Evolution of the 6973 tube

[Rhodesplyr]

Many popular output tubes have a development history, like how the 6L6 eventually led to the 7581A, I know the story of how the EL84 became the 7189 and then the 7189A, but I've never heard much about where the 6973 tube came from. Does anyone know the story?

[g1]

Not much out there for history that I can find. Supro calls it the 'classic American jukebox tube'.
Came out around 1957 according to this: http://www.r-type.org/exhib/abn0118.htm

I'm currently looking at a pristine National Val-Trem that is probably the most gorgeous and best condition vintage amp I've ever seen. It uses a pair of these. If it sounded as good as it looks, I'd never give it back. Sadly, the sound is nothing overwhelming.

[Enzo]

That is where I mostly saw them - in Seeburg jukebox amps. SOme power tubes spawned industrial versions of themselves, but a lot of tubes were simply designed for a need. We need a compact power tube so we can build stereo amps in spaces wher mono amps used to be.

[Leo_Gnardo]

Many popular output tubes have a development history, like how the 6L6 eventually led to the 7581A, I know the story of how the EL84 became the 7189 and then the 7189A, but I've never heard much about where the 6973 tube came from. Does anyone know the story?

I don't have any answers about the 6973 for you, but I'm in a similar way curious about the development of the 7591. All I ever heard about it, it's one of the rare tubes that doesn't have a military backstory but was developed intentionally for hi fi use. Whether that's a fact or not, I dunno.

6V6 then 6AQ5 so I've heard were miniaturized, then sub-miniaturized versions of 6L6. I'm looking forward to trying out some experimental dinky-power amps with 6AQ5. I happen to like the sound of the few amps I've run across with these mini 7 pin wonders in push-pull output circuit.

Would be nice if you could share your stories on the 7581A and 7189, or point out where we could read up on these, TIA.

[g1]

Car audio was also something that originally needed miniaturized tubes.

[Rhodesplyr]

I looked at the datasheets comparing the 6973 and 7189A. Max plate & screen voltages are similar. 6973 draws less heater current (450mA).

The 6973 is described as a Beam Power Tube, not a Pentode. Plate resistance is higher than 7189, and transconductance is lower. Push-Pull Pairs of 6973 and 7189 can produce about the same output power, but 6973 requires a larger amplitude drive signal.

The mention of the 6AQ5 gave me an idea. Looking at the datasheets, it seems to me that the 6973 may have been developed from the 6V6 (same heater current and other similar characteristics). Maybe the 6973 and 6AQ5 represented two directions of evolution from the 6V6 with the 6973 in a 9-pin envelope with higher voltage ratings and the 6AQ5 in a 7-pin envelope with lower-voltage ratings.

Edit: A little more research shows that the 6CM6 and 6CZ5 were both 9-pin tubes with basically the same pinout as the 6973, but lower voltage ratings. So I'm thinking that there were a few 6V6 equivalents in 9-pin envelopes, and from one of those, the 6973 was engineered with higher voltage ratings specifically as a HiFi audio output tube (as claimed on the datasheet).

I used to subscribe to the magazine Vacuum Tube Valley, and they had quite a few articles about the development of certain families of vacuum tubes.

[mozz]

Scratch built a amp with 4 6aq5's as outputs. Man that thing growls. Put in a half power switch, a 500ohm in line with the B+. I've bought a few full tube caddies for $50 just because i spotted a pair of NOS 6973's in there. Think i mentioned this once before, there is a certain major brand piece of older hybrid test equipment (kind of useless these days) that has 4 7189A's as outputs in it. I confess to parting out 2 of them. I have yet to use them in a build but when i find a decent power transformer with about 440v that will be my voxish AC48.

[tubeswell]

Scanty published information but the article by John Atwood starting at Page 12 of VTV Issue 15, attributes the 6V6GT as the 'prototype' of the 6973 and the 6CZ5

https://www.tiffe.de/roehren/VTV/VTV15.pdf

[Leo_Gnardo]

Scratch built a amp with 4 6aq5's as outputs. Man that thing growls. Put in a half power switch, a 500ohm in line with the B+.

Kool! I'd like to hear more about it - any info on OT, PT, whatever. TIA! Growls.... I'm likin' it already.

[mozz]

Damm, upgraded the hard drive on this and all my old files are here, somewhere, shortcuts don't work too well.
Appears to be a 6g3 fender with a 5879 and/or 12ax7 channel. My schematic uses that as a base. From what i remember, 3 years ago, the output transformer, which is square, was out of a old RCA PA amp which had 4 of the 6v6's. I think it pushed 30 watts. I don't remember why i didn't just use the power trans also, must have been real high voltage? Found the sams, 740vct so i had over 500v with diodes. I still have it here somewhere. Power trans from the photos looks like a Tektronix. Comparing the pictures to the schematic, i may have used a 6j6 7 pin tube where the schematic shows a 12at7, i think it's just a schematic typo. 10k was switched in to bring the power down, that's why it growls, pre amp plate voltages must drop pretty low.

[Leo_Gnardo]

Thanks for the photos & tech memories mozz. Yeh, that's the stuff! Tube covers with "windows" on the outputs - whooee! Way kool look!

[g1]

I'm wondering where the weed whacker line comes into play.

[mozz]

You use it to bias the tubes.You run it wide open and touch the weedwacker line to the output tubes, crank the fixed bias until it melts and starts smoking. That's the exact point you leave the bias at. If it's cathode biased, you put 1 drop of 2-stroke oil on the tubes, when you lower the resistance, there's a point where it smells like a 76 cr250, that's the exact point you leave the bias at.
My amps sound real good until they blow up.

[g1]

there's a point where it smells like a 76 cr250, that's the exact point you leave the bias at.

Ah, the Elsinore method.