Never assume

[Enzo]

Had an amplifier in here this evening. Low muffled output and the controls had litle effect. The gain controls worked, but the posts didn;t. Signal into the phase inverter showed a strong power amp stage. Into the FX return gave zip. One tube stage between there and the PI. Sure enough, 300v on the plates of this tube and zero on the cathodes - tube not conducting. Can't see into the tube, but the others are working and the preamp tube heaters are all four wired in series, so one works, they all must, right?

Grabbed a brand new tube from a box of 100. (In fairness there are only about 20 tubes left in it) Put it in, and... same thing. Must be something in the amp then. Checked the voltages, same as before. Checked the heaters. Heaters were funny. That one tube had about 6v across the 12v heater. But it was in series with the others. No other parts in the circuit, just +/-24v rails and a small value resistor at each end to trim it a little. The other three tubes had an extra volt or two across each.

OK, I put the original tube back in and found the same thing. I can;t find anything wrong with the amp circuit, yet this tube also only comes up to about half its heater voltage. And yes, that voltage was evenly divided on either side of pin 9.

What could possibly be causing the amp to drop half as much across whatever tube was in V6?

I won;t stretch the story any farther. I took yet another new tube from the box, installed it, and the amp screamed like it ought to.

I spent way too much time sorting out a bad tube. My bnew tube was bad also, in the same way. The point is that even though I hand out the advice that "any tube can fail at any time," it still confused me. Of course it is a pretty large coincidence that the failed tube had a weird heater issue, and the new tube I selected from a bunch had the exact same weird problem. (Gee, what are the odds...) And no, the tubes were not even the same brand, and the box of 100 is several years old, while the amp's tube was recent. Can't blame anything on a bad batch.

SO once again, never assume a part is good, just because it is new.

And never assume that two parts are good because they act the same way in a circuit.

Assuming can bite you, even if you have 50 years of experience.

[txstrat]

(Gee, what are the odds...)

With this luck today you should definitely buy a lottery ticket. Who knows..?

[Steve Conner]

You know what they say: Never assume. It makes an Ass out of U and Me.

Also, some mathematician once argued that the odds of winning the lottery are so slim, that it hardly matters whether you buy a ticket or not. I remember my mom once proved this by finding a winning lottery ticket lying in the street. It was only a $50 jackpot sadly.

(Any real mathematician would shudder at my above use of the word "prove")

[tim]

Enzo,

Great post!

I've been burned like that a few times in my 35 or so years repairing electronics.

Nowadays, I don't assume that the part I installed is good (it probably is, but...) what with all the counterfeit parts floating around these days, even from reputable parts suppliers.

[Amp Kat]

Had the same thing happen about a year ago and fortunately is was only at 10 years of experience not 50. Bad part is that it was two days later before I figured it out. That's what that extra 40 years will get you.

[gbono]

Yes, had a similar issue with the PI tube/stage in a Sunn 200s. Also would add that tube testers don't always help to solve these type of problems.

The odds of winning the MegaMillions play in the California lottery is 175,711,536:1. What would Carl Friedrich Gauss have to say about this game of "chance"??

[52 Bill]

I've had similar things happen, so I keep a known working 12AX7 sitting in my pin straightener that's bolted to the bench. Whenever I want to swap out a tube, I just reach for it and pop it in. If the amp does need a new tube, then I'll walk across the shop and pull one from new stock.

Ok the truth is I'm lazy and I really keep the tube on my bench to save me from walking across the shop every time I want to swap out a preamp tube, but it works for me.

[CharlieP]

I've had similar things happen, so I keep a known working 12AX7 sitting in my pin straightener that's bolted to the bench. Whenever I want to swap out a tube, I just reach for it and pop it in. If the amp does need a new tube, then I'll walk across the shop and pull one from new stock.

Ok the truth is I'm lazy and I really keep the tube on my bench to save me from walking across the shop every time I want to swap out a preamp tube, but it works for me.

Thanx for being truthful 52 Bill!

[Chuck H]

I haven't had this "coincidence" problem yet with an amp... But I expect it. That is probably what keeps it from happening.

Very early on in my life I noticed that whenever my car would break down I had to fix the problem... And whatever problem the problem caused. OR... Whatever happened to be wrong at the same time. In other words, I found out early on that the problem at hand should never be considered exclusive. There was seemingly ALWAYS something else wrong that kept any repair to the original problem from working. This is not unlike your recent problem and my strangely multiple experiences with this sort of thing have (I think) helped to prepare me for a WORLD of chaos. Where I, unfortunately, expect nothing less than the worst, most difficult scenario I can imagine associated with the current circumstances.

Scarred for life in an unhealthy way.?. sure, you betcha. But if that's the only hand you get dealt, you'd better learn to live with it.

Chuck

[Mars Amp Repair]

No kidding,
Speaking of being 'honest',
I recall one guy posted that in like 5yrs he had only had one rework. I don't think anyone even contested that as it was so unrealistic & frankly ridiculous. I recall you could guess who it was that evaluated the 'reworks' for him

SH#T, whether or not we like it, we're all stuck in this thing called 'the human condition'...that's why pencils STILL have erasers , tho come to think of it the way technology is going, at some point there might be no more pencils

[txstrat]

tho come to think of it the way technology is going, at some point there might be no more pencils

Or worse, no more erasers...