[Roger Jones]

Hi, Vacuumlanders,
I'm seeking advice and opinion on a vintage Heathkit audio amplifier – the W7-M.

I was given one a few weeks ago by an old gent who built it back in the day. It was a bit grubby but reasonably well constructed... although one resistor (6AN8 plate) was the wrong value for the last 50 years! Now fixed. Of course, I am compelled to refurbish it to add to the “toys” for use once in a while...

This is an odd duck... it uses s/s Si diode doubler B+ rectifiers from a 180 VAC PT secondary and a 1/2 wave Se fixed-bias rectifier from a 40 VAC secondary. Tubes are two EL34's and one 6AN8 (pentode amplifier, triode phase splitter.) It has a physically large OPT, actually larger than the PT, and tests about 3500 ohms impedance plate-to-plate.

The schematic is at http://www.vintage-radio.info/heathkit/index.htm

To keep this short (various unproductive farting about omitted!) ... B+ caps replaced (two were dead, two reformed OK); fixed bias filter caps reformed OK; all paper coupling/decoupling caps replaced; EL34's very weak (pity!), so replaced with 6L6's to hand; 6AN8 tested good; DC voltages look about right.

Variac-powered up carefully... no smoke! B+ good, fixed-bias voltage good, AC power draw OK, signal gets to speaker. So far, so good...

But, max power output is lousy! The W7-M is listed as a “55 watt amplifier” but the most I can get at 200 Hz is a bit over 10 watts. At that level, one half of the waveform “soft clips”.

More forensics done (with NFB off)... it clips asymmetrically in the 6AN8. With the o/p tube grids driven directly with a P-P source (a centre-tapped audio transformer, primary driven by a 10 watt s/s utility amplifier), I can just about get 25 watts into an 8 ohm load at clipping – but at least it's symmetrical, hard clipping, i.e. evenly maxed out 6L6's. Confirms poor 6AN8 performance, so that needs fixing in due course.

Here's some test data from the o/p stage-only test using new, Russian-made 6L6's, all just at clipping, fixed bias balance and level optimized:

115 VAC in:
B+ at max load = 464 VDC
Across 8 ohms: 38 VAC, peak to peak, 200 Hz, for 22.5 watts RMS

120 VAC in:
B+ at max load = 478 VDC
Across 8 ohms: 40 VAC, peak to peak, 200 Hz, for 25 watts RMS

Bias around -30 VDC per tube... not much effect on the above, left at max.

Changed Se rectifier for 200 volt s/s Si diode, gives a couple of volts more available bias. Repeated tests:
115 VAC in:
B+ at max load = 457 VDC
Across 8 ohms: 40 VAC, peak to peak, 200 Hz, for 25 watts RMS

120 VAC in:
B+ at max load = 481 VDC
Across 8 ohms: 42 VAC, peak to peak, 200 Hz, for 27.5 watts RMS. Just a smidgeon better, but way off 55 watts!

Older, used but good 6L6's give results just a bit below the above, but very close.

So, where to from here... some of my printable thoughts to date:

1. W7-M is not even a modest collectible... so, dump it “for parts or repair”, no big loss to the world! After all, it's only a old PA amplifier.
2. The W7-M is as scarce as hen's teeth, a priceless collectible, so keep it “as is” for posterity... Ha, ha, had to say that!
3. W7-M was never “55 watts” - that was pure hype, so settle for 25 watts with a rebuilt 6AN8 front end.
4. Ditto, but re-design and rebuild the front end as a true Williamson, i.e.. double triode amplifier/phase splitter and double triode driver stage (there's space for an extra tube base hole in the chassis.) Fix the expected VLF instability (addressed elsewhere) as OPT, although large, is certainly not up to Williamson specs (plate-to-plate primary inductance likely nowhere near 100H – that costs money!)
5. Put it on the shelf “as is” and wait for inspiration... at this point, an attractive option!
6. Any other ideas?

Just thought... was Heathkit playing the IHF (?) “peak power” game. IIRC, there was a promotional hype back then to advertize amplifiers at around twice the true RMS power rating to sell basically crap equipment. But I don't think HK did this.

All replies gratefully received, recognizing, of course, that none of this is life changing. After all, it's a hobby... but I hate unfixed stuff!
Cheers,
Roger