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In Reply to: RE: Heathkit W-6 posted by mark.korda@myfairpoint.net on April 05, 2015 at 14:46:07
The most under rated of the vintage amps IMHO. Properly set up , even stock. it can run the mot difficult speaker load w t to it. Simple great transformers.
A
Follow Ups:
Hi, Shovel,thats perfect,your a great teacher! BCGuitar,(HolyCrap) thanks for verifying that wiring job.When you said that I knew I was not dreaming. Steve,thanks for the picture,it sure looks like the one I saw. Eli, you know a ton about tube amps and I want to learn from you.I still have copied answers from you on how to remove tone controls from the Dyna PAM's and am almost ready to start that project after a slight delay. Eli, that Heathkit was a giant 70 watt Colossus and maybe the rectifier tube could not maintain the AC to DC current or voltage transfer as do the diodes in a bridge....I don't know and I wish you would tell me. I do know most tube guys want the (soft start) of the diode tube. Uncle Stu for your input now and later too as I consider you a great teacher of the tubes. Thanks all you guys.....Mark...Shovel,I'll try your instructions tomorrow.
The B+ PSU in a W-6M is a "full wave" voltage doubler, not a 4 diode bridge rectifier. The diodes are series wired pairs, to increase PIV capability.
That topology is cost effective and can be HIGH performance. It's found in some McIntosh models, the Harman/Kardon Citation 2, and the Marantz 8B. :> D While nothing more need be said, superior bass behavior is a big benefit of SS rectified B+. The better amps employ a choke between the doubler stack and the reservoir position. The W-6M is cheaper.
Only vacuum rectifiers with cathode sleeves, like the 5AR4 and 5V4, exhibit a "soft" start. Directly heated types like the 5U4 and 5R4, turn on almost as quickly as SS diodes do.
I see some obvious areas of improvement in the W-6M.
Extra heater current is available. So, rewire pins 4,5, and 9 of the I/P 12AU7 socket and install the much more linear 6CG7/6FQ7.
Replace the OEM B+ diode pairs with 2X 600 PIV/2 A. Schottky diodes. Install a CL-130 inrush current limiter in the line between the rectifier winding and the center of the doubler stack. Increase the value of the doubler stack capacitors. Install a low DCR 1 H. choke between the doubler stack and the reservoir position. Increase the capacitance in the reservoir position. An additional B+ PSU refinement is inserting a "hash" filter between the doubler stack and the main filter choke. Increases in the decoupling capacitance value could also be of benefit.
Improve the bias supply by installing a 4X Schottky diode bridge and CRC filtering the rail.
Eli D.
Hi Eli,
Why are their so many stages in the driver section?
Dave
It's Williamson style, with an added "twist". Cathode followers are DC coupled to the "finals". Blocking distortion can't occur and a bit of positive g1 current, for which KT88s/6550s are rated, can occur.
Considering that the differential gain block is a 'X7, cathode followers are a damned good idea. The 'X7 triode "can't fight it's way out of a wet paper bag".
A 2nd look at the schematic suggests that reducing the open loop gain might be reasonable. Notice the attenuating resistor network, right at the I/P jack. FWIW, I prefer a simple 100 Kohm grid to ground resistor, along with a CC grid stopper.
I like high gm in circuits that employ GNFB. Open loop gain could be reduced and gm improved by changing the tube used in the differential gain block to a 12AV7. Heck even a 12AY7 would (IMO) be an improvement over the OEM 12AX7.
Eli D.
Now it makes sense to me. The cathode followers are to give the 12AX7 enough drive to power the KT88's/6550's, which I understand are difficult to drive. It gets the job done, but it does seem like a lot of circuits go out of there way to use 12AX7's when another tube would be a better choice.
Is this circuit more or less stable than a traditional 6SN7 Williamson without the CF? (I suspect less.) will the traditional 6SNZ Williamson drive PP KT88's/6550's sufficiently?
TIA!
Dave
"Is this circuit more or less stable than a traditional 6SN7 Williamson without the CF? (I suspect less.) will the traditional 6SNZ Williamson drive PP KT88's/6550's sufficiently?"
The number of RC poles in the W-6M is the same as in a "textbook" Williamson setup. Therefore, no significant difference in stability is present.
A "routine" 2X 6SN7 Williamson setup is quite capable of driving KT88s. OTOH, the W-6M is Class "AB2". That's good for a bit of extra power and immune to blocking distortion.
Overall, the OEM W-6M is a fairly decent effort. A corner was cut in omitting a PSU choke and the gain structure (IMO) could have been thought out better. The weaknesses are shored up easily enough and I'd be glad to own a pair.
Eli D.
I had not considered the extra demand of class AB2, even though I had read this before about the Heath/Williamson circuit. They also run the tubes UL instead of triode, if I remember correctly.
Dave
Good morning,
The Heath W-6 Mr. Korda mentions has been shown on E-Bay previously.
The seller listings are cash only - local pickup only.
Another W-6 recently sold for $1,050 dollars on E-bay. It, also,
was in very nice condition. Both amplifiers were offered with no tubes.
Readers should realize the Heath W-6 utilizes a solid state
bridge-diode and does not have a tube rectifier.
One internet source mentions about 2,200 of these amps were
produced.
Regards to all,
John
I have seen that ~2k production number too. Any other independent source that either backs it up or says otherwise? It would be nice to know just what the production was...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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