Heathkit W5 triode build

[zackthedog]

I've been tinkering with variations of the original triode Williamson and had thought to use a pair of working Peerless 16309's pulled from Heathkit W5's. But with the traditional Williamson I've run into a lot of strange behavior which I assume comes from oscillation/instability, such as jumps in the output tube bias, extreme driver stage imbalance, wobbly voltages, etc. This leads me to believe that the 16309's really don't have the right specs and were crafted specifically for the Heathkit W5 circuit. I hate to waste them but I'm also leery of going full-steam on the W5 build because they were known to fail in that circuit. Plus, I'd prefer a triode amp.

It occurred to me to duplicate the W5 front end, reduce the B+ on the output stage to about 440vdc, triode-wire the KT66's and adjust the cathode bias values accordingly. The KT66's are running at 60mA, everything else dials in, it seems stable and it sounds lovely. Barring anything I'm not aware of, I thought this was a nice way to employ the 16309's under the assumption that they would behave well and might not be so stressed under these conditions. The standing current is higher than the original, but the B+ is lower and the power demand cut in half. The coupling caps are safely overrated to avoid any problems there. Would it be correct to assume that, under these circumstances, the 16309's might be less prone to problems?

I also assume that changing the outputs to triode mode reduces the feedback, but that doesn't *seem* to be a problem. I haven't found any reference to modifying the W5 to triode operation, so I thought I'd ask if any of the experts here see any problem with this. Appreciate any feedback, thanks!

 

[c_dk]

In searching thru these forums I have been led to believe that the 16309s failure mode was due to the under voltage 400v coupling caps. I thought that changing to 630v caps is the solution.

Is this correct or is there a additional problem that would need such a major circuit change to protect them.

 

[zackthedog]

In searching thru these forums I have been led to believe that the 16309s failure mode was due to the under voltage 400v coupling caps. I thought that changing to 630v caps is the solution.

Is this correct or is there a additional problem that would need such a major circuit change to protect them.

That seems to be the consensus, along with a notoriously under-spec'ed power tranny. Dave Gillespie, in one of his threads, mentions the "turn-off thump" as another possible culprit, as it sends some serious LF info through the power tubes and OPTs. I've definitely heard that in some of my projects but I'm not experiencing it here.

The 12AU7 is not a tube I've ever been terribly fond of, but I dug up some Telefunkens I had stashed away and this is about as smooth and sweet a triode amp as I've ever built. I've done a lot of SE amps and played around with triode PP, but always avoided feedback as beyond my skills. I like the Gillespie EL34 triode amp based on the W4 a lot, but this is really special.

 

[trobbins]

Maybe it would be best to identify all the differences you have introduced compared to the original W circuit. Or are you just trying to copy the Heathkit W5 circuit, and if so then what differences have you made to that circuit?

Can you also identify what measurements you have made - for starters you haven't identified the feedback level, or the OPT primary inductance.

 

[zackthedog]

Maybe it would be best to identify all the differences you have introduced compared to the original W circuit. Or are you just trying to copy the Heathkit W5 circuit, and if so then what differences have you made to that circuit?

Can you also identify what measurements you have made - for starters you haven't identified the feedback level, or the OPT primary inductance.

The only differences from the original W5 are:

1) Lower B+ on the output stage (plates at 440vdc as opposed to 480vdc, cathodes at 38vdc)
2) Lower cathode resistance on the output stage (330 ohm cathode resistor replaced with 180 ohms) and removal of cathode bypass cap
3) triode-wired KT66's
4) slight reduction of driver-stage decoupling resistor from 6.8K to 5K.

I did not change the feedback resistor or compensation cap. The OPT primary is 10K, I don't know what the inductance is. The original feedback factor, according to the manual, was 18.1dB.

I notice that I'm missing a decoupling stage (100 ohms, 20uF), between the OPT taps and the driver stage. I'll add that in to ensure stability.

Everything else is the same, front end voltages measure well, according to the manual. I guess I can't ask for too much technical guidance, given that I can't do any sophisticated measurements, though I have a friend with a scope and he's offered to help take a look at how it's behaving.

 

[trobbins]

Changing from pentode to triode will change the output stage mid-band gain, as well as low-frequency and high-frequency corner responses, both from tube gain and not using UL, and hence the loop gain response, and hence the level of feedback and stability margins. Substantial changes like that are best confirmed with relevant testing, as your stability is in 'no mans land' at present.

The 100 ohm 10uF filter between B+ and driver/preamp stages is not there for standard feedback stability, but more to attenuate B+ signal ripple from interacting with driver/preamp stages.

 

[c_dk]

What or where, how does the 16309 fail?

 

[zackthedog]

What or where, how does the 16309 fail?

Typically you'll see one primary winding open or compromised in some way. I have one that measures 310 ohms on one side and 2200 ohms on the other.

