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Tales of a Heathkit W7-M amplifier



 
 
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  #1  
Old September 29th 12, 11:32 PM posted to rec.audio.tubes
Roger Jones
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Posts: 39
Default Tales of a Heathkit W7-M amplifier

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
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  #2  
Old September 30th 12, 12:38 PM posted to rec.audio.tubes
flipper
external usenet poster
 
Posts: 2,366
Default Tales of a Heathkit W7-M amplifier

On Sat, 29 Sep 2012 15:32:57 -0700 (PDT), Roger Jones
> 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 “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


I doubt Heathkit was 'playing games', at least not by 100%. A better
'guess' might be that the 6L6s simply aren't 'right' for it but I
don't really know what the 'rating' of that amp is.

The UL taps should be enough to tell you it isn't a 'P.A.' amp. It's
the most powerful of their 'W', so called, 'Williamson' amp line
except the W7-M is obviously not a Williamson circuit while the
others, W3-M. W3A-M, and W5-M, are. They're supposed to be 'High
Fidelity on a budget' (what else from a 'kit' company? Excepting
Harman Kardon, of course) and, if I recall correctly, the W7-M was
touted as the first to achieve a 'Watt per Buck'.

I can't find a proper 'spec sheet' for the W7-M but can find a manual
for the W5-M, which goes through excruciating detail on the frequency
and power response, and the 'summary' spec says "Rated Power... 25
Watts, Maximum Average Power... 32.5 Watts, Peak Power... 47.2 Watts."

"Rated" seems to be what they consider the 'Hi-Fi' range (perhaps OPT
limited because they brag 'able to get 20 Watt at 20 Hz without
"overloading"), and where they spec, for example, hum and noise (-99dB
from 25 Watt). Maximum Average Power is, I think, rather self
explanatory: (mid freq) max continuous power (albeit with distortion
outside what they consider 'Hi-Fi'). "Peak" would be, I presume,
transient before B+ sags and the hairy edge of clipping.

Okay, so which of those '3' is the "55 Watts" people say the W7-M is?
Even if we take 'worst case', that it's 'peak', you should still be
getting over 38 Watt and I've seen posts where people claim they
'measured' 45 Watt. Of course, I take what people 'post' with a grain
of salt but still...

So I get back to the 6L6 which, even if nothing else, the OPT
impedance is probably not right for.

As for the 'peak power' game, I don't recall many, if any, U.S.
manufacturers do it. That, and the patently absurd PPMO nonsense was
usually (or always) cheap Japanese, now Chinese, junk. Now, U.S.
dealers sold it, and sometimes under a U.S. 'store brand' but, as I
said, I don't think U.S. manufacturers did, at least not those with
any reputation to speak of.

The 6L6s, if they take more drive than the EL34s, 'might' be the cause
of the 6AN8 clipping but, past that, I'd recheck resistance values, to
see if any have drifted, or maybe a leaky cap altering bias, gassy
tube? etc. It 'ought to work'.

  #3  
Old September 30th 12, 02:32 PM
John L Stewart John L Stewart is offline
Senior Member
 
First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 301
Smile

Quote:
Originally Posted by Roger Jones View Post
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
Hi Roger- The 6L6GC's you have sub'd in have nominally half the G of the original EL34's so will need twice the drive voltage from the 6AN8. Not impossible but more difficult. The bias will need to be reset quite a bit as well.

The 6L6 cathodes are smaller than EL34 & cannot deliver the current required into your load as designed.

The FW Doubler yields a good SUF for the PT. But you should replace the selenium bias rectifier with a FWB Si Rectifier in aid of reliability.

As a working example, I built two UL amps based on PP 6L6GC's around 1960 driven by a cct similar to this. With a 475 volt FWCT rectifiers PS they deliver just over 25 watts steady state. 50 Watts peak?? Maybe!!

Those are my starting thoughts anyway, Cheers, John
  #4  
Old September 30th 12, 08:03 PM posted to rec.audio.tubes
Doug Bannard
external usenet poster
 
Posts: 50
Default Tales of a Heathkit W7-M amplifier


"Roger Jones" > wrote in message
...
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


Hi Roger:

I'm in agreement wqith both Flipper and John Stewart on this one. Your 6L6s
have approximately half the gm of the EL34s and will consequently require
about twice the driving voltage for the same plate current.

As far as Heathkit specmanship is concerned, I have never known them to play
games with "peak music power" etc, so I wouldn't rule out the possibility of
55 watts out, although in UL configuration the EL34s are probably being
flogged pretty severely.

It would be a good idea to check for problems in the 6AN8 stage.

