Fender Reverb Head Scratcher

[Alex R]

Did you try following the signal through the driver tube grids/plates and then through the transformer to the transducer? I mean with a scope, or failing that an audio probe? You'd find out where it stops that way.

[Alex R]

Oh also those UL twins sometimes have a plate-to-ground cap on the reverb driver tube, might be worth lifting that just to eliminate it, it will work fine without it, probably better.

[tubeswell]

I see now. I presume it works okay on the dry side?

A couple of thoughts occur to me

It could be that you just ain't driving the pan hard enough. Have you tried putting in a 12AT7 driver tube? A 12AX7 (even in parallel) isn't going to pass very much current, and you want as much current as you can going into the RT so that you get even more out the other side to drive the pan. Besides that, the 500pF coupling cap really suits a 12AT7 driver's BW, and it might not be passing a wide enough BW for a 12AX7. (Also, use a 2k2-4k7 cathode resistor if using a 12AT7)

And maybe try putting the RT supply in as-high-a-voltage situation as the amp can provide (say closer to 400 if that's possible?)

[jcollins]

I'll do that next.

[Rhodesplyr]

Looking back over your first post, I just saw the bit where you say you've only got 16V across the RT primary. You need about 400ish+

And you need to learn how to subtract 395V (12AT7 plate voltage per the schematic) from 410V (supply voltage from the schematic). Here in the Northern Hemisphere, that gets me 15 Volts.

You also advised using a 12AT7 instead of a 12AX7, though I don't see anywhere in this thread that an AX7 is being used as the reverb driver tube. The stock tube is an AT7.

Maybe I'm old-fashioned, but I think that you might want to know what you're talking about before offering advice.

[txstrat]

...that gets me 15 Volts.
Maybe I'm old-fashioned, but I think that you might want to know what you're talking about before offering advice.

There's talk of 16v in the first post. Nothing about subtract 395 from 410.
As far as I'm concerned I know that tubeswell knows what he's talking about. Trying to help in the first place.
And we don't want to offend anyone, do we?

[tubeswell]

And you need to learn how to subtract 395V (12AT7 plate voltage per the schematic) from 410V (supply voltage from the schematic). Here in the Northern Hemisphere, that gets me 15 Volts.

You also advised using a 12AT7 instead of a 12AX7, though I don't see anywhere in this thread that an AX7 is being used as the reverb driver tube. The stock tube is an AT7.

Maybe I'm old-fashioned, but I think that you might want to know what you're talking about before offering advice.

Sorry Rhodesplyr - the schematic in the link that was posted (which I just realised was the wrong one! (gulp). http://music-electronics-forum.com/a...tic_reverb.jpg) showed a 12AX7 driver, and the plate voltage at about 300, so with a 16 V drop across the RT primary that would make 316, hence my comments. Otherwise you are right about 'what I am talking about?'. I'm taking a holiday.

[Rhodesplyr]

There's talk of 16v in the first post. Nothing about subtract 395 from 410.
As far as I'm concerned I know that tubeswell knows what he's talking about. Trying to help in the first place.
And we don't want to offend anyone, do we?

The schematic for a 135 Watt Twin Reverb shows 15VDC across the primary of the Reverb Drive Transformer--a difference between the 410V supply voltage and the 395V at the 12AT7 plates. And, yes, the first post mentioned a 16V drop there, so I'd say that that's not the problem. Tubeswell said that you should see about 400V across the Reverb Drive Transformer primary, and that's simply wrong. Then he suggested putting a 12AT7 in a circuit that already has a 12AT7.

How do you help someone if you can't even take the time to look at the circuit being discussed?

The solution, as Alex mentioned, is to get out the scope and trace the signal to see where it's being interrupted.

A link to the 135 Watt Twin Reverb:

http://www.schematicheaven.com/fende..._135_schem.pdf

[madkatb]

Maybe it's shorted?

[txstrat]

How do you help someone if you can't even take the time to look at the circuit being discussed?

It wasn't the content but more the chosen words that made me jump in. I myself would have asked if somebody had a look at the right schem, instead of what you wrote.
Maybe I'm old-fashioned too, and mabye too sensitive.

OK. We need 400ish on the RT, a grid leak resistor of 1M, a cathode resistor of 680 ohms. I would also check if the plate to cathode cap is OK (not shorted) That should make the tube amplify the signal.

[Enzo]

Hey, relax folks. Everyone has a bad day now and then. It is one thing to scold someone for a mistake, it is another thing to scold them in general over how they do things. Making a mistake is not the same thing as plowing ahead in ignorance. COnsider one's overall posting track record.

[Rhodesplyr]

My apologies if I was too harsh. I was just trying to make sure that jcollins didn't get sent in the wrong direction with his diagnosis.

[tubeswell]

Never mind. I deserved what I got leaping in where no mouth has dared to tread.

[Alex R]

Well I read that little slip and passed over it because, I guess like you tubeswell, I spend my days measuring voltages 'across' things with my neg lead clipped to the chassis, going, yep, 400 on the reverb tranny, that's ok... and I'm sure that you know every bit as well as I do what voltage drop across a transformer is. 2,909 posts and 2,908 good ones ain't a bad score.

[jrfrond]

BTW- a little not-so-novel trick for troubleshooting reverb driver circuits without a pan is to plug the drive signal into a 8Ω speaker so that you can hear what's going on directly. Only works with transformer-coupled circuits like these, as they are nothing more than a Class A power amp. Once you do that, you can lift the 500pF coupling cap at the input to the grid and completely isolate the circuit. Then you can inject signal into the grid and troubleshoot. If you have subbed a known good transformer and tube, that leaves a shorted grid resistor, shorted rolloff cap or open cathode resistor OR an open ground. Perhaps even a bad tube socket.

You can go high-tech if it makes you feel better, but after a check of the tube socket, I'd just replace the two caps and two resistors comprising the circuit.

You didn't mention what test equipment you have, but a scope and audio generator would be really handy here, or at least a generator and signal tracer.