an appeal to some designers.
Thanks for posting.
DT 667
Jeff Korneff also sent me the following e-mail. He also told me to share this and his schematic as well.
***********************************************************************
Kevin,
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!
Jeff
JDM said he has been trying to contact you regarding the TAL/Korneff 45 amplifiers.
DT667
No one has contacted me.....
Have him e-mail me at kdbmeb@gmail.com
"No one has contacted me....."
Have him e-mail me at kdbmeb@gmail.com
CT's contact info has been forwarded to JDM per request, sir.
DT667
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