CV73 / KT44 valves

[Andy M0FYA]

Has anyone got any views, or even better, practical experience, of using CV73 valves for the output stage of an audio amplifier?

As everyone knows, KT66s sell for a lot of money these days, however CV73s can be obtained at sensible prices. The CV73 is equivalent to the 11E3 and the KT44, and appears to be electrically very similar to the KT66.

It has a 4 volt heater, 7-pin base and a top-cap, but these differences are not an issue for a new-build.

Opinions welcome, please!

Many thanks,

Andy

[Skywave]

A quick wander around the 'Net shows that others have been thinking along the same lines.

My thoughts: I cannot see a reason as to why the idea isn't worth a serious try. Depending on the configuration, single-ended or class B push-pull, automatic or fixed bias, you might have to do a bit of tweaking of the control grid bias and possibly O/P matching, but I'd be surprised if these were critical issues. With all valve power amps., the really important factor is the dynamic linearity of the Ia / Vg characteristic of the O/P valves, the quality of the O/P transformer and of course, if class B push-pull, valve characteristic matching.

Give it a whizz - and let us know how you get on - thanks.

Al.

[tiv48]

i have never tried using cv73 so would also like to know how you get on
i have used el34's easy and pl519's more dificult as they need 40 volt heater
rob t

[Andy M0FYA]

I don't think I'll be building anything in a hurry, I have so many radio projects queueing up, but I like to think these things through and plan them in my head! Maybe one day.........!
I think the only parts I would need to acquire specifically would be the output transformers.
The CV73 is certainly an impressive-looking valve, a bit like an 807.
Andy

[Paula A]

Output transformers will be fun for valves never intended for audio use. I know how to do the numbers, but really it's more a case of doing it all on paper, then running into a dummy load and seeing when everything smooths out.

I have changed my opinion of valve power output stages over recent years. Now it's easier to run from individual isolated heater supplies and run on a split supply as used in totem pole transistor/mosfet amps. Thats just a matter of personal taste for me as big expensive lumps of ironwork are to my eyes impractical for new build gear.

[Andy M0FYA]

According to the National valve Museum website "The KT44 beam tetrode on a B7 base is described for both audio use and television line output service. In the television role the valve can withstand a positive anode surge of 8,000 volts. The KT series are mainly known, however, as audio devices. The KT44 is a four Volt version of the KT66 with the anode connection taken to a top cap so that the valve can withstand high peak anode voltages"
So I wonder if an o/p transformer specified for a pair of KT66s would be correct for a pair of KT44s?
Has anyone seen the load resistsnce specified for a pair of KT44s in, say, a Williamson circuit?
Andy

[Paula A]

http://mike.saunby.net/2010/07/kt44-push...ifier.html

http://mike.wepoco.com/Home/retro-geeker...-Amplifier

ra is completely dependent on the actual operating point set in a class b or ab1 or various modes. I usually just treat the figure given for audio use single ended and consider that as "to centre tap" and you aren't far off IMHO. Lots and lots of manufacturers and designers got it completely wrong as well. Apparently triode strapped they are identical to the PX25.

There is no reason why pretty much any triode or above can't be used for an audio output stage. I built a thing way back using 18 EF80's (had a box full of them, a chassis from a relay era bandit and a spare output transformer) Chucked out about 10 watts of rather mean sounding guitar (by ear), was probably miles off anything like sensible operating conditions !