Changing from pentode to triode will change the output stage mid-band gain, as well as low-frequency and high-frequency corner responses, both from tube gain and not using UL, and hence the loop gain response, and hence the level of feedback and stability margins. Substantial changes like that are best confirmed with relevant testing, as your stability is in 'no mans land' at present.

The 100 ohm 10uF filter between B+ and driver/preamp stages is not there for standard feedback stability, but more to attenuate B+ signal ripple from interacting with driver/preamp stages.

Thanks, that confirms what I thought. "No man's land" sums it up well. ;-) I'll try to run some tests. I appreciate your response.

 

[c_dk]

Since this thread has been idle now for 4 days can we call this a partial hijack?

It would seem that the the general consensus is that the Altec/Peerless 16309 transformers are excellent transformers up until they fail.

It sounds like the OP in a effort to protect these transformers suggests converting the amp operation for it's original distributed/quasi Ultralinear operation mode to a triode mode of operation.

Fine sounds like quite a project that i believe he has melded together with a W4M upgrade thread.

A simple question then....what mods should be done to a W5M operating in it's original configuration to protect these now getting rarer transformers?

 

[zackthedog]

Since this thread has been idle now for 4 days can we call this a partial hijack?

It would seem that the the general consensus is that the Altec/Peerless 16309 transformers are excellent transformers up until they fail.

It sounds like the OP in a effort to protect these transformers suggests converting the amp operation for it's original distributed/quasi Ultralinear operation mode to a triode mode of operation.

Fine sounds like quite a project that i believe he has melded together with a W4M upgrade thread.

A simple question then....what mods should be done to a W5M operating in it's original configuration to protect these now getting rarer transformers?

I've put this aside for the time being out of concerns that *I really don't know what I'm doing*. ;-) Right now I'm focusing on the EL34 triode amp. A friend is very interested in these and we have a pair of working (supposedly) W4s on the way that we picked up quite cheaply.

My plan is to come back to the W5 shortly and just duplicate the original screen-tapped version, because I've actually never heard it! It seems you can protect the 16309s to a great extent with a better power transformer and properly rated coupling caps. Dave's B+ shut-off mod for the older Heathkits might be a good idea as well. I have some spare 16309s as well so we'll see what happens.

I'm still interested in an original "naked" triode Williamson and considering either the Heyboer Peerless S-265-Q rebuilds or the Sowter "Williamson" OPTs. I still have an original Patridge WWFB but my expectation of finding a mate is pretty low.

Perhaps at some point Dave or someone else will discuss the W5 and suggest mods and improvements. There's a lot of talk about these elsewhere but it would be great to have something in-depth like Dave has done with the earlier Heathkit amps. There's so much great information in those threads that I'm still re-reading them!

 

[c_dk]

I was basically gifted a pair of these by a long time friend, client and fellow AKer, when his wife had him thin his stash. Originally built by his neighbor a Lear Siglear engineer in the 50s one has a replacement Stancor power transformer the other a original which had the nichrome wire and contact soft start switch.

The 12au7 tubes have white tape on them with the engineer's best drawn lettering that they tested good in 1963 and 1968. They still test good today.

I have corresponded with Sheldon Stokes and installed two of his power supply cap boards and with the original rectifer tubes, various 12au7s, and new Chinese sourced (can't remember the brand) KT66s they make a solid 25 watts.

The power transformers run a little hot I believe and I have talked to Heyboer about getting some new replacements that are wound for today's AC supply.

Last time I played them was to drive a pair of Quad57s I repaired for another friend.

The Quad owner was licking his chops but I can't let them go......if you guys knew how many Mac units I shipped off to Asia over the last 40 years you would run me off this site.

Past regrets makes me want to get these in their best long term shape.....I might even be able to find some pictures.

 

[zackthedog]

I'd love to see them. Working examples are hard to find. There are several "needs repair" models on eBay but that's a big risk. I was lucky to find these working 16309s a year ago.

 

[trobbins]

With respect to protecting the OPT, there are a few subtle changes that can lower the risk of a part failure causing collateral damage to the OPT, and the benefit of regular maintenance can also lower the risk of part failure creeping up on the amp. But be aware that prevention additions and maintenance can become very technical topics to drill in to, and many end up rationalising that a certain amount of effort is appropriate but going any further is not needed (perhaps from personal technical judgement or experience, or a lack of awareness or skills or tools).

