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I spent today trying out my sIII with some different tube configerations and was wondering how balanced do tube have to be in a cathoid biased amp. I tryed 3 sets of used old stock power tubes and although they all were a little low at pin 7 330-339 each pair matched within 2 volts is this OK.

Additionallly I did some toying around with pre-amp tubes mostly old stock some used some NOS and here's what my ears told me. Number one if you have a tube thats even a little microphonic V2 will let you know it when you turn up the gain. I kept a good Mullard or RCA in V3 and switched tubes in V1 and V2 all 12AX7s. I found that with the Mullard in the PI there was more crunch and dirt as opposed to the RCA.

RCA's clean, more headroom, later breakup, less crunch, less Marshall

Tungsols (old) nice sound, quicker breakup creamy with lots of sustain when they broke-up but less crunch.

RFTs quicker break-up, smooth distortion, good sustain less crunch but definatly British sounding. I love these in my original Super Champ makes the amp much more usable when cranked.

Mullards all old stock used but tested good, crunch, crunch and more very much Marshall sounding, these seemed to bring out the Plexi sound. Less sustain than the Tungsols and RFTs.

I also tried some brand new EH 12AX7s really didn't cut it out a set of 4 2 were very microphoinic.

Nothing technical here just changing tubes and playing, I kept the mullards in as I love the Plexi tone.

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JGK

Thanks JGK
Good to know when i walk down to the tube store.
I'm lucky it's only 1/2 block from my Apartment.
Dave in Portland Oregon

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Love is a small Cat on your amp purring as you play

Thanks for the info. I have a friend mailing me 4 Brimars tubes I want to try in my sIII plexi. Also, an ANOS Telefunken 12ax7 but I am mostly interested in the NOS Brimars. I have always used Brimar with good results in my old Fender amps. This will be my first time trying them in a Marshall type circuit. I

Try some new Tung Sols too. They are proving to be very nice. I put them in the JTM45.

I move a set into a TC15 and it made a big difference.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

My Brimmers and the Telefunken 12ax7(s) came today. I'll try them maybe in the morning while my daughter is at preschool and I can crank the amp. Now I can also install the NOS Phillips EL84s.

I want to measure the preamp voltages before and after inserting the NOS tubes. I have not taken any measurements since my amp was first turned on and it should be easily approaching 100 hours by now and thus breaking in.

Do the voltages obtained at various pins offer any means of characterizing the tube or the tone of the amp? Can I conclude anything useful regarding these measurements? Can they characterize ANYTHING about the tube such as GAIN or TRIODE MATCHING or whether the tube is fresh or spent?

For instance what do the values at pins 1 and 6 mean?

What do the values of pins 2 and 7 mean?

How does any of this vary by position, like v1 vs v3? Can this help me quantify and select a tube for v2 vs v1? Stuff like that....

Can you use these values to determine if your phase inverter tube is "balanced"?

Any info you guys have on this would be usefull? Maybe links directly related to this?

Thanks

You can try the "Resource" section here for starters. Lots of useful info:

viewtopic.php?t=1210

Happy reading!
Joe G

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Some assembly required,
Brain sold separately...

I would assume that readings on the 12AX7s at P1 and P6 would give you some idea on what each side of the tube is putting out and how balanced the triodes are. On the power tubes P7 would also give you some idea on power output and how close they are balance. This is just my obsevation and I am definatly not a techno guy. JGK

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JGK

From what I read in the above suggested info these measurements would not really provide sufficient criteria match a phase inverter tube. It would another method of matching with one criteria such as output and thus a type of static matching. The info put out by Groove Tubes claims this to be insufficient. Appropriate matching has to be done across time and on a scope. So they say. I saw some traces of tubes that were
'static matched' and you could see how it was a little off when looked at on a 'scope but to my eyes it was close enough for rock n' roll (especially if their estimate of only 1 in 50 12ax7s have matched diodes is true).

You'd need to be using precision resistors to get any meaninful matching measurements and we're not there by a long shot.

It can be beneficial to have matched triode sections in the phase inverter because of how the tube is used in the circuit. Ideally you want to balance the drive to the tupower tubes. For other parts of the pre-amp stage in guitar amps this is not necessary.

Do you think Fender or Marshall did that?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps