[morzh]

This week first I tested KT88 on my Hickok 600 and they are fine.
These are Russian made.

Today I decided to check 12AX7 and 12AT7. All tubes marked as Mac, all are Chinese.

Q1. The very first 12AX7 tube I tested first goes very bright for a split sec, then goes normal. Tests good. The rest of the 7 tubes do not do that, they go to normal glow, no flash.
Why?

Q2. All four 12AT7 test 1700-1900 at 4500 table value. Is this normal or...?
Q3. All tubes when tested for gas, exibit large (1 full division) needle movement only when the 1st triode is being tested. The second only moves the needle by 1/4 div. Why?

[TV MAN]

Mike, I have seen tubes flash like that before, usually in ac/dc sets, esp the miniatures. I haven't had one fail. Not sure what causes it but in my experience it hasn't caused any problems.

[morzh]

Tim

Yes, seems like it is normal.

I do not understand the "gas" situation, I also suspect it could be normal, as it is uniform across the 7 tubes tested, present on the 2nd triode.
And another thing that confused me is, all tubes are borderline good/replace for 12AX7 and high end replace for AT7.

Probably normal. The amp is 7 years old and the tubes are all Chinese. With heavy usage it is likely the case.

I have little to no experience with the miniature tubes, so have little idea as to their behaviour.

[TV MAN]

A thought on the 'low' 12AT7 tests, may be that Mac chose tubes with slightly less emission for lower noise and matching, rather than amplification.

[morzh]

Not sure, the tube if marked AT7 should satisfy the datasheet. There are compatible tubes with lower gain as new. Would be a bad engineering practice, don't think Mac would do such thing.

[TV MAN]

Probably right. I also didn't realize it had several years of use. Do you have any 12AT7 tubes NOS or otherwise to test for comparison? Now I am curious.

[morzh]

No. I ordered a few of each, according to the vendors (specializing in tubes, 100% feedback, so theoretically trustworthy) testing very nicely, but not NOS. Also mfrd by Ratheon and RCA, don't want Chinese or Russian.
Will compare.

[morzh]

While looking for another tube I discovered two 5751, one Sylvania, one GE.
5751 is pretty much a 12AX7.

Both test fine, above the minimum, and Sylvania does not show any gas while GE shows some on the 2nd triode. I start suspecting it is normal for them to have it at this value.
Gosh, had I known I had them...

[Phlogiston]

Not sure, the tube if marked AT7 should satisfy the datasheet. There are compatible tubes with lower gain as new. Would be a bad engineering practice, don't think Mac would do such thing.

Some manufactures actually ran some of the audio stages at less than 12V to keep noise down. They would also run the filaments in series and use the string to generate a bias voltage for the output tubes. I think the last one of these I saw was a Fisher.

Some of the expensive, foreign 12AX7 tubes flash like this. I have heard all kinds of explanations, none of which that I care to quote.

[morzh]

Russ

I never liked serialized filaments idea: for one, one tube dies and the rest extinguishes. And you have to figure which one.
Then in some places it can result in some bad scenarios. Like in Zenith portable radios.
I am fond of the idea using things as they are intended. Need a bias, use a resistor.
These off label usage is fine for cheap stuff where quality is traded off for space/cost.
Not in high end audio.

[Phlogiston]

Russ

I never liked serialized filaments idea: for one, one tube dies and the rest extinguishes. And you have to figure which one.
Then in some places it can result in some bad scenarios. Like in Zenith portable radios.
I am fond of the idea using things as they are intended. Need a bias, use a resistor.
These off label usage is fine for cheap stuff where quality is traded off for space/cost.
Not in high end audio.

OH YES! I agree. Once anyone sees the meltdown caused by a open filament, they will too. I also wonder how these tubes run at 10V or less keep from going to sleep - seems like it would be hard on the cathode. The fisher I was referring to uses the 6BQ5 version that runs at 400V B+. Expensive and a quick failure with no bias.

[morzh]

Oh, but if you are TRUE audiofool and CANNOT LIVE without them 6BQ5 at low voltage needing changed every other week, you will cough up the necessary amount and feel blessed while doing that.
It's a religion.
Until you convert into some religion

[Phlogiston]

Oh, but if you are TRUE audiofool and CANNOT LIVE without them 6BQ5 at low voltage needing changed every other week, you will cough up the necessary amount and feel blessed while doing that.
It's a religion.
Until you convert into some religion

For anybody who want to see what I'm talking about - a fellow has posted the schematic. Pay careful attention to the filament string (center . not near the rectifier). See what the filament voltage is? Also see what happens if a filament opens - POOF!

Yeah, this one is 7591s but it is similar.

https://fisherx100a.files.wordpress.com/...r101_2.gif

His site:

https://fisherx100a.wordpress.com/

[codefox1]

Many small signal tubes "flash" initially from the filament winding, but surprisingly last a long time. Whatever winding mechanism stretched the filament a bit too much upon manufacture stretched it a bit on one end. For stuff left on for long periods or constantly, no big deal, it fails or not usually without consequence. Otherwise replace/return. Larger current tubes, don't take chances.

[morzh]

Well, the tubes that I received, NOS NIB CV4024, a mil spec Mullard 12AT7, flashes worse than other. All 4 of them at the same time. They work fine.