Need advice for a 93 Fender "The Twin"

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I'm looking for Ideas on for a 1993 Fender "The Twin" amp. It works fine except the reverb quit. Is there anyone in this forum that is familiar with the circuit for this amp. My basic question is this: There are 2 12AT7 tubes that appear (to an amateur) to be part of the reverb circuit. There is also a transformer for this circuit. Is this true? If it is true can you tell me how to test the transformer. One of the tubes looks to be barely lit.

My experience: obviously I'm not an electronics expert. However, I did replace the reverb pot on the board several years ago. I'm good at soldering and all the connections are a nice shiny silver, not a cold joint. And the reverb worked just fine. I've replaced my power tubes and properly re-biased the amp before. I understand that the capacitors are dangerous. And I'm ultra careful, never touching wires, or ever working on the amp when it's plugged in!

When using the amp, the reverb has 3 areas where there could be a problem. 1. There is a selector slider switch for channel 1 channel 2 or both. 2. There is the reverb level knob. 3. there is the reverb on/off on the footswitch. So, with the amp on and warmed up, channel selector tested in al positions, and reverb turned all the way up there is no reverb. RCA wires from the head to the spring unit are plugged in. There is a noticeable hiss and a bit of different sound but really no reverb. Footswitch is plugged in and reverb is on. When reverb level knob is turned to off, the hiss goes away and the amp gets quieter. I have the schematic for this amp but to me it's just a bunch of spaghetti. Now if there was someone who knew this circuit, and could tell me to just replace the tubes, or how to carefully test that transformer it would be greatly appreciated.

Beyond that the amp works just fine, I have a pedalboard that can simulate reverb but prefer to have everything on the amp working properly.


Hi Jim -- It sounds like the reverb drive tube may have died on you. There are two connections to the reverb tank -- one is the drive signal, one is the spring signal. The fact that you get more hiss when turning up the reverb control is a good thing, and indicates that the spring preamp is working.

1. Remove the reverb footswitch. If it is shorted out, you will get no reverb.

2. remove the two reverb RCA cables from the back of the amplifier. Remember which wire goes to which jack. If they are reversed, you won't harm anything, but the reverb won't work.

3. With the wires removed, advance the reverb control slightly. Try plugging in each reverb plug only part way so that the ground does not make connection. One of the plugs should cause a loud hum from the amplifier. If so, it proves the reverb preamp is working properly, and shows which jack is the spring input jack.

4. If the amp has a master volume control, turn it down, and connect a small speaker to the other spring jack. If you advance the channel gain control and provide an input to the amp, you should be able to softly hear the sound in the speaker. That proves the reverb drive signal is working. If not, try replacing the 12AT7 reverb driver tube as mentioned above.

5. Don't rule out the fact that the reverb tank itself went bad, or the cables to it. Use an ohm meter to check for continuity across the terminals of both both spring plugs. One will be very low (say 10 ohms or so), the other around 1000-2000 ohms. If either are open, you've found your problem.

This should get you started. Good luck!

I doubt the little transformer is bad, they can fail, but other things way more likely.

Remove the spring unit from the bag and unplug the cables. Now measure resistance across each little jack. Both should show continuity. The jack marked OUTPUT should be about 150-200 ohms or so, and the INPUT jack very low resistance. It will either be correct or open, it won;t be "wrong."

If either end shows open, then look at the tiny wires inside, usually green and black, are any broken off?

If those both check OK, then turn the amp on and set to play, with reverb control up midway. The two cables we pulled from the reverb pan jacks, touch the tip of each with a finger. One of them should make HUM out the speaker when touched. That is the one to plug into the OUTPUT jack on the pan. Make sure they were plugged in correctly.

If you did get the hum, then the recovery stage is working. And if the reverb pan is OK, you should get noise. With the pan conected again, and the amp running, rock the amp a bit to crash the reverb springs. Get a loud reverb crash noise?

The reverb also needs a drive side circuit. The 12AT7 V8 is the driver tube, and it forms a tiny power amp with that little transformer. If this circuit works, you can conect a small speaker to the drive cable and hear sound when you play. The cable to the INPUT jack of the pan now. The other 12AT7 is next to the end power tube, and is your phase inverter, V4.

