Why do all my valve amps keep breaking?

January 5, 2011 5:54 PM

Why do all my valve amps keep breaking?

I have had a few different heads in the last year or so and they all seem to suffer the same problem eventually.

One of my current heads (Peavey XXX) has just started doing the same as the ones before which is where the amp turns on fine, works fine, even sounds fine...it's just not loud enough. Even if I turn it to volume level 10 (which should be enough to deafen me and knock me over in my small room) it still sounds like it's on volume level 1.

This happened with my 5150 in the summer and the guy that repairs my equipment said it was a power supply problem and fixed it. It also happened the XXX previously and he told me it was a faulty pre amp valve - however i'm not sure if it was ever fixed from before as I didn't have a chance to test it at a very loud volume until this weekend.

What am I doing wrong here? I did notice that I occasionally don't use the correct speaker lead and might use an instrument cable if I can't find my speaker lead. Would this be enough to cause the same problem? Could it be to do with transporting the heads or the temperature?
I do know I need to warm the head up and put it on standby after use etc. Is there any basic tests I can do to find the cause of the current problem?

Any suggestions would be helpful, I can't keep taking it back!

posted by rinsemedown to sports, hobbies, & recreation (7 answers total)

Do the speaker leads and the instrument cables have the same connectors??

But, this is why they invented transistors. Tubes suck.

It absolutely could be the jostling, and it could be the effects of a number of temperature shocks. It probably isn't the cable, though, I think it would get hot before it would wreck the amp. Or at least hit some kind of internal limiter. (If it is new enough to have that circuitry.) Could also be your repair guy not using the right kind of caps in his repairs and they blow out again.
posted by gjc at 6:00 PM on January 5, 2011

Are the amp and the preamp sections separate? Can you disable one of them and feed a different source through the power side?

And/or hook up a meter to the preamp side? If I'm not mistaken, the preamp output is line level, so even a volt meter could act as a vu meter, I think. You'd have to check annd see what the line level voltages are- I think it is a couple of volts. If it is much lower than that, it is the preamp.
posted by gjc at 6:04 PM on January 5, 2011

If you stack your tube amps on your speaker cabinets, or use combo amp/cabinets, the vibration generated by the speakers can be detrimental to the life and performance of your amp. Vacuum tubes (valves) are physical things; their inner metal parts and connections can be damaged by vibration, and their working life substantially shortened. Moreover, some tube amp designs remain "hand wired," meaning there are essentially no supporting circuit board for mounting electronic components. Resistors, capacitors, chokes and diodes are often just soldered between pins on vacuum tube sockets, and other vacuum tube sockets, or sometimes, to short strips of standoff junction strips, depending on circuit design. Finally, the high voltage elements of your power supply, especially output tubes (valves), power supply components like capacitors, and transformers, are degraded by heat, and sometimes, just ensuring that the amp chassis is dust free, and that there is clear air circulation goes a long way to keep things working. Sometimes, a gentle fan can move enough air to keep heat buildup to a minimum, but tubes (valves) do like a reasonably warm operating temperature, for nominal performance.

Some tube types are available in "ruggedized" or "military" versions. Generally, these will have metal enclosures, rather than glass, and some different designs of internal plates and grid elements, often with mechanical di-electrics as insulating and isolating elements, that their "commercial" glass tube variants don't. Often, you can just pay more for the ruggedized version of the tube (valve) and plug it right in, with improved results, but other times, the ruggedized version requires a different tube socket, or a retainer to work as intended.

As for substituting instrument cables for speaker leads, generally, this wouldn't damage your amp, or speakers, but, because the wire guage for some instrument cables is pretty light, the additional impedance could limit the power your speakers get, creating distortion and low output from your speakers.
posted by paulsc at 6:22 PM on January 5, 2011

Vacuum tubes (valves) are physical things; their inner metal parts and connections can be damaged by vibration, and their working life substantially shortened.

Like light-bulb filaments, they are much more fragile when they are hot. Wacking the cabinet with a spoon could be enough to damage a hot valve, it would do nothing if the unit is powered down.
posted by StickyCarpet at 8:56 PM on January 5, 2011

In the case of the 5150, which normally uses 6LGs, you can try substituting 5881s. The 5881 is the military version of the 6L6 with the same pinout but slightly different specs so expect some changes in the sound. I've also heard people talk about the 7581 but they appear to be a military version of the KT-66 which may not be as simple a swap as the 5881.
posted by tommasz at 6:24 AM on January 6, 2011

Oh, and for the 12AX7 try a 6681, which is intended for mobile use and more resistant to vibration and filament voltage fluctuations.
posted by tommasz at 6:28 AM on January 6, 2011

Test equipment time, amigo.

Get a cheap signal generator and a decent DVM. Get a 100 Watt 10 ohm resistor.

Resolve the connections of all this so that the amp input gets the output from the signal generator and the 100 Watt resistor is wired to the speaker output. This means go get jumper wires (Radio Shack) and appropriate connectors, as needed.

Using the AC Volts measurement on the meter, measure the input signal voltage to the amp and see that you have a 10 or 20 mV signal... (.010 to .020 V ac).

Turn the amp on and let it warm up. Set the volume to 1.

Measure the AC voltage across the 100W resistor. Note it.

Change the volume to 2. Verify that the voltage on the output goes up and down with the volume control adjusted to different numbers.

If it's working OK, then you can blame your speakers, cable, or signal source (guitar?). If it's not working OK, then you can blame the amp.

Verifying the parts in isolation will allow you to troubleshoot the problem. If you note the readings you get, the next time you have a problem, you'll be able to compare the readings you got the first time and see if the amp has changed or something else has.

Audio generator recommendations - HP 204C
DVM recommendations Amprobe AM-160
100 W resistor recommendation Digikey p/n TEH100M10R0JE-ND


The DVM is pricey, but it's my current favorite. You can use it for the next 50 years. A veritable suite of test capabilities, but if you are strapped for cash, there are lots of alternatives. See Radio Shack.

Point is.... you've got to get numbers and get away from 'subjective'. Once you find what the problem is, you can experiment with changes in your usage pattern to prevent recurrence.
posted by FauxScot at 7:29 AM on January 6, 2011