Last year, I picked up a new Fender Reissue '65 Twin Reverb for a gig that I started with a band. I was looking for that classic high-power, clean, make your ears bleed and empty out your sinus cavity Fender sound. The twin was the ticket. It served me well down to Nashville and back - I'm sure my chiropractor loved my decision as well because that beast is heavy.
Anyway, a few months ago, I discovered when I turned up the reverb control past 4 or so, I would get a high pitched whine. I thought that it was funny but I chalked it up to a problem with tubes or whatever. However, shortly after, I put the amp away and began playing with some of the other amps I own. It was only recently I decided to put it up on the bench and figure out what was wrong.
In the course of troubleshooting, I went through a few different methods beginning with the easy and ending with the slightly more difficult, but still easy.
I first checked the tubes. One of the problems that can often appear in a tube amplifier is a microphonic tube. This is a condition where the tube actually becomes a sympathetic input source based on external vibrations. In simple terms, it's a case where the tube vibrates because of something happening (noise from the guitar playing through the speakers, cabinet gets bumped, vibration in the floor, etc.) and the tube begins to inject its own noise into the circuit. This can often be a bad thing if it gets out of control, but can also be manageable if it's not too bad. The easiest way to check for a microphonic tube is to tap it with a pencil lightly while the amp is on and running. It's pretty easy to tell if something is amiss.
Happily, the tube was ok. In fact, all the tubes were fine in this regard. So I moved on to a different test.
The Fender Twin Reissue is loaded from the factory with 4 - 6L6 power tubes, 4 - 12AX7 preamp tubes, and 2 - 12AT7 preamp tubes. The 2 12AT7 tubes are used in V3, which is the Reverb driver / send position and in V6, which is the Phase Inverter. A quick test I also performed was to swap the AT7 from V3 with the one in V6. The logic is that if there was something else wrong with the tube, it would have changed the behavior of the amp.
After doing this, the reverb still whined in the exact same way. So, I moved off of playing with tubes and tried the next thing before cracking the chassis and fiddling with scopes and meters and pc boards and all that fun stuff. I unplugged the reverb tank from the chassis. Problem stopped. The new problem is that the Reverb also stopped, so while it stopped the symptom, it wasn't a cure.
While there are other things going on that I will not go into in a simple blog post, here is the troubleshooting chain of events. I played with tubes that send the signal to the tank, which didn't do anything. Next, I moved to the tank itself. After that, we have to get into other components such as the reverb signal return (V4) and the controls, transformer, and everything else that deals with the circuit itself.
For those of you who haven't seen the workings of a reverb tank, it's a pretty simple device. It basically gets a signal from the amp on one side, runs the signal through two springs (or more), which vibrate and modify the signal, and then return the modified signal back to the amp to run through the rest of the preamp and power amp and then on to the speakers for the enjoyment of the listener. Similarly to the tubes, the reverb tank can become microphonic and vibrate sympathetically with external vibrations is it does not have enough isolation from the cabinet.
I pulled the reverb tank from the cabinet and set it down outside the tolex bag on the bottom of the cabinet. This helped. The control got up to around 7 before the whine started. This was definitely a step in the right direction. I then suspended the reverb tank above the cabinet using shock absorbing material. Cranked the reverb to 10 and voila - no runaway whine. Problem isolated and solved.
In the lean six-sigma world of Fender manufacturing, although they sell a reissue of a 1965 amplifier, it's fair to say that any resemblance to the original is simply cosmetic. It has a black faceplate and a clone of an antique circuit, but it's heart is truly one of manufacturing efficiency, cost per unit, and maximized profit. As such, its padding to isolate the reverb tank from the cabinet is a very simple piece of corrugated cardboard. Over time, with travel, changes in temperature and humidity, and use the cardboard stiffens and loses its ability to buffer the external vibrations.
I've seen some people recommend placing the tank on a soft towel. It accomplishes the same thing, which is isolating the tank from external vibration, but I chose to handle it slightly different. I used two pieces of foam provided to me by two dollar store foam paint brushes. I installed these at both ends of the tank by attaching them to the original cardboard. The advantage is that the foam does not have the potential to interfere with the springs if the amp gets jostled or otherwise moved aggressively, as often happens when traveling.
I reassembled everything by setting the tank on its isolation board and sliding it back into the tolex bag. Probably the most difficult part of this operation was finding the screw holes under the tolex bag. I then played it and made sure the problem didn't reappear. Everything checked out so I reinstalled the back panels and once again have a happy Fender Twin Reissue.
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