So to recap, isolate the problem: the reverb pan itself (uncluding the cables), or the drive circuit, or the recovery circuit.

Dave, this is a "The Twin," the footswitch is one of those Fender one cord does it all things, not a reverb short like the old Twins.

Thanks for all the great input. I will look at these tomorrow and make a determination. The footswitch uses a stereo 1/4 inch TRS jack. One is to switch between channels and the other is to turn the reverb on and off. One test I did make was with the tubes. There are the 2 12AT7's . One lights up pretty bright the other almost not at all. When I switched them between the sockets it was the same. The one that lit brightly also lit brightly when plugged into the other socket. When the reverb initially quit, I replaced one of the 12AT7's because I tested it and it was dead. I didn't look at first but later saw that there were 2 of the little buggers. Unfortunately, my local music store has their tube tester at one of their other stores (about 40 miles away) and won't have it back until next week. Trust me, fixing this isn't life or death! I just want it to work properly.

The reason I mentioned the transformer is that this transformer has a bit of corrosion on it. whereas the rest of everything on the amp is almost pristine.

Although I have not performed any of the above mentioned tests yet. I can tell you that when the amp is jostled a bit you can definitely hear the springs through the amp.

I think that I will order up one tube anyway, they are only ten bucks, and then perform the above mentioned tests.

FYI: Yes, this amp is "THE TWIN" (red knobs) 100 watts 2 channels. Clean channel is Volume, Treble, Mid, Bass. Treble and mid switches pull out for boost. Channel 2 is Volume, Treble, Mid, Bass, Presence, and Gain. Treble, Mid, Bass and Presence pull out for boost. There are 4 inputs. High and Low for active pickups. The 2nd set is High and low but combines both channels. Actually a very versatile amp.

Thanks again.

Jim -- One easy way to test the dim 12AT7 tube that was in the reverb driver socket, is to swap the two 12AT7 tubes between their sockets, and note how that effects the operation of the amp. If the dim tube is truly bad, it will now likely affect the normal operation of the amp, since it is now working in the power amplifier section of the amp. If there is no significant change, and the reverb still does not work, then something else is still amiss, and the tube is not the problem.

The fact that you can hear the reverb springs bang around definitely makes this a reverb drive problem, and at least localizes the problem to that area and associated components.

new info

Hello again.

Here's what I did. I swapped the tubes. No difference in Amp operation, although one doesn't light as hot. I attribute that to one being a Fender 12AT7 and the other being a Mesa 12AT7 ECC. (just a guess there)

I pulled the reverb tank and touched one of the RCA leads to get the hum, and put it to the output jack on the tank. No change. I carefully checked all the small green and black wires on the input jacks on the tank. All seem to be very good. Put the footswitch on and tried switching between on and off with no success. Pulled the footswitch, no change. The tank and springs do make an echo when moved.

Or as far as checking Continuity and Open??? Ok now I'm getting out of my element here. I do have a good multi meter if you can tell me what to set it to and what to parts to touch with the leads and what readings I should get then I'm capable of that. I 'm only an electronics student only by necessity.

I am trying to include photo's here of the transformer in question and the tube socket for the 12AT7 as there is some green corrosion on that as well.

I did an attatchment of the photo's I'm new here so I don't know if both worked. If they don't post look me up on facebook. James Daubney, my profile pix is of a 7 year old kid. I'll load them there as well.


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Jim -- Set your meter for the lowest ohms scale. The ohms symbol look similar to a "n", being the Greek symbol for omega. Unplug the reverb pan cables from the amp and and place the probes across the pins of each plug -- one to the center pin, and one to the outside connector. It makes no difference which lead goes to which plug connector. One of these cables should read less than 20 ohms, while the other could read 200 ohms or more. This may require going up to the next higher scale to cause a reading to be indicated. As long as you are getting a reading anywhere around these numbers on the respective cable, (other than a completely shorted reading), the reverb pan and cables are good.

If your readings are good here, the next test is to connect a speaker to the reverb output jack on the amp, and see if a signal exists there.

