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Saturday, September 22, 2012


Most people don't use the standby switch on their amp correctly, at least in the way that I understand things. I wonder why these are still being put on amps. Yes, I understand the reasoning, but I haven't really seen tubes wearing out that much faster. Maybe it's a subtle difference.

The idea behind a standby switch is to give the tube heaters a chance to warm up to operating temperature before you apply the B+ voltage and the tube starts working. Failure to do so leads to cathode stripping (whatever that is), and is detrimental to tube life. What I've read, and believe enough to follow myself, is you should
  1. turn on the power switch
  2. wait at least 30-60 seconds
  3. turn on the standby switch, and make noise.
Powering down is the opposite:
  1. turn off the standby switch,
  2. wait at least 30-60 seconds,
  3. turn off the power switch
But where most people go wrong is the time in between those two events. At the end of the first set, don't use the standby switch. Leave the amp on. Unplug your guitar or turn the volume down at the end of a set, but leave the amp on the whole night. Component failures are most likely to happen at start up--caps are uncharged and there's a large current surge, things are cold and need to heat up, etc. Think about a light bulb. How many times have you seen one fail when it's been on a while? Most fail when you turn them on.

Leaving your amp on the whole night is easier on the amp because there's only one startup. After everything is warmed up and ready to go, it coasts along until the end of the night. If you're turning it off because it's getting warm and you don't want to cook the components, consider installing a small fan. Even a little air movement will help immensely. I've installed fans in my Fender amps because I don't like the idea of the downward hanging tubes cooking the components in the chassis above them. I think this is a drawback for combo amps, but then it can't be too big a deal because there are so many combo amps doing just fine. If you what to install one, you can run a 12 volt fan off the 6.3 volt heater supply. Sometimes you can hook it up directly, while other times you may need to add a diode and capacitor to give it a DC voltage. It will run slower than it was designed to, but if you can feel any air movement, it will move the heat out. Running it slower has the benefit of making it quieter.

If heat is a concern, you may have bigger problems. The tubes will get hot enough to burn you, but nothing else should be so hot that you can't touch it for a few seconds. Especially watch the transformers, as they are the most expensive items in the amp. Heat means too much current is being drawn, and you need to find out why. Sometimes it's as simple as a bias adjustment, other times it might be rusty laminations in the transformers causing eddy currents and robbing power.

Friday, September 7, 2012

Fender Pro Tube series Twin repair story

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