[[email protected]]

On Tuesday, August 29, 2017 at 6:48:14 PM UTC-7, leadfinger wrote:
On Monday, August 28, 2017 at 9:20:30 AM UTC-4, wrote:
On Saturday, September 29, 2012 at 3:32:57 PM UTC-7, Analogdino wrote:
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 €œtoys€� 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 €œ55 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 €œsoft 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 €œfor 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 €œas is€� for posterity... Ha, ha, had to say that!
3. W7-M was never €œ55 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 €œas is€� and wait for inspiration... at this point, an attractive option!
6. Any other ideas?

Just thought... was Heathkit playing the IHF (?) €œpeak 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

Hi Analogdino (Roger):
I know this was a long time ago and lots of time has passed,, but I am also working on one of these units and am looking for another to pair it with. Any news of people who might be looking to get rid of theirs or if you are interested in parting with your Heathkit W-7M, please contact me.

Mike

Hello Mike,

I have 3 W-7M amps which have been setting under my workbench for far too long. I can let one go so you can have a pair to enjoy. They are complete with covers and in pretty good condition although I havent looked at them for many years. I believe they came out of the NASA Lewis Research Center many years ago. They appear to be factory built. They are located here in Northeast Ohio. Let me know if you are interested and I can send pics.

Dave Bodkin

Hi Dave:
Thanks for reply.
Personal sent to you.
Thanks