Help me identify this thing

[fred.violleau]

Hey guys,

Picked this on the local adds.
The guy selling it said it came form a jukebox and sported 6BQ5... Would love to reuse these and build a small stereo amp? Or should I keep the three way output? Would definitely make an interesting guitar amp.

I've been trying to identify this, but without success...
The stamp on the chassis seems to date the amp to Nov 6 1965. That would correlate with the blue molded caps era.

Sticker from California regulations indicates that this thing has travelled as I am located in Montreal, Quebec, Canada.

Using a tube rectifier, 3x A_7 and 4x 6BQ5?
Looks like there are three output transformers. And could not find any reference to these numbers on the web about their specs.
I did a quick check on the voltages, looks like everything is working, but since one of the tube socket is completely destroyed, I stopped my investigation there.

Will try to trace a schematic in the coming day, but if someone has some info, that would help me figure out what are the possibilities with this.

Thanks in advance!

Fred.

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[maxkracht]

Is the center transformer sharing one 6bq5 from each side?

[Stevem]

Looks like the big Ot is driven push pull from the center two most BQ5s, the other outer ones are driving the other two smaller OTs single ended class A.

The two brown 5 watt resistors are the cathode resistors for the class A 6BQ5s.

By the way Rock-ola’s where made in California.

[maxkracht]

Looks like the big Ot is driven push pull from the center two most BQ5s, the other outer ones are driving the other two smaller OTs single ended class A.

The two brown 5 watt resistors are the cathode resistors for the class A 6BQ5s.

Two SE and one PP would make way more sense, but the cathodes are tied together in pairs connecting to those resistors and It looks like there's a brownish wire and a blue wire coming out of each smaller transformer going to 6bq5 plates. Could a SE and PP amp share cathode resistors? I can't see enough from the picture, so I should stop guessing...

[fred.violleau]

Finally founds some time to draw the layout, next is schematic

jukebox.png

if anyone what this power amp is or where it is coming from, let me know!

JUKEBOX.pdf

Thanks for your help;

F.

[nuke]

It's an amp that can run two channel stereo, or one-channel mono at twice the power of a single channel in stereo.

The middle 12A...7 is wire about the same as the other two, a gain stage and a concertina phase inverter. But its output is sent to a pair of tubes on each phase, rather than just one, like the left and right side 12A...7 tubes.

So you can get mono or stereo. Mono being sort of akin to bridged mode on a modern solid state amplifier.

Should be a nice guitar amp platform, but I'd probably single-purpose it as either a 30 watt mono amp, or a 15 watt per channel stereo amp.

[statorvane]

Stereo amp (2 x 6BQ5) with center channel woofer / subwoofer (2 x 6BQ5)?

[fred.violleau]

It's an amp that can run two channel stereo, or one-channel mono at twice the power of a single channel in stereo.

The middle 12A...7 is wire about the same as the other two, a gain stage and a concertina phase inverter. But its output is sent to a pair of tubes on each phase, rather than just one, like the left and right side 12A...7 tubes.

So you can get mono or stereo. Mono being sort of akin to bridged mode on a modern solid state amplifier.

Should be a nice guitar amp platform, but I'd probably single-purpose it as either a 30 watt mono amp, or a 15 watt per channel stereo amp.

Hey nuke,

Thanks for your help;

I wanted to repurpose this into a stereo british power amp (15 watts per channel) powered by a Vox AC30 PT.
The actual PT looks way too big and seemed to power other things because of other connectors that would feed another circuit ?)..
Concertina PI means a 6BQ5 PP setup if I undertand correctly;
The big on seems setup as a PP as well but only on one of each 6BQ5 pair when they would already receive a signal from the stereo inputs ?
Or maybe you're not supposed to use both at the same time, it is either mono (and usind 1 6BQ5 on each side), OR stereo and using both pairs ?

And what is the purpose of the caps on each side of the Big OT that then feed the Center Tap of the smaller OTs.

Again thanks for your help;

F.

[fred.violleau]

Stereo amp (2 x 6BQ5) with center channel woofer / subwoofer (2 x 6BQ5)?

Hey Statorvane,

That is what I thought, but wouldn't that send too much signal on these two tube that receives the stereo input gain stages ?

Thanks for your opinion;

F.

[nuke]

Hey nuke,

Thanks for your help;

I wanted to repurpose this into a stereo british power amp (15 watts per channel) powered by a Vox AC30 PT.
The actual PT looks way too big and seemed to power other things because of other connectors that would feed another circuit ?)..
Concertina PI means a 6BQ5 PP setup if I undertand correctly;
The big on seems setup as a PP as well but only on one of each 6BQ5 pair when they would already receive a signal from the stereo inputs ?
Or maybe you're not supposed to use both at the same time, it is either mono (and usind 1 6BQ5 on each side), OR stereo and using both pairs ?

And what is the purpose of the caps on each side of the Big OT that then feed the Center Tap of the smaller OTs.

Again thanks for your help;

F.

It would make a great low power stereo amp! In stereo, you'd get about 12-18 watts per channel. In that case, you can delete the large output transformer and the center 12A...7 tube.

A "concertina" phase splitter is also called a "cathodyne phase splitter". It allows for inverting and splitting the phase for push-pull output with only a single triode stage. (1/2 of a 12AX7 for example). You'll find these in lower power amps, especially 6BQ5/EL84 push-pull amps, as they don't need much drive voltage to achieve full power.

There's a good reference page here:

http://www.valvewizard.co.uk/cathodyne.html




This is opposed to the "long-tail phase inverter" you'll find in things like most Fender guitar amps from the 60's onward. The LTP has advantages in providing greater gain and better linearity.

Your amp is most likely a juke box amp chassis or some-similar device. Back in the mid 1960's, subwoofers as we know them today were not at all common. However, *mono* was very common. It appears to be designed to allow functioning as either a 2-channel stereo amp, or as a single-channel mono amp. In mono, it would provide the same total output power into a single speaker channel. You get your choice of either mode of operation. It would do some interesting things if you drove all three inputs at once, but I don't think you'd get a pleasant sound out of it.

This works a lot like modern two-channel solid state amps for bass guitar and PA that can operate in "bridged" mode.

The 1µf 400vdc caps from the screen supply nodes, I'll have to think about a little. But it appears to be there help stabilize operation.

I tend to think they may have intended the speaker connections not used to be jumpered from common to one of the taps.