output transformer for a heathkit w5m

[trobbins]

Primo, not sure why you bring 1kV in to the discussion?

The 5AR4 was likely more expensive in the day. 5AR4 is a good choice nowadays (even if its a Chinese variety with added ss diode protection) as the delayed turn-on would alleviate initial stress that the directly heated cathode 5R4GY causes.

But I can't see any benefit in changing the power supply otherwise. Yes, from a risk perspective, the output stage coupling caps should be changed to a modern type, and with a 630VDC rating.

 

[Steve O]

Here's a link to an article written by a guy named Charlie Kittleson written for Vacuum Tube Valley. In it he states the 16458's were the early for the W5M and the 16309's were the later ones. Charlie was well known in tube audio circles back in the 1970's, 80's, 90's, etc.
http://dalmura.com.au/projects/VTV02 extract.pdf

I'm familiar with that article: second issue of VTV. In the early days, VTV frequently got details wrong and didn't always fess up. I suspect the statement regarding dates of 309 vs 458 may be one of those instances. I base this on the fact that I have an original assy manual dated 1956 that shows the 309 in the pic on the cover page. This is one year prior to the date VTV states the 309 began to be supplied by Heath. Also, numbering system of the two OPTs would tend to support the 309 being the orignial issue and 458 being a later replacement. Unfortunately, AFAICT, Heath never changed their PN when they changed OPTs so there may never be a definitive answer on what truly came first.

My take is that the PT which puts out about 1000v on the secondary is designed for choke input filter. A better design is a 2 choke LCLC power supply with a 5ar4 rectifier.

<snip>

Per Heath (and my meas), the PT was rated at 910VAC CT w/117VAC on primary. This would be ~ 950VAC on today's higher mains voltage; a high value for sure. Heath was fully aware of the implications of such a high voltage and dealt with it by specifying the 5R4 rectifier that's capable of a higher PIV than most high vacuum rectifiers inc 5U4 and 5AR4. They also utilized seriesed electrolytics w/bal resistors for the first three filter sections with ratings of 900VDC and 700VDC, a decent safety margin for peak unloaded DCV encountered during cold start.

Heath goes into uncommon detail about their PS design decisions in the circuit description portion of the assy manual. Probably worth a read prior to undertaking a PS redesign. Also, there is anecdotal evidence that the Bakelite socket for the 5R4 was prone to carbon tracking thus overloading the PT. I haven't seen this on W5s I've worked on but it's worth looking out for. The fix is a quality porcelain socket in this position.

If Heath can be accused of committing a major design error with the W5, it was spec'ing 400V rated caps for coupling drivers to finals. And not spec'ing a "surgistor" as original equipment.

 

[cocobolo95]

Thanks for all the replies so far. This has turned into a really interesting discussion. Keep that info coming.

On a different track, I received the w5m yesterday. When I examined the underside, I found engraving by the original builder. Seems the kit was finished on July 21, 1955. I was 1 & 1/2 years old when kit was built. Bet most of you weren't even born yet.

Amp seems to have spent all it's life in Texas. This explains the lack of corrosion of the parts.

 

[primosounds]

Primo, not sure why you bring 1kV in to the discussion?

The 5AR4 was likely more expensive in the day. 5AR4 is a good choice nowadays (even if its a Chinese variety with added ss diode protection) as the delayed turn-on would alleviate initial stress that the directly heated cathode 5R4GY causes.

But I can't see any benefit in changing the power supply otherwise. Yes, from a risk perspective, the output stage coupling caps should be changed to a modern type, and with a 630VDC rating.

 

[primosounds]

Well, it gets really close to 1000! and if you got weak tubes the voltages get even higher. Anyway since the w5 is pretty much the only vintage amp with such high primary voltages It makes you wonder why they used such a high voltage unit? Maybe, they got a good deal for a ham radio PT and decided to build an amp around it. The Allen 20 (pp 6l6gc) which is similar the w5 uses a high voltage pt but use a choke input ps and a resistor cap voltage dropping filter. The final B+ is 430vdc and it use a 5v4 for the slow turn on. But it doesn't use a whole bunch of series hooked up capacitors to handle the high voltage surge. That seems like a much saner/easier approach.

 

[primosounds]

I Per Heath (and my meas), the PT was rated at 910VAC CT w/117VAC on primary. This would be ~ 950VAC on today's higher mains voltage; a high value for sure. Heath was fully aware of the implications of such a high voltage and dealt with it by specifying the 5R4 rectifier that's capable of a higher PIV than most high vacuum rectifiers inc 5U4 and 5AR4. They also utilized seriesed electrolytics w/bal resistors for the first three filter sections with ratings of 900VDC and 700VDC, a decent safety margin for peak unloaded DCV encountered during cold start.

