Help identifying an amplifier from the ≈50s?

[dtauris]

Hey all!

First time, long time. Always been a reader, but this is my first time joining in.

I’ve been in possession of an amplifier of some kind pulled out of a tech room at a closing recording studio a few years back. I recently started trying to figure out what it was. I expected it was some kind of compressor/limiter from the names on the knobs. But after looking at the tube compliment, I realized it was a 5 tube PP hifi of some sort. I finally traced out the schematic this week and it’s honestly left me with more questions.

Some highlights from the mystery amp are:

-Giant ‘Ward Leonard/Sprague’ ceramic variable power resistors for the power supply. No chokes in sight.
-Hum balance pot tied directly to the power cathodes from a virtual center tap of the filaments. Even tho there is a CT from the OT.
-A 4 pin connector labeled “Pre Pow” that carries its own plate supply, ground and filament voltages to some preamp(?) device that was not with it.
-Input tied direct to grid. No DC blocking cap.
-A UTC A-24 wired in reverse to the “Input”. I have a feeling this was a later addition.
-Tone section that includes a “Treble Cut” and a “Bass Boost”
-Negative feedback that ties to the driver section instead of the input tube.
-Driver stage I cannot make sense of.
-UTC S-16 output that’s wired to two different outputs, 500Ω and 8Ω. Although, it seems to actually be connected for 16Ω.
-A Simpson ballistic meter with an on/off switch wired to the 500Ω output.
-Screen grid tied to cathodes with one of the a variable power resistor. (This guy is open unfortunately so I can’t tell the actual resistance. But measuring with calipers say it’s about 6.2kΩ-ish.

Most of what I’ve gathered from drawing it out is common in certain types of amplifiers I’ve seen. But it seems to be a stew of ‘best of’ sections from a variety of designs. Mixes between a hi-if, guitar amp and potentially a mic preamp missing the first input stage(?).

A Gibson GA-20 comes to mind. As does a Bassman 5B6. A just a general Williamson ultralinear hifi of some type. I found a schematic by a Wes Kinsler he claimed was based off of a Scott receiver from the 50s. Nearly identical tube compliment but the layout is very different. The meter and OT choice along with the 4 pin connector is what really throws me off.

I’m still learning about the nuances of older tube amplifiers so maybe this is obvious and some one here may spot it. Or maybe this is truly a custom design. I’m interested in the insights this community may have.

Regards,
~Drew

[sluckey]

Looks home made to me.

[PRR]

By the (well  drawn!) schematic, and the provenance, I'd guess a disk recording amplifier. Not Top 40 grade, a utility job for such as General Eisenhower commencement speech (I had that disk) or school concert recordings to raise money from parents.

None of the circuitry is the least bit novel.

Certainly all the parts were off-the-shelf at the radio/TV jobber supply or (as today) mail-order from Allied. 

[Latole]

Not a home made , but hand made like we build industrial amps in the '50 / '60

[dtauris]

a disk recording amplifier. Not Top 40 grade, a utility job for such as General Eisenhower commencement speech (I had that disk) or school concert recordings to raise money from parents.

Now that makes a lot of sense. I find that use case very interesting. There’s definitely been some add-ons over the years. I know this UTC24 was an experiment. In general though, it makes me wonder if amp building was more common knowledge back then? I know everyone knew what a tube was and had a place down the street to test them. Whoever built this unit first, appeared to be quite experienced in designing, wiring and the various circuits of the day. The layout is decent too.

Looking back, I realized I made a couple of errors in the schematic. The input impedance is set to 270kΩ, not 37kΩ. I thought 37k sounded low. Also, must of had a color blind day while I was doing the input section because the plate resistor wasn’t 330k but 220k.

I’m really curious about a few things and hoping someone has some guidance.

1. The G2 to K resistor. My beginner to intermediate understanding of tube amplifiers is at a loss. I don’t quite understand the purpose of this. If the fluctuations in voltage at the plate are mirrored at the cathode, isn’t it enough to tie the screen to the next stage of the power supply? Why electrically bind it to the cathode as well? I’m especially curious because this resistor has opened and I would like to replace it. But I can’t find any similar circuits to guide choosing a value. Any resources for learning more would be appreciated.

2. Does anyone know some examples of the kind of device that would precede the input? I assume a mic preamp stage like a 6J7. The 2W 4.7kΩ seems just right for that. I may try to repurpose some other unit I have in the pile for this. Of course, I can imagine a lot of things, but wondering if anyone can’t point to specific devices that have a front end disconnecting stage like this?

3. Is it possible to use the same transformer as a mix bus AND program bus simultaneously the way it’s organized here? My understanding is that you should never use two taps off of the OT at the same time. Perhaps it’s different when it’s 8Ω and 500Ω vs 8Ω and 16Ω?

4. Lastly, are those handmade inductors on the variable power resistors intentionally small inductors? Was this some sort technique to remove unwanted oscillations from occurring in wire wound resistors? Surely, there’s a purpose to these curly Qs.

Thanks very much for the insights so far.

[HotBluePlates]

1. The G2 to K resistor. My beginner to intermediate understanding of tube amplifiers is at a loss. I don’t quite understand the purpose of this. ...

RDH4, pages 496 and 506 where it mentions supplying the screen from a "voltage divider."  Must have been so common in the old days they didn't bother drawing a diagram for it.

In your amp, the "voltage divider" is placed between Ground, Node B at the positive terminal of C3, and Node A at the positive terminal of C2.  It is made up of R21 (6.2kΩ screen to cathode) and R20 (163Ω? cathode-to-ground).  But these 2 resistors provide a current-path for more than just screen-current to be pulled through R2 (1.8kΩ between Node A and Node B).

The cap at Node B feeding the screens is kinda small at 10µF.  The designer can rig a ratio of voltages for Node A, Nobels B/Screen, and cathode by juggling the relative values of R2, R20 and R21.  They can get a steadier screen voltage than the smallish filter cap would suggest due to feedback between screen voltage & cathode-bias.  They can also change the bias voltage, get a steady cathode bias, and thereby make C15 seem like a larger value because there is additional current (not tube cathode-current) passing through R20 because of its connection to R21.