# Minature Vaccum Tube Question

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#### Mark A. Gulbrandsen

I am aware of the minature vacuum tubes(the ones with wires comming out the bottom) that were designed back in WW-2 for aircraft avionics and that these are used today in some of the Groove tubes microphones. Would any of these tubes be sutable for building a Grounded Grid line stage, were there medium-mu versions of these tubes, which is what is needed, and if so what are the numbers of these tubes. If anyone has any other info on them that might be of help I would greatly appreciate it.

Thanks,
Mark

#### Mark A. Gulbrandsen

Thanks for the link guys! I can get some of these locally so I thought I'd see what they could be used for.......

Mark

#### Mark A. Gulbrandsen

I picked up a dozen 6021's today for 10 bucks apiece. Most are RCA and some are Sylvanias, all are NOS.

Mark

#### agent.5

You may want to consider the Aikido amp

In the safer version, at the bottom of the page, you can replace the 5687 and 6N1P tubes with 6021, and just change all resistors with value of 430 to 181. The rest of the circuit remains the same.

My math is

Rk = rp (mu +1)
rp = 6500, and mu = 35. thus
Rk = 181

#### kevinkr

Paid Member
Note that the 6021 may not be happy with 300V applied to any of its elements as I found to my chagrin in my submini tube based phono stage. The 6021 has a maximum plate voltage rating of just 165V and although the voltage across either section does not exceed this value in the aikido design, the clearances are such that leakage paths across the mica can become a problem with anything over 250V.. Some 6021 work fine well above this value and others don't. I would use the version designed for 220V as agent.5 suggests - it ought to work well.
You may have to tune the cathode resistances in order to get the desired operating points, the equation he used is not usually all that accurate.

Kevin

#### RE 604

Hello ,
this is my 6021 RIAA - pre with 6111 as line stage .
The circuit and pcb layout are on my homepage under "Vorverstaerker"" .
This is the pocket- version ;-)).
Regards , Alex .

P.S.: The resistor , feeding the riaa - network , must be 68 k if you use the 7963 tube ( difficult to find here in Germany ) .

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

#### Mark A. Gulbrandsen

What I am really after is a simple line stage to build. I was hoping that the 6021 might be sutable to use in a grounded grid line stage similar to Rosenbilts.... but these tubez may not be able to work at the voltages found in that preamp. Will have to do some more investigating on this.....

Mark

#### Robert Morin

Mark A. Gulbrandsen said:
What I am really after is a simple line stage to build. I was hoping that the 6021 might be sutable to use in a grounded grid line stage similar to Rosenbilts.... but these tubez may not be able to work at the voltages found in that preamp. Will have to do some more investigating on this.....

Mark

I don't see why the Rosenbilts "grounded grid" (I put hat in quotes because the configuration Rosenbilt uses is a diff amp in my book" would not work but you will have to do some serious tweaking of voltages...

Tally ho!
Robert Morin

#### wa2ise

These are "Sub-mini"

More precisely, these little tubes are called "subminiature". "Mininature" tubes are the conventual 7 and 9 pin tubes like 6AU6 or 12AX7s.

As for medium mu submini's, the 6021 mu = 35, the 6111 mu = 20. And for a high mu tube, there's the 6112, mu=70. Aside for the smaller package (and lower max B+) these tubes are quite similar to the 12A_7 series of triodes. Not identical, but similar.

I used some of these for the analog sections of CD players (just after the DAC chips). See my web page tubedac.htm . Some submini tube specs can be found here . Also I built AM radios using these little tubes, and I have some Spice 3f4 models, all clickable from my web site

#### jleaman

Ive seen in some mag's that i havethat these little tubes are used for the pre-amp stage and input - out put stage.. Any one got some schematics for such thing ?

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