Do I need new tubes? Not glowing in DR

Hey Guys,

So I recently (about 2 months ago) purchased a used 3-channel Dual Rectifier...and it's my first tube amp. I was extremely careful when I purchased the amp (made the store literally take everything apart to make sure it was in excellent condition). However, I just realized that two of the matched tubes aren't glowing when on. The head has two sets of matched Ruby 6L6's, and one set of Mesa 6L6's (which are the ones that don't glow).

My question is, is this normal for Mesa tubes to not glow as much as Ruby (or other brand) tubes? Or, is this a sign that I need new tubes? I haven't noticed any difference in sound, and none of the channels sound extremely different than the others.

Here are some pictures of the tubes.

Thanks!!

the two that aren't glowing are the rectifier tubes...many times you won't see any glow in a rectifier tube...if you are getting sound they are working (unless your amp has a tube/silicon switch and you have it set to silicon)

I do have mine set to silicon though. Does that mean that they should be glowing?

npo12354 said:

I do have mine set to silicon though. Does that mean that they should be glowing?

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Nope, that means that you´re using Silicon Diode rectification istead of Tube rectification. Flip the switch to "Tube" for comparison...

So-called tube "glow" is not a reliable indication of functionality, because each and every tube exhibits this glow differently.
You could have two identically "glowing" tubes, and one could still be defective.

npo12354 said:

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If you look at this picture very closely you can see the tops of the heaters glowing orange in the tube on the right. Look for four orange lines between the top of the grey plates and the mica spacer.

Put the amp in a very dark room, you'll be able to see the heater glow better.

The Rectifier (5U4GB) tubes should show heater glow regardless of Rectifier Select switch position (they are always in the circuit).

And as mentioned, heater glow IS NOT an indicator of tube function.

Dom

domct203 said:

npo12354 said:

Click to expand...

If you look at this picture very closely you can see the tops of the heaters glowing orange in the tube on the right. Look for four orange lines between the top of the grey plates and the mica spacer.

Put the amp in a very dark room, you'll be able to see the heater glow better.

The Rectifier (5U4GB) tubes should show heater glow regardless of Rectifier Select switch position (they are always in the circuit).

And as mentioned, heater glow IS NOT an indicator of tube function.

Dom

Click to expand...

First I "would" switch to tube rectifier and see if both tubes glow. My rectifier tube glows!. If they do great. If you still do not see any glow on the tube on the left I would suggest swapping the 5U4GB rectifier tubes (that is put the current right one in the left socket and the current left one in the right socket). If you see the one in the left now glowing then that tube is good and I "might" suspect the tube that is not glowing. If the tube you placed in the right socket is now glowing I would suspect the left socket's solder joints and/ or circuitry at some point. Only a reputable tech could tell for sure at that point.

Dennis

The Rectifier Select switch has nothing to do with 5U4GB heater operation. The rectifier tube heaters are always connected. None of the amp's tube heaters are switched (see attached schematic below).

Also, the rectifier tube heaters (what creates the "glow") are AC, and they are wired in parallel directly from the 5V tap on the Power Tranny. There is no "circuitry" involved with the rectifier tube heaters (see attached schematic below). Being that we can see the heater glowing in the tube on the right in that pic, that would suggest that the 5V tap in the tranny is OK.

Put the amp in a dark room and power it up. If you only see one of the 5U4GB's heater glowing shut down the amp and swap the 2 tube's position as suggested to see if it is the tube or the amp.

If the problem appears to be the amp, I would start with the socket contacts. Give them a good cleaning and re-tension the contacts to assure a good connection to the tube pins.

If it is a bad tube, replace it. With rectifier tubes you can change just the bad one, but IMO being that it is a used amp I would change them both.

Dom

Thanks everyone!! When I looked at the tubes in the dark, they do all appear to glow...very slightly, but never the less glow. This combined with the fact that I haven't had any concerns regarding the quality of sound that's coming from the amp, I'm assuming that everything's fine with the tubes. Since the amp is used, I'm planning on swapping all of the tubes out with new ones relatively soon.

Could anyone recommend any websites or books that would get me on my way to understanding tube rectifier amps a little more? Obviously, anything relating to a DR would be preferred. This way I would have a better idea of what's going on under the hood.

Don't change tubes just because the amp is used. They may be quite new and they're probably fine, certainly in the pic there is no evidence of problems. (Although that doesn't always mean much.) The Ruby power tubes would most likely show signs of discoloration of the red paint, or of the silver 'getter flash' in the top of the tube, if they'd been used heavily and were on the way out.

How to test the rectifier tubes: remove one, set the amp to Silicon Diode and play at low volume. If the amp works, the rectifier tube is fine. Remove it and repeat with the other one. Don't crank it full without both tubes in.

The tube rectifiers produce more power supply sag than solid-state rectifiers. This lowers the voltages in the amp, changes the tone a bit, and changes the dynamics when the amp is pushed. Each rectifier tube has a current rating, and one 5U4 is not sufficient for a 100W amp, so Mesa use two. Nothing more complicated than that. Rectifiers don't really wear out like power tubes do, they either work or don't work, so if they work there's no need to change them. Rectifier tubes aren't 'matched' like power tubes, but you should generally use two from the same manufacturer if possible since they can have different voltage drops, which will load them unevenly.

But any tube can fail at any time so it is worth keeping a spare of each type, or a complete set of power tubes (since it's a good idea to keep matched sets of those). You don't absolutely need a spare rectifier tube since you can always get through a gig by switching to Silicon Diode.