Transformer trouble?

[Lloyd Fisher]

Hi folks,

I’m new to amps, and looking for help.

I have a tube amp from a 1940’s film projector and I’m trying to determine the output impedance.

When I input 38VAC from another device’s corded power supply across pins #3 of the two 6L6 sockets, I measure 10.7V at the speaker output. If I’m doing all of this plus the math right, I think I should be shopping for a speaker with an impedance of about 300 ohms : )

The output transformer's primary resistance is 515 ohms, and 250 & 265 ohms from the center tap. The secondary is 23 ohms.

Can anyone please offer an idea of what I am am seeing here?

Thanks very much for any help!

[Leo_Gnardo]

That transformer may be intended to drive what's called a 70 volt system. That's how many "distributed" PA systems are wired, like those with lots of speakers in the ceiling. Like in an office, or a department store. Possibly your projector came from a drive-in movie theater? Lots & lots of speakers in that situation. There's a little transformer on every speaker that takes the high voltage line down to a level where it can safely drive an ordinary speaker voice coil. This kind of system allows very long speaker runs done with skinny wires. Imagine how much copper it would take to run 8 ohm speakers directly at distances of hundreds of feet.

In any case, it looks like that OT would be unsuitable for driving any ordinary speaker load we encounter in guitar amps.

[drewl]

You know the make and model?

There might be info or manuals/schematics for it.

I have some old Bogen tube power amps that have a 70v tap for long speaker runs as well as regular 4/8/16 ohm taps.

[Lloyd Fisher]

Thanks guys. This is helpful.

Model 40B is the amp, from a Victor Animatograph projector. I do have a schematic, but haven't figured out yet how to make it available here for viewing.

I don't see taps on this. Just a 2-spade receptacle for a speaker plug.

Can I follow up here and ask for some perspective on buying an output transformer for such an amp. My box fits a10" speaker and my aim is for the home amp that breaks up at lower volume.

Thanks again!

[Helmholtz]

https://en.wikipedia.org/wiki/Consta...speaker_system

[drewl]

Is this it?

https://www.ebay.com/itm/Vintage-Vic...-/381159641251

You can read the schematic if you hover over it.

[Jazz P Bass]

Here is a 'snag' of the schematic:

[Jazz P Bass]

Found the manual.
Page 1 & 2 attached.
VICTOR40.pdf

[The Dude]

Tweaked slightly.

[Lloyd Fisher]

Yes, that's the schematic.
I didn't find anything like a tap there, or on the amp itself either.
Can I ask for a nudge in the right direction for finding a suitable output transformer for this conversion?
Maybe you guys have a favorite book, or even a favorite transformer?
I'm new to amps, but old enough to appreciate a shortcut : )

[Leo_Gnardo]

Yes, that's the schematic.
I didn't find anything like a tap there, or on the amp itself either.
Can I ask for a nudge in the right direction for finding a suitable output transformer for this conversion?
Maybe you guys have a favorite book, or even a favorite transformer?
I'm new to amps, but old enough to appreciate a shortcut : )

The schematic shows no taps on the transformer's secondary. Looks like it's indeed a 70V system or something similar. Funny thing another correspondent on another thread has us transformer hunting today also. Here's a good dealer, selling good transformers at reasonable prices, a whole bunch of them.

https://shop.amppartsdirect.com/Output_c5.htm

Your choice - many offer a selection of useful secondaries. Since you're dealing with a pair of 6L6, 40-50W is your power range. And if you can find a transformer that has the same mounting bolt pattern as the one you have in the amp, you're in seventh heaven.

While you're at the website, you might go shopping for some fresh filter caps. An amp this old is bound to need them. No harm in increasing the values a bit from 16, 16 and 8 microfarads. I'd suggest 500V ratings, and up the values quite a lot to keep hum levels to a minimum. Say 47, 47 and 22 uF.

Before you hit the buy button - let's figure out what's going on at the far end of the power supply - components 43, 6, 5, 4, 3, 46 and 1. Looks bizarre to me. Can anyone explain what in the world of sports is going on here? Maybe a variable supply for a pilot or projector exciter lamp? Might be able to snip out these bits without any harm.

And of course you're welcome to shop anywhere else. This supplier happened to come up handy today in another thread. No salesmen here, we don't have any particular need to recommend any shop. Though we do have some favorites that have done well by our members.

[mozz]

Lucky to see 30w in a old amp like that. I can even bet they are 6l6 metal, or the original ones were anyway.

[Leo_Gnardo]

Lucky to see 30w in a old amp like that. I can even bet they are 6l6 metal, or the original ones were anyway.

Good point. Maybe nice big bulbous 6L6G's too. It would be nice to know what's the B+, but I wouldn't stress those old caps by surprising them with high voltage after they've been sleeping for decades. Could lead to a smoke test. One thing's a relief - this doesn't look like a back bias amp thank hevvins.

[Lloyd Fisher]

I can help with id.
The PE is the projector input, photo electric cell, from the audio edge of the film. The cable had a coaxial look and feel, if I remember right.
Pot 6 is on the input panel, and it's labeled P. E. Cell.

[Leo_Gnardo]

OK, that makes sense. And you can dispense with those components I mentioned. Likely you're not going to use the mic input either. The resistor between the input jack and first tube's grid can be changed to a standard "stopper" resistor, between 10K and 100K, best placed right on the tube socket. I'd go for the low end of the range. Input jack, change it to a shunt jack, Switchcraft 12A, so that the input's shorted when nothing's plugged in, as in many guitar amps.