I have a presto 3 channel tube mixer from the late 40's early 50's and it didn't come with the accompanying power supply. I can find no info or schematic on it, any help would be greatly appreciated.
A Presto three channel mixer. Not much to go on without a model number or a picture. Presto was in Paramus NJ and became part of Bogen I think. I do remember a model 90B (below and here) that had three channels. Never owned one so don't know if it had a separate PS on not. In any event you don't just power these old things up because of there age. Any electrolytic capacitors it has will be pretty much dried out and need reforming or replacement. I can't help you with a schematic and Presto is long gone. As far as any needed PS, you'll have to improvise. I'd start with just a simple 250 or 300 volt source plus filament voltage. This is assuming you can do things like trace wiring and make judgments based on what you can see and measure. If not then take it to someone that is more experienced.
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Does it have a power stage, or is it purely a preamp/mixer? I have a Presto 39-B which is just a preamp/mixer with 6J5 output stage, it has a large dropping resistor on the way in for 400VDC input, I've lowered it's value substantially so it's happy with 250-300VDC, absolute value non-critical. If I have 300VDC after the dropping resistor, I then have 155ish on the output stage plate. Hope that gives some idea.
That's going to be a record/playback amplifier with bias/erase oscillator, which could be challenging to reverse-engineer. A series of high-rez photos of the innards might be enough for the gang here to suss out power supply requirements.
Just to make sure - you only have the mixer without the deck, and the tubes in the mixer are 6J7, 6SL7 and 6SN7, correct?
No more guesswork: I found the schematic in an old audio book. 900-A1 and 900-A2 are identical , except that the A-1 has only one input channel and the A-2 has three (passive mix) mic inputs.
The low-level stages tubes (6J7/6SL7) are wired separately and probably DC powered for lower noise.
To power this preamp you'll need to supply the following voltages:
(referred to the Jones power plug)
+ 355 VDC @ 0.1 A (pin 10)
+ 19 VDC @ 0.8 A (pin 11)
6.3 VAC @ 1.5 A (between pins 7-8)
Supply GND (pin 9)
I don't have the schematic of the original P.S.U but it should be very easy to duplicate with modern parts.
The low-level stages tubes (6J7/6SL7) are wired separately and probably DC powered for lower noise.
To power this preamp you'll need to supply the following voltages:
(referred to the Jones power plug)
+ 355 VDC @ 0.1 A (pin 10)
+ 19 VDC @ 0.8 A (pin 11)
6.3 VAC @ 1.5 A (between pins 7-8)
Supply GND (pin 9)
I don't have the schematic of the original P.S.U but it should be very easy to duplicate with modern parts.
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Man you guys are totally awesome! Thank you so much! I've had this thing for years and could find no info on it except what is on the Presto History website. I also know Sam Phillips used a similar 5 channel mixer with a PT900 for early Sun recordings. I have an Eico 1030 and will use that for PS. Thanks again, I will keep you guys posted.- Status
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