Shall we compile a list?

[Jim McShane]

The bad ones:

KT90 Bad idea...clay pigeon target ....Bin-it. Unreliable.

This is a bit of a moot point as the supply is gone, but did you ever hear a properly made KT-90 Ei tube? I think the Ei KT-90 type II is the finest sounding power pentode or beamie ever made. But after the NATO action in the Balkans the tube quality at Ei dropped way off. Virtually every KT-90 made after that had misaligned screens.

And FYI, back when the tube was available you would get people who just raved about the tube, or you'd get people that just thought it was awful. I found a couple things about the "hate 'em" crowd that might be of interest...

1. They ran them too cool. They'd bias them at 40-50 ma with a 450-ish volt power supply. That's far too cool.

2. They seemed to prefer the "warm" tubes. The lack of any top end energy was seen as a virtue by this group, so the extended top end of the KT-90 was often described as "harsh".

If it was me and I was lucky enough to have a chance to obtain type II or pre-NATO intervention type III EIs I'd grab them and never let go. They are particularly wonderful in triode BTW.

 

[kevinkr]

This is a bit of a moot point as the supply is gone, but did you ever hear a properly made KT-90 Ei tube? I think the Ei KT-90 type II is the finest sounding power pentode or beamie ever made. But after the NATO action in the Balkans the tube quality at Ei dropped way off. Virtually every KT-90 made after that had misaligned screens.

And FYI, back when the tube was available you would get people who just raved about the tube, or you'd get people that just thought it was awful. I found a couple things about the "hate 'em" crowd that might be of interest...

1. They ran them too cool. They'd bias them at 40-50 ma with a 450-ish volt power supply. That's far too cool.

2. They seemed to prefer the "warm" tubes. The lack of any top end energy was seen as a virtue by this group, so the extended top end of the KT-90 was often described as "harsh".

If it was me and I was lucky enough to have a chance to obtain type II or pre-NATO intervention type III EIs I'd grab them and never let go. They are particularly wonderful in triode BTW.

I used EI KT-90 and the even earlier VTL KT-90 in my heavily modified Citation II, I thought they were about the best sounding tube I had encountered for this application, and the early ones were fairly reliable. I have a friend who still runs a Citation II based on my redesign and he absolutely swears by the EI KT-90. When I went SE I sold him my remaining stock, and afaik he is still running them - good ones are definitely long lived.

 

[7N7]

My suggestions

I have spent a lot of time fooling around with valves...

Suggestions?

6SN7 - not much to say here, a super valve but the loctal 7N7 is quite often better and cheaper and the PTFE Loctal sockets are better (low leakage) Dont forget 12SN7 and 14N7 - cheaper also

12AY7/6072. The best of the B9A dual triodes - low distortion and very linear

2C51/WE396A another good dual triode, mu = 35 or so

E282F Frame grid high quality pentode. Strapped as a triode has high gm and unusually, medium mu (28). So low Ra and very low dstortion - I tested it)

D3A - super pentode, used as a triode offers mu of 80 and gm <40. Ca-g is quoted in triode mode (2.7pf - not bad)

E810F/7788 makes a super triode when trapped; mu 60, gm <50.

WE416D A nightmare to install and run, this was the ultimate valve in many ways, with mu of 250 and gm of 65. I made curves for this, it's insane - bias with an IR LED and use a current regulated filament supply. Gold-plated all over - Exotic! Designed to run at 3GHz...

A2134 Obscure GEC 7-pin 9W Pa pentode. Makes a super triode - ideal as a driver. I used it many times in different circuits. Low Ra.

For power...

Look at the dual beam tetrode transmitting valves:

829, 832, QQVO3/20, QQVO6/40, QQVO7/50 and YL1071. The last is the best. Strap them as triodes (I have the curves). YL1071 offers 60W Pa (but really with a fan) and only 11W heater. Ra 600 ohms...

YL1150, super, but rare beam tetrode (Mullard/Amperex) 75W pa. A pair in Class AB1 offers 200W at 2% THD as beam tetrodes, would make a super triode

813 does make a super triode: but the HT is expensive (1kV). Easy to drive, and ra is like 845 - about 1800 ohms. 50W to light it up though.

13E1, another very high gm and attractive-looking beam tetrode. Designed for regulator service, has very low ra. I have tested these at 500V as triodes running 100mA and never blew one up - got hot though... I made a good amplifier with a pair. It delivered 33W with only 290V HT.

(Edit) Finally, looking for a high-current decnt pentode sink? Try EL822, a bit pricey, but has lovely curves and works (I have used it) A good alternative might be the Russian 6P15P

Loads more if you want!

Paul

 

[richwalters]

Perhaps the "wish list" needs requalifying. i.e biassed towards tubes that are still around in sufficient quantity that are worth using and not just exotic types. Try searching for the D3A or 7721 and 7199 are examples that are becoming so rare and pricy that any development work is unjustified as they will never be manufactured anymore. Can one be honest in getting an unused tube ?
Daresay the above comment applies to scores of obscure tubes manufactured that never found a home in projects. If I can't get a decent quantity of one particular type then hunt for others that can work in lieu. So that implies I'm staying with more recent versions.
If one finds a box of 50 up of a particular brand (as was in my case) in a disused cowshed then one is in xxx luck. Over 60 years the mice had the packaging well and truely sorted out..

Comments welcome.

richj