Purpose of 3 12ax7s in 18w tmb ef86

[antm88]

Hi, I've recently bought a ceriatone 18w tmb ef86, and I noticed in the layout that all of the 3 12ax7s that are in it are wired in different-looking configurations (I don't really have much of a clue about electronics as you may have guessed).

The layout

I'm basicly wondering what would happen if I were to use different tubes in each slot (ie all 12ax7s, but different models) - do the 3 tubes kind of work together in the same way or does each specific tube do a different job? If the latter is true then how important is it to buy a set of three matched tubes, and are there specific recommended combinations of different 12ax7 tubes which are regarded to sound much nicer? Or is it better to buy all of the same make?

I figured that 12ax7s would be the best improvement I could make to the amp at the moment - I've heard they last ages (correct me if I'm wrong )

Also, final question - can you acheive more gain with different 12ax7s? (sorry bout all the qs - I'm new to all this tube business)

Any replies would be much appreciated!

Thanks, Ant

[Alexo]

You don't need to match your preamp tubes (12ax7's) at all. You can stick any make 12AX7 from any year in any slot you want and it will all still work splendidly.

Try it, it's fun to experiment with different brands and see what affect they have.

Some brands do have slightly higher gain (Mullards for one, I think) but technically they should all be about the same. You can try substituting a 12AU7, 12AY7, 12AT7, etc, for different amounts of gain, but the 'AX7 has the most gain of all of them.

[antm88]

Thanks very much for the quick and helpful reply! I was hoping that would be the case about using whichever ones you want! Off to look for some tubes now

[tarzanalog]

Here's what I plan to try when I get my 36W EF86 next week.

(I have many different 12A*7 variants to try in different parts of the circuit.)

I plan to try rotating CV4024/12AX7/ECC803S/7025 (in order of gain low-high) at the input of the TMB channel to get the input gain the way I like it. The Realistic 7025s I have are made in Japan w/ gold pins and are the highest gain tubes I have. The Mullard CV4024s are just amazing tubes and while perhaps not the best for taming the TMB channel will probably work great as a driver.

Changing out the PI (V3) will change the way the EL84s function more than any of the other substitutions so be aware of that.

[briane]

every tube sounds different to my ears, even those from supposedly the same batch, and the same manufacturer.

I consider a critical part of bring up a new amp is matching ALL the tubes to the sound you desire. This is not a matching process, it is a selection process. Sometimes I use 12ay7, sometimes 12at7, sometimes another substitute to get the sound I am after. Sometimes I actually use 12ax7's.

Also, I would say your choice of power tubes is as critical to the sound of the circuit as your pre-amp tubes. In my cases I often prefer the chinese 12ax7s matched with some old,old, old sylvania power tubes. And old philips or sylvania 12ax7s are often the bomb in my book..but like I say it is circuit by circuit, amp by amp.

And obviously my choice of tubes changes if I change the speaker.