Heath goes into uncommon detail about their PS design decisions in the circuit description portion of the assy manual. Probably worth a read prior to undertaking a PS redesign. Also, there is anecdotal evidence that the Bakelite socket for the 5R4 was prone to carbon tracking thus overloading the PT. I haven't seen this on W5s I've worked on but it's worth looking out for. The fix is a quality porcelain socket in this position.
.

And yet for all of that it is quite evident that their decisions was not best for the long term reliability of the amp.

 

[Steve O]

And yet for all of that it is quite evident that their decisions was not best for the long term reliability of the amp.

Well, it seems Heath took care of most of the PS issues associated with uncommonly high PT sec voltages but cheaped out elsewhere (possible OPT misapplication - whatever OPT it was, low coupling cap voltage rating and maybe over stressed rect socket). They issued an interesting service bulletin in '66 (well after W5 left the product lineup) outlining running changes made while it was in production. Lots of changes but nothing in the PS except for "optional" addition of surgistor to PT primary circuit. Like most things in life, hindsight is 20:20. And this is a 60 yo implementation of a 70 yo design that's stiil held in fairly high regard by many tube amp aficionados, warts and all.

 

[primosounds]

Well, the fact they had issued bulletins explaining their choices and further service bulletins really points to a need on their part to address complaints from either users or technicians in the field. There are many other amps from the 40s and radio amps from the 30s which are still operating with their stock transformers. The only transformer that i have not seen replaced on the w5 is the choke. I am not saying that the transformers are crappy but that they did not adequately address issues that caused the trans to fail.

 

[trobbins]

They issued an interesting service bulletin in '66 (well after W5 left the product lineup) outlining running changes made while it was in production. Lots of changes but nothing in the PS except for "optional" addition of surgistor to PT primary circuit.

Would you be able to locate or identify that bulletin?

Primo, every Williamson or close variant (of which there were thousands and thousands sold) used up to a 425V-0-425V power transformer secondary, so yes the W5M was a standout with circa 10% higher. That said, B+ wasn't going over 700V. Going to a directly heated dual diode certainly stresses the parts more - even Williamson changed from his in-house U52 to the 5V4 in the late 1940's. Given the sheer quantity of those amps around, and ageing of 1950's parts, and no over-current protection on the secondary side, its easy to see why failures seemed high. But we know better now how to manage those weaknesses, without going down a redesign path.

 

[Steve O]

Would you be able to locate or identify that bulletin?

<snip>

The bulletin is titled "W-5M SERVICE BULLETIN", has # 597-344, is dated 1/20/66 and is four 8-1/2 X 11 pages inc std schematic. No other identifying marks. I got this as part of a W-5/WAP2/FM3 package at a flea mkt. Purchased for ~ 20USD mainly for the GEC KT66s in the W5 but also had prime condition instruction manual that had service bulletin stuffed into it. I'd never seen the bulletin before this and have never seen one afterwards inc online. If you aren't successful in locating a copy online, let me know and I'll see if I can get my scanner to work and post a copy here or elsewhere.

Regarding rectifier selection, there can be no dispute that an IDH unit would have been a great choice for all the obvious reasons. However, in Heath's discussion of the PS design, it's implied that consumer grade rectifiers such as the 5V4 and 5U4 of the era would have been overly stressed. Thus the high PIV/high voltage drop transmitter/industrial 5R4 and everything else that made it work in this app. 5AR4s weren't avail at the time.

 

[trobbins]

Kevin Chant has a transcription of what appears to be the bulletin you have:
http://www.kevinchant.com/heathkit.html

If you get keen, then I certainly would appreciate a scanned copy as a reference article, as I don't think Kevin's info is a complete record. I'll have a look at the resistor power ratings - that is is good feedback.

Yes I can see that the GZ34 would have been the only better valve available at the time of the W-5M introduction, but may not have been available at the time of settling on parts.

Interestingly I have a circa 1959 Williamson variant using a Partridge TD2183 power transformer with 470-0-470V secondary - it uses a GZ37 which would have only been recently released then.

Ciao, Tim

 

[mech986]

If you have a good phone or regular camera, a good close pic is almost as good as a scan for some.

