SE Tube mic pre

[tardishead]

Have spent some time trying to research class A single ended tube mic pre schematics. I found the RCA BA1a BA2A and BA11A but the rest of the schematics I could find are push pull Collins, Altec, Ampex, RCA BA21 or cathode follower Pultec MB1 and Altec 1566.

I have 2 Pultec MB1 preamps which are great but I want something that you can push with pleasing distortion. I have a V72 aswell but again these dont seem to distort much either. Both preamps I use if I want quality pritine hifi program. I want something a bit more dirty when pushed.

I dont need an EFX pedal like some of you may think, I am just looking for a mic pre that has a bit more character and a nice break up sound.
Is there a simple vintage schematic out there Class A with octal tubes perhaps????

 

[ciminosound]

Well there's some Gates stuff with 5879s or EF86s or NY Dave's with the 12AV7 kicking around. Not Octals I know.

A lot of the pres are Class A gain stages with cathode followers to DRIVE the output. Cathode followers generally are thought of as "neutral" sounding.

Aren't the Altecs Class A Balanced and not push-pull??

Didn't CJ have some octal versions of Altec stuff or was that just the 436 limiter?

 

[PRR]

> a mic pre that has a bit more character and a nice break up sound.

Reduce the supply voltage. Seriously.

 

[emrr]

Collins 6Q-1 is a no/very little feedback circuit that's pretty easy to reproduce with other parts. Same with RCA BA-2A, but needs the right type output xfrmr. Both can be pushed with a vocal and it's hard to tell if the singer got gravel in their throat or the pre clipped a little. BA-2A has a volume, 6Q can have one added in the obvious spot.

Most of the units with negative feedback loops will have 20-30 db of feedback and do not clip nicely. SE doesn't really have anything to do with the sort of clipping aspect you are talking about. The Gates EF86/5879 units are in that category.

PP Altec 458A has no feedback loop, and clips nicely.

BA-2A fits your description the best.

 

[tardishead]

Thanks for the info guys. Thats great. Has anyone got the Collins 6q-1 schematic and the Gates schematics you're talking about.
They appear on old threads but the schematics are long disappeared.

If I minimise the feedback on my Pultecs I assume that will give me more of "drive" capability I am looking for. At present they are minimum gain and max negative feedback.

 

[emrr]

I meant those particular Gates don't break up very smoothly.

 

[tardishead]

anyone got the Collins 6q-1 schematic?

 

[emrr]

I sent it to you already

 

[abby normal]

[quote author="emrr"]
BA-2A fits your description the best.[/quote]

Doug, could you post (or send me) this schematic? I've been looking for this one for a while. It was around here but the links are dead.

 

[emrr]

http://www.coutant.org/ba2c/index.html


RCA BA; always search for example BA-2, BA-2A, BA-2B, BA-2C. And even BA2, etc.

 

[rafafredd]

Interesting. Both triode connected. I wonder what 9pin duals would work somewhat like the 1620 in triode mode. That would lead us to one more nice one bottle.

 

[emrr]

12AU7 closest standard 12xx7 type offhand without looking it all up. Distortion #s generally perceived as better with octal types. I had a BA-2A once that had been re-engineered for triode connected EF86's. Socket changes only, no component changes. Pretty much worked the same.

 

[rafafredd]

12au7, mmm. Have plenty. Might give it a try. What about R11/C8??? Never seen such a thing in a simple circuit like this. What´s the point of this filter?? Some FR compensation maybe? I should off coarse use other transformers types, so I´ll rip these extra parts in there.

 

[emrr]

R11/C8 is a slight treble boost. Makes it look like a classic Neumann top end. Certainly added to compensate for extreme top end roll off. Probably not needed with modern iron. Not really needed in these either. Common in RCA broadcast circuits.

 

[abby normal]

[quote author="emrr"]http://www.coutant.org/ba2c/index.html


RCA BA; always search for example BA-2, BA-2A, BA-2B, BA-2C. And even BA2, etc.[/quote]

Ahhh, I have that one (BA-2C), I did not know that they were all the same. What's the difference between the A,B,C?

Doug could I check out that Collins 6Q-1 schematic? I love that old school stuff. :thumb:

 

[emrr]

Electrolytic mounting style.

 

[flaheu]

this BA-2 c looks very nice :grin:
I don't have any SE yet in my setup, that would be great to add one like this.

What would be the specs needed for these TX, is there any TX ready available from usual manufcaturers ?

What means MMF ? micro-micro Farad,I mean pico farad ?

 

[flaheu]

BUMP :grin:

 

[emrr]

The transformers used in the BC-2B clone would work fine. Several Cinemag as well. I'm sure there are others. Cheapest somewhat similar sounding common vintage iron; maybe A-10 and A-25. If you find the post about BC-2B voltages there's some commentary about transformer impedances; bottom of this page:

http://www.groupdiy.com/index.php?topic=21504&postdays=0&postorder=asc&start=0

mmfd = pfd

Note this circuit has low output headroom by modern standards, but still perfectly useful. Same with the Collins 6Q-1. Both come from the time when preamps generally put out 0 dbm to +10 dbm.

 

[flaheu]

Thanks for the links, I'm going to read those posts and see what I can find.