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#1
Posted to rec.audio.tubes
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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
Posted to rec.audio.tubes
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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'. |
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#3
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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 |
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#4
Posted to rec.audio.tubes
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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 |
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#5
Posted to rec.audio.tubes
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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 |
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#6
Posted to rec.audio.tubes
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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 |
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#7
Posted to rec.audio.tubes
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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 |
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#8
Posted to rec.audio.tubes
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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? ;) |
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#9
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Quote:
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 |
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#10
Posted to rec.audio.tubes
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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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#11
Posted to rec.audio.tubes
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Tales of a Heathkit W7-M amplifier
On Mon, 1 Oct 2012 09:52:20 -0700 (PDT), Analogdino
wrote: 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! First let me correct myself; the W-6M is the 'most powerful'. That raises the point, though, that it depends on how you define 'Williamson'. Heathkit refers to them as Williamson 'type' and I get the impression their emphasis is on the 'basic design requirements', as Williamson described it. Which, I think, explains the previously mentioned "rating" question. Williamson says 'negligible distortion' up to the 'maximum rated output' so that's where Heathkit 'rated' that one even though it could put out more continuous Watts. 'Rating' lower that 'maximum' may seem like a 'cheat' but, remember, Williamson specs "15-20 watts" so Heathkit could argue that theirs not only does more 'Williamson' Watts (the 25 Watt rating) but can do over TWICE the power before clipping and it's arguably better to have a 'teensy' bit of distortion at 45 Watts vs the waveform chopped off at 20. I tend to agree with your interpretation that a 'Williamson' has the 'drivers' but if we go by the actual 'Williamson' design then none of the Heathkits are true 'Williamsons' because the original uses triode outputs (KT66 wired as triode) and the Heathkits are UL. Williamson explains why (he 'added') the 'driver' but that doesn't necessarily apply if you have more gain in the outputs, needing less drive, and sufficient overall gain. He simply says a pentode front end doesn't have enough gain (not surprising with triode outputs), not that there's anything else 'wrong' with using one, and I suspect Heathkit would have argued they're using 'better' tubes (EL34, for one) and UL to achieve the 'same thing' consistent with the 'type'. Btw, Williamson also offers the alternative of using a self balancing paraphase, which I'm sure would horrify some people, and explains that this eliminates the need for the (additional) drivers, so even Williamson doesn't 'lock' the 'type' (assuming there is such a thing) to one topology. The W-6M goes another step and operates Class AB2 using cathode followers, which isn't mentioned by Williamson. I didn't think you'd change OPTs. The point was really to use the 'right' tubes because the load impedance isn't correct for the 6L6 operating Class AB at that B+ You mentioned just 'living' with the lower Po but distortion will be higher too, because it's too low an impedance for the 6L6, although you could use a lower speaker impedance tap to raise it. The low end would suffer somewhat but probably not enough to notice. |
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Input gain stage uses triode 2 while the LTP consists of triodes 1 & 3. No need for another hole in the chassis, just enlarge the existing hole to accomadate the duodecar socket. A much better drive for the 6L6's or whatever. Cheers, John |
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#13
Posted to rec.audio.tubes
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Tales of a Heathkit W7-M amplifier
On Mon, 1 Oct 2012 17:01:53 -0400, "Charles"
wrote: "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? ;) Even funnier, I read it as "solder on" the first time around. |
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#14
Posted to rec.audio.tubes
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Tales of a Heathkit W7-M amplifier
On Tuesday, 2 October 2012 21:41:59 UTC-4, John L Stewart wrote:
