Altec 342b Tube Mixer

[RSRecords]

Hi, I recently bought an Altec 342b tube mixer. I want to add a line output.
Any ideas on how to achieve a 600 ohm line out?  I was thinking of using this Jensen 10k61-1m transformer. I also have a sowter 9530x
Links to trannies:
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-10k61-1m.pdf

http://www.sowter.co.uk/specs/9530.htm

The schematic/datasheet for the 342 is attached.

Thanks for any help!

 

[RSRecords]

link to 342 schematic:

http://www.greatplainsaudio.com/vintage_altec/342B_mixeramp_oper_inst.pdf

 

[PRR]

There is no low-level point suitable for tapping.

If you remove the 7027, DC voltages will soar. But 7027 are scarce.

Remove the link at 7027 pin 1 (but keep the connection at pin 4). 7027 _IS_ a 6L6 except they put that pin 1 strap to G2. They said better G2 cooling. But it also blows-up if you use other tubes. This amp does not need better G2 cooling. 6L6 are readily available. Convert it. You can still use 7027, but you can also use 6L6.

LOAD the speaker output! Parts ConneXion has big dummy loads.Tap the Speaker Out. Small pad resistor, 10K:600 transformer.

The suggested connection keeps the big nasty power amplifier far below overload.

 

[RSRecords]

There is no low-level point suitable for tapping.

If you remove the 7027, DC voltages will soar. But 7027 are scarce.

Remove the link at 7027 pin 1 (but keep the connection at pin 4). 7027 _IS_ a 6L6 except they put that pin 1 strap to G2. They said better G2 cooling. But it also blows-up if you use other tubes. This amp does not need better G2 cooling. 6L6 are readily available. Convert it. You can still use 7027, but you can also use 6L6.

LOAD the speaker output! Parts ConneXion has big dummy loads.Tap the Speaker Out. Small pad resistor, 10K:600 transformer.

The suggested connection keeps the big nasty power amplifier far below overload.

I see...that's a great idea!
Actually the previous owner modified the 1st 2 channels to be a guitar input.  This would allow me to keep that
Mod.

Thanks for  the drawing.

Quick question, the 10 or 8 ohm...are you saying either a 10 or 8 ohm 50 watt resistor between the 8ohm output and ground (0v)?  What's the purpose of this?  Load the output of the amp?
What wattage would you recommend for the 470 ohm? Any recommendations for brand or type?


Edit:

So, I'm thinking of putting the dummy load circuit on a switch. Does this seem like a good idea?
Any obvious pitfalls I'm missing?


Thanks again.

 

[Dmichel123]

Used one of of these as a mixer for well over a year. We removed the power section and IIRC, changed the last 12AX7 stage to a cathode follower... Very, very similar to the extremely overpriced 1567A. Voltages were very high, but all that seemed to do was give extra headroom. Used it for a long time like this with zero issues.

 

[Dmichel123]

Comparing the two schematics again, I probably wired the PI tube (6CG7) in parallel as a cathode follower. We reversed the mod and sold the PA to a local club a couple months back.... RCA BC3C and GE BA6B mixers have replaced it.

 

[gridcurrent]

my suggestions.

first of all, delete the 180 ohm resistors hanging on the mic inputs.

an ambitious modification but I've had good results with similar pa amplifiers:
convert the output tubes to 6L6GC, wired as triodes.  use a 470 ohm resistor with silicon diode (1N4007) from plate to screen.
lower the B+ by inserting  suitable power resistor from rectifier to first filter capacitor.
derive the B+ to the output tubes from the decoupled screen supply.
take note of the limits for 6L6GC when operating as triode.
convert from fixed bias to cathode bias.  250 ohms should be about right.

even with the cathode bias' lower power, there is plenty of level from the 16 ohm tap (over +20 dBu).
a transformer load is prudent, yes, use the existing output transformer.
though the output is unbalanced, not an issue feeding differential inputs.

 

[CJ]

might have pwr supply 120 Hz in your mix, pretty dirty on the output tubes, some cancellation of course,

ripple might be 1 volt ac feeding the output transformer,


ripple s/b  less than 10 mv after a good pi filter,

 

[RSRecords]

my suggestions.

first of all, delete the 180 ohm resistors hanging on the mic inputs.

an ambitious modification but I've had good results with similar pa amplifiers:
convert the output tubes to 6L6GC, wired as triodes.  use a 470 ohm resistor with silicon diode (1N4007) from plate to screen.
lower the B+ by inserting  suitable power resistor from rectifier to first filter capacitor.
take note of the limits for 6L6GC when operating as triode.
convert from fixed bias to cathode bias.  250 ohms should be about right.

even with the cathode bias' lower power, there is plenty of level from the 16 ohm tap (over +20 dBu).
a transformer load is prudent, yes, use the existing output transformer.
though the output is unbalanced, not an issue feeding differential inputs.

Sorry some of that was a bit over my head. I'm trying to research now .  Could you maybe help me out with a quick sketch?

 

[gridcurrent]

CJ points are well taken.  my overlook.
revised the  suggestion to include moving the point at which the output transformer ct gets it B+,
i.e., 2 stages of filtering.

 

[RSRecords]

So...looking for a suitable replacement power transformer for this guy. Any suggestions? It has some goo leaking out and is humming more than acceptable. Thanks

 

[dmp]

So...looking for a suitable replacement power transformer for this guy. Any suggestions? It has some goo leaking out and is humming more than acceptable. Thanks

Not exact, but close, Hammond makes EI power transformers with a 700v CT, that has a 50v tap for the bias. Both 5v and 6.3v windings.
Does NOT have the 21.5V winding to make 12v filaments. So you'd have to rewire v1, v2, and v3 for 6.3v filaments. But they are 12AX7 so that should be no problem.

 

[dmp]

You're probably going to want to service it before putting in a new transformer. The selenium diode(s) should be replaced with silicon. Replace the electrolytics. And make sure no one put in a larger fuse than it should have.
Might be why the transformer overheated.

 

[RSRecords]

Good call. It's been recapped and all the selenium diodes have been replaced with silicon. When I purchased it, it was modded for guitar input on channels 1/2 with a clean/distortion switch. I essentially use it as a bass amp now. AND IT SOUNDS SICK. I'd like to maybe redo the mod as I don't like the way it's behaving. I'll need to trace out what exactly was done to it but removing the 1st 12ax7 reduces hum considerably even though with the mod it's not in the signal path. The transformer is working ok but there is that goo leaking down the twisted pairs. I ran it pretty hard on a session in an amp closet and probably didn't help the situation. rock n roll I guess.