• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Jukebox Iron

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mosfetmaniac

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Joined 2009
#1
As a christmas holiday project I'm thinking of building a PP el84 based amp.
I have in the shed a chasis from a Rock-ola 429 jukebox, which I got working last weekend and I was thinking of stripping the chassis for the OPT's to use in my el84 project.

Does anyone know what the specs the 429 ( chassis no.41056-A) transformers are?
They have 2,4,8 and 16 ohm speaker taps as well as a 70volt line tap.

Without pulling them out of the chassis i'm after the primary impedance, freq response etc.. if anyone knows?

Cheers.
 
#2
My advice would be pull the transformers and check manufacturer and model number. If you're lucky, they'll be A&R or Ferguson etc, as their specs are easy to find. Also, you could check what tubes go into it and have a rough shot.
 
#4
mosfetmaniac said:
Do you think there could be a label underneath the transformers, there's certainly nothing on the sides or the top.
Theyre driven by EL84's.
Click to expand...

If there's nothing on the top or sides, you may be out of luck in that respect. That said, worth a look. Uhh off the top of my head, I think EL84's in push pull would be around 8K to 10K? You'd have to check some specs on EL84 amps to make sure. Also, just to make sure, do some serious internet trawling to make sure you can find info on the amp. Getting deep in the web can reveal some great stuff.

PS, just realised you never said whether it was Puch Pull or Single Ended (if you don't know what that means, check how many tubes go into each transformer.
 
#7
Look at it this way, the amp the transformers are in is based upon pairs of EL84s, so it would seem an appropriate use would be in an EL84 based design. Juke boxes generally sound pretty good, so the freq response would be more or less hifi. Even though they often were connected to strings of 8" cieling speakers through a 70v system, the Rockola people and other brands really did try to make a nice sounding box. And certainly you'd have no trouble matching any load you can come up with with all those secondary taps.
 
#9
Hah, what are you like at painting? 😛
For some stupid reason I just ordered the timber for a 30 metre T&G fence to do as well.. along with the stairs for an attic and the paint for the lounge...

Nothing like a good long relaxing Christmas holiday 🙂
 
#10
Use a variac (or the mains fused in a pinch) across the plate winding. The ratio of primary secondary is the turns ratio. Turns ratio squared * 8 will give you the approximate plate-to-plate impedance @ 50/60hz with an 8-ohm speaker on the 8-ohm tap.
 
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