Imho the following three changes are relatively simple and surreptitious ways to help protect against collateral damage: fusing the power transformer secondary; adding series ss diodes to the valve diode anodes; and adding a MOV across each OPT half-primary winding. It can get quite technical to appreciate how they can assist, and how to select the most appropriate parts for a particular amp - which may stop a few in their tracks due to uncertainty, especially as the Williamson amp itself is a very technical beast to appreciate, even after reading dozens of articles and threads on the topic. The first link presents a technique for selecting secondary side fusing for a typical Williamson amp power supply, and the second link discusses OPT protection in general, with info on MOV selection:
https://dalmura.com.au/static/Williamson design info.pdf
https://dalmura.com.au/static/Output transformer protection.pdf

There are other changes that can reduce risk to the OPT. Any change is likely to be somewhat specific to the amp circuit being used, so the advantage of forum threads is that sort of specific detail can be worked on, whereas books and magazine articles typically can only cover general concepts or just one or two specific circuits.

Unfortunately it seems the various Heathkit assembly and operating instructions don't go to the topic of maintenance. Some books and magazine articles of the era did, but I'd be guessing that more than 95% of amps were just run until some gross distortion or lack of signal output or smoke heralded an internal problem.

 

[zackthedog]

Thank you very much for those links. I'll take a look!

 

[c_dk]

While reading through many of the attached articles was interesting they still did not get to the root of the failure mechanism of these 16309 transformers other than they are a smaller form factor than the replacement.

As a corollary to this does anyone know where these fail, that is is it the primary tap sections or the the "ultralinear" tapped sections that short and fuse together.

If ignoring the UL windings because they are the weak link, the OPs triode proposition would make sense......not so much if it is the primaries that fail.

Did Heath cut some corners with this design.......building what many have described as a great sounding unit but with undersized transformers for the KT66s output when used in a semi ultralinear design?

 

[trobbins]

The issue will be a lack of anything but anecdotal 'my OPT failed', or 'the amp came with a replacement OPT'. Even if someone had been able to tear down a few failed units, that information has to be aligned with a report of what if anything had failed in the amp, or what in the amp was not operating nominally. Even if you could find a few 1st hand reports, and could sift through the uncertainties, that won't provide a statistically significant outcome, and so may only point to one or two failure mechanisms, but that wouldn't mean they are the only mechanisms, or the dominant % mechanisms. So imho it's back to engineering assessment (and adding belts 'n braces protection) and maintenance.

 

[primosounds]

While reading through many of the attached articles was interesting they still did not get to the root of the failure mechanism of these 16309 transformers other than they are a smaller form factor than the replacement.

As a corollary to this does anyone know where these fail, that is is it the primary tap sections or the the "ultralinear" tapped sections that short and fuse together.

If ignoring the UL windings because they are the weak link, the OPs triode proposition would make sense......not so much if it is the primaries that fail.

Did Heath cut some corners with this design.......building what many have described as a great sounding unit but with undersized transformers for the KT66s output when used in a semi ultralinear design?

The 16309 OPT is supposed to be rated at 25 watts, which is right at the output of the 2 kt66 . So, any kind of an issue like a voltage surge or a problem with the coupling caps leakage can drive the output past the point of safe operation of the 16309. The 16458 are much larger and presumably have a higher wattage rating but i don't recall running across any technical specs to verify that.

 

[zackthedog]

The 16309 OPT is supposed to be rated at 25 watts, which is right at the output of the 2 kt66 . So, any kind of an issue like a voltage surge or a problem with the coupling caps leakage can drive the output past the point of safe operation of the 16309. The 16458 are much larger and presumably have a higher wattage rating but i don't recall running across any technical specs to verify that.

I think they're rated for the same output wattage but more sturdily built. After all, the Partridge WWFB is pretty hefty but still only rated for 20-25 watts because of the complexity of the windings. But there's no question the 16309 is undersized. They're tiny. ;-)

 

[zackthedog]

While reading through many of the attached articles was interesting they still did not get to the root of the failure mechanism of these 16309 transformers other than they are a smaller form factor than the replacement.

As a corollary to this does anyone know where these fail, that is is it the primary tap sections or the the "ultralinear" tapped sections that short and fuse together.

If ignoring the UL windings because they are the weak link, the OPs triode proposition would make sense......not so much if it is the primaries that fail.

Did Heath cut some corners with this design.......building what many have described as a great sounding unit but with undersized transformers for the KT66s output when used in a semi ultralinear design?

I have a bad one and one primary reads about 2200 ohms, as opposed to about 300 for the other. I don't know enough about OPTs to know what that means (except that it's probably no good). Does it tell you anything?

 

[zackthedog]

The issue will be a lack of anything but anecdotal 'my OPT failed', or 'the amp came with a replacement OPT'. Even if someone had been able to tear down a few failed units, that information has to be aligned with a report of what if anything had failed in the amp, or what in the amp was not operating nominally. Even if you could find a few 1st hand reports, and could sift through the uncertainties, that won't provide a statistically significant outcome, and so may only point to one or two failure mechanisms, but that wouldn't mean they are the only mechanisms, or the dominant % mechanisms. So imho it's back to engineering assessment (and adding belts 'n braces protection) and maintenance.

True enough, I suppose.