Hi GrantRay:
The suggested use of added 20 Ohm Rs across each end of the pot, to the center wiper has little, if anything, to do with the adjustability of hum. In a good amp, a well designed one, there should be NO user adjustments what so ever ( IMHO, IME ).
In the case of Orest12, his improved
hum adjustment was NOT the main goal of those two added Rs .....at all.
As stated by Palustris and I, a hum pot alone is a poor design, because the mechanical interface of the pot's adjustable wiper arm, to the pot's internal resistive element, which needs to be avoided. It acts as a diode-generator. ALL the 2A3's instantaneous peak current return to ground is running SOLELY through that wiper arm interface, a mechanical connection. The two added 20 Ohm 2 Watt high quality resistors, ( from each end of the hum pot, to wiper arm's center tab-connection ), transforms a " wiper-arm-only " path, to a DUAL signal path, consisting of the funky diode-like wiper arm AND paralleled and nicely-soldered 2 Watt resistors.
A soldered joint, in the cathode return of a 2A3 tube to ground, ( as one can imagine if thought about ), is MUCH better.
Orest12 heard it as
increased dynamics and drive through the SE amp. Of course !! That is exactly what is has, with better
internal transfer efficiency !!!
To answer your question about Tadashi Asano, adding Hum Pots to his Dad's, Isamu Asano 's amp design, here is what " I " think :
Again, this is ( IMHO, IME ) :
I say Tadashi made a BIG blunder !!! ( Isamu might now be rolling over in his grave. ) The son made the amp worse-sounding, poorer performing, on two key levels. Adding a wiper-arm in the 2A3 cathode's to-ground return path is a blunder, for the reasons as fully outlined above.
Adding those two 30 Ohm Rs is a second error IMHO.... equally serious. He made a 80 ohm about 6 Watt pot ( total value hum pot, 30+20+30 Ohms ) with 20 Ohms of fine - adjustment in the center. An E.E.'s dream approach. However, when Tadashi added those two 30 Ohm resistors, he also added three solder joints in series, which is added circuit complication. Resistors added in series, to get a value, should be AVOIDED in a good SE amp, due to unwanted solder joints.
Resistors in parallel, on the other hand, if matched to each other to 0.01%, ( no skews) CAN be used very beneficially applied in a SE amp. See my Avatar .
I always find ( IMHO, IME ) that with un-needed parts added to a SE 2A3 DC amp, the
worse it sounds !! KISS rules, when applied using a GOOD thoughtful implementation.
We did found above, that intelligent "R-doctoring" of a hum pot improves
transfer efficiency in the Cathode's return-to-ground path. clearly audible, over time, to Orest12 ( and me, and I suspect, Palustris).
I would carefully and safely measure the 2A3's filament voltage, AT the tube socket, which is the point-of-use, and see how close it is to 2.5 VAC. Also measure the same 2A3 filament voltage across the 80
and 20 Ohm pot spans. YES, you really should add soldered Rs to ameliorate the wiper's sad mechanical-only diode-like interface. Optionally, you may want to add a single 100 Ohm pot, in place to the three-some.
Seek to "doctor" whatever pot is used there, be it across the stock 80 Ohm threesome, or across a new 100 Ohm pot, ends to the center / wiper's terminal.
Best sound, Grant Ray, will be what Isamu, the father did, NO pots, use the center tap and well wired ( leads ) of a 2.5 VCT filament transformer winding. Less is more !! Also, Tadashi added a return path for negative feedback, after the 3K 2A3 Rk, bias resistor, which ( negative feedback ) should be avoided like the plague in a SET amp !!
Back to modding the hum pot, the amp will sound better, because it will have much better
transfer efficiency in the cathode return to ground.
Another possible alternative is to use a properly - rated Hammond Ef transformer, for each tube, below deck.
( IMHO, IME ) the best sounding 2A3 SE amp implementation, is to never ever use a hum pot, and rewire the Ef ( and power transformer ) lead outs, with higher quality lead out wire, ( I use Mil Spec wire with silver content, wide-band ) to improve
the amplifier's transfer efficiency in
all possible ways. Its audible, if a good listener-builder just would think and act on it. An inch of bad wire can ruin the musical experience.
For now, ONLY carefully do 2A3's Ef VAC measurements, and report back to us !! Cool thread.
Ohh, by the way, the son Tadashi has " little clue " as to how to best lay out and, in particular, how to properly wire a SE amp ( IMHO, IME ) .
You NEVER EVER bundle wires as you have shown us in your second photo - with tie wraps, if you want best possible sound. The fields from each wire interact, one upon the other.
Do this : Cut off and discard ALL the that amp's tie wraps, and then, carefully position and float the wires three-dimensionally under the chassis, not touching the chassis if possible, nor touching each other. Wherever possible, if wire paths have to cross each other, try to do so at right angles. Wait till you hear that !!.
Finally,
untwist, and float separately, the two Red-Red wires, from the power transformer to the tube rectifier socket, but only after you do the other wires.
Take PHOTOS, prior to snipping out the tie wraps, so you can simply return the amp to stock, should it be necessary in the future !! Have fun, LISTENING to that !! Please report back to us all . Have fun listening to the four ( good quality ) Rs and separated wire runs.
Dowto1000
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