Stupid Simple Octal Mic Pre

[schmidlin]

A friend of mine is going through his I-need-a-tube-preamp-now stage, and this popped into my head.  I thought about running the triodes in series with a vol in between, but all-balanced appealed to me, and I like the warm octal sound.  Any speculations on gain level?  Any snags you see?  Go dual pentodes?
Thanks!

 

[analag]

This is a rudimentary design and your results with correlate with that. Research classic differential designs to upgrade how you implement your ideas.

 

[PRR]

6SL7 has a high plate resistance, maybe 50K at this ~~1-1.5mA operating point.

You've loaded it with 5K per plate. Maybe a bit more with 1K||10K termination.

Tube gain is 7 or so.

IT and OT ratios cancel.

Gain is about 17dB. This may be insufficient.

Oh, you may as well combine the cathode resistors. Then you may omit the cathode caps.

Few input transformers will really "balance". You might change the 1Meg to 10K, and force balance for bass and midrange, but treble balance will be uncertain (one end of the winding is earthier than the other).

Build it, but expect to have to follow it with another preamp for gain, and don't expect much "warm flavor" at this low operating level.

 

[emrr]

Search user 'newyorkdave' and 'rca 76', look at the schematic, and examine the one tube preamps in the mixer.  Good for 30 dB.  Then look at the RCA BA-2, good for 50 dB.  Only problem with those being expensive output transformer.  If you want balanced and 'simple', look at the Altec 458A; rework it for a pair of 12AU7's or 6SN7's.  Dual 6SN7's (PPP) would work in a variation of what you've shown.  I can't think of any reason to use pentodes.  Spend some money on some real transformers, and forget the cheap stuff. 

 

[schmidlin]

You gents truly are amazing!  Just what I was looking for.

I thought the gain might be shy, and the newyorkdave series'ed 6SN7 makes sense, just what I had in mind if I needed to go the unbalanced route.  Will this drive a VU meter to zero at output?

Got the idea from the Federal, stripped to the output stage so I thought maybe...it uses 6SN7 10k/600 out so it seemed to be a match.  I see now the it has a much different plate resistance than the 6SL7: very surprising & a poor assumption on my part.  I was looking for gain.

I know you guys are heavy hitters, and I'm not shooting for the moon here by any stretch.  I'm thinking of this for a guy who has been running raw mics into an interface then into his computer.  ANYTHING that boosts the signal (even stuff with the name ART on the front) gets him excited about how much better it sounds.  So maybe I'm just being lazy for a pal who would love a good tube boost.  And while I'm embarrasing myself, I'll fess to using Dukane octal ITs on this project ("cheap stuff").  Trying to use "what's around" and stay simple.

Rudimentary is OK in this case, for me, here.  And thanks again for your (as usual) stellar input.

Also, couldn't find the RCA BA-2, Altec 458A, or the RCA 76 (same as the newyorkdave pre?)

 

[gyraf]

A variation of your first idea - the DISA micpre: http://gyraf.dk/schematics/Disa_91_b21_micamp.gif

Jakob E.

 

[PRR]

> different plate resistance than the 6SL7: very surprising & a poor assumption on my part.  I was looking for gain.

Voltage gain, current gain?

There's no free lunch. The 6SN7 stayed in production for a reason. 6SL7 has high voltage gain in high impedances. The 6SN7 has decent voltage gain and more current, suitable for lower impedances.

And here's a guide. Wideband audio transformers are not high impedance loads. 10K or 20K is as high as they really get. And before resistance-coupling got common, everything was transformer interstage. And everything was "General Purpose Triode", a family which extends to 12AU7 and 6SN7. Mu is a lot more than unity, but kept fairly small so that plate impedance is not high; specifically, so that audio transformers work fine.

> using Dukane octal ITs

They are not awful (though the ART mike input stage IS clearer). Balance is not a goal. Wire just like DuKane did, ground and one grid. Quick and dirty small studio work, take the output unbalanced. The DuKane tranny has high voltage gain, the load is only 10K-22K, so use low gain low-Z 6SN7 with 10K plate resistor and 1K cathode resistor from a 150V DC supply (120VAC winding). You get 1:10 in the iron, 1:6 in the vacuum, gain of 60 or 36dB. Which may be "too much" so omit the cathode cap (save a buck). Gain around 30dB. Output cap has to be 200V 0.5uFd for respectable bass.

> ANYTHING that boosts the signal ...gets him excited about how much better it sounds.

Anything boosted always sounds better. He'll get over that illusion.... in several decades. Maybe. I still get fooled by small level differences.

 

[schmidlin]

Thanks PRR.  I did not know about the lower impedence for audio.

This all makes perfect sense, but am confused about the solution you describe.  Are you talking about using both 6SN7 triodes in series on not?  The 0.5uFd cap threw me since you shouldn't need it with an OT and only one stage.

Thanks to you too, Jakob.

 

[PRR]

> you shouldn't need it with an OT

The situation you describe, you shouldn't need an OT.

It is a -small- studio. A 6-inch cord is plenty to get into his interface. No need for balanced.

I suggest you try the DuKane alone before you add more iron. First, he may not know a difference. Second, to my rather un-fussy ear, an ART mike input (a Tube Channel but with the tube bypassed) was more clear than the DuKane plus one BiFET chip, and conversely the DuKane had "color" enough so I could tell in an A/patch/B test.

> Will this drive a VU meter to zero at output?

No. Why?

 

[schmidlin]

Just for yuks.  I had some spares.

Thanks again, I'm off to see if I have all the parts handy...