Analogdino;963098 Wrote: 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 An intriguing solution to the drive problem can be found in the 6U10 triple triode. Triodes 1 & 3 look like a 12AU7 while triode 2 is similar to 1/2 of 12AX7. All that makes possible a drive cct as used in the Mullard 5-10 & 5-20. Input gain stage uses triode 2 while the LTP consists of triodes 1 & 3. No need for another hole in the chassis, just enlarge the existing hole to accomadate the duodecar socket. A much better drive for the 6L6's or whatever. Cheers, John filename: Avater.gif | |Download: http://www.audiobanter.com/attachment.php?attachmentid=303| +-------------------------------------------------------------------+ John L Stewart Interesting idea, John, but there's a lot of chassis space for an extra hole. Also, that would give a bit more wiring space (the 6AN8 is already "tight".} Anyway, now it's all back together with "borrowed" EL34's (from my Leak TL/25) I now get over 40 watts RMS, max, so I'll not be redoing the front end as planned, will just leave it "as-is" (but with a 470K screen dropper replacing the original 1.5 Mohm) - this slightly increases the 6AN8 plate current to, IIRC, just over 1 mA - better, IMHO. BTW, I've got a line on a couple of new Russian EL34's (that I can afford!), so the Leak TL/25 will likely get them and the W7-M will have to live with the present weak ones! Loud enough for me, for now... Cheers, Roger |
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#15
Posted to rec.audio.tubes
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Tales of a Heathkit W7-M amplifier
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 €śtoys€ť 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 €ś55 watt amplifier€ť but the most I can get at 200 Hz is a bit over 10 watts.. At that level, one half of the waveform €śsoft clips€ť. 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 €śfor parts or repair€ť, 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 €śas is€ť for posterity... Ha, ha, had to say that! 3. W7-M was never €ś55 watts€ť - 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 €śas is€ť and wait for inspiration.... at this point, an attractive option! 6. Any other ideas? Just thought... was Heathkit playing the IHF (?) €śpeak power€ť 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 |
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#16
Posted to rec.audio.tubes
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Tales of a Heathkit W7-M amplifier
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 €śtoys€ť 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 €ś55 watt amplifier€ť but the most I can get at 200 Hz is a bit over 10 watts. At that level, one half of the waveform €śsoft clips€ť. 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 €śfor parts or repair€ť, 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 €śas is€ť for posterity... Ha, ha, had to say that! 3. W7-M was never €ś55 watts€ť - 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 €śas is€ť and wait for inspiration.... at this point, an attractive option! 6. Any other ideas? Just thought... was Heathkit playing the IHF (?) €śpeak power€ť 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 |
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#17
Posted to rec.audio.tubes
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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. 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 €śtoys€ť 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 €ś55 watt amplifier€ť but the most I can get at 200 Hz is a bit over 10 watts.. At that level, one half of the waveform €śsoft clips€ť. 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 €śfor parts or repair€ť, 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 €śas is€ť for posterity... Ha, ha, had to say that! 3. W7-M was never €ś55 watts€ť - 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 €śas is€ť and wait for inspiration.... at this point, an attractive option! 6. Any other ideas? Just thought... was Heathkit playing the IHF (?) €śpeak power€ť 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, All, Much time has passed... it's now 2017! My W-7M is now fully fixed. "Good+" EL34's are in place (my 6L6 efforts were all a waste of time.) The 6AN8 stage sorted out... the HK schematic has wrong plate load - it is 330K (not 33K.) I now get over 55 watts at clipping with pretty low THD. This is a good amplifier, low hum, works well. Mike, if you still want it, email me at Cheers, Roger |
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#18
Posted to rec.audio.tubes
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Tales of a Heathkit W7-M amplifier
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 €śtoys€ť 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 €ś55 watt amplifier€ť but the most I can get at 200 Hz is a bit over 10 watts. At that level, one half of the waveform €śsoft clips€ť. 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 €śfor parts or repair€ť, 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 €śas is€ť for posterity... Ha, ha, had to say that! 3. W7-M was never €ś55 watts€ť - 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 €śas is€ť and wait for inspiration... at this point, an attractive option! 6. Any other ideas? Just thought... was Heathkit playing the IHF (?) €śpeak power€ť 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 |
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