12au7 vta board question

I recently installed 3 new mullards 12au7's (not matched) on the VTA ST70 board.

Ive read reference in some threads to putting a different type of 12au7 in the center position of VTA board... im not sure what that is about. Please explain.

There's a wide array of tubes you can use in any of the three positions. The center position tube is the primary voltage amplifier for both channels and will have the most influence of the three on what you're hearing, so it's your best bet and most bang for the buck. You SHOULD get a triode matched pair for the outside tubes, as those handle phase splitting duties and you'll want to balance the output as close as possible.

By now, you may have noticed there's a LOT of options in the 12AU7 family (5814, 5963, 12BH7, ECC82 etc), so a lot of possible choices. That can be trial and error to get the sound you want, and there, some of the member suggestions for what works for them can point you in the right direction. I do quite an eclectic mix of music here on my ST120, and have settled on a nice pair of Westinghouse 5963's with a GE 6SN7 (with adapter) in the center. I was a habitual tube roller till I stumbled across that combination, and have been locked in on those since.

PS ... you'll probably want to avoid the 6SN7 and 12BH7 varieties if you're running an original Dynaco unless you've upgraded the iron. Those tubes will load the amp harder. The Latino and other new production tend to have much stronger iron and can handle the additional load.

Swapping is used to fine tune to audio balance. Not enough bass? Swap in a center tube that complements bass. Too much silibance in vocals? Find a center tube that highlights what you like.

But as Skizo stated the two outside tubes should be balanced. But they can affect sound as well. Even power tubes can affect sound. But center tube seems to have the greatest influence.

When swapping in tubes let them run in for a while before judging. Sit back, relax, and give it a good listen.

Thank you both for the explanation.

It sounds super right now only after a few hours and I am enjoying it. I am also new to tube audio and certainly not doing super critical listening nor do I have any reference.

How important is this to me at this point?

I like to match the two outside triodes (gains) for each 12AU7 the best I can.  The outside 12AU7s need to drive their respective output tubes with equal gains or distortion will go up.  The center input tube gain should be close as well otherwise you may have channel imbalance of the left or right.  But I have not really noticed any imbalance or distortion over one tube versus the other in actually use.  

The matched tube comment is usually directed at the output tubes where with the VTA board you can individually set bias on each tube.  The old design had one bias control for each pair of output tubes.  This made output tube pair matching a priority in the old design.  

I started out on the ST120 with no reference for what it should sound like.  I experimented and found what liked and what I did not.  I kept what I liked and tried something different when I did not.  And I found what was recommended by others may not satisfy my tastes.  So tube rolling was necessary for me to open my horizons on what the amp is capable in my system.

A new amp should run-in before experimenting anyway. If you like the performance then there is no reason to experiment.  Enjoy what you have!

Can the amp be powered up with two KT88's in the front left and right channels, two KT120's in the rear left and right channels? If so in what way do you think the sound will be affected?

Jim McShane just weighed in on this yesterday:
https://dynacotubeaudio.forumotion.com/t3399-run-away-bias-on-my-right-front-kt88-power-tube-vta120
I would add my tuppence-worth that no tube or capacitor (there are probably other components) can be evaluated in anything less than 100 hours. New output tubes especially go through some real changes during that time, whether NOS, or new-issue Russian, European or Chinese.

Dogstar wrote:Can the amp be powered up with two KT88's in the front left and right channels, two KT120's in the rear left and right channels? If so in what way do you think the sound will be affected?

The front and rear are not individual channels. They are 2 halves of one channel that operate in a push-pull configuration. They should be the same tube type and hopefully matched properly.

I mean there's nobody stopping you from doing that but it's asking for trouble.

The most common direct subs are:
ECC82
5814A
5963 (not 5363)
CV4003

The 12BH7 is often found to be quite nice on the VTA board center position. It is not technically a 'direct" sub, as it draws twice the heater current, but the power tranny can easily handle it. Nice 12BH7s are plentiful - TungSol, RCA, Sylvania, are all nice and every single tube will give a somewhat different flavor.
But these guys are correct - give the amp at least a month to settle in.
Oh, and always clean pins carefully before you plug 'em in. Those skinny little pins should be immaculate!

Not surprisingly, I do things ass-backward. I have Sylvania 12BH7 in the phase inverter sections, and a Brimar CV4034 in the center position. Sounds real good to these tired, old ears.

Do whatever makes you happy!! It's just a pleasant hobby.

Gregg R.

It's all about making the tubes work together to give you what you want. The 5963's (not 5363's   ) were actually a bit too dark for me with any one of several center tubes - I compensated for that by pre-processing the signal with a brighter tube in a tube buffer (my Maverick TubeMagic DAC). That was a major improvement. The 6SN7 in the center (with adapter) really brought it all together.

And ya ... don't discount the effect your power tubes will have on the overall sound. Each family has their own strengths and weaknesses. My Psvane KT88's ain't bad, but the TAD KT88str's were better, if a bit clinical. The TungSol KT120's ** are as good as it gets for me. Your results will vary.

** Once again, the Latino amps are plenty strong to handle these, but I wouldn't use these with stock old iron.

BTW ... the "clinical" sounding TADs have opened up a LOT over time. Apparently those take a while to break in. Now that I'm thinking about it, I'll swap those back in (AND re-tension the sockets!) for awhile to log some more hours and see what happens.

sKiZo wrote:It's all about making the tubes work together to give you what you want. The 5963's (not 5363's   ) were actually a bit too dark for me with any one of several center tubes - I compensated for that by pre-processing the signal with a brighter tube in a tube buffer (my Maverick TubeMagic DAC). That was a major improvement. The 6SN7 in the center (with adapter) really brought it all together.

sKiZo,

In your opinion or anyone else who wants to chime in, what is the brightest tube for the center position?  My system is a bit treble shy.  I have an ST70 with JJ 6L6GCs.  The 6L6s made the sound much better over the EL34s. I currently have the 5963s that came with the kit.

Tim L.

Try an RCA clear top yet? Those are quite tight at the top and inexpensive to boot. Pick up three and give those a listen.

sKiZo wrote:Try an RCA clear top yet? Those are quite tight at the top and inexpensive to boot. Pick up three and give those a listen.

sKiZo,

Just so happens that I got a quad of them recently, but hadn't tried them yet.
I installed them in all three positions today and gave it a listen.

You're right!  The high end snaps nicely now.  Just what I needed

Thanks!

Tim L.

Slightly off topic but since others are talking driver tubes I'll post this here.

Back when I was running Tung Sol KT-120's I remember that I distinctly had tubegasms. OK not quite as messy as a standard orgasm but while listening to good recordings with the KT-120's there were times that the sound became more everything. More 3 dimensional. More lifelike. More open. More.

I'm using Genelex Gold Lion KT-88's and though the sound is good I just haven't seemed to experienced the tubegasm experience. I'm even using a Genelex Gold Lion GZ34 rectifier where with the KT-120's I primarily used a Webber SS rectifier.

Has anyone else had experiences like that?

Incidentally I'm still using the TungSol 12AU7's that are now about 3 1/2 years old and just recently have been experimenting with the 6SN7 center position tube.

System synergy is a complex thing. I believe the word is inscrutable.
By all means, try using the Weber again. It may give you that extra little punch which you crave. If you have the time delay relay installed, try the Weber WS-1. It lacks the resistors and will give you a higher B+. To be on the safe side, monitor the B+ voltage at the test point at the center rear of the driver board. It should not exceed 410 VDC.
And make sure you keep an old rag at the ready. Just in case....