Diagnosing OPT (Quad Reverb)

[92Volts]

Hi all,

I'm working on a very neglected Quad Reverb (think a Twin reverb with a 4x12 combo cab)... the amp appears to have been modified to AB763, had water damage, numerous bad tubes, etc when I got it. It would not surprise me for any or many different things to be wrong with it.

After fixing some other issues (preamp, PI, and reverb related) I decide to measure the OPT impedance because the speakers/cab appeared to total to 2 ohms when I got it, even though they appear to be original and the original spec was 4 ohms.

I put 107 volts across the plate leads with my isolation transformer (in the circuit but no tubes installed) and get 211mv out on the speaker leads, with the voltage at the speaker output slowly falling as I leave it hooked up for longer. Only load on the output is a 470ohm "just in case" resistor but it's not driving a speaker.

This voltage ratio would be 500 or impedance ratio of 250,000.

This pretty much needs to be a dead transformer, right? It's acting like there's some capacitive coupling between the primary and secondary but no solid transformer-like voltage relationship. 

The confusing things are:

It makes a decent amount of volume and the only reason I checked the transformer was to verify the impedance ratio.
Neither the secondary, or either half of the primary is an open circuit.
No measurable primary-to-secondary short (I know high voltage brings out shorts where multimeters don't, though).
The OPT is heating very slowly, barely noticeable to the touch, even with 107 volts across its primaries for 20 minutes.
No funny smells or smoke even with 107 volts across the primaries for 20 minutes

So if there's an internal short, where is the voltage/power going? If there's an internal open circuit why do I measure reasonable and consistent ohms across each winding? Each half of primary around 42 ohms (total 84 ohms), secondary under 1 ohm.

Thanks for any advice! Hope everybody is doing well and healthy.

[92Volts]

Figured it out -- I just had to post here first to look silly 

Added a resistor in series with the voltage I was putting on the primaries and it's drawing about 360ma. That's huge, when it should be very near zero.

It has an internal short, and the only reason it wasn't heating up quickly was it's a big ol' hunk of iron.

EDIT: Nope, it had an EXTERNAL short. Anybody familiar with these amps should know what I'm talking about. Can I delete this post out of embarrassment?

[sluckey]

You should be able to delete your own posts.

[shooter]

Can I delete this post out of embarrassment

I wear ALL mine proudly 

[PRR]

> it had an EXTERNAL short. Anybody familiar with these amps should know what I'm talking about.

Is this a guessing game? Spill the beans. Folks come here to learn.

[DummyLoad]

> it had an EXTERNAL short. Anybody familiar with these amps should know what I'm talking about.

Is this a guessing game? Spill the beans. Folks come here to learn.

guessing he had his speaker or dummy load connected to the jack labeled external speaker and not the internal - the internal speaker jack is a tip switched wired ground shorting jack when unplugged.


--pete

[92Volts]

DummyLoad’s got it, and I suppose I should mention it— this amp has a shorting output jack. If you have no dummy load at all connected (my “safety resistor” was inside the chassis) it shorts the output to avoid the risk of running without a speaker connected.

Of course you have the other risk of running into a short which could have damaged the OPT doing what I did, but I got lucky and it was fine.