Tube combination

[mudhoney]

Hi!
I've got a '94 Fender Twin Amp and i've got some new and nos preamp tubes. The question is: what tube you'd put in what slot?

The tubes layout is as it follows :

V1 12ax7 1st gain stage for all channels.

V2 12ax7 2nd gain stage for the Vintage Drive Channel.

V3 12ax7 2nd gain stage for Channel 2.

V4 12at7 2nd gain stage for Channel 1;
3rd gain stage for Vintage Drive Channel & Channel 2
effects loop return amplifier.

V5 12at7 Reverb Driver.

V6 12ax7 Reverb Return Amplifier.

V7 12ax7 Linked to the Preamp Out/Power Amp In

V8 12at7 Phase inverter.

And i've got these tubes:

2 RCA 5751 (NOS)
2 Tungsram 12AX7 (NOS)
5 Sovtek 12AX7wa
2 JEN 12AT7 (NOS)
1 Telefunken 12AT7 (NOS) [working, but slightly microphonic]
1 Electro Harmomix 12ax7eh
1 Groove Tubes GT-ECC83-S

Best combination?

[Chuck H]

Tubes vary greatly. You need to plug them in to see what they sound like and then it's still hard to know what your listening to unless you can swap them one at a time in the same position, leaving all else equal, to hear the specific differences. Amps are also variable. With +/- tolerances on components and ao many different designs, speakers, etc. Once you asses the properties of each tube you then have to determine what those properties will do in each socket position. Even then you'll have specific interactions as you change tubes around that alter the ideal, ie: tube A in V1 sounds good but now with tube B in V2 tube C sounds better in V1. If you want to tweak the amp with preamp tubes it can be an all day chicken with as much trial and error as actual scientific observation. Tweaking with preamp tubes is something I do. But I use simple, non master volume, no reverb type amplifiers that have only three preamp tubes. I also have a good understanding of circuits and experience with what tube properties I like for certain positions on certain types of amps. That said...

You can divide and concur. The AT7's have limited application and arent used in any critical amplifying roles. Plug those in however you like and maybe swap them around a little to see if there is any subtle differences. Odds are good the JEN's will be similar with the exception of microphony. Which may not matter in the applications they're used.

The two 5751's are lower gain tubes. Don't use them in "drive" circuit high amplification stages. Probably the first and second stage of your "drive" channels. They may sound good in the early stages of the clean channel.

Burn the Sovtek WA's... Not really, but IMHE those tubes have no tone and only two uses, They are good as early gain stages in very high gain amps that tend to be microphonic with most tubes. The WA's are nearly always completely non microphonic. And they're OK for rolling the ice pick highs off in amps that are too bright because they are dull sounding. Use them only if you must for one of the above reasons.

Start with the least microphonic Tunsgram in V1. Great sounding tubes usually and if stable will set the voice in the early stages. If not stable then you must try the other 12ax7/ecc83 tubes and a WA as a last resort.

I have no experience with the EH ax7 and GT types change like the weather so I can't offer commentary. I'll guess they'll find use as V6 and V7. Less critical amplifying stages.

So, I'd start with: V1 least microphonic Tungsram, V2 other Tungsram, V3 ?, V4 JEN, V5 JEN, V6 GT, V7 EH, V8 Telefunken

V3 will have to be a WA or 5751 to start with. You may find microphonic issues that require moving less microphonic tubes into sensitive positions and so end up subbing the microphonic tubes into less sensitive positions. With eight friggin preamp tubes and a somewhat limited selection I don't envy your task.

[mudhoney]

Thank you for the clarity of you'r answer!