how to know a tube that redplated is bad?

[iknowjohnny]

I had one of the 220k bias resistors come loose and i played the amp for a couple minutes before i noticed the hum and looked to see one 6V6 was redplating big time. Turned it off but it was a good 2 if not 3 minutes it was on i think. When i resoldered the resistor i turned it back on and everything sounded fine and upon checking the bias the tube was only about 1 mA lower that it usually is. I had checked the bias several times before this happened and that tube was always about 1 Ma less than the other and now it was 2 mA difference. So apparently it WAS affected by the trauma. I realize the current is not a issue, it's only slight so thats not what i'm concerned about. I'm just wondering if somehow it should be considered bad even tho the current draw is well within range with respect to the other tube and the tone is no different?

[xtian]

A) If it sounds good, it is good.

B) A difference of 1mA in current draw is below the threshold of measurement error, not even approaching an indication of health!

C) Rock on!

[iknowjohnny]

Thanks. I just mentioned the 1mA difference not to suggest it might mean it's bad, but to just show it did change. I have checked the bias many times recently and they're always 1mA apart so now that for the first time there 1 more mA difference it just shows that it DID affect the tube even if it's still fine.

[xtian]

Thanks. I just mentioned the 1mA difference not to suggest it might mean it's bad, but to just show it did change. I have checked the bias many times recently and they're always 1mA apart so now that for the first time there 1 more mA difference it just shows that it DID affect the tube even if it's still fine.

How are you measuring and what's the accuracy of your meter?

[ToneMerc]

Thanks. I just mentioned the 1mA difference not to suggest it might mean it's bad, but to just show it did change. I have checked the bias many times recently and they're always 1mA apart so now that for the first time there 1 more mA difference it just shows that it DID affect the tube even if it's still fine.

Well, did you check the line voltage before and after? How do you know that the 1mA delta isn't from line voltage variation.

TM

[iknowjohnny]

Measuring with 1 ohm cathode resistors and it;s accurate because for months i have been tweaking it with the chassis out and i have checked it many many times and it's always been the same. And line variation doesn't matter because i'm talking 1mA with respect to the other tube. So if line voltage fluctuated they will both still be 1mA apart. Now they're 2 apart.

[Stevem]

I would keep a spare tube on hand as the screen grid is now very likely to fail in that tube and then short to the heater as they mostly do , and then of course blow your mains fuse!

[iknowjohnny]

Thought i'd update this. I guess when they redplate they can sound ok then soon go south. Thats what happened, tho it didn't die it just started sounding lousy all of a sudden. I couldn't figure out why the amp didn't sound right almost instantly and then it hit me. I remembered the redplate incident and swapped the tune out for a known good one of the same brand and there was my tone again the way it should be. What a relief. I spent a few days trying to figure out what was wrong !

[Stevem]

This why it pays to place a 1 ohm 1% 2 to 3 watts resistor in series to ground off of each output tube Cathode.
You then place a meter set of D.C volts across it and your reading the current the tube is pulling, be it idle or cranked.
You then can make note of how much idle current the tube is at and then as the tube ages you can tell how fast it's giving up the Ghost.

[iknowjohnny]

This why it pays to place a 1 ohm 1% 2 to 3 watts resistor in series to ground off of each output tube Cathode.
You then place a meter set of D.C volts across it and your reading the current the tube is pulling, be it idle or cranked.
You then can make note of how much idle current the tube is at and then as the tube ages you can tell how fast it's giving up the Ghost.

I already have them. All the amps i've had for years have had that. The tube still reads only 1 mA different than b4 it redplated but the tone sure changed for the worse.

[martin manning]

Now that's interesting. I wonder what physical damage occurred that allows normal emission (DC behavior) with poor sound (AC behavior)?

[trobbins]

The 1952 linked paper on valve reliability gives a nice discussion on your likely situation, if the redplating was substantial and was the ultimate cause of failure. Red plating can cause valve farting, and the circuitry loses control of the grid voltage!

http://www.radiomuseum.org/forumdata/us ... bility.pdf