modifying an 1567a altec

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alexidoia

Well-known member
Joined
Jan 31, 2006
Messages
85
Location
The Bask Country
hi there,
The subject says it all but here are more precise questions with reference to the schematics:
http://www.idoia.com/studio/1567a-08.jpg
My altec as such is working good but:
1) certain mics seem to overload the first stage tube, I guess I could use pad before entering the unit but could I add a pot to  dose the entry level in the tube, as such I could remove pot 2.3.4 and 5.
2) considering a use with no more than 2 input, I was thinking breaking the circuit after pot 3 and 4 but I would unload the circuit and create a unbalanced load, am I right ? Is there a simple way to do it ?
3) I would like to add an output attenuator allowing me to drive the output a little and keep a reasonable gain. Could I use a balanced OA after the Transformer J5 and if so would a balanced 600 Ohms do the trick ?
Thanks in advance
 
1. I'm not sure what ratio the input transformers are, but placing a 1M pot at each input would give you gain control. Keep in mind, that you don't want to attenuate much here as noise will become a problem. You could just add a switchable pad there.

2. The output impedance from the 12AX7s would be high (tens of KOhms), and you need to use a coupling cap from the plates as they are connected to B+.

3. Sounds reasonable. Check the power handling of the attenuator. Otherwise, again you could use a switchable pad.
 
I've got two of these...

Planning (down the road a bit) to scrap the 1567a and do four 1-bottle 12ax7's type pre's using the pultec mb1 w/out cathode-follower as a basis for the circuit....

Prolly with both a balanced trafo out and unbalanced hi-z out... all terminal strip to be sure... forget about those damn xlr's or trs's (plus, i've already got pigtails for this)

i've got 6 or so of the green altec mic-trafo's (1:15 or 1:10?), a couple of the red altec output trafo's, a couple jensen (115k?) mic-ip's, and i'll prolly plan to use edcor's (after comparing between bal/un-bal out) for the rest of the outputs...

use the existing psu transformer with new ~mb1 requirements.

add in a polarity and +48v switch along with ip pad, neg-fb gain, op-atten...

i've got most of the parts and plan to do it P-to-P so it should be a relatively inexpensive eight more useable mic channels (sorry, haven't found the stock 1567a too useful in my sitch.. ie; 4/5-in-1-out)

I'm just not sure whether the stock power transformer will provide enough current to replace the existing 6cg7 with a 12ax7 in an mb1 configuration... If there are issues I could just drop the 4th channel...

Cheers,
j
 
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