Do JJ KT-77's really need 500V of B+ for best tone?

[Nigel]

I had some in my Zinky and they died at 560v, but were the ideal sound I'd been searching for forever.

Peter, what is your take on optimum B+ for maximum tone from this valve?

 

[duesentrieb]

http://www.jj-electronic.com/pdf/KT77.pdf

Limiting values:
Ua =800V
Ug2 = 800V
Ua,g2 = 800W
Wa =25W

 

[Nigel]

No they don't.

Thanks for the feedback, it's interesting. I currently have a pair in a Rivera M60. Plate voltage is 440, tubes are biased @ 26 watts, with the OT mismatched to provide 7K to the plates. It's close to the magic I experienced, but not quite there. Different circuits, speakers and cabs, as well as 120v difference. The clean tone is to die for, distortion is close but not as awesomely dynamic and robust.

What voltage and impedance values have you found inspiring?

 

[Nigel]

I'm against using the impedance selector as a "tone tweak". That's not what it's for and it's very hard on the valves when you mismatch it. Therefore I always keep it matched to the cab configuration I'm using. I run 16R Marshall cabs.

Moreover, I don't play with power amp voltages for the sake of tone tweaking either. I do it all off of a plotted load line and match the impedance ratio of the OT to the B+. This is how true engineers/designers do it. Amps are not "all tone". There is an electrical side to them that also must be adhered to if you don't wanna let the magic smoke out and want a reliable design.

The KT-77's "Ra" value is 23, like a 6L6, so 7K for a pair's primary Z is proper. The mismatch is to get closer to the ideal spec. The Rivera is 3.5K, so running the 8 ohm tap into a 16 ohm load does the trick. To that point, we are in accord: "it's very hard on the valves when you mismatch it." Precisely; I believe JJ respecified the tube as a 25 watt valve due to it being fitted in EL-34 circuits, which generally have around half the ideal impedance, to keep them from over-dissipating under heavy drive conditions and prematurely failing. This does not provide best tone in my experience. It shifts the frequency response up in a way that's not to my taste. I much prefer the valve biased as a 32 watt tube, as it does not open up and sing for me unless it's in that range. The JJ E34L and JJ 6L6GC behave similarly in my experience.

 

[Peter Diezel]

Jon ... thank You but it seems the question was on me.

Nigel:

This idea of mismatching all over the internet is nonsense.
The power tubes are always in the load of the primary
windings of the output transformer.

The JJ KT77 maximum plate voltage is no problem up to
800 volt.

The issue can happen only on the screen grid stoppers.
The data sheet recommends 100 ohms but I would go
to 470 ohms minimum.

The KT77 works very well on EL34 transformers because
there is low distortion factor. On the other hand there is
maybe a need for more treble response.

On the EL34 transformer we had an output on 130 watts
with a plate and screen voltage for 470 volt. 115 watt with
the KT77.

I will get a prototype transformer with 25% more load on
the plates but only more 10% on the inductivity.

Since then our transformers works very well with the KT77.

Please send me an email for more technical information.

Cheers,
Peter