12U7 - any users out there?

[lordearl]

I recently came across a sleeve of unused 12U7 (Zenith) valves.
Here's the datasheet.
http://frank.pocnet.net/sheets/137/1/12U7.pdf

Anyone ever built a line stage using these?
I thought of using them as the gain stage in an Aikido (don't think the output stage would suit them. What plate current would be appropriate for a B+ of 24v?

 

[dsavitsk]

I built a little guitar amp out of them once: ecp audio diy: Low Voltage, All Tube, Guitar Amplifier for Headphones One issue in a line stage is going to be getting sufficient grid bias.

 

[lordearl]

I built a little guitar amp out of them once: ecp audio diy: Low Voltage, All Tube, Guitar Amplifier for Headphones One issue in a line stage is going to be getting sufficient grid bias.

Neat little amp!!

Yes, getting the operating point correct is going to be the key. The curves show decent linearity in some parts. I think I'll try the Aikido circuit as the parts count is small and output impedance is low.

Broskie biases the 6GM8 at 2mA (using 180R resistors) with a B+ of 24v DC.
Looking at the curves of the 12U7, just need to change the heater arrangement and it should theoretically be a drop in replacement, perhaps using bias resistors of 300R or so?

http://frank.pocnet.net/sheets/127/1/12U7.pdf
http://frank.pocnet.net/sheets/137/1/12U7.pdf

Here's Broskie's discussion of the 6GM8:
http://www.tubecad.com/2006/04/blog00

 

[ballpencil]

B+ of 24v DC in Aikido means 12V per tube, right? It's actually even less than 12VDC as you need to spare a bit for the cathode resistors.

Not much you can get with only 12VDC on that 12U7.

An externally hosted image should be here but it was not working when we last tested it.

Cathode resistor at that bias point above is 0.5V / 0.5mA = 1K.

 

[lordearl]

Yes, increasing the B+ to 48v would be better (24v at each plate).
Would give 1.2mA using a cathode resistor of 416R (0.5v/1.2mA) - is this a better operating point?

We would also need a tube with higher transconductance at the output stage, so I might use the 6GM8 there, but we would need to ensure the DC coupling between stages is still in check....