Presto 39b 3 channel mic preamp

[ombudsman]

Anyone have experience with one of these ? I picked one up for cheap-ish and my tech likes this sort of thing so I will be sending it to him soon.

Basically I was wondering if it would make sense to find a couple of suitable output transformers (and if so, what characteristics they would have) in order to convert it to 3 separate preamps with direct outs rather than the stock configuration where they are summed into a single output.  Or if there is anything we should watch out for that anyone else may have run into when restoring one of these.

My hope is that it will sound like what it is, an old tube preamp of the type that you might hear on a 40s movie soundtrack, not too hi fi, which might be good for guitars and vocals.

 

[PRR]

Is this the electronics from the 900 deck?

If so: there's no "3 preamps". There's one preamp, and a funky *passive* mix network in front of that.

 

[ombudsman]

Hi PRR

I wasn't aware of it being related to a tape deck but that could be. It looks like this:

http://i.ebayimg.com/images/g/EpMAAOSwLsBZRd1b/s-l1600.jpg

http://i.ebayimg.com/images/g/BJcAAOSw~rpZRd1C/s-l1600.jpg

For each of the 3 inputs (or at least, this is an accurate count for what is on the chassis),  it has an input transformer, a 6sj7, with a 3 position impedance switch and also a Daven attenuator below deck, and then there is a single 6j5 and output transformer. It has no onboard power supply. So, I was assuming that those are separate channels with a single gain stage each.

 

[gridcurrent]

Anyone have experience with one of these ?

"our" own Doug (emrr) has drawn out the circuitry.

direct outputs?
sure, with exceedingly high ratio step-down transformers.

 

[ombudsman]

thanks gridcurrent.  Do you happen to have any figures, or a ballpark estimate of the kind of ratios I should look for ? Then at least maybe I could determine if there is any current production option or if I should look for something vintage. (I'd prefer to go vintage really.)

I do see a Kenyon p-401 on ebay (listed in the title as a P-400) which seems like a candidate (since the existing single OT is a p-400), but it's only one and I don't know where I could get specs.  It doesn't cost a lot less than what I paid for the whole unit, but that's the pattern, I get a good deal and then I turn it into a so-so one eventually, but that's OK.

 

[emrr]

Consider it an alternate Altec 1567A with impedance switching on each input.  There's no sane way to change the circuit without totally butchering it.  You'd need 2 more tubes and 2 more output transformers.  You can't really fit additional parts in that chassis anyway.  Rare beast, don't be that guy.  It's cool as-is. 

I guess 'search' wasn't your friend:

https://groupdiy.com/index.php?topic=58678.msg776462#msg776462

 

[gridcurrent]

Do you happen to have any figures, or a ballpark estimate of the kind of ratios I should look for ?

Altec/Peerless 4722 to the 150 ohm winding, 18:1
perhaps UTC A-27 to the 200 ohm strapping, 22.4:1

not to discourage your idea but am not thrilled with the concept of these direct outputs.
use it as a simple mixer or build a few discrete channels based on the design that can drive a line.
in agreement with emrr.

 

[ombudsman]

thanks guys.

I was imagining just running signal leads out of the chassis (there are some existing holes that would allow that) and putting additional output transformers and connectors on a separate rack shelf. I would not hack an old chassis. But adding tubes does sound like too much.

I did search on presto 39b and presto 39-b but got no results on either.  Search titles only was not on and it was pointing to all forums. Just tried it again to make sure. Am I doing something wrong ?

 

[lathetroll]

This is a mic preamp that is designed to go to a Presto 92A amp for cutting records with a Presto 6N record cutter.

It is mentioned in this manual:

https://www.lathetrolls.com/viewtopic.php?f=9&t=3152&p=18807&

I have that amp.  I believe it has a power out that is designed to power that preamp.

I agree with the poster who said, basically, keep it in preserved condition.

 

[emrr]

Yep, I still have the 39B, sold the 92A I got with it years ago. 

 

[1000tinyempires]

I have one of these that was raided years ago for its OT and they simply rewired the 6j5 to a 6SN7 which added a cathode follower output to the original circuit in place of the OT.

I built a power supply for it and have debated adding individual OT's for each channel on the Power Supply chassis as there is room. I started with the idea of simply keeping the cathode follower 6SN7 circuit and adding 2 more tubes and some Edcors to balance the outs.

Now I wonder if I should in fact dig out 3 input transformers and wire them backwards. 

The Edcor route is straight ahead, but 6j5's are cheap and I think I can find 3 Jensens from a console pull.

If you had one hacked and missing the OT what would you suggest.

P.S. this is my first ever post here and I am glad it is in this thread. I have learned a lot over the years from this forum.