Sub mini tubes reverb recovery

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Tubetec

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Nov 18, 2015
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5,951
I just scored two slieves of 5 Raytheon sub mini pentodes , all JAN mill spec, astonishingly good quality .
A few euro's each ,and probably a lot cheaper by the 100 .
Its barely 10mm in diameter with 7 wire ended leads, and  only about 25mm in lenght .
Various different current ratings are available to cover any need , I chose a 6.3 volt heater version ,that does about 7ma anode current with a gain of 70 in triode mode . These tiny tubes have great rigidity so microphonics is low compared to larger older styles. I have read that heat dissapation needs to be accounted for preferably  direct contact to the valve glass especially in the higher powered ones  . Heater cathode voltage  as well as maximum anode voltage varies quite a lot from type to type as well .
Looks like they will make a very nice low level input stage either as cathode follower or anode loaded . I was thinking of a built in tube recovery module for a regular spring reverb tank , cathode follower input with MOhms of input resistance , I might  try driving the springs from both ends in  antiphase ,then in the join between the two pairs of springs on the longer tanks attaching a tiny rare earth magnets which will excite a two pole cut down guitar pickup positioned underneath , maybe seperate recovery coils for each spring could allow more flexibillity . I can wind my pickup coils with very much higher turns cause the spring itself doesnt have to support it , the lams and coil on the end previously used for recovery  could be rewound easily to function from the usual 4 or 8 ohm speaker output transformer on a fender champ type output stage , a small multicore and cheap n cheerfull mini aviation plug will connect the pan to the driver amp , I have a pair of C core output single ended transformers off ebay  en route , 3500:8:4 ohms , the driver amp  can double as either a headphone or monitor speaker amp when required .

It might also be possible to combine  both drive and output stage at the same time ,of course that kind of reverb is potentially unstable but Ive found when the tank drive  is trimmed with with a wirewound pot, or maybe  lightbulb limiter it works nicely and a 4 or 8 ohm tap gives tons of line level signal to your mixer or recorder .Simple switched coupling cap could allow the bass weight to be dialed or out of the sound and passive top cut could be added to the signal somewhere .

I know the pros will scoff at  a reverb machine thats on the verge of oscillation ,feed forward or feedback or a combination of both I cant fully understand exactly  , trying to borrow some of the ideas in the Akg springs . Seeing as Im doing it might as well do a stereo set , maybe even with some kind of matrixed/patched drive to multiple tanks .A mid/side drive/recovery arrangement of the tanks seems plausible in my head but  but Im not smart enough to be able to visualise it .

Nice and simple also,just two tubes .Ill scribble out a schematic/ layout drawing later just to make things more clear .
 
Sounds like a great project. I have several times been tempted by the small size and low microphonics of these tiny tubes but I have always been put off by their availability.

Cheers

Ian
 
There's several classes of "sub miniature".

Hearing aid tubes --- battery costs set a limit on how long a tube must live. (If batt cost over 5 years is $500, a $5 tube is a small added cost.)

Small full-time stuff, semi-moble radios -- these might be designed to last as long or longer than premium ordinary types.

Bomb/missle timers -- may be rated for massive shock, but only have to live 10 minutes. (Radiosondes, ballon-lofted weather transmitters, are similar.)

Because Tubetec has not revealed what type he got, no way to help.
 
5899 was what I got ,as used in the B&K cathode follower preamp .
EF731 is the Uk type , its a vari mu pentode , but as far as I know when used triode connected it just functions like a normal amplifying tube.
 
Remote cutoff pentodes become remote-cutoff triodes. The bend is only mildly mollified by triode mode.

For small signals like reverb recovery, the distortion will be negligible.

This series of subminiature tubes is "ordinary" guts in a real small bottle. Kept cool it will have long life. Packed too tight in a too-tiny chassis run in the desert you may be down below 1,000 hours, depending how critical your application is. If gain falls off with age, you turn-up until it ages so much you can't get happy; then replace. You want to think how much it costs (part and labor) to replace, who may be shooting at you, or if you may be flying into a mountain blind. (These are real military/airline tubes, not grandma's radio.)

I think you are fine.
 
The reference chart is great thanks Rec Eng ,

the tiny electrode structures in these valves mean theres not much resonance at audio frequencies to worry about , 
even Neumann used a very special wire ended valve with a grid top cap connection ,Ac701k .

I did work on a boutique guitar pedal one time alright ,had a couple of 6111 in it I think ,according to the owner ,the gain had sagged on it compared to new ,that was after 5-6 years of professional useage and touring ,and that its a distortion pedal so things were driven . It did require resoldering to replace the tubes . I have some very tiny m2 threaded circuit board mounting posts with screws on top ,I might make the mini tubes 'screw in' on a small bakelite strip and use m2 solder tags to connect the wires . I'll close the cathode follower loop back at the Drive amp which means only the valve itself will be mounted in the tank  all the other components associated with the cathode follower can be placed in the control box ,no grid resistor will be needed ,as theres a dc path to ground through the output transducer of the reverb . Im playing with the idea of an over and under humbucking coil arrangement with small moving magnets attached to the springs .
 

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