I have a Peavey Windsor running Ruby EL34B-STR's. One of the power tubes went bad.

I can order another Ruby EL34BSTR and re-bias the amp (idk if it has a bias for each tube or not) and I assume it'll be fine. However, everyone's talking about matching pairs. I understand that if I end up getting a tube that doesn't match the pc and tc of the other 3 tubes, the bias and breakup will be completely off. But I can't seem to order based on these figures.

So if that's so important, does that mean you need to buy a matched quartet every time a single power tube goes out?

4 Answers 4


To oversimplify slightly, assume that matched tubes all have the same "bias point". Meaning, if you have unmatched tubes, the bias adjustment that works for one of them might not work very well at all for the rest. If you have matched tubes, they are close enough that one bias point will work for all four.

You only have one bias adjustment dial on that amp, so you can't even adjust it in pairs to accommodate two different matched pairs.

Besides all of that, either the tube was really old when it failed or it was maybe less than two years old. If it was that young, then there might be something wrong with your bias control, the amp, or maybe they are really cheap tubes. Power tubes should last a couple years without failing. If the tube was really old, then consider changing your tubes more often. Tubes don't sound as good when they get old. And they definitely age. Just having the tube heaters warmed up causes the tube to slowly wear down as electrons are slowly boiled off the cathode (the cloud of electrons is what makes the tube work). Consider changing your tubes once a year or so if you play frequently.

If you've tried other brands and you prefer Ruby tubes, then go ahead and stick with them. If you've never used another brand, you may be surprised at how much better your amp can sound with a nice set of tubes. The Ruby (actually Shuguang) tubes are rated four stars at The Tube Store but there are other tubes rated higher for not much more money, and the JJ EL34L is rated higher at the same price.

When you get a new matched quad, you probably will need to at least check the bias on your amp, or have it checked. Your amp does have a bias test point, which allows you to check the voltage between the test point and ground with a multimeter. One source indicates the correct bias voltage for EL-34s is 44 volts. You can more accurately set the bias point for the exact tubes you have by using a bias probe, like the Weber Bias Rite. There is at least one handy online bias point calculator based on plate voltage. Looks like a Peavey Windsor has a nominal plate voltage of 470 Volts. So with a bias probe you might go for 26 - 37 mA bias current depending on how you like it to sound. I've read that Peavey amps sound a lot better run hot, so you might go closer to 37 mA.

Since you're asking this question, my assumption is you haven't done a lot of work on amps. If that's true, I recommend you get your tubes replaced and bias checked by a pro. If you insist on DIY, please read the following:

  • There are dangerous voltages inside your amp!! Use extreme caution!
  • Obtain a non-conducting tool or set of tools to adjust the bias pot in your amp.
  • Keep one hand in your pocket or behind your back whenever your amp chassis is opened up and especially when poking around.
  • Note that in order to keep one hand in your pocket, if you use the voltage bias test point you'll have to clamp the ground lead of your tester to the chassis somehow. Do this with the amp unplugged.
  • Also note that dangerous voltages can remain in the amp (in charged up capacitors) even when it is unplugged from the wall. Always assume the amp is live.
  • A bias probe is the easier, safer, and more accurate way to go, since you can look at the display as you turn the bias pot with one hand while your other hand is behind your back, and you don't have to put a test lead inside the amp chassis to check the voltage. Note that the tubes and the bias probe may get very hot. Turn the amp off and let everything cool before disconnecting tubes and/or a bias probe.

Assuming the amp originally contained a matched quartet, you can get away with a matched pair, and install both of them with the following setup in mind:

The four output tubes in your Windsor are used in pairs to achieve the combined amplification (schematics here).

Your tubes are placed in row A,B,C,D (thats V4-V7) and A,B is amplifying the positive output and C,D is amplifying the negative output.

The matching of tubes is done to ensure that the positive and negative output is amplified equally. In other words the amplification factor of A,B and C,D should be the same.

So if tube A is out, you change A and C, If tube B is out, you change B and D etc.

(this is mainly theoretical though - in practice even a matched set is not perfectly balanced and over time the tubes changes, so an amp tech would most likely measure all of them and reposition tubes according to A,B and C,D currents.. or so he should).


Old and new is not the only difference. If these four tubes really must be balanced, you need a tube testing tool for selecting the matching group from much more than just four tubes.

However selecting the matching pairs was much more critical for transistor electronics, as individual transistors vary in parameters much more dramatically than tubes. Maybe the pair matching is not required for your amplifier; then replacement of the single failed tube is ok.


Good answers above, I'd just like to add that the amp bias circuit needs to be checked closely to determine if the tube failed because the bias circuit had a problem. Otherwise the new tube will probably fail very quickly, if put into an amp with a faulty bias circuit.

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