1950's Moviola Amp Load Mismatching ~ Power Transformer?

This 1950's Moviola "Squawk Box" amplifier has a headphone output that can be used to power a (x2) 12" cabinet but I popped it into a single speaker cab with what I thought was the correct ohms rating and cranked the volume. It worked for a minute, now only the light comes on with no audio output whatsoever, any thoughts guys?


:-[

Tube fried? Output transformer Fried?

Tube fried? Output transformer Fried?

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Thanks, that's what I'm thinking. I have a cheaper old "quick" tube tester, I'll check that out. I have a multimeter, how could I safely test the power transformer? Total newb here.

I did a quick check and all the tubes seem to test good, but it says it only checks if the filament is good, so I guess that leaves the transformer

put an ohmmeter on the primary of the output.

As a matter of fact, I fired it up and as it turns out, there IS a faint audio signal passing. I plugged my guitar into it and I when I tried cranking it, the very weak signal started to distort, so I'm thinking that the preamp section is good.

put an ohmmeter on the primary of the output

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You mean, on the headphone/ext-cab output or the speaker wire? What would it tell me?

is that a normal speakeror a powered magnet speaker?

if the output transformer is blown you still might hear a faint something. IS there even an output transformer? Do you have a schematic?

Is that a normal speaker or a powered magnet speaker?

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I do not believe that it is a powered magnet but the only thing I know for sure is that it's an Alnico speaker.

If the output transformer is blown you still might hear a faint something. IS there even an output transformer? Do you have a schematic?

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Thanks for your help, there is a very faint signal, it's VERY faint. I'm assuming that the only transformer in there is a power transformer for the output, but I'm a much better audio engineer than electronics technician, so perhaps you could tell me,,

T1 is the pwr trans, T2 is the output,

to check the output, unplug the pwr cord, let the caps drain a few minutes,  simply remove the 6aq5 and measure ohms from pins 3 to 8
s/b around 300 ohms, if shorted, could be the cap across the pri,
screen to plate.
same circuit as a fender stand alone reverb almost,

6aq5 is a nice tube that more people should use, very cheap, if you use 8 of them in push pull you would have a cool amp, kind of a poor man;s ef86,

CJ said:

6aq5 is a nice tube that more people should use, very cheap, if you use 8 of them in push pull you would have a cool amp, kind of a poor man;s ef86,

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That's funny CJ, I was thinking the other day about how many 12AX7s it would take in PPP to make a useable power amp 

OK there is an output transformer 8) I wasn't sure because its hard to tell in the 1st picture of the unit, only looks like there is one transformer. Yea chances are the output transformer might be blown, if it was drawing too much power to power the other speaker, I know the older tube amps can be really touchy when it comes to loads. goodluck!

PS good news it replacements are probably pretty cheap.

I would bet the wax coupling caps are shot.

The DC voltages are on the plan. (Grids are all near-Zero.) Go through and see what's wrong. Any non-Zero grid voltage means the coupling cap to it leaks; replace cap. Replace all just-because.

Electros are probably at end of life. Even if they work today they may fail soon.

50:50 chance all tubes are *fine*. 12AX7 is the most popular tube on the planet, EZ to replace. 6AQ5 is a little 6V6 and not in production but once popular and surely still sold at good price?

Power output 3 Watts. OT primary 5K (3K-7K will be fine; a Champ OT is readily available albeit twice as chunky as you need).

Thank you SOOO much guys, you guys are always awesome on here. 

to check the output, unplug the pwr cord, let the caps drain a few minutes,  simply remove the 6aq5 and measure ohms from pins 3 to 8, s/b around 300 ohms, if shorted, could be the cap across the pri,

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That tube is only showing 7 pins. You mean, put one lead from the multimeter on the tube-socket pin 3, and the other on the last pin, (7), while the tube is removed? I did that on pin 7 and got a read of 1.7M, probably way too much.

I did check all the caps & resistors on this thing and they all seem to check out, plus I noticed that the "headphone jack" on the back is clearly connected to the OT.  Please excuse my newbiness, thanks all for the help!

Looking at the datasheet (6AQ5), measuring resistance between pins 5 and 6 should check for primary continuity.
amp off for several minutes, 6aq5 out, of course.
Laurent.

6, 8, they all  look the same,  ???

yes, you got it,


maybe if i used my own tube dat link that is in my signature,