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Post subject: Yet moreTube questions!!!!!
Posted: Tue Oct 15, 2013 2:22 am
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I'm sure I can see some rolling eyes at the sight of yet more Tube questions, however....

Can the 12AX7A in my 79 SF Champ be replaced with a 12AX7? It still has the original tube, and I have tried some replacements (ECC83) with spectacular results in some cases...squeal, bright glowing, sound dropping off at high volume, etc. I'm not a tube guy, but I understood that the 12AX7A is a ruggedized 12AX7? Am I just unlucky & getting bad tubes?
The 12AX7A's are harder to come by, and therefore more expensive.

I do have a box full of vintage ECC82's (Philips, Amperex, Mullard, National, RCA, etc, etc), and I know they are different to the ECC83's (lower gain & not really audio tubes). Does the Champ circuit need to be modded to take ECC82's?


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Post subject: Re: Yet moreTube questions!!!!!
Posted: Tue Oct 15, 2013 3:41 am
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Mike73JM,

Originally, the 12AX7A was another designation for low-noise spiral wound heater filament 12AX7 (simliar to the 7025). Tungsol and Raytheon made some of the best 12AX7A for the military in the 1950-60's. Suffixes on most new 12AX7 do not adhere to strict guidelines. The only two 12AX7 types with spiral wound heaters are the Sovtek 12AX7LPS and the Gold Lion 12AX7/B759.

The 12AX7 will work fine in a 12AX7A position. So will the ECC83, the Euro designation for the 12AX7.

The ECC82 is the Euro designation for the 12AU7. It will work in a 12AX7, albeit with less gain and different response curve.

ECC81 = 12AT7.


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Post subject: Re: Yet moreTube questions!!!!!
Posted: Tue Oct 15, 2013 8:45 am
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I suspect a problem with the amp itself, not the tubes. No 12AX7, 12AX7A, or 7025 that tests "good" or better should exhibit the symptoms described.

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Tue Oct 15, 2013 9:10 am
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Arjay,

I would think that also, if the original fender 12ax7a didn't work, but it works just fine. I have a Philips Miniwatt ECC83, when I put that in, it glows really bright at the base for a second or two when power is applied, and then works ok (sound dies down at higher volumes though)


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Post subject: Re: Yet moreTube questions!!!!!
Posted: Tue Oct 15, 2013 10:05 am
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Mike73JM wrote:
it glows really bright at the base for a second or two when power is applied, and then works ok (sound dies down at higher volumes though)


I would take a look at the cathode bypass caps tied to pins 3 (V1A) and 6 (V1B). The associated resistors may also be suspect.

Arjay

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Wed Oct 16, 2013 12:32 am
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Will do. May give the ECC82 a go also, now I know I'm not going to kill the amp by putting one in!


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Post subject: Re: Yet moreTube questions!!!!!
Posted: Wed Oct 16, 2013 12:51 am
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" I have a Philips Miniwatt ECC83, when I put that in, it glows really bright at the base for a second or two. "

This is normal for Euro made Philips ECC81, -82, and -83 tubes (Miniwatt, Mullard, Amperex, etc). No problem.


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Post subject: Re: Yet moreTube questions!!!!!
Posted: Fri Oct 18, 2013 4:10 am
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BMW2002Ti wrote:
" I have a Philips Miniwatt ECC83, when I put that in, it glows really bright at the base for a second or two. "

This is normal for Euro made Philips ECC81, -82, and -83 tubes (Miniwatt, Mullard, Amperex, etc). No problem.


+1

I have a few tubes that do this as well. Among them are an older Ei 12AX7, and a newer Ruby 12AX7. Every time that I turn them on I expect them to blow, but they are still going strong and sound great.

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Sat Oct 19, 2013 12:54 pm
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I never notice the glow. I just plug em' in and play and listen. Good tone, good tube! Some tubes with spiral filaments will not show much of a glow but, if they are working they are good. Glow is not the be all end all! Flash is normal.

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Sat Oct 19, 2013 1:31 pm
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Are you saying that we should spend more time in front of our amps and less time in the back? You're probably right. :lol: :wink:

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Sat Oct 19, 2013 1:42 pm
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shimmilou wrote:
Are you saying that we should spend more time in front of our amps and less time in the back? You're probably right. :lol: :wink:
I am saying people are too busy fixing what ain't broke! They get a tube amp and it works and then they look at the back and start re-engineering what they really don't understand. I see people changing out their Sovtek LPS tubes all the time because they don't glow so to them they must be bad. All the while the amp is still working fine. Go figure! :roll:

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Sat Oct 19, 2013 1:47 pm
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Cap47 wrote:
I am saying people are too busy fixing what ain't broke! They get a tube amp and it works and then they look at the back and start re-engineering what they really don't understand. I see people changing out their Sovtek LPS tubes all the time because they don't glow so to them they must be bad. All the while the amp is still working fine. Go figure! :roll:


In my case, you're preaching to the choir.

:wink:

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Sat Oct 19, 2013 9:22 pm
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Some people just have a fascination for learning. :lol:

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Sat Oct 19, 2013 9:52 pm
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AaronK wrote:
Some people just have a fascination for learning. :lol:


Until that learning curve becomes so steep that they're forced to consult a real amp tech who didn't learn his trade from reading some blog or watching youtube videos made by other clowns just as clueless as they.

:lol: :lol: :lol:

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Post subject: Re: Yet moreTube questions!!!!!
Posted: Sun Oct 20, 2013 8:05 am
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The Mullard CV4024 (12AT7) also glows very bright when you first turn on the amp...

I like my tubes full of Blues :mrgreen:

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