Little tube mixer

[dspruill]

This is a little tube mixer I put together over the holiday. I built it from a schematic I found on the web. It was originally for 2 high impedance mics and a "phono" input. Through much trial and error because of my noobness I altered it so I could use some input transformers I had. So It now has 3 high z inputs and 3 low z inputs. I really like the sound, definitely has some raw low- fi character. Funky meter is from a Auricon sound film recorder thingy.

David

 

[lassoharp]

Nice work! - Looks good.

Have any problems with  PT hum pickup? 

I had issues with those ceramic grid caps 'popping inside out' and losing tension.

Running a cap coupled output stage?

 

[dspruill]

I was having some hum problems with one of the input transformers I tried. It was not shielded sufficiently. The output stage is transformer coupled via a Hammond 842 fed from a 6C5 tube. The input tubes are 6J7s The power supply is a kit that I got from Welborne labs. It is very well filtered and it provides a DC filament supply.

 

[lassoharp]

I meant were you cap coupling to the 842 or direct coupling.

 

[schmidlin]

Very cool.  Great lo-fi look to match the flava.

Care to post a schemo?

 

[dspruill]

Oh, it is direct coupled to the 842.

David

 

[dspruill]

This is the schematic and parts list, I put the mic transformers outputs in parallel with the high z inputs and added a 6j7 section so I could have 3 low z inputs. I also gave each 6j7 its own cathode resistor and bypass cap.

David

 

[Schmilsson]

Very neat. A lot of stuff I haven't seen before. Hi-z input to 1(!!) triode straight into output trafo.
Shared cathode caps?

 

[dspruill]

I went with separate cathode caps, and cathode resistors.  The Hi-z straight to the triode was originally for a phono input, which I presume would have already gone through its own preamp stage.

David

 

[emrr]

How's the Hammond 842 doing?  It's not supposed to have any DC through it.  Take a measurement?

 

[dspruill]

Thanks Doug I didn't realize that. What should I measure? So far I have been using the mixer for about a week with no problem, should I run the B+ straight to the plate and capacitor couple to the Hammond?

Thanks,
David

 

[emrr]

Frequency response, Hi-Z input to output, full gain, source set low enough to use full gain.  Expect degradation of lows with DC.   It may or may not be fine.  

http://www.hammondmfg.com/560.htm

You really need something like an A-25 there.  You could add a plate resistance and cap couple, but your operating parameters will change, with less output available.  

 

[dspruill]

Thanks Doug, I will keep on the lookout for an A-25.

David

 

[emrr]

Measure it first. 

 

[Schmilsson]

emmr(or someone), what are the pros and cons of using a shared cathode bypass caps?

 

[emrr]

Short answer 'pro' is cost savings of a resistor and a cap.  Note in this case the screens are shared too.  Sometimes you still find shared screen feed in fairly high dollar vintage equipment. 

'Con' would be the resulting channel interaction and crosstalk.  Does it matter?  The channels are being mixed anyway.  What's the usage? 

 

[Schmilsson]

Was simply curious, had not seen it before and didn't even know you could do that.
Also, OP mentioned it being lo-fi, so I thought maybe that had something to do with it.

 

[emrr]

I think such cathode connection is fairly common in guitar amps, and the #1 mod suggested is to separate them. 

 

[lassoharp]

Was simply curious, had not seen it before and didn't even know you could do that.
Also, OP mentioned it being lo-fi, so I thought maybe that had something to do with it.

I have a 60's stereo hi-fi amp that uses common cathode resistor/cap on the output stage.  Have seen other consumer ("cheap") designs that do same thing.  As emrr said, channel interactions are there for better or worse.

Here's a not-so-cheap modern hi-fi design that does the common CR thing.  Some say it has 'charm'.  I'm not crazy about it on the amp I own.


http://www.decware.com/se84cschematic.pdf

 

[PRR]

> The Hi-z straight to the triode was originally for a phono input, which I presume would have already gone through its own preamp stage.

No. Dynamic pickups for 78rpm had a large "flat" output, could be run right to the volume pot of the nearest radio. No gain, no EQ.

The preamp-needed cartridge you are thinking of is for microgroove, which goes back to the oldest film-sound systems, but was not widely used until after the 1940s. The 6J7 (not 6SJ7) suggests the design is that old.

6C5 and 6J7 were introduced 1935, VERY popular. 6SJ7 is 1938, used a lot in new designs, but 6J7 remained popular (also there was a war on) into the 1950s. The industrial-number 6J7 was used for broadcasting Eisenhower's inauguration. 6J5/6SJ5 is an improved 6C5 and became common.

You want a sweet triode, and a smaller spares-inventory? Triode-strap a 6J7. Clearly 6J7 6C5 and 6J5 use very similar guts, but I like the 6J7 triode best.