Low output on JTM45 build

[pdf64]

It’s disappointing that the JJs are no better than the Chinese KT66.
Please report back on the Gold Lions.

[g1]

It’s disappointing that the JJs are no better than the Chinese KT66.
Please report back on the Gold Lions.

JJ sells a tube as "6550" with a 35W max. dissipation rating. It seems to be that modern manufacture is pretty loose with the specs.
Who knows what the proper transformer impedance would be for either of those KT66's. Maybe it would have been instructive to run 16R tap into 6R load.

[Helmholtz]

Maybe it would have been instructive to run 16R tap into 6R load.

Maybe, but decent 6 Ohm speaker would be hard to find. Also I'd expect even lower output power than with 8 Ohm into 16 Ohm tap.

[tubby]

JJ sells a tube as "6550" with a 35W max. dissipation rating. It seems to be that modern manufacture is pretty loose with the specs.
Who knows what the proper transformer impedance would be for either of those KT66's. Maybe it would have been instructive to run 16R tap into 6R load.

AFAIK the original 6550 is actually a 35W plate dissipation tube as can be seen on old RCA or Tung-Sol datasheets for that tube. It was only when the 6550A came out that plate dissipation was upped to 42W.

To that extent JJ does a historically "correct" 6550.

[Helmholtz]

AFAIK the original 6550 is actually a 35W plate dissipation tube as can be seen on old RCA or Tung-Sol datasheets for that tube. It was only when the 6550A came out that plate dissipation was upped to 42W.

To that extent JJ does a historically "correct" 6550.

Also "upped" plate dissipation or different rating sytems don't change the tube characteristics.

[pdf64]

I recently got a 100:1 probe, so I’ll investigate the Vsat of my Groove Tube and Chinese KT66.
It seems that the issue must be excessive Vsat, as the higher than typical JTM45 OT primary impedance should be easy to ‘pull down’.
I think 6550 have a 35W design max anode limit, whereas it’s 6550A that have the 42W limit.
But maybe the 35W of the JJ is under the design centre system, which would align with a 42W design max rating?

[g1]

AFAIK the original 6550 is actually a 35W plate dissipation tube as can be seen on old RCA or Tung-Sol datasheets for that tube. It was only when the 6550A came out that plate dissipation was upped to 42W.

To that extent JJ does a historically "correct" 6550.

That is a fair point, but I don't think anyone is looking for authentic old spec 6550's. The other manufacturers who make 42W rated versions are not using the 'A' suffix either.
I think most people buying 6550's are looking for power, and many buyers do not notice the lower power figure. I was very surprised when I saw the 35W rating on the JJ datasheet.
I would be just as critical of marketing a 6L6 that meets the original spec., at least without a big notice that says so. Most people now consider a '6L6' to be a 30W plate diss. rating tube.

Maybe, but decent 6 Ohm speaker would be hard to find. Also I'd expect even lower output power than with 8 Ohm into 16 Ohm tap.

I would like to believe that, but I don't think I have that much faith in the modern tube manufacturers. Measurement is free anyway. Trust but verify and all that.

[Helmholtz]

.. as the higher than typical JTM45 OT primary impedance should be easy to ‘pull down’.

Not sure, what exactly you mean as the plate load together with the Vgk = 0 plate curve define Vsat.
When scoping the plate voltage at some clipping, Vsat is the instantaneous voltage at the minima.
One might say Vsat corresponds to the internal plate DC resistance at the maxima of the plate current.

I could do same Vsat measurements with original KT66s.

[g1]

But maybe the 35W of the JJ is under the design centre system, which would align with a 42W design max rating?

I don't want to derail too much, but I think it's more akin to their 6V6 that is not really a 6V6. There are no real industrial applications that tube manufacturers are accountable to anymore. The applications are basically hobbyists, and we will continue to take what we can get, with no recourse other than griping about the situation.

[Helmholtz]

FWIW, the original KT66 datasheet shows that output power is essentally constant between 5.5k and 8.4k (see Fig. 7):
https://frank.pocnet.net/sheets/084/k/KT66_GEC.pdf

I assume that doesn't include OT losses.

Also see design example on bottom of page 109 here:
https://drtube.com/datasheets/kt66-osram.pdf

[pdf64]

FWIW, the original KT66 datasheet shows that output power is essentally constant between 5.5k and 8.4k (see Fig. 7):
https://frank.pocnet.net/sheets/084/k/KT66_GEC.pdf
…

What does the ’(o)’ on the chart signify?

[Helmholtz]

What does the ’(o)’ on the chart signify?

I think it means at idle. And the chart is likely for cathode bias.

[pdf64]

I did some tests, comparing GrooveTube KT66 to TubeTown KT66, using a 100:1 probe on one of the anodes, on my JTM45 type build, with a 6k OT.
It looks like the Vsat is about 50V for the GT (bottom of red trace wave, scale on the left). The blue trace shows the VAC across the 8ohm resistive load.
Whereas with the TubeTown, it's about 100V. Sorry it's displayed a bit differently, I'd worked out how to set the scope up a bit better by the time I did the GT test.

[pdf64]

With either the GT or TT valves, if the loading was adjusted to reflect a 3k load back to the output valves, the Vsat increased to about 140V with the TT, 160V with the GT. The TT put out a fraction more power into the 3k load than the 6k load, whereas the GT put out a bit less.

[Enzo]

So is it fair to say the difference between 3k and 6k was "a little bit"?