Heath W5M class of operation

[goldear]

Does the Heath W5M run in class A, or Class AB1? A different thread had indicated that all Class self-biased amps are class A, and since the W5M is self-biased, I'm assuming that it must be class A.

But here is what confuses me: How can it have 25 WPC in class A when a Push-Pull 6L6GC amp only generates about 30 watts when driven in AB1? I always thought that class A amps caused the power output to be lowered by about 75%.

 

[jazbo8]

A different thread had indicated that all Class self-biased amps are class A...

Whoever wrote it was sadly mistaken. Either self-biased or fixed bias output stages can operate in Class A, or Class AB depending on the bias voltage.

I always thought that class A amps caused the power output to be lowered by about 75%.

That's probably too much of a drop, 40-50% seems more likely depeding on the design.

In any case, the W5M has a class AB1 ultralinear output stage.

 

[GordonW]

AB1.

Any amp with a cathode bypass cap, can run Class AB. The cap allows for the varying total plate current draws inherent to Class AB operation, to take place (it "holds" the voltage on the cathodes relatively constant, for all intents and purposes).

Regards,
Gordon.

 

[Dukiedook]

I think the W5s also ran KT-66 tubes instead of 6L6GC tubes, but good luck finding a reasonable pair of NOS KT-66s.

 

[jaymanaa]

I think the W5s also ran KT-66 tubes instead of 6L6GC tubes, but good luck finding a reasonable pair of NOS KT-66s.

You will definitely feel classy running GEC 66s in w5s.

 

[Tom Bavis]

With cathode bias, the operating point begins to shift once the tubes begin to cut off and the cathode current starts to rise. It'll be a compromise - tubes have to be biased hotter to avoid crossover distortion at full output.

True class A push-pull will deliver about twice what a single tube does in class A.

 

[goldear]

With cathode bias, the operating point begins to shift once the tubes begin to cut off and the cathode current starts to rise. It'll be a compromise - tubes have to be biased hotter to avoid crossover distortion at full output.

True class A push-pull will deliver about twice what a single tube does in class A.

Perhaps I misunderstood you previously. I had thought that you had indicated that the sliding operating-point for a cathode biased P-P pair made it to where the apposing tube never completely entered cut-off.

 

[goldear]

I think the W5s also ran KT-66 tubes instead of 6L6GC tubes, but good luck finding a reasonable pair of NOS KT-66s.

Well those are not affordable by mere mortals. And so far as I know, the KT-66 was the british-equivalent to the 6L6GC wasn't it?

I've also got a box of those old Soviet 5881 equivalents which people were raving about a couple of years ago which I'm itching to try-out. And I hope that these can handle being biased that hot.

 

[Tom Bavis]

Class AB has each tube going into cutoff for part of the audio cycle. (not completely cut off in triode or UL, but near zero). As signal level increases, cathode voltage increases as average current increases. If it increases to the point that both tubes are cut off for part of the cycle, there will be crossover distortion. If you bias the tubes hot enough, crossover distortion won't occur until it's past the clipping point, above rated power.

 

[Pio1980]

The thing with tubes in tapped screen output ckts is that the screens have to be rated at least the same voltage as the plates to avoid arc over self destruction as is too often the case with max screen voltage operation. The KT66 seems pretty immune to self destruction.

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[goldear]

The thing with tubes in tapped screen output ckts is that the screens have to be rated at least the same voltage as the plates to avoid arc over self destruction as is too often the case with max screen voltage operation. The KT66 seems pretty immune to self destruction.

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Are you sure about that? Wouldn't the screen just need to have at least as high of a rating as the B+? So in the case of a 7027A if the B+ voltage was under 500V you'd still be fine, wouldn't you, despite the plates having a 600V rating.

Does anyone know what the B+ voltage is rated for a W5M?

 

[Pio1980]

Are you sure about that? Wouldn't the screen just need to have at least as high of a rating as the B+? So in the case of a 7027A if the B+ voltage was under 500V you'd still be fine, wouldn't you, despite the plates having a 600V rating.

Does anyone know what the B+ voltage is rated for a W5M?

That works for lower rated screens if the screen source is lower than the plate source. That doesn't happen with screen tapped opts like ultralinear or Wulliamson. OPTs wirh separated screen windings can have lower screen voltage as also with straight pentode operation.

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[goldear]

That works for lower rated screens if the screen source is lower than the plate source. That doesn't happen with screen tapped opts like ultralinear or Wulliamson. OPTs wirh separated screen windings can have lower screen voltage as a kso with straight pentode operation.

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I'm not sure how well we are communicating here. I realize that UL places the screens at the same DC voltage as the plates. But what I don't know is if this still presents a problem when the B+ voltage on an amp is less than the maximum screen-voltage for the output tube.

My assumption is that if the screen voltage stays below its ratings, it doesn't matter if the screens have a lower voltage rating than do the plates.

 

[Pio1980]

I'm not sure how well we are communicating here. I realize that UL places the screens at the same DC voltage as the plates. But what I don't know is if this still presents a problem when the B+ voltage on an amp is less than the maximum screen-voltage for the output tube.

My assumption is that if the screen voltage stays below its ratings, it doesn't matter if the screens have a lower voltage rating than do the plates.

Agreed. It always seems how that it's the screens arcing over when the tubes go 4th of July.

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[Dukiedook]

Are you sure about that? Wouldn't the screen just need to have at least as high of a rating as the B+? So in the case of a 7027A if the B+ voltage was under 500V you'd still be fine, wouldn't you, despite the plates having a 600V rating.

Does anyone know what the B+ voltage is rated for a W5M?

I would not even think of putting NOS 5881Tung Sols in there if I had a W5.
6L6GC minimum, NOS US if you can. I included a voltage chart I scrounged up on the net.

 

[goldear]

I would not even think of putting NOS 5881Tung Sols in there if I had a W5.
6L6GC minimum, NOS US if you can. I included a voltage chart I scrounged up on the net.

I agree. I'd never do that either. But 6L6GCs seem like they should be safe though, and 7027As seem like they might be even safer.

When I said the Soviet 5881-like tube I was talking about the: 6N3C-E tubes, which I have seen claims of people running these as high as 700V. I have an entire box of these, and I'm assuming that they should be safe to operate in the W5M.

 

[Dukiedook]

Yeah, if they are older military Russian tubes it sounds like they should be ok.

 

[Pio1980]

I'm trying running regular Russian 6L6 at 300 for screens and a bit over 400 for the plates in the Fishwin and it sounds incredibly good thru the Advent 2W E-waves.

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[jaymanaa]

I would not even think of putting NOS 5881Tung Sols in there if I had a W5.
6L6GC minimum, NOS US if you can. I included a voltage chart I scrounged up on the net.

I've done it, it all depends on the set up of course. My thinking was I love the sound of the TS 5881 and knowing the W5s have PT issues, I thought I'd ease things up a bit. They sound ok to me, but to be honest, I haven't run them a ton.

 

[Pio1980]

I blew up a TS-5881 in the Fishwin at ca 405V on the screens and plates. Don't see a good reason to run the screens full-on in straight pentode operation if it's optional.

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