Altec 342B conversion

[Toxophilite]

I have an Altec 342b, cool amp, looks to be nice quality!
It works quite well
I've replaced most of the electrolytics (due dilligence and the main one was oozing a little of what looked like blk rubber inside)) and replaced the plug in SS rectifier a PO had made using a octal tube base with a similar one but with a 110ohm resistor added to simulate the voltage drop of a GZ34 . This was because the VDC was about 30 volts to high everywhere, Still some troubleshooting to do regarding that in the preamp area.


I'm thinking of using it as a guitar/bass amp and maybe(if possible) keeing a couple channels as mike inputs and possibly adding lines out on these channels(I still have a couple of the really skookum looking northern electric input transformers it came with.)


It has a channel input modified for guitar(1/4" in 1 meg to ground 33k resistor to grid of 12AX7) and it sounds quite nice
However I'd like to add a mid control so I can scoop some mids when I wish.
It'd be really cool if I can accomplish this on just the two instrument channels (I was contemplating adding a fender tone stack too) and leave the two mic channels with the original(I'm guessing a type of baxandall 2 band)


here is a link to the schematic   http://www.greatplainsaudio.com/vintage_altec/342B_mixeramp_oper_inst.pdf


Could this be possible??


Also would it be possible to have lines out on the two mic channels?


THanks very much in advance

[sluckey]

You could use one of the 12AX7s to build a Fender AB763 preamp/tone stack. Send the preamp output to V4 pin 2 thru a 220k mixing resistor. The mic channels would need a mixing resistor also. Line out for the mic channels could be picked off just prior to the mixing resistor.

[Toxophilite]

Thanks a lot for responding


To make sure I understand. For the added preamp. Would it require using another triodes? . Like opne of the ones used by one of the other channels?


and would it be possible to bring it back in with a mixing resistor prior to the master volume before pin 2 of V3  or even right at pin 2 of V3?




Thanks again
 I appreciate it. I am learning some of this stuff , but in fits and starts

[sluckey]

To make sure I understand. For the added preamp. Would it require using another triodes? . Like opne of the ones used by one of the other channels?

My idea would involve stripping all the circuitry around V2 and using that tube for the AB763 preamp.

and would it be possible to bring it back in with a mixing resistor prior to the master volume before pin 2 of V3  or even right at pin 2 of V3?

You could do that. You would have to pad down the output of the AB763 preamp to offset the extra gain of V3. But maybe you want a hi gain preamp? I mentioned this preamp because you wanted scooped mids. There are other preamps that you could copy rather than the Fender. Just use this general idea to plug it into the existing circuit.

This all sounds easy enough when just looking at the schematic. But it could be very difficult depending on the physical layout. And if this circuit lives on a pcb it may be more trouble than it's worth. Even true ptp can be difficult.

[Toxophilite]

Thanks again
That's very helpful
I really like the fender tone stack, that's what I was thinking of. I'm a mostly clean player not much of a high gain guy.


Out of curiousity, and this might be a completely dumb question, but if I go back in after the tone stack before pin 2 of v3 can I then get away with out re-utilizing V2? (I really only need another1/2 12ax7  for a tone stack right?(for one channel))


Here's a couple pictures, as you can see, in the preamp section there's a fair bit of room.


Also on the face plate a previous mod( reversed before i got it) had installed 3 500k pots with knobs(now all disconnected) handy spot for a tone stack if I put the right size little pots in


By the way I'm really enjoying my S. Lucky deluxe light build/conversion with added tremelo. I've used it in recording sessions and rehearsal and many people have commented on it's great clean sound(I usually put it through a JBL D130)
So thanks a lot for that and the help I received building it

[Willabe]

Wouldn't that unit bring a good price on ebay?

Then you could build what you want with the money from selling it to someone who wants it as is?


             Brad   

[Toxophilite]

Don't worry Brad!!
I sacrificed half of my extended family to the GREAT GOD ALTEC   to appease his righteous wrath in this matter of AUDIO BLASPHEMY



Bahahahaha!


I actually plan to use it as an amp, with channels for Microphones, guitar and then a line channel for bass
Very close to it's original intended function as a PA mixer, you can sleep at night!
As you can see from the pictures someones been at it already anyway
On ebay they fetch btwn $250-$350 without transformers and then you have to ship the 22lbs item
 

[sluckey]

but if I go back in after the tone stack before pin 2 of v3 can I then get away with out re-utilizing V2? (I really only need another1/2 12ax7  for a tone stack right?(for one channel))

OK, I see what you're talking about. Should work just fine.

[Toxophilite]

Cool! Thanks
I appreciate you having a look at it all!


I can try that as an experiment with very little alteration, just build a tone stack where those pots are and run a line up to them and then back through a mixing resistor to the master volume, It'll be fun!
If the exposed location of the pots makes for too much noise I'll look at other options

[Toxophilite]

So I installed a fender tone stack where the three added pots are ( A PO addition) and tried sending to the pin 2 of V3
It worked but had very little volume (likely as S. Lucky imagined) So instead I hooked the output to the pin 7 of V3 and though I'm now effectively putting the added fender tone stack in front of the original tone stack. The results were much more useful and I can get a fair amount of tonal variety from the interaction of the two.


As I just put the other input transformers up for sale on ebay I might go the other route and sacrifice a channel or two towards having individual EQs on a couple of channels but all in all the experiment up to now has been a success.
Thanks S. Luckey for your help
A negative feedback control might be cool too or a "presence" Either that or I put in the 120pf brightness switch.

[Toxophilite]

I'm resurrecting this post because I can't seem to get rid of this buzz this amp has.
With the preamp volumes turned down and the master cranked it's not si bad. mostly hiss. But as soon as you start bringing the preamp volume up it starts to get really loud really fast. The power supply has been recapped and have all the electrolytics EXCEPt for the 4 50 mfd 5 vdc electrolytics on the cathodes of the preamp tubes (they're couple with 2.7 k resistors)


I haven't as yet replaced any of the decoupling caps


Since monkeying with it I brought it back to stock to see if my monkeying had caused any buzzing


It's loud and not super low more of an angry buzzing! but with some bottom end to it


If anyone has any ideas I'd appreciate it
The schematic can be found here(page 4):
http://www.greatplainsaudio.com/vintage_altec/342B_mixeramp_oper_inst.pdf

[Toxophilite]

Oh yes and it had some buzz before when i bought it
I change the filter caps cause one was leaking black goo and swollen looking on the bottom
still buzzed
Did all the other caps and it still buzzed
It's also a very loud amp!