There's another thing that you should check however, as I have run into this
problem on other amplifiers. You should check the primary inductance of the
OPT. The simplest way to do this is to measure the exciting current on the
full primary, feeding it from an adjustable 60Hz source such as a variac
while monitoring the voltage across it and the ac current through it. Make
sure that there is no load on the secondary. You will be measuring the
magnitude of the impedance (Vpri/Ipri). The dominant term in this impedance
is the primary inductance, so to a first approximation, at 60 Hz,

Lpri=Vpri/(377*Ipri) in henrys.

You can start off at around 10 V rs and increase from there. You want
enough ac voltage to raise the flux density in the core high enough that the
core has some decent permeability. You should see the inductance increase
substantially as yoiu increase from 10V to 100Vrms.

If the OPT has no shorted turns, I'm sure that you will see inductances well
in excess of 100H even with only 10 V rms input.
If however the inductance is way down in the mud, then you have a shorted
turn problem.

I remember a few years ago evaluating a Quad 2 power amplifier for a friend
that he had just paid big bucks for on Ebay. The amplifier would only
produce ~6 watts at clipping, and the problem did indeed turn out to be a
shorted OPT.

Good luck and best Regards: Doug Bannard



  #5  
Old October 1st 12, 10:52 AM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,964
Default Tales of a Heathkit W7-M amplifier

On Sunday, 30 September 2012 08:32:57 UTC+10, 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 “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


You should find that at some low value RLa-a you will get 55Watts with fixed bias abd class AB1. The PO amounts you quote would be high % class A1 but do the load line analysis and all would be revealed. Maybe the amp will have enough heater power for 6550/KT88/KT90 and then you can get what you get now but Ea will swing lower with low load, but if you want hi-fi, just keep Ia and Ea so Pda at idle = 25Watts approx, and then you can settle for the 22W in nearly all pure class A1 - even if you connect the 6550/KT88/KT90 in triode.

The Heathkit input driver stage looks like an RCA or Dynaco schematic with bean counter minimalist use of two tubes, pentode input, triode CPI. The pentode could generate lots more gain if its anode load is bootstrapped off the cathode load of triode CPI, see the RCA typical designs used. As it is, if the amp had about 15dB GNFB, then it'd be fairly insensitive because the pentode has anode load of a low 33k,and if gm = 0.002, A = 66 approx, so for say the max 24Vrms at anode, you need approx 0.37V Va-k, and if there was 20dB GNFB then Vin = 3.4Vrms approx. The pentode input tube needs to make slightly more anode signal voltage than applied to each EL34 grid. The added gain using bootstrapping results in a lower amount of GNF, ie, ß is lower, for the same amount of GNFB, because open loop gain without GNFB mght be 4 times greater.

Patrick Turner.


Read my website for many more ideas about how to improve the amp using LTP driver stage with CCS for cathode, and triode input, and for all info on how to stabilise the amp unconditionally, ie, make it LF stable with say 20dB GNFB even with no load at all connected, and stable with any value of pure C across output terminals, between say 0.01uF and 3uF.

www.turneraudio.com.au

  #6  
Old October 1st 12, 05:52 PM posted to rec.audio.tubes
Analogdino
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Default Tales of a Heathkit W7-M amplifier

On Saturday, 29 September 2012 18:32:57 UTC-4, Analogdino wrote:
> Hi, Vacuumlanders,
>
> I'm seeking advice and opinion on a vintage Heathkit audio amplifier – the W7-M.

(snip)
> Cheers,>
> Roger


Hi, Flipper, Doug John and Patrick,
Many thanks for the most useful replies. I've made some progress to date but first some replies to the comments made:

From Flipper: "The UL taps should be enough to tell you it isn't a 'P.A.' amp. It's the most powerful of their 'W', so called, 'Williamson' amp line except the W7-M is obviously not a Williamson circuit while the others, W3-M.. W3A-M, and W5-M, are."
Me: Agree. I identified it loosely as a PA amp due to the 250 ohm o/p option but the UL format implies it was meant to be Hi-Fi . Not "Williamson", of course, as no driver stage. Odd that they used the "W" designation... IMO, the W4 and W5 are clearly higher quality amplifiers (I don't know the W3 but did refurbish a W4-AM.) Your points on the 6L6 sub are taken... but see below! Also, 3,500 ohms P-P is a given... I'm not about to but a new OPT!

From John: (re. my 6L6 substitution.)
Me: Right! I put in two "somewhat tired" EL34's from my Leak TL/25 and, with a bit of bias twiddling, got 49 watts RMS at clipping, but still with my kluged transformer drive to EL34 grids. Getting there! Now looking for two "good+" EL34's. I sense that this amplifier runs them hard. I plan to try a pair of 5881's that I have (but they're not matched)... not done yet.

