High Slope Power Pentodes

[Cressy Snr]

I've been having a ferret around Frank's electron tube datasheets and have found quite a nice selection of these high gain, power pentode, forerunners to the ubiquitous EL84.

Quite an education all'n'all.

The 7 pin British based ones are the most interesting to me, as hopefully they won't be as expensive as the octal based 6V variety.

The ones with funny heater voltages are at the top of my list of valves nobody would want.

Brimar 7D6 (40V), 7D8 (13V) look like good candidates for 10W push pull amplifiers. In pentode mode, the 7D8 has a gain of 600.
Without feedback, you could drive it straight off a CD player.
With feedback, a single stage of ML4 will drive it.

There's the KT41, KT33 with 4V heaters, that also look promising.

If any Brit valve experts have any other suggestions, I would welcome them. It's a nice little avenue for me to explore as I like the sound of pentodes.

[Alex Kitic]

The 7 pin British based ones are the most interesting to me, as hopefully they won't be as expensive as the octal based 6V variety.

The ones with funny heater voltages are at the top of my list of valves nobody would want.

There's the KT41, KT33 with 4V heaters, that also look promising.

If any Brit valve experts have any other suggestions, I would welcome them. It's a nice little avenue for me to explore as I like the sound of pentodes.

I checked on the KT33 and KT41. If the KT33 (C) can be run up to 25W dissipation, like the datasheet suggests, it would probably be a good candidate for use in a RH Universal circuit (octal socket, standard connection, you just need to supply a variable heaters supply for the output tubes).

The KT41 looks like a perfect candidate for an RH84 with exotic tubes. The sound quality might turn out even better, depending on the intrinsic quality of the valves used.

Besides a variable heaters supply for this exotic experimenters RH84, I would suggest the use of an LT108x instead of the LM317, as it will require less voltage drop, i.e. cathode bias voltage to operate (1.25V for the reference plus drop across regulator, I guess it would work perfectly well with 2.5V total cathode bias). Regulation of current draw (you can use a switch to choose a pre-defined value) and zener drop from B+ should give a reasonably controlled environment. Swapping rectifiers should allow for easy B+ adjustment.

What startles me is the implication that such exotic tubes might be available for "peanuts", so to speak?!

[ACPEN]

Could I recommend the Mullard PENB4 as an excellent 4v 2.1A,B7 based candidate. I prefer them to the more common PEN4VA etc. They are a true Pentode unlike some of the given alternatives.

Regards,

David.

[Mike H]

Seem to be a few KT33 on eBay for not too silly money, hardly any KT41 though.

[Cressy Snr]

Cheers guys!

I'll certainly look into the PENB4.
Meanwhile' I've scored a quad of Brimar 7D8 for a reasonable price.

The 6P25s are sounding bloody marvellous, driven by Telefunken ECC81s with plate to grid feedback. They are stunning little beam pentodes, for sure, sounding far better as pentodes than they do as triodes
I do hope these B7 based types have a family sound, that is as good as the 6V octal ones. I can't see any reason why they shouldn't.

[izzy wizzy]

I've been having a ferret around Frank's electron tube datasheets

Steve,

How do you go looking for something when you only know the characteristics (roughly) of what you're after? Strikes me it must be very trial and error or is there something I'm missing here?

cheers,

Stephen

[Cressy Snr]

I've been having a ferret around Frank's electron tube datasheets

Steve,

How do you go looking for something when you only know the characteristics (roughly) of what you're after? Strikes me it must be very trial and error or is there something I'm missing here?

cheers,

Stephen

Well I started with the 6P25, which is equivalent to the KT61, so searching on Frank's using the Vade Mecum section leads to sheets of equivalents, which you may or may not get a full data sheet of, on the main part of Frank's site.

So you can, using the Vade mecum find equivalent types once you have found a reference, such as the KT61. This yielded the KT41 and the 7pin base types. It just grew from there.
Similarly, I knew the EL33 was a similar type of hi gain pentode. This then yielded another load of equivalents such as the 7D6, 7D8, 7A3 and so on.

You do need to know at least one designator of the type you are looking for, to get a list of equivalents, otherwise you are flying blind, and could wander out there for years.

Steve

[simon]

Duncan Amps TDSL free download can be useful too for finding similar valves.

[steve s]

Dont go buying any of these i have them all .. And more,

[Cressy Snr]

Dont go buying any of these i have them all .. And more,

Now I really need a visit in March Steve.