Um, Can I do this?

[john65b]

Hey all you smart tube guys,

I have been getting real interested in Balance Connections to amps lately. I have a Pass Aleph 5, Krell Clone, that have balanced inputs, and eventually building a Aleph JX...

Anyway, I have been shorting the neg input to ground for single ended connections, but have been thinking of this idea to build a tube pre Concertina style to balanced XLR connections - see attachment.

Is there any reason this won't work? No values are set yet, just proof on concept only. Note - I do not have a balanced source.

Thinking a 6SN7, 6922 or 6N1P Input tube...

If a horribly dumb idea, go easy on me, as I am very thenthitive.

[hemgjord]

Off the top of my head:
Sure it will work and as long as both outputs are equally loaded the concertina has approx 1/gm output impedance so it should behave... if equally loaded, which probably would be where I would have my doubts-cable impedance, input impedance etc need to be perfectly balanceed otherwise both signal balance and output impedance is compromised.

/Olof

[Shoog]

Since you have two triodes why not just a LTP. You get gain with that configuration.

Shoog

[john65b]

OK, so I haven't heard that it cannot, or should not, be done.

LTP version attached. Feel free to advise on additional components. Afterwards we can look at tubes and component values. I am leaning toward a 7N7 (loctal 6SN7)

Again, I have read that Balanced arrangement is a waste unless the entire chain is balanced (source/Pre/Amp).

Am I spinning my wheels since my source is single ended??

[bigwill]

Might need a topology with a lower output impedance to drive a cable + whatever is on the other end of that XLR cable, although with a short cable length and light input impedance it might work OK. If the load is heavy, you can cheat and feel good about it by buffering the outputs of the LTP using some MOSFET source followers

[john65b]

XLR Cables will be made short (a foot), and should be feeding a few amps with input impedance of between 47k and 100k

Maybe an ICEpower ASP1000 (20k-ish?)

Am I correct in thinking the Concertina type would offer lower output impedance, but lower gain, while the LTP would offer higher gain and higher output impedance?

[bigwill]

Originally Posted by john65b

Am I correct in thinking the Concertina type would offer lower output impedance, but lower gain, while the LTP would offer higher gain and higher output impedance?

You are correct, although driving a 47k load from the anodes of a 6SN7 in an LTP is really pushing it - a typical anode resistor for the 6SN7 is already somewhere in this order of magnitude, and if you go by the rule that the actual load resistance should be 10x that, then you will be way off!

The Concertina will probably drive the load just fine, but then each phase has a different output impedance (Very likely someone will correct me on this)

How about going the whole hog and using two 6SN7s, one as an LTP and another as two cathode followers (one for each phase) to provide a good strong drive?

[bigwill]

For clarity here is a diagram (NOTE: Grid stoppers omitted)
You might want to float the heater supply on the cathode followers too, since the cathodes will be sitting at quite a high voltage, and a big heater to cathode differential is a Bad Thing

[bigwill]

The follower 6SN7s could be replaced with suitable MOSFETs too, if you want to save on tubes, which is understandable.

[leadbelly]

Originally Posted by Shoog

Since you have two triodes why not just a LTP. You get gain with that configuration.
Shoog

? You'll get more gain going common cathode + concertina than taking those same 2 triodes and making an LTP.