RM50 power tubes wearing unevenly..

[withmittens]

When I have a new set of tubes they read pretty well matched. they bias really well and evenly too. then a few months down the road, or in the case of a new set of KT77's a day, I go to re-bias them and they are 10 Mv off of one another.

when I removed them to try my older but fine last time I used them, 6CA7's they were 10 Mv off of one another.

I'm scared to check my GE Black plates tat I had in there for a while.

they sound fine but I don't think it should be happening

what gives?

 

[Jaded Faith]

In short, welcome to the state of current production tubes. As anyone in our industry will tell you, it's a crap shoot. There are no "magic brands" immune to this and quality will vary from batch to batch. It's a necessary evil these days. I buy about 1000+ tubes annually for the shop, all brands and from several distributors that do not sell to the public. It can be all over the map and it's maddening! :lol:

PS: It has likely has NOTHING to do with the amp and would have shown the same symptoms on a burn-in rack.

 

[withmittens]

would it be safe to say that my 51 year old GE 6L6's, that have been fine for years, test 10 Mv off of one another after a month of use in this amp then it may be the amp?

thanks for the reply

 

[withmittens]

I meant to say that IF this happens, would it be safe to assume it's the amp?

 

[Jaded Faith]

That depends on a bunch of things. It sounds like you do not have a tester and are using the readings on your amp to determine matching. This is not a good system. The amp could be off in its bias circuit for example. The tubes WILL EVENTUALLY WEAR, but it's going to happen at sometime in some amp. This is a 100% accurate truth. It doesn't ensure any amp is at fault because it's going to happen. Finally, your MTS amp is hitting the plates around 500V. Many 6L6's from the early days were good up to 450V. I'd expect wear and tear in that scenario for sure.

 

[withmittens]

nope I have a tester, it's just that when the tubes go in the amp they can be biased almost identically, then after some time or in the case of the KT77 a day , I can't get them to bias within 10mv of each other. it's consistent, this

 

[Jaded Faith]

Interesting. What's the tester and what are the metrics you are using to evaluate the tubes on it?

 

[withmittens]

Superior Instruments Model TC-55 with chart.

 

[bruce egnater]

Clearly there are a number of factors that could affect this. Tube wear/drift being the first. If you measure the tubes in the amp and then swap them into the other sockets, does the reading follow the tube or stay on the same socket?

 

[kc2eeb]

Had a similar problem with the RM50 (besides arcing on the PA board) When I changed the OT it stopped. Yes, it was the same socket every time. Do what Bruce and Rob say.

 

[withmittens]

just checked mine. it follows the tube. If I check the strength of the tube on my tester they are the same. Maybe this isn't the best way to test a tube but they read 95% on my tester. I'm having a local tech look at another amp soon, I'll ask him to test the tubes on his machine

 

[bruce egnater]

The idea is to test the tubes in the amp using the bias test points on the rear. Put your known matched set in the amp. Measure each tube at the test points and write down the readings. Now swap the tubes into the opposite sockets and measure again. See if the readings follow the tubes or stay on the same sockets. Does this make sense? Also, bare in mind most tube testers do not apply nearly as high a test voltage as an amp. You tubes will surely respond differently to the 200-300 volts your tester might use compared to the typical 450-500VDC in the amp. Does your tester measure idle current directly in DCmilliamps? That is what you are measuring in the amp. If your tester is just a good/bad or % reading, it may not correlate directly with the idle current measurement in the amp.

 

[withmittens]

well, I had previously thought that all of my tubes were fairly matched. I bought them that way and my tester shows that they are indeed reading the same percentage "good". BUT, I guess it is not the kind of tester that is reliable for this. I understand that wear would contribute to this, but this phenomenon is consistent through all my sets

That said, my sets that are showing matched on my tester, biased evenly the first time in the amp, and then upon retesting the bias, they test 10Mv or more off. the reading follow the tubes when I swap sockets. Put them back in the test and they read the same as the first time

My local Tech will verify whether the tubes are in fact matched.

 

[kc2eeb]

A good tube tester allows you to vary the B(+) (Plate voltage) Bias(-) and read the current (ma) draw. Varying the bias voltage while maintaining the same Plate voltage will allow you to calculate transconductance. Also, most tube testers do not have the voltage available to test output tubes at the voltages they are actually operating at. I have a Longhin 460 (Made in Italy, no longer made) that will go up to 460 volts. Having said all that, the best test of a tube is in the amp it's going to work in.

 

[withmittens]

....and while I wait for that, I think the real question in, if it is tube wear, why does the amp consistently wear the tubes unevenly? should the amp not wear them at least together? and are the tubes no longer good if they can't be biased reasonably close? for instance my Winged C L6 set is not reading 33 and 20 in the amp....

 

[kc2eeb]

Most importantly, don't jump to conclusions. First, and the easiest to check are the tubes.
Something you could try with out taking the amp apart is start with a pair of matched tubes, sent the idle bias, then, leaving the voltmeter connected, play a steady note through the amp, like holding a note down on a keyboard and measure the current on each test point. The current should go up relatively even at both test points. If there is a big difference like 40 or 50 ma between one test point and the other, shut the amp down, reverse the output tubes and do it again. If the measurements switch, it's the tubes. If it doesn't, it's something in the amp. A good tech will know what to check. It could be anything from a "cold" solder joint on the tube socket, screen or plate resistors out of spec., Output transformer, etc.
Also, play the amp in the dark and observe the plates on the output tubes. (the gray boxes in the tube) They should NOT glow red. Play loud. If you see any trace of red glow, shut it down, the tubes are drawing too much current. That could be a too high bias current, bad resistors, etc. If one tube glows, but not the other, that could be screen grid resistor, or OT. Also, check for any thing that looks like burnt carbon. That's a sign of arcing. MOST IMPORTANT: The voltage in the amp can KILL you. So, if you have no experience with high voltage circuits, DON'T go in there.
Last thing, in all the old amps they never matched output tubes. There was not even a spec. for that.

 

[withmittens]

thank you for that input.

I have started with known matched sets. I trust Tube Depot and Tube Store to match well. each set I have bought were purchased as sets from either of these sources. The KT77's were bought on this here forum as brand new matched set, unused. So I know they are going into the amp matched, also evidenced by them biasing nearly identical the first time.

The problem arises much too soon for considerable wear to occur, a few months of normal use, not gigging every night at loud volumes. a few gigs, a lot of band practice and a lot of bedroom volume practicing

For each brand new set to be 10-12Mv off, in re-biasing, of one another after this kind of use tells me the amp is wearing them this way. It may be jumping to a conclusions but it seems a logical conclusion to jump to. That the bias numbers follow the tube to a new socket seems to back this up as well.

So my question to any tech would be, why is that happening and how much is it going to cost me. I have a guy in town that is outrageously priced, others agree, so I want to know what I'm talking about as much as possible before I go.

 

[withmittens]

I would be fine with it being the OT as I could upgrade it and feel ok about that.

 

[kc2eeb]

Did you play the amp in the dark to see if it's "red plating?" Reason I'm asking is that once a tube "red plates" it's basically shot and if that happens in the same socket all the time the first likely cause is a bad screen grid resistor. That's relatively inexpensive to fix.

 

[withmittens]

tonight