From Doug: "Your 6L6s have approximately half the gm of the EL34s and will consequently require about twice the driving voltage for the same plate current... It would be a good idea to check for problems in the 6AN8 stage... You should check the primary inductance of the OPT.
Me: This suggests I need to add a driver stage (6SN7 or 12AU7) and redesign the whole front end to have a true Williamson, i.e. my option 4 in the original post. I'd expect to end up with a very good 25 watt amplifier to rotate into my vinyl tube stereo system... have to reliable it "Ex W7-M". OPT is a given but I'll check for shorts (I'm pretty sure it's OK.)

From Patrick: "You should find that at some low value RLa-a you will get 55 Watts with fixed bias and class AB1. The PO amounts you quote would be high % class A1 but do the load line analysis and all would be revealed... The Heathkit input driver stage looks like an RCA or Dynaco schematic with bean counter minimalist use of two tubes, pentode input, triode CPI. "
Me: Thanks for the excellent analysis and advice. I'm stuck with the 3500 ohms P-P OPT, but I'll likely still give around 25 watts RMS from "average" 6L6's (have several!) and, see above, close to 50 watts RMS with good EL34's (none to hand!) Re. 6AN8: IMO, likely not worth fiddling with... redesign stages.

So, briefly, to date, I plan to:
1. Find some good EL34's, or settle for a decent 25 watter with 6L6's
2. Rebuild front-end to Williamson all-triode architecture. Sadly, the W7 is not a "valuable collectible" like the W5, or even my refurbished W4, so it's not a big loss to the world! Input sensitivity then not a problem and VLF stability is easily fixed (see the thread on this from a while back.)
3. It will run with a WA-P2 pre-amp (as does my home-brew "near Williamson" and my W4.)

Again, thanks to all for the thoughtful replies... we are not alone in our challenges!
Cheers,
Roger
  #7  
Old October 1st 12, 09:12 PM posted to rec.audio.tubes
Analogdino
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Posts: 5
Default Tales of a Heathkit W7-M amplifier

Quick update:
Tried a pair of good 5881's, bias optimized, even balanced the cathode currents to around 1.5 mA.
No good!
Max p-p volts across 8 ohms = 40 VAC, 200 Hz.
Thus, power = 25 watts.
Pretty much same as 6L6's, not really surprised.
We soldier on...
Cheers,
Roger

  #8  
Old October 1st 12, 10:01 PM posted to rec.audio.tubes
Charles[_7_]
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Posts: 7
Default Tales of a Heathkit W7-M amplifier



"Analogdino" wrote in message
...

Quick update:
Tried a pair of good 5881's, bias optimized, even balanced the cathode
currents to around 1.5 mA.
No good!
Max p-p volts across 8 ohms = 40 VAC, 200 Hz.
Thus, power = 25 watts.
Pretty much same as 6L6's, not really surprised.
We soldier on...

Did you mean "solder" on? ;>)

  #9  
Old October 2nd 12, 02:52 AM
John L Stewart John L Stewart is offline
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First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 301
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Quote:
Originally Posted by Analogdino View Post
Quick update:
Tried a pair of good 5881's, bias optimized, even balanced the cathode currents to around 1.5 mA.
No good!
Max p-p volts across 8 ohms = 40 VAC, 200 Hz.
Thus, power = 25 watts.
Pretty much same as 6L6's, not really surprised.
We soldier on...
Cheers,
Roger
The 3500R presented by the existing OPT with 8R on the 8R tap is rather a steep load for the 6L6 family run in the present conditions. Should be more like 6K which I had used.

Try increasing the secondary load R on the 8R tap gradually, say 10R, than 12R & so on. You may see an increase in developed power, a useful experiment. Better still, do something similar on the 16R tap where the OPT copper is better utilized.

I like to set the load lower than the published optimum. The loudspeaker load is complex with its frequency extremities at quite a bit above that published. Some tests reveal the 3H developed in a PP OP is less when the load is set lower than the optimum published.

We don't listen much to ideal resistance loads!! All easy to try.

Cheers, John
  #10  
Old October 2nd 12, 09:02 PM posted to rec.audio.tubes
Analogdino
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Posts: 5
Default Tales of a Heathkit W7-M amplifier

On Monday, 1 October 2012 17:01:57 UTC-4, Charles wrote:
> "Analogdino" wrote in message


(snip)

>
> We soldier on...
>
> Did you mean "solder" on? ;>)


Good one, Charles! Both... in the "63/37" light infantry!
Cheers,
Roger

 




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