6CG7 PP Microphone Preamp

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That won't have a ton of gain, around 30-40dB before the output transformer?

(somebody correct me if I'm wrong, which I am a lot.)
 
I was thinking with input transformer(20), V1A (19)and V1B(12) gain would be around 50 before V2, but I don't know how to calculate the output section (or possibly the input section for that matter ;)).
 
V1b cathode and biasing circuit needs revising as it will not act as a phase splitter as drawn but the basic concept is OK. There will be zero gain from V1b. V1a and the output stage together should get you close to 40dB gain and with another 20dB from the mic transformer you are close to 60dB overall.

I am sure I have seen an old mic pre design with a push pull 6CG7 output stage. I'll see if I can find it.

Cheers

Ian
 
Look at the RCA BA-21A and the Altec 428B, both may give you ideas.  Ignore the tertiary feedback. 
 
Thanks Doug and Ian, I found this schematic for an old UTC PA that uses 6sn7's on the input, cool stuff.  Definitely need to get rid of the cathode bypass cap on V1B and match plate and cathode resistors.  Since there is no interstage transformer with centertap I would need to reference the grids of V2 to ground.

Link to UTC PA schematic:
http://www.triodeel.com/images/utc20w.gif
 
your v1B still needs a bias resistor with the grid R tied to it. Right now your grid is in the cutoff range of the tube.

http://www.valvewizard.co.uk/cathodyne.html
 
So on V1B, I need a cathode bias resistor around 4K above R6 and a grid resistor at 470k coming off between the two.  An input cap at V1B will need to be added(1 to 22nF) but I can't figure out the effect of having a pot behind it and another cap in front of the pot.  I would probably need to remove gain control out from between V1A and V1B and put it somewhere else maybe?  But where?  It seems like to many parts at this point and I don't know what the payoff sonically will be as compared to just strapping the output tube and using a cheaper, regular 4:1 off the shelf output transformer like Dave did in the MILA.  Thanks ChrisP
 
Yeah, I'm not sure how adding a pot in front of a cathodyne will effect everything either. I'm on mobile right now, so I'm pretty useless. I imagine a 50k pot would work, 1M grid leak on v1b, calculate caps for those values and see if it makes sense.
 
I have a prejudice against these sort of tube phase splitter circuits in preamps.  I haven't heard one yet that I thought was as good as pure SE or PP.  Any reason you aren't building pure PP?  If you are after sonic perspective/comparison, that'll be more obvious than in-betweensy. 

So, C3/8/9 all look awfully large.  I might expect motorboating, particularly from C3. 

The DI:  I see this sort of DI repeated all over the place, and I have found it common that a lot of keyboard outputs get really noisy when plugged directly into a tube grid like this.  I don't know if cap coupling solves it, it may well.  Anyone else notice this? 
 
emrr said:
The DI:  I see this sort of DI repeated all over the place, and I have found it common that a lot of keyboard outputs get really noisy when plugged directly into a tube grid like this.  I don't know if cap coupling solves it, it may well.  Anyone else notice this?

I haven't ever noticed, plugging iPods and such into them. What kind of noise?

I can certainly picture a ground loop from a common unbalanced keyboard or RCA style output. Keyboard ground direct into the preamp ground and some ill luck with their (now shared) PSU ground set up. And a cap won't cure it. But many guitar amps do this exact same thing and share this issue.

A badly designed RCA/keyboard unbalanced output that expects a <10K load might also behave weird, but I can't think of any real world examples.
 
Hiss and light buzz.    I've had everything on the same ground, lifted ground between devices, lifted one or the other power ground, basically every variation I could think of.  I can move to an active SS DI and be quiet as a mouse.    I've wondered, in the heat of battle, if one device is biasing the other, but never taken the time to think through it or experiment.  It's not every keyboard, but it's been enough to be a concern. 
 
chrispsound said:
Not that I know what the hell I'm talking about but maybe a grid stop resistor might help?

Speaking of which, what's the 10k resistor doing in series with the center tap of the output transformer? If it's for voltage dropping, it should have a decoupling cap going to ground.

Peace,
Paul
 
My dad used to tell me something similar to this:  "Its better to not post an idea and appear stupid than to post one and remove all doubt".  I'm glad I didn't listen to him much.  Thanks everybody for posting and I have learned a tremendous amount.  When I have time I will repost the schematic with the corrections mentioned and will get some other ideas on the board.  Still hungover, ChrisP
 

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