Got a Fender Professional Tube Twin for repair, the owner says it's lost a lot of its volume. Here's how the repair unfolded.
Most common cause of failure is the tubes. Checking the bias on the power tubes will show if they are all working (a dead tube doesn't draw any current), and also see if the amp is set up correctly. The amp is set up a little cool (460V plate, 26-30mA/tube), but this isn't terrible and isn't the cause of the problem. It's time to look inside.
This amp is a full-featured amp. Lots of settings and switches. 8 preamp tubes and 4 power tubes. Inside there are 4 circuit boards, 2 of which are mounted component side down so you can't see the parts. A schematic would be helpful, but I couldn't find one online so I started with the basics.
Checking the plate voltages on each tube is a good diagnostic test for the overall circuit. Pins 1 and 6 are the plates on 12A_7 tubes, and you don't need a schematic to find those. You should also get a pop as you touch the probe to the plate to indicate that the signal is going through the amp. If it doesn't pop, something is wrong downstream. Unfortunately (or fortunately), all the plate voltages were reasonable for a Fender amp.
This amp has two channels, plus a gain boost on one of the channels which probably adds in another tube stage. Since the low volume problem is happening on both the channels and with the gain boost, the problem has to be either after the preamp or in a shared tube stage. Often the first preamp tube stage is common to all the channels. Checking the first tube would just involve substituting a known good 12AX7 tube to see if the problem went away, but I decided to check the power amp first for no good reason--had to start somewhere.
This amp has Preamp Out and Power Amp In jacks. There are switches in these jacks that can oxidize, and it's possible that some resistance built up which could cause a volume drop. Usually this problem shows itself as variation in volume, or crackles, not a consistent volume drop. A quick check is to hook a guitar cable between these and get around the switches. If this brings the volume up, then some DeOxit will usually clean the switches. Doing this had no effect, so that wasn't the problem.
I have some experience with the plate resistors on the phase inverter of some newer Fender amps opening up. If one opens up, you lose half the power. This would have shown up as no voltage on the plate when I checked it earlier, but I still re-checked it. The failure on the amps I saw was due to what I think is a too-low power rating for the resistor. (As a side note, I've learned that if one of these resistors opens, BOTH plate resistors should be replaced while you're in there. It's usually just a matter of time before the other one goes.) But on this amp, the plates are 1 watt resistors which is a good design upgrade on Fender's part.
What's interesting on this amp is it seems that the two tube stages needed for a long-tailled phase inverter are split over two tubes. Normally one 12A_7 tube with its two tube stages is used. I wasn't sure what the other tube stage in each tube was doing, so I spent a little time trying to follow the circuit path with a continuity/resistance meter. In doing so, I found one resistor that looked fine but was open. In looking at the circuit, it appears that this tube stage is a cathode follower, and this is the cathode resistor. That made sense because the plate had voltage on it, but the signal is coming off the cathode, something that I hadn't checked earlier. This is where a schematic would have saved some time in identifying this.
Since the resistor was open, I bridged it with a good resistor and the volume came back. Looking at other Pro Tube schematics, this tube stage is possibly the recovery tube for the effects loop.
To repair it, I decided not to remove the circuit board to get at the solder traces underneath. Due to the complexity of the amp, this would have added time (raised the cost), and there is also some risk to flexing the ribbon cable connectors as you remove the circuit board. By clipping out the resistor, leaving as much of the leads intact as possible, you can form the leads into loops, and use these as sockets for the replacement resistor. The replacement resistor's leads wrap around the loops and provide a good mechanical connection. When soldered, the repair is very strong. I feel good about doing it this way on my amps, so I think it's the best way to go. Total billable time: 2 hour