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Post subject: PreAmp Tube question
Posted: Mon Sep 16, 2013 1:13 pm
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Looking at the recomendations of tube supplers you guys have given me, Thanx much again, the preamp tubes on my 73 VR; mixture of 7025's, 12ax7, 12at7. The curious but clouded mind I have, There dosnt seem to many 7025 around, or is this the same as an 12ax7, and looking at the reverb driver tube,12at7, why wouldnt this be a 12ax7 as well? Just my curious nature that wants to know why I guess.
Also, looking at the schematic..... what is 1/2 7025?


BTW, just loaded same amp with two Celestion Gold 10's, cant wait to crank this muthr


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Post subject: Re: PreAmp Tube question
Posted: Tue Sep 17, 2013 11:23 pm
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The 7025 is a low-noise version of the 12AX7. Originally there were some minor construction differences to lower noise, but most 12AX7's now incorporate those features. Many tubes are marked "7025/12AX7". Other than noise, all other specs are the same and the tubes are completely interchangable. So you can use a 12AX7 in place of a 7025, though you may want to look for a 12AX7 that's advertised as being low-noise.

While all the 12__7 tubes are kind of interchangable, their specs and performance do differ. Not just the amount of gain - AT7's can push a lot more current than AX7's can. Spring reverb circuits need a lot of current, and AX7's will usually die quickly if used as a reverb driver tube.

12__7 tubes are "dual triode" tubes - literally two independent tube circuits housed in one glass envelope. Often, one triode will serve one function while the other one does something entirely different. Sometimes, an amp only needs an odd number of triodes instead of an even number of triodes, so one of the triodes inside one of the tubes will be unused. So "1/2 7025" means that only 1/2 of the tube is being used (if there are two places on the schematic each showing 1/2 7025, it means both triodes in one tube are being used but for two different functions).


Last edited by strayedstrater on Wed Sep 18, 2013 9:38 am, edited 1 time in total.

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Post subject: Re: PreAmp Tube question
Posted: Wed Sep 18, 2013 12:24 am
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IMO, probably the best current production 7025. One of the amps that thetubestore used for testing was a VR.

http://www.thetubestore.com/Tubes/Preferred-Series/Preferred-Series-7025-12AX7

Those Celestion Alnico Golds are very clean and solid sounding. Let us know what you think of them.

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Post subject: Re: PreAmp Tube question
Posted: Wed Sep 18, 2013 4:45 am
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As far as I know, the only two modern 12AX7 types that have spiral heater filament (to reduce heater induced hum) --- the Sovtek 12AX7LPS and the Gold Lion 12AX7/B759.

Both versions have long anode plates, so should be selected for low-noise quality. And even then they become noisy in combo cab.


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Post subject: Re: PreAmp Tube question
Posted: Wed Sep 18, 2013 12:21 pm
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thanx for all the responses, but one last ?, I see long plate and short plate advertised, whats the difference?, or is that the same thing as long tail? Please forgive me for sounding like a rookie but really getting into the guts of these things is something ive always left to a pro and never ask questions, now im wanting to know, like why is the grass green?....not because I want to change its color, LOL


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Post subject: Re: PreAmp Tube question
Posted: Wed Sep 18, 2013 1:31 pm
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"Long/short plate" = physical length of the anode plates in a tube.

http://www.ebay.com/sch/i.html?_nkw=12ax7+long+plate



"Long-tailed" = type of phase inverter circuit topology. Long-tailed PI typically use each half section of a duo-triode (like a 12AT7 or 12AX7) to drive the top and bottom halves of the signal sine wave. Almost exclusively used by Fender for their push-pull amps.

http://www.valvewizard.co.uk/acltp.html


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Post subject: Re: PreAmp Tube question
Posted: Wed Sep 18, 2013 1:34 pm
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Short plate 12AX7/ECC83:

http://www.tubemongerlib.com/gallery2/d ... ngland.jpg


Site only allows two URLs per post now.


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Post subject: Re: PreAmp Tube question
Posted: Wed Sep 18, 2013 1:51 pm
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I get some strange limitations sometimes too. Sometimes, a long post will result in the "technical difficulties" screen. Once, I just removed a "/" (slash) from between two words and then the post went through. :?

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Post subject: Re: PreAmp Tube question
Posted: Wed Sep 18, 2013 2:47 pm
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shimmilou wrote:
I get some strange limitations sometimes too. Sometimes, a long post will result in the "technical difficulties" screen. Once, I just removed a "/" (slash) from between two words and then the post went through. :?


+1

The server also barfs occasionally when using *some* special character keys.

Arjay

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Post subject: Re: PreAmp Tube question
Posted: Thu Sep 19, 2013 5:30 am
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so being a long plate or short plate, its dosnt matter which you use , or is there different specific uses for each, and the "long tail" has a specific use in a fender?


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Post subject: Re: PreAmp Tube question
Posted: Thu Sep 19, 2013 7:10 am
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The only criteria for short plates that I know of, is early military specification for tubes to handle excessive vibration & G-forces. This criteria led to the creation of the 5751, in the early 1950's. A tube that has close to 12AX7 electrical qualities (albeit lower mu), but higher vibration and G-force tolerance for use by the Air Force and Navy. 5751 plates are shorter than the typical 12AX7 of the 1950's.

I know of no military or consumer specifications for long plate tubes. Tone criteria are specific to each tube. And not a direct function of the plate length.

The "long-tailed" is another name for a type of differential amp, phase inverter configuration. Phase inverter is usually the tube just before the output or power tubes, in the circuit of a push-pull amp. The inverter converts the signal from the gain stages into two parts. The upper & lower portions of the signal sine wave. For amplification by each specific tube or set of tubes of the push-pull pair(s).

Typically, a duo-triode like the 12AT7, 12AU7, 12AX7, 6CG7, 6SN7 or 6SL7 are used as long-tailed phase-inverters. If you look at a typical Fender amp that uses the long-tailed PI --- you'll see the symmetric layout of this section. Check the schematic and the 12AT7 PI, just before the 6L6GC output tubes. Note the symmetry in the layout.

http://www.webphix.com/schematic%20heav ... _schem.pdf


Valve Wizard has a good section on the AC-differential amp or long-tailed phase inverter:

http://www.valvewizard.co.uk/acltp.html


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Post subject: Re: PreAmp Tube question
Posted: Thu Sep 19, 2013 2:54 pm
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Thanx Beemer, and to all the other responders, that clears up a lot. This helps with searching for replacement tubes and somewhat knowing what im looking at and what im really needing.
With the Gold 10's mounted, You think a pair of side draft webers would add more bottom en :lol: d without loosing tonal clarity?


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Post subject: Re: PreAmp Tube question
Posted: Thu Sep 19, 2013 4:27 pm
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Side draught Webers have much better response than the stock side draught Solex.

:D

PS: A 12AT7 that is strong testing, quiet, and balanced across the two triodes makes a big different in amps that use a long-tailed 12AT7 PI.


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Post subject: Re: PreAmp Tube question
Posted: Thu Oct 03, 2013 10:31 pm
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BMW2002Ti wrote:
PS: A 12AT7 that is strong testing, quiet, and balanced across the two triodes makes a big different in amps that use a long-tailed 12AT7 PI.


+1

These attributes are also critical to the operating parameters of a standard 12AT7 when used as a phase inverter in a class AB configuration.

Arjay

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