Amp for vocals

[txstrat]

Hi folks,

I'm plaing on building a stereo tube amp for vocals.
Have two 50w OTs (with UL taps) lying around on my workbench and thought I'd give it a go.
Well it actually doesn't have to be stereo, I just wanna place the speakers left and right on the (imaginary) stage and use one OT for each side.

Does anyone have experience or maybe a schematic for an amp like that?
I thought it would be best to build two cabinets for the speakers and one for the amp in order to place them separately.
What kind of speakers would I need? Will guitar speakers be OK? Do I need a crossover to different speakers?
I know that's a lot of questions - hope someone will chime in...

Matt

[txstrat]

While searching on the net I found the Shure Vocal Master very interresting. Anybody have a schematic for that one?

Please help!!

[black_labb]

guitar speakers wouldnt be ideal due to their very unlinear nature. they are more sesitive than speakers that are quite linear though, so if you want higher volume they could be ok. a mid/woofer and a middriver or tweeter (depending on the specific units) would probably be best, but i'm not a professional on these things. i dont know how far up the spectrum vocals extend either.

[MWJB]

I've sung at jams through Fender Twins at small/medium venues...at one jam the pA packed up as we were setting up, switched the PA mic (with Hi-Z transformer into the low gain #2 input) into a Twin, with stock Oxfarts, and everyone thought they were singing thru the PA.

Ideally, you might want speakers with better hi end than typical guitar speakers (which usually peak at 5.5-6K), you might find some full range drivers with whizzer cones? Some guitar speakers are more than likely up to the job...others less so. Not all PAs sound great and not all guitar amps & speakers are crap as PAs in the real world. If you're talking about larger venues then things get more complicated.

My immediate concern would be how are you going to mate the Lo-Z PA mic to the amp?

Check out the Fender 100W/135W PA amps.

[txstrat]

No larger venues. Just clubs.
The Fender PA amps look great. Thanks for the hint. Only thing is, I wanna build the amp with two 50w OTs (each with two 6L6) instead of one 100w OT.
Wadda ya think? Can I just split up the signal into two before the PI and feed two PIs.
That would give me two power stages with the same signal (at least in my theory).

[MWJB]

You don't need 2 PI's, just have one PI plate feeding the 2 in-phase power tubes & OT primaries, the other PI output feeding the out-of-phase tubes & primaries. E.g. like a twin, between PI & tube sockets, but with an extra OT & primary wires.

OT's will want to be mounted at 90 degrees to each other.

[txstrat]

Do you think this would work?
Assumed I use the PA 135 schematic of the Fender amp. From where would I get the NFB? From either one of the OTs? Or do I have to use both, each with half the NFB resistors value?

[MWJB]

Yes, that would work. Perhaps my preference would be to have OT 1 fed by tubes 1 & 3 (rather than 1 & 2) and OT 2 fed by 2 & 4 (rather than 3 & 4). This would keep the power tube grid wires shorter & lend itself to a better layout. Perhaps also a stage of filtering between the OTs with a small value dropping resistor between them, say a couple of hundred ohms @ 10W?

[txstrat]

I thought each OT needed one "push" and one "pull" tube.
Wouldn't feeding OT 1 with tubes 1 & 3 (and OT 2 with tubes 2 & 4 respectively) cancel out the signal, since they are push pull?
Any thoughts of the NFB?

[MWJB]

Wire the grid wires from the PI so tubes 1 & 2 push, tubes 3 & 4 pull (like in a twin). So 1 & 3 are push pull for OT 1 and 2 & 4 are push pull for OT2.

NFB from a typical BF/SF LTPI will work fine. Let's assume each speaker is 8ohms, at 50W per OT we're expecting 20VAC at each OT secondary, or 40VAC total (both in phase OT secondaries feed one nfb loop), so I'd double the typical Fender NFB dropping resistor value as a starting point & look for a 20:1 to 30:1 feedback ratio...e.g. 47ohm load as per BF/SF and try a 820 to 1500ohm NFB resistor, or whatever sounds best in between those values.

[txstrat]

Got it, great, thanks so far.
Think it'll take me some time to get it working, though. (haven't even started yet)