Here's what I got. RCA Cables plugged into the reverb spring unit. Ohms has only one setting on Micronta 22-193 multi meter. one side was .20 the other side was 2.4. Then I plugged a powered monitor speaker (has RCA connectors and low impedance) into the RCA cables coming out of the amp. Reverb turned all the way up, wife listening on the speaker, nothing.

So, would that lead you to think the transformer? Or something else.
If transformer. I found one from an eBay retailer Triode Electronics pn# 40-10834
$16. From what I'm understanding here. (like following the flow of water I guess) the cables and tank are ok. The tubes are functioning, so unless it's something in the switching blocking the output, it would then me a power problem i.e. transformer? Hey I'm just guessing here, but looking at the condition of that transformer is what pointed me to that. Ok well what are your thoughts? Oh and if I haven't said it before, thank you so much for your guidance and expertise.
Jim -- here's a couple of quick easy things that can help clue us in on the reverb driver transformer's condition:

1. Use your ohm meter to see if you get any continuity at the jack on the amplifier that provides the drive signal to the reverb pan. Connect one probe to the chassis, and one to the inner portion of the jack. If your probes can't make a good connection to the center connection, just plug in a known good RCA cable to the jack and then make your measurement at the other end of the cable. You should get a reading that is virtually a dead short, because it is connected to the output winding of the driver transformer. If you do, that is good. It doesn't indicate so much that the winding is shorted, as it does continuity of the winding. I'm guessing this exercise will test out ok.

2. To test the primary of the transformer, take the shield off the the 12AT7 reverb driver tube. Now turn the amp on and let it run a while -- long enough that the tubes get good and hot. 15 minutes should be more than enough time. Now grasp the 12AT7 driver tube to determine it's temperature. It should be noticeably hot (can't keep fingers on it). If it is only warm (can hold onto with fingers), it likely indicates that the primary winding of the driver transformer has opened, and needs to be replaced. There is a resistor that could open inside the amplifier that is associated with this tube that would cause the same symptom -- but with the obviously poor storage environment this amplifier has endured at some point in it's life, my money is on the transformer for sure.

If the tube is getting hot, and you get a good reading of continuity at the drive output jack, then obviously something else has failed inside the amplifier, that will take a schematic and signal tracing to determine where the fault has occurred.

I don't want to lead you in over your head. At this point, you could have a bad transformer, but they are still down my list. I'd be inside looking to see if plate voltage was present on the reverb tube, and something other than zero on the cathodes.

Power off, plug an RCA cord into the reverb jack on the amp and measure resistance between tip and shield. One jack should read very low resistance, the other jack should read 220k ohms or so.

Borrow a set of patch cords from your stereo, and connect the reverb pan to the amp with them. ANy change?

Don't you just love technology. Simple reverb circuit and they have to make it complicated. Remind me not to put in channel witching in my amp.

From footswitch, through some IC's that light up some leds to change the resistance value of photoresistors, then there is the reverb switch and the channel switch on the amp to contend with. Oh what a wicked web we weave.
Tested the RCA's coming out of the head amp off. They tested basically the same numbers coming out of the spring unit 2.4 and .200. Held the reverb tube (not the phase inverter) and though it was lit and bright it wasn't very hot. I could have held onto it for several minutes. This was after about 15 minutes with the amp on, playing the guitar and power setting to high. I think I'll go ahead and order the transformer.
Another question. Is there any good product for cleaning up that green stuff and keeping this lubed and protected in the future? I'm comfortable with marking and removing the tubes, removing the head from the chassis and spraying something if its available. I have no problem removing and re-soldering the transformer, or a resistor (if I knew which one) But, big But... I don't want to mess with anything that could shock me. I know where the capacitor's are and won't touch them. I will also unplug the amp for a least a day before doing any service. Any other thoughts or "tricks of the trade" (I'm an offset printer by trade and part time musician) before I start?
That tube should have been quite hot after 15 minutes -- much too hot to hold onto, so it is a very good bet that the primary of the driver transformer has opened. Since the amplifier otherwise plays normally, it's also a safe assumption that the B+ for the reverb driver tube is available, and the tube itself has been eliminated, so a bad transformer is a likely suspect.

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