Vari-mu compressor with a 1T4 tube anyone?

[joe-electro]

I just got 6 1T4s in a lot of tubes and discovered they're vari-mu tubes designed for use in car radios. Might be interesting... anybody ever tried a design with these?

http://www.r-type.org/exhib/aaa0044.htm

Thanks,

---Joe

 

[emrr]

So many choices.  Be curious to hear what could be eaked out of these lower voltage battery tubes.  Bias range looks lower than some of the standards, maybe? 

 

[lolo-m]

A friend of mine gave me a lot of those 1T4 tubes too... And like you, I was thinking I could try to do a varimu comp or a DI...
I'm still working on my PM670 (time constants section) so I didn't try to make anything yet with those tubes... But I will... Keep in touch, I'm with you on this ! ;D

I think that a good start can be a modified PRR varimu with 600/10K transfos. The sidechain control amp is quite simple to do and to modify. Using 1T4 wired as triodes (maybe using 4 of them for an easier thump control), the main thing is to find the good polarisation for the maximum gain without too much distortion...

 

[Larrchild]

I remember those tubes.

I'd build up a circuit with a pair of tubes and a pair of xfmrs and just manually control the grid bias and plot out the dB's of GR first.
That will define your sidechain requirements.

 

[joe-electro]

I wonder if we built some totally esoteric and exotic compressor that used say 8 of these tubes and maybe about 20 other tubes and 10 or 12 transformers if we could get the price of the 1T4 up to over $100 each. (he smiles greedily while rubbing hands together) 

 

[emrr]

go for it    ;D ;D ;D

 

[Larrchild]

I think 12 is the right number to push-pull-parallel.
That way you can series the fils for a nice even 12v. ;D

 

[lolo-m]

12 for a mono channel is cool !!! But I will have to quit the building to speculate on my NOS tubes !!! ;D ;D ;D

 

[PRR]

> you can series the fils for a nice even 12v.

I think you are joking.

12 per side is going in the right direction. The tube has OK curvature, but is scaled for SUCH small current that it aint a lot of good for audio. It will be down near 0.1mA for a useful amount of GR. In a radio IF stage, the interstage can be tuned-up to >>100K AC load, so 0.1mA peak is >10V and it is in no danger of starving and distorting. In audio, the only way we can give 100K load is with a resistor, and the 3mA idle current can't happen. If a shoe-box is available, massive parallel can drive ~~10K loads, which we can do with an audio transformer, while keeping DC drop small. We also get high idle Gm meaning potentially low idle hiss.

It is inevitably a low-voltage input, and unless MASSively paralleled, will need a post-amp. (P-P 1D8 will make +26dBm.)

Twenty 1T4 and two 1D8 will heat with just 1.2A at 1.4V. You can get highly efficient low-V power modules for CPUs. 1.4V would be Pentium-class power and rather large/costly. 3.3V 1.2A and a 1.5 ohm 2W resistor will heat your tungstens. CUI Inc FSK-S5-3R3U, $20 at DigiKey. Or a 6VCT PT and monster caps, but size and losses will be higher.

A single D-cell would power the heaters through an entire side of an LP.