Tube blues (not blues tubes)

[mikepukmel]

I haven't been working with tube amps long enough now to see something like this happen personally. These are valveart KT66's with 10 or 15 minutes runtime.


fla2F.flv


Is this a problem with KT66's, or just modern day tubes? In all the years, way back when, when we were kids in the 70's using those old beat up fender, marshall, vox, ampeg tube amps, I don't think I ever remember this happening.

[alexradium]

How is the voltage, it looks pretty high on screens.
I used Tungsol with success in a 480v b+ Amp.

[mikepukmel]

Thanks Alex, don't know, it was posted by a guy on a facebook marshall amp forum. Seems like, by his description that he builds lots of amps, and the build photos he has look very professional. he said the tubes had about 10 minutes on them. I found the reissue schematics put a second big resistor between the hv feed and screen grids, 1K5 in some schematics, then the 'standard' 470R after that. Im guessing the old tubes, the screen grids could handle the voltages better?

[g1]

In the old tubes, everything could handle everything better.
It's pretty difficult to speculate on that video without seeing the schematic. Could just be a bad tube, could be a modern tube trying to hold up to old tube specs (or beyond as was often the utilization).
Screen grids do seem to get much more attention in modern designs though, so perhaps they are a particular weakness.

[mikepukmel]

In the old tubes, everything could handle everything better.
It's pretty difficult to speculate on that video without seeing the schematic. Could just be a bad tube, could be a modern tube trying to hold up to old tube specs (or beyond as was often the utilization).
Screen grids do seem to get much more attention in modern designs though, so perhaps they are a particular weakness.

Thanks G1. Ive been emailing around to find the tube sellers that provide a 24 hour burn in. A few say they do. Couldn't get much info. One seller said "our suppliers provide that service". Hmm. I remember something back in the day about a big machine, they could select socket types, etc. Bunch of those big phenolic dials all over, could select heater voltage, plate voltage, various resistors etc. Not sure of modern day burnin doesn't mean leaving the tubes out in the sun for a while. All I know is: if tubes come from China, and they say "burned in 24 hours" on the invoice, your guess is as good as mine what that means.

[bob p]

... it was posted by a guy on a facebook marshall amp forum. Seems like, by his description that he builds lots of amps, and the build photos he has look very professional. he said the tubes had about 10 minutes on them.

I think it's important to know the context in which that video was posted. Why did he post it? Was the builder demonstrating what a bad tube looks like? Or was he trying to get help identifying what was wrong in his new build?

It's easy to jump to conclusions that the tube is bad (I agree it is highly suspicious) but to prove that the amp build was not defective we need to see the amp operating after swapping the tubes in their sockets, or the amp operating with a different set of tubes. If the problem follows the bad tube then it's the tube. If the problem stays in the socket then his build is defective, which could be caused by any number of problems, such as bad wiring or a defective screen resistor. Unfortunately we don't have enough information to reach any definitive conclusion.

Tube quality is a problem. There are plenty of threads on this forum where we've complained about it. The bottom line is that you need to test/burn in your new tubes so that you can immediately identify/reject the bad ones. You don't ever want to buy new tubes and put them on a shelf without testing them first. FWIW I don't trust the word of any vendor about burn-in. Because none of them are willing to offer an unlimited guarantee, upon receipt I do a resistive-load burn-in to immediately identify and reject bad specimens.

Regarding why modern tubes fail -- the biggest problem IME is that screens aren't what they used to be. But screen failure is more of a problem in pentodes than in beam tetrodes because the beam tetrode is designed to dissipate about 5-10% power in the screen, compared to 20% for the garden variety pentode. Regarding modern pentode failure: there are two groups of thought on this -- one group suggests that classic amp designs need to be modified when using modern tubes to limit screen dissipation, another group considers that heresy.

As big as the problem is with modern tubes, there's also a problem with self-proclaimed amp building experts who may not be all that they claim to be. A nice-looking build doesn't say that much about the knowledge of the person who built it, beyond their assembly technique. In the old days Fender used minimally trained staff to stuff and solder. Today the world is full of people who can build an amp by looking at pictures, but can't even read a schematic. A disturbingly high number of people fancy themselves as amp builders, while their level of expertise is actually limited to parts assembly.