A friend of mine found a deal on parts to make a Loftin-White based, directly-coupled 6SL7 / 2A3 stereo amplifier, and brought it to me hoping it might make an interesting... mic preamp. I'm not so sure about that idea, but I told him I could build the stereo first, and would try my best to also make it usable as a mic preamp (with some additional parts), maybe more of an unusual character thing than anything very good.
So, for the stereo, I'm looking at two schematics that seem like interesting designs to explore, and which would use the parts I have:
(Schematics show one channel, but this is a stereo and I plan to build two channels.)
My problem now is that, while I can find plenty of mic pre schematics and some that use 6SL7s, I've never built something like this before (so far I've been tinkering with guitar amps) and don't really know what the important aspects of mic preamp design are. How much gain should I be aiming for? How do I choose suitable input and output transformers? What other concerns am I unaware of?
My first thought for the topology is to set up some switching to take both 6SL7 preamps out of the stereo, and connect them instead in push-pull on either end of the mic input and output transformers. Any advice on whether that's a good starting point?
I'm thinking capacitor-coupled designs would be more complicated because they'd require switching the last stages to act as cathode followers, but maybe that SRPP stereo design could work for something like that. Other options I suppose would include using just a single 6SL7 from one channel instead, or incorporating the 2A3s somehow.
So, for the stereo, I'm looking at two schematics that seem like interesting designs to explore, and which would use the parts I have:
(Schematics show one channel, but this is a stereo and I plan to build two channels.)
My problem now is that, while I can find plenty of mic pre schematics and some that use 6SL7s, I've never built something like this before (so far I've been tinkering with guitar amps) and don't really know what the important aspects of mic preamp design are. How much gain should I be aiming for? How do I choose suitable input and output transformers? What other concerns am I unaware of?
My first thought for the topology is to set up some switching to take both 6SL7 preamps out of the stereo, and connect them instead in push-pull on either end of the mic input and output transformers. Any advice on whether that's a good starting point?
I'm thinking capacitor-coupled designs would be more complicated because they'd require switching the last stages to act as cathode followers, but maybe that SRPP stereo design could work for something like that. Other options I suppose would include using just a single 6SL7 from one channel instead, or incorporating the 2A3s somehow.
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You don't want to use this as a mic preamp. It will simply be too noisy, especially if you plan to AC heat the 2a3.
Build it as a stereo amp, and then either use it that way, or sell it and build a proper tube mic preamp.
Build it as a stereo amp, and then either use it that way, or sell it and build a proper tube mic preamp.
Well, I suppose it wouldn't be difficult to rectify DC for the heaters if necessary, but the 2A3s wouldn't even need to be part of the mic preamp. My thought is to use just the 6SL7s as the preamp for either the stereo or the mic, with switching to connect them into one signal path or the other.