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Old 4th October 2010, 02:43 AM   #1
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Default Another odd-ball PA amplifier

Model D-78621-A

I was given this amp by a good friend of mine. It's from a large retail store. It's built in Illinois by Automatic Electric. I haven't been able to find any information at all about this amplifier. I can find all sorts of info about Automatic Electric but nothing about audio amplifiers. They built telephones mainly. This could have been a PA amp or maybe an amplifier for some sort of internal telephone system. It has 3 large relays in it and 2 mic inputs up front. The mic inputs lead me to believe it's just a PA amp but the relays have me stumped.
It's in very good condition and probably worth a bit to someone who collects old telephone equipment but it's a nice parts find for me. 7 octal sockets and a power transformer anyways. There's 2 transformers that could be wired as single-ended output transformers. Good for a few watts maybe.

It's mono and contains the following tubes.

6SN7 x2 (sweet find but not matching)
6SJ7 x2
6J5 x1
6V6 x1
5Y3 x1
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Last edited by lazzer408; 4th October 2010 at 02:59 AM.
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Old 4th October 2010, 03:55 AM   #2
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It actually looks like a bit of telco equipment Some of the transformers in telco equipment can be high quality with nice thin laminations .
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Old 4th October 2010, 03:58 AM   #3
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Yikes! That's a rat's nest.
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Old 4th October 2010, 12:29 PM   #4
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Looks like some UTC output transformers, very sweet!
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Old 4th October 2010, 06:34 PM   #5
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Quote:
Originally Posted by raypalmer View Post
Yikes! That's a rat's nest.
You can say that again. All the wiring is the same shade of black.

Quote:
Originally Posted by SigloOne View Post
Looks like some UTC output transformers, very sweet!
I looked around in there for awhile and I don't think those are output transformers. One has 3 wires coming from it and the other has 5. I think the output transformer is that round thing in the middle in front. It's not a capacitor. It has posts on the bottom labeled 1-8 and seems to go to the terminals in back and wired to the 6V6.

What this does have is everything I'd need to make a small 5~10w practice amp. I have a bunch of 12AX7s from that National I've been playing with. Perfect for guitar.
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Old 4th October 2010, 07:21 PM   #6
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What is listed on the outputs?
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Old 4th October 2010, 07:36 PM   #7
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I'm only guessing but I believe that this has a 600 ohm output. It was probably designed to feed a power amplifier.
A single 6V6 won't produce enough power to be a PA amplifier. The little round thing at the front of the unit looks like a microphone matching transformer. It is certainly not an audio output transformer.
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Old 4th October 2010, 07:53 PM   #8
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It could be powering an intercom system or internal telephone system. In that case it would only need a few watts for small desktop speakers. There was a Shure in-line matching transformer connected to one of the mic inputs. I haven't powered it up yet but I will to try and figure out what does what on the 8 terminal output strip.

Oh also, the mic inputs go straight to the 6SJ7s. One each.

I've sent a couple emails to to some telephone collectors to see if they might know what it is.
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Old 6th October 2010, 03:25 AM   #9
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I sorted out some of the components. The two large transformers in the middle are one choke and one output transformer. The output it tapped at 500ohms and rated for 10w. Not much for a PA but fine for intercoms. The round transformer must be a matching/mixing transformer for the 2 mic inputs. Of the 3 large relays, the middle one has to be pressed to turn on the output. The front one partially mutes an input on the back but left the mic gains up. The third cuts the audio completely. There's an input on the back I'd guess for background music at speaker levels. I'm stumped on how the relays are activated.

Of the 8 terminals on back:
1 and 2 (audio in)
3 unknown
4 common for relay coils (ground?)
5&6 500ohm output
7&8 500ohm output (parallel with 5&6)

The 3rd pin could be for signaling the relays but I have no clue how it would signal all 3. Does anyone know how these systems worked? It's more for curiosity sake then anything.

I have a good power transformer, tubes, output transformer, and misc other parts and sockets. Today I found a newer Fender 8" practice amp at a music store for $40. It's modeled after the older Fender amps like the Princeton with numbered knobs and the gray/silver grill cloth. I think I'm going to gut it and make a mini tube practice amp out of the parts from this intercom/PA amp. I might look for an old Fender schematic to base it off of. Might be a fun project!
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Old 6th October 2010, 05:50 AM   #10
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3rd pin was DC in to activate the relays. Took awhile to trace. They are delay relays. One activated the other activated the other. Weird setup. The muting relay is voice activated when someone starts speaking on the mic. Interesting little amp... that is now gutted to serve my tinkering.
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