Hi, vacuumlanders.
I wonder if anyone can shed some light on the following...
I rescued an Electrohome "Concord IV" amplifier from a junked console
(at the end of a neighbor's driveway - there were no tubes in it and
the console had been in a basement for decades holding up one end of a
workbench!)
It's a nominal 8 to10 watt PP amp using two 6V6GT tubes with an
integrated preamp for a cheap record changer (called "hi fi" but
not really!) It was clean enough top and bottom when dusted off.

After checking for no B+ shorts, I plugged in the rectifier tube
(only) and started to bring up the B+ on a variac; got to 135 VDC
(less than 1/2 way up) and left the caps to form for a bit... so far
so good But while poking around with a DVM to check the B+ to all the
tube bases, I measured 135 VDC on one 6V6 plate pin and only 124 on
the other. I immediately smelled a rat, powered it down, discharged
the caps (slowly) and checked the OPT primary. Worst fears
substantiated - one half of the primary was open. Never mind, we're
still ahead of the game (good chassis, power transformer and other
stuff,) It's still slated to be the stereo partner to my "Scratch 10"
small monobloc - see ABSE.

I pulled out the OPT and checked the primary again: 296 ohms from one
plate terminal to CT, 5.5 Megohms (ugh!) on the other! OK, so we
replace it, no big deal. But with what? What was the original
plate-to-plate impedance used? Regardless, what do we really need
for two 6V6's?

On the variac, I put ran the good half of the primary up to 100 VAC
and measured 9.96 VAC on the secondary. That's a 10:1 turns ratio, or
20:1 turns ratio Plate-to-Plate to speaker, so impedance ratio 400:1.
But what was the speaker impedance?

Well, as an avid pack-rat, I had pulled the 3 speakers and X/O as well
! (12 inch woofer, DC res: 10 ohms, 8 inch mid range, DC res. 7.4
ohms and a horn tweeter, DC res. 9.3 ohms; all look like RSC (Radio
Speakers of Canada) brand.) If they are nominally 8 ohms, that makes
the P-to-P impedance 3200 ohms, IMHO very low for PP 6V6's. But if
the speakers are 15 ohms (distinctly possible for that era), the
P-to-P impedance would be 6000 ohms, more reasonable. The separate
6V6 cathode bias resistors are 470 ohms each. Compare this to 510 ohms
in each cathode in my "Scratch 10" 6V6 PP amp. where I use a 10000
ohms P-to-P impedance OPT and get 10.4 watts max with 325 VDC on B+
and 9.6 watts with 315 VDC on B+. The power transformer is 320-0-320
(open circuit) with a capacitor input filter but I've not measured the
B+ on a simulated load.

So, what primary impedance should the replacement OPT have? 6000,
10000 ohms, other? I don't mind re-engineering the O/P stage to get
a genuine 10 watts in class AB1, or close enough. I've not yet drawn
load lines on 6V6 plate curves, it may come to that, but two 6V6's are
such an established design that it should be a no-brainer to pick the
best OPT primary impedance.

What do people think? BTW, I'm also open to sales offers of a
suitable PP OPT.
Cheers,
Roger