6SL7/6SC7 Mic Pre Ideas?

[tablebeast]

I have a Stromberg-Carlson AV-38 mixer that I would like to convert. Currently it has three mic inputs and a phono input with a low and hi EQ and no master volume. The four channels are each based on 1.5 sections of 6SC7 tubes running into a 6SC7 phase inverter tand then to a 6N7 power tube in push-pull going to an output transformer. It has a gigantic amount of gain for each channel as-is. What I want to do is gut it and start from scratch. I would like to have 3 mic pre circuits that use 1 to 1.5 octal tubes and use the fourth channel as a direct recording guitar amp. Each mic input has a center tapped input transformer that can be set up for 1:14 or 1:28 use. Obviously I'd like to use them as 1:14 to preserve bandwidth. Putting the guitar amp circuit aside for now, I'd like to develop the mic pres first. I'd like to base the mic pres on either 6SL7 or 6SC7 tubes. I figure the main difference between these tubes is the common cathode in the 6SC7. I'd also like to have unbalanced outputs without output transformers. I searched the archives but came up with little info on using these tubes as mic pres. I'm not particularly looking to clone a commercial device from the past, I just want something cool and funky for guitars and maybe vocals so a slightly compressed/distorted sound would be cool. I have the Sam's photofact of this that I can scan if that would help. Thanks in advance.

Jesse

 

[skipwave]

Broad question, lots of options, lots of compromises.

If you want to keep it simple, a few cascaded plate-loaded stages and a cathode follower as the final stage to provide a usably low output impedance. Basically an Altec 1566 circuit (nobody groan just yet), which I mention since the output is paralleled 12AX7 triodes and the 6SC7 triodes seem ready made for that configuration.

Ring of three + cathode follower. You will probably want feedback around the three, since you have so much gain available, too much wasted gain (like that Altec) and you've got a hiss machine. Maybe use cathode bypass capacitors and use a switched feedback arrangement like NewYorkDave used in his MILA preamp.

With 1:14 input transformers it is hard to bias the first grid negative enough to avoid clipping with loud mics on loud sources, or even medium sources. I've had this problem, and ended up using pads on the inputs which is kinda of self-defeating in a way. The distortion sounds cool, but it is not consistent enough when the playing dynamics only clip the loudest passages. I think I need a line amp to put after a compressor for a full-time rougher-upper.

 

[tablebeast]

Yes, I know my question is broad. I'm really looking for a couple of ideas to fit inside my parameters, and then go from there and refine the path I choose. Since i have three pres int he one box I may even try different topologies for each channel, so the more ideas the better. Your idea to look at the Altec 1566 is a good one. I already have the scehematic for it and I have actually used one of these guys before on some sessions. Fro what I remember it sucked on kick drum and snare, but rocked hard on guitar and slightly less on vocals and bass. I think it used the same 1:14 type input transformer so your comment about distortion with high signal levels makes total sense now. In any case the 1566 idea sounds like a good one and I can use the 6SC7 or 6SL7 in place of the 12AX7's.

A few questions though:
1.

Ring of three + cathode follower.

What exactly does this mean? Ring of three what? Cascaded stages? I only see two in the 1566.

2. Why does the 1566 use a parallelled 12AX7 as a cathode follower? Can't I just use one of the stages for a cathode follower?

3. If I'm going to use a both sides of a duo triode as the output stage, why shouldn't I use it as a cascode? What are the plusses/minusses of these three topologies (single cathose follower, paralleled cathode follower, and cascode)? Don't all three give me the low impedance output I want for recording?

 

[skipwave]

[quote author="tablebeast"]Yes, I know my question is broad. [/quote]

Please don't take offense. I have a knack for stating the incredibly obvious. Who knows, it might not be that obvious to some of the people reading this. It sure wasn't to me until I read a bunch of preamp design development threads.

Ring of three:

Dave gives an example drawing:
http://www.groupdiy.com/index.php?topic=8537&postdays=0&postorder=asc&highlight=ring+three+tubes&start=15

The 1566 has three stages, but ditches any feedback in order to make that weak silver microphone on the gooseneck loud enough for everybody in the store to hear. :razz:

The paralleled cathode follower is capable of more (maybe even double :wink the power. Since the CF is all about current gain, that's cool. Paralleled, plate-loaded triodes provide the same benefit, but also double the grid capacitance, thus Miller effect is a greater concern.

I forgot you mentioned 6SL7, too. I was thinking with the 6SC7's common cathode, you couldn't do a cascode or White Cathode Follower, so you were stuck with the basic CF if you want a CF at all. I figured you needed a CF, since with these high gain triodes, getting the output impedance to play nice with even a 10k input might be difficult. The octals are pretty fat, though. You could try calculating the resistor values for a plate out, and see if you can get enough current at a low enough output impedance. IIRC, neither of the tubes you mentioned would like it.

 

[tablebeast]

Please don't take offense.

No offense taken. It takes a LOT to offend me. So no worries there. I was really just reiterating that it was in fact a broad question and I was looking for a lot of broad answers.

So, cool, you've given me a lot of ideas so far. I understand the ring of three concept now. This in fact was kind of what I was looking for. So, the ring of three is two triodes cascaded, followed by a cathode follower and NFB going back to the first triode's cathode. I assume the gain control goes between the first and second triode? If I do the ring of three what kind of ballpark parts values am I going to need for the cathode follower triode using 6SL7's?

Jesse

 

[pstamler]

No, the gain control doesn't go between the first and second triodes; that would only control the open-loop gain, while the closed-loop gain is controlled by the NFB. Gain control should either be in the feedback loop (controlling the feedback resistor) or afterwards.

The 6SL7 and 6SC7, as high-mu, high-impedance, low-current tubes are actially crappy for cathode followers, as are 12AX7s (despite their use in certain high-end hi-fi preamps). For a good cathode follower with some balls to it, you need a medium- or low-mu tube. 6SN7 is the classic, 12AU7 will work but not as well.

Peace,
Paul