 

[PakProtector]

Call Heyboer and get them to wind you a pair of S265's. Same a-a load, but quite tolerant of the service they'll encounter. I am running my pair at 110 mA idle current with 813's at 600V. You can also specify the U-L taps in 10% increments, so you can get 30 or 40% locations( they have 10 layers on each half of the primary and end-of-layer taps are quite easy to install). Lt me know if you're going to contemplate this, we can get 4 made at the same time ).
cheers,
Douglas

 

[Steve O]

Kevin Chant has a transcription of what appears to be the bulletin you have:
http://www.kevinchant.com/heathkit.html

If you get keen, then I certainly would appreciate a scanned copy as a reference article, as I don't think Kevin's info is a complete record.

<snip>

Yes, Kevin's "bulletin" is an incomplete transcript of the actual service bulletin. I'll try and get something scanned and posted in the next day or two.

Interestingly, Kevin's info does shed more light on production dates of the 309 vs 458 OPTs. If I read it correctly, Mike Lefevre of present day Magnquest states his records of Peerless info indicate the 309 was designed in 1955 and the 458 in 1957, both for the W5. This tends to support the idea that the 309 was Heath's original issue W5 OPT and the 458 was a later production change and that the VTV article got things backwards.

 

[primosounds]

I have had 6 w5 in my shop and 3 had trans problems. 1 PT and 2 OPT. Not a large sampling size but considering other anecdotal information that number of bad amps is not unusual for this model. So, rebuilding the amp can be an opportunity to address issues such as the high voltage surge. However, one addresses that is a different matter and can be approached in different ways. It is known that a choke loaded power supply has advantages and one being the ability to cut the volts down substantially. As an example my dual rectified separate chassis ps has a 450-0-450 pt. The filter is LCLC but I installed a switch that can add a 5uf cap before the first choke. With the 5uf cap B+ is 450 to 490 volts for a PP amp depending on the load. When i cut out the cap to make the ps LCLC the B+ is 350v for my SE amps. A lot less complicated than series connected caps with balancing resistors. Less parts, higher reliability.
If you select the appropriate choke, making the ps choke loaded will allow the safe use of the 5v4 rectifier which would be a good thing as well.

 

[Shadowdog]

On the output transformers, that's what I've always thought from my reading over the years.

 

[Brice]

I had a pair of W5 and the PT blew up without reason. So yes they tend to be weak also on the PT side as well. And they run hot. very hot which might explain the failure rate.

 

[nerdorama]

Call Heyboer and get them to wind you a pair of S265's. Same a-a load, but quite tolerant of the service they'll encounter. I am running my pair at 110 mA idle current with 813's at 600V. You can also specify the U-L taps in 10% increments, so you can get 30 or 40% locations( they have 10 layers on each half of the primary and end-of-layer taps are quite easy to install). Lt me know if you're going to contemplate this, we can get 4 made at the same time ).
cheers,
Douglas

I could be tempted to go in for a pair. Any idea about approx cost from Heyboer?
John

 

[soundmotor]

I had a pair of W5 and the PT blew up without reason. So yes they tend to be weak also on the PT side as well. And they run hot. very hot which might explain the failure rate.

Here too. I had one pop with no more abuse than low-level listening. Somewhere I have a Heath service bulletin recommending the Stancor 8412 as a suitable replacement for the one on the W-5M. That one is 400-0-400VAC @ 200mA. I think it will even fit into the W-5M can if you are truly ambitious.

Re: 16309 vs. 16458, the timeline for which came first always bugged me but it became more or less settled science after VTV weighed in. I'm sure I parroted that factoid as well. You'd really need to dig into the mags like Radio & TV News and look for the first Heath W-5M product releases or ads and see if they note what OT was there or a show a pic.

Later reflection though, the scarcity of the 16458 was more likely because mono tube gear ran out its string. The W-5M was at the tail-end of that era. After the 16458 revision, were the W-5Ms discontinued soon after when Heath made a line change? AFAIK their final mono basic amp was the AA-81 as they changed over to stereo gear which came right after the W-5M.

I second changing over to S-265 if the rewinds are good. That's one of the original "Musician's Amplifier" spec types and easily handles the KT66.

 

[dcgillespie]

Sound -- To your point, I have an article published in the October, 1955 issue of Audio-High Fidelity magazine, authored by Eric Leslie, entitled "Acquiring a Hi-Fi Amplifier". The article basically documents the author's build of a "just being put on the market" (his words) Heathkit W-5M amplifier. On page 2 of the article, (page 59 of the magazine), a top side view of the amplifier is shown with the cage cover removed. It clearly shows the small (16309) OPT installed.

Dave