Presto 92-B ( 92-A ) amplifier [ speech amp ]

[Opcom]

I could sure use some assistance here. I checked out the Presto 92-B amp, and it's obvious it will need some components replaced but nothing major apparently.

I would really like to have the schematic diagram or manual for this, and would be willing to pay something for it. The alternative is to draw it out myself, which I usually don't mind, but this amp is more complicated than most, having several feedback circuits and what seems to be some dc-coupled circuts. The issue does not stop there, becuause I would like to know the correct voltages in the curcuit.

The output circuit is interesting with four 807's in push pull parallel and cathode bias. Plate voltage is 500V and the screen voltage is made by dropping 150V across a VR tube to make 350V. There is an adjustment (big ww pot) for balancing the cathode current, an adjustment for the cathode bias resistance (tapped resistor), and also an adjustment for AC signal balance which does something with the 6SJ7 driver tubes. The output transformer secondary is 600 ohms and also has what appears to be an 8 ohm winfing. I believe all this fru-fru is necessary because this amp is really an old record-cutting amp, and therefore fidelity would have been a requirement. The OPT is an ADC A5349 and is about a 7x7x7 cube, huge for a 60 watt amp, if anyone has info on that rare beast.

I want to put this back in like new performance, and any help w/ schematic or other data would be appreciated.

[Pete, WA2CWA]

Did you checked here:
http://www.televar.com/grshome/Presto.htm
The 92A is shown under Amplifiers along with some other info:


and here:
http://www.televar.com/grshome/Presto4.html
it says:
"We have reprints (photo copies) of the follwing catalogs, schematics, etc from PRESTO available":

A bunch of Presto models, including the 92A are listed.
A name on this web site, probably the contact person is:
Alan C Graves. 5365 Cobblestone Lane, Eugene, Oregon 97402.

[Opcom]

Thank you!

I'm going to send the gentleman $6 for the manual copy.

Bear is going to trade Altec 1570 transformers with me, he's got a 600 ohm one from a 1570BX so I'll be converting my spare 1570B to a BX.
I'll eventually try both amps and see what the result is WRT driver regulation.

On another note, I have been told by another friend, a new friend, that I should use a more appropriate amp than the Presto. It looks appropriate to me, but there are those who want to put together the old record cutting gear and that's what the Presto is from, and needed for.

And because I tend to research things to death,

On yet another note, I found decent OPTs for improving the 1570B's weak bass response so might as well post it here.
The amp would need to have the 930V HV supply redone with capacitor input to get the 1200VDC required.
Builders would put in SS HV rectifier and use the space from the 5R4's for large electrolytics.

Plitron PAT 4149-00 Application: 211 150 W, This transformer is specially designed for two 211 or Svetlana SV572 triodes in Push Pull configuration, grid driven into the positive cathode-grid voltage region. The primary impedance is 8 kOhm. The anode voltage should be at 1200 Volt. Power bandwidth starts at 14 Hz and a -3dB (1W) frequency range up to 41 kHz, all without applying negative local or overall feedback. Secondary impedances of 4 Ohms, 8 Ohms or 16 Ohms are included as well as a separate center tapped 64 Ohm feedback winding.

Plitron PAT4135 Application: Svetlanta SV572 Triode 150 W, A special design for the SV572-10/30 graphite high-power triode tube. And developed in close cooperation with the tube manufacturer, Svetlana. Intended for push-pull triode amplifier design, this transformer has a bandwidth up to 120 kHz, a primary impedance of 8 kOhms and secondaries at 2, 4 and 8 Ohms. The
-3dB power bandwidth starts at 20 Hz.

Plitron also states:
Toroidal power transformer and toroidal chokes for this design are also available [for building from scratch]. Plitron.Toroidal power transformer : 6632-X0-00 (730VA power rating 2 X 120V at 50 Hz input; multiple secondary outputs)
678VAC @ 0.65A
342VAC @ 0.30A
0-65-130-250 @ 0.30A
5.8VCT @ 6A
5.8VCT @ 6A
--> my question-how do you get the 1200VDC B+ from this?
With a doubler it makes 1500V at 600mA, enough for 2 channels with 811A's or 572B's. Is this appropriate for the transformers offered?
A question for Plitron.

Toroidal chokes: 6619-X0-00 ( 20H at 0.4A); 6620-X0-00 (10H at 0.15A)

[W1DAN]

Hi:

I tend to agree that the Presto amp is kinda special for the cutters (not many around), but probably could be used as a modulator. If it were mine, I just not make any permenant changes to the unit so it could be used as a cutting head amp later.

Good luck!

Dan
W1DAN

[Opcom]

I won't make any changes on the Presto. I am always against cutting up good equipment. I fired it up and reformed the caps. It could use 1 or 2 replaced and might need a couple of 807's, but it works and sounds great in a speaker.

I'm going to put it aside for now since an 500 ohm OPT for the 1570B is on its way. Yessir, I'm so close to having a good station on the air.. I am all excited about this. Just need the dipole put up, and the local club guys want me to put up a tower. I've been collecting things for that, a couple discones and the like. People in this hobby have been more helpful than in any other hobby.