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Post subject: does Pro Jr require balanced phase inverter tube?
Posted: Sun Aug 30, 2009 9:19 am
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Aspiring Musician
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i'm gearing up to buy some tubes for the PJ (want to have some spare tubes on hand). i'm leaning towards the standard Retube Kit from Eurotubes (all JJ's). some of you may know Bob automatically gives you a "balanced" tube for the V2 spot. He claims the PJ can be fussy and requires V2 (phase inverter) to be balanced.

i've been reading many posts that deny any need for balanced PI tubes.

part of my question is does the PJ utilize the Schmidt Long Tail design? (which as i've read, totally defeats all need for a balanced PI tube)

i'm only trying to learn if the extra $3.00 for a balanced tube for the Pro J is necessary. thanks


Last edited by msvolpe on Sun Aug 30, 2009 5:01 pm, edited 1 time in total.

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Posted: Sun Aug 30, 2009 9:57 am
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Aspiring Musician
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Joined: Fri Aug 21, 2009 1:45 pm
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From everthing I've read about phase inverters, they seem to be the harderst working tube in an amp. I have asked several tube amp repair guys if it's worth the extra cost and they all seem to agree. Therefore, I would say cough up the extra $3 and get it (you could skip lunch one day to make it up :D ). I purchased 12ax7's with matching triodes for my gain stage and phase inverter on my pignose and they do make a difference. I am currently looking to purchase tubes for my twin reverb and will be doing the same, balanced triodes for v1 and v6. good luck and enjoy.

If you have time, here is a link that can explain almost any question you might have regarding tubes. Read the section "Technical article on the importance of a matched output section and matched phase inverter / driver tube"
http://www.guitaramplifierblueprinting. ... tinfo.html

Hope this helps.


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Posted: Sun Aug 30, 2009 4:59 pm
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Aspiring Musician
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Joined: Mon Jul 16, 2007 9:58 pm
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thanks for that article

so this excerpt should be what addresses the question...

quote: "When I scope an amp in the lower frequency region, the vast majority of the time, the upper and lower parts of the sinewave are not even close to
equal. This is more disparent than just a slightly mismatched set of output tubes. At this point, I install a matched phase inverter / driver.

The problem with phase inverters, is finding a matched tube. You have to remember that a 12AX7 / 12AT7 etc., is NOT a single function tube as an
output tube. It is TWO tubes (two triodes), sharing a single bottle.

VERY FEW TUBE COMPANIES MATCH THE A AND B SIDES OF PREAMP TUBES. They warrant the tubes to work, and warrant them not to be
microphonic, but do not say they are matched. This is not any bad commentary on tube suppliers .... to do this matching is time consuming and
requires specialized equipment. If you can find somebody that has a Tektronix tube curve tracer, and bring them a bunch of tubes, maybe you will
be lucky and find a match. There are a few tube vendors on my feeble little website that do offer matched preamp tubes, mostly those dealing in
high end audio applications.

Matched phase inverters and output tubes are one of the reasons some amps "sing" and others are pedestrian compared to their brothers and
sisters." (/ quote)

now, let me know if i'm understanding this right...the 12ax7 is two different triodes in the same bottle, so technically its like doing two things at once. in the case of the phase inverter, each triode in that tube is responsible for driving one of the power tubes. it splits the signal in half, and i'm guessing each triode is responsible for each half. henceforth, if the 2 triodes are not balanced, then the power tubes are experiencing different wavelengths (compared to one another), if thats the right term to use here.

the thing that confuses me is tubes can lose balance overtime, and on top of that, the amp circuit itself can be rather unbalanced by nature, so perhaps it's all a waste to use balanced PI tubes?

now granted, i realize there are already debates about this showering the internet, and the true answer has yet to be defined, but i figured "what the hell i'll start another discussion like evryone else" :roll:

NOTE: Thread title changed...I figured the first title would scare people away lol


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