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RE: One thing about Yamamoto A-08

Posted by drlowmu (M) on November 24, 2015 at 18:49:03

In Reply to: One thing about Yamamoto A-08 posted by C.Y on November 24, 2015 at 16:35:19:

The Yamomoto A-08 we used had exquisite parts. It was a FACTORY built unit, and it was neatly wired, for sure.

The Yamomoto tube sockets, as one example, were lovely, and it has C-Core Output trannies, power and chokes as I recall. The speaker wire's binding posts are special also, very nice feel with a fine-thread so tightening is progressive to the touch. Lovely.

If you look at the A-08 schematic on line, you will see they specify " RN65 " resistors, vintage American military spec types I presume these to be. Where they use an RN-65, for - say, the plate resistor or cathode resistor of the front-end tube, I would now very carefully (and expensively ) employ TWO closely-matched Caddock MK-132s or TF-020s, and such resistors' end-result is HUGE in "tone, transparency and purity".

The Yamomoto sounded gray and artificial to us both, narrow band like a bad AM radio station signal playing through a cheap radio. It missed playing one octave higher and lower, versus my prototype's implementation. Also, it was TOTALLY outclassed in dynamic contrasting, and taking control of the large ALTEC A-4 speaker system. This is due to what holds almost ALL other SET amps back,

(1) poor internal wiring,

(2) high DCR chokes ( above 20 Ohms ) , and

(3) high amounts of capacitance in the power supply.

For example, the A-08 sports a "typical" / undesirable 10 HENRY ( about 80 to 100 Ohm ) choke in a C1/L1/C2 filter, ugh. Its DOA, by design, or should I more properly call it mis-design. I used L1/C1/L2/C2 - 10 Ohms and 8 Ohms for L1 and L2, 500 mHY and 1.8 HY respectively. Low Stored Energy Supply, aka LSES.

One other thing, the A-08 used an electrolytic, (as I recall) to bypass the Rks. And noisy zeners on the Pentode's extra grid. No good. However, I used about EIGHT carefully chosen Rk bypass film capacitors in parallel - across each Rk, to ARTFULLY get the amp to play wide-band and with proper musical intent.

Yes, Yamomoto-San, ( had he been in front of those ALTEC A-4s ), may have rolled over and simply puked, if he heard the A-B. But maybe, he would learn something too, to apply into the future.

This amp build has been FUN so far, and I look forward to retrieving it from those A-4s, 55 miles away, and getting it playing on my now-silent A-7s, fifteen feet away. !! Need to finish re-wiring those A-4 woofers first, ( wanna' hear them properly ) that task coming-up this Sunday I hope.

Each ALTEC 210 bass horn weighs 440 pounds, unloaded, and each ALTEC 515C must be about 30 pounds, two per bass horn, so, we need to move about 500 pounds when loaded, just to access the enclosure's rear woofers' panel. Ugh.

Jeff Medwin

Edits: 11/24/15 11/24/15 11/24/15

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