12AX7 Preamp Tubes

[blueswaite]

The preamp tubes say in a Vox AD120VT will it take out some of the monophonic ringing say using a 12AU7 with less gain. What is the advantage or disadvantage of certain tubes. My understanding is basically less gain.

[UnklMickey]

I think it's not a good idea to change the type of tube in that amp. That tube is not used in a conventional manner as a preamp tube. It's used to emulate a power tube. The signal that tube sees is relatively large compared with a tube in the early stages of a pre-amp. If it shows signs of being microphonic, I suggest getting a better quality tube of the same type.

[sumgai]

bluesy,

Unk's right, as usual. If you change the gain factor, you also change the harmonic tonality of that whole section of the amp. The stock 12AX7 is just about the highest gain tube available in a dual triode package, the gain ≈ 100. For the 12AU7, however, the gain factor ≈ 20, yielding only 1/5th of the original gain factor! Lowering the gain by that much will probably "mellow down" the overall sound so much that the amp will very likely no longer sound like any of the emulations found on the top panel switch.

If you have a ringing sound problem (a good indicator of a microphonic tube), don't replace the tube just yet........ Instead, first tap the tube physically (with your fingernail - it's hot!) while the there's no signal at the amp input jack. If you hear anything more than that tap out of the speaker (like it suddenly squeals, or the tap carries on like a reverb), the tube is probably bogus. If the replacement tube does the same thing, buy/borrow a different brand of 12AX7, and see what happens.

For most cases where a tube will be operating at moderate or high gain (like this scenario), I recommend a 7025, a direct replacement for the 12AX7, but it's more rugged (nearly military spec quality). It costs more, and doesn't last any longer, but it should be quieter, and withstand the rigors of Rock N' Roll more easily than the run-of-the-mill 12AX7.

HTH



sumgai

[blueswaite]

Thanks for your replys. I went to Guitar Center and checked out several Voxes, Fenders, Ect. and the ringing was there from using several different guitars. years ago I had to have my ear drums removed so I may be hearing this frequency alittle louder than normal. Several individuals at GC said the amp and other amps sounded normal to them. i will be changing the tubes to a better quality one though.

[sumgai]

bluesy,,

blueswaite said:

.......
years ago I had to have my ear drums removed

Errr, excuse my non-medically qualified education, but aren't ear drums sort of like, necessary to the hearing process? Or did you mean that you had them removed and replaced, as in updated to the newer Version 2.0 ear drums?

Damn, wish I could afford to do that! ;D


sumgai

[blueswaite]

You can still hear I for get what the inner part of the ear is that actually lets you hear.But as long as that is not damaged to bad you can still hear. I have about 50 %. Thats why people can use hearing aids to get the level of hearing back.

[sumgai]

bluesy,

You can still hear I for get what the inner part of the ear is that actually lets you hear.But as long as that is not damaged to bad you can still hear. I have about 50 %. Thats why people can use hearing aids to get the level of hearing back.

Holy Smokes, Fireman! You mean I can get back up to the 50% level? Really?

;D

Nah, just joshin' ya, I"m not too bad off. But I truly didn't know about that eardrum thing, I'll have to look into it. Just for curiosity, mind you, I have a set of expensive ear plugs I carry at all times to all places. Hope they work.... the way I hear it, they aren't gonna issue me any new ears until they do a recall on my entire bod.

Say, how're you comin' along on that microphonic tube problem?



sumgai

[blueswaite]

It comes and goes. Got ahold of Seymour Duncan and with it happening with all my guitars it is more than likely a microphonic tube in the Vox. I talked to Groove Tubes and they felt the same and suggested for my amp to put GT 12AX7-C's in. It comes and goes. If I play with a liter touch it is barley noticable. But will be changing the tubes.

[GuitarTechCraig]

I have a friend with an AD50VT and his preamp tube (an EH 12AX7) was microphonic as well. Replaced it with a JJ ECC83S and it hasn't been ringing since.

[sumgai]

I've never been impressed with Electro-Harmonix's tube offerings. Don't know what it is, and I don't really care, but they just don't seem to "get it", quality-wise. Fortunately, there are other vendors out there, and for all the GT bashing going on, Aspen Pittman is still one of the best in the business.

Tubes are one of the rare areas wherein it ain't necessarily true that you get what you pay for! There are con-men under every rock, ready to take your money and give you junk in return. Just be aware of that, as you search aboot while looking for a reasonable value in your tube purchases.

And it also ain't necessarily true that all foreign-built tubes are junk, either. While most Chinese tubes are of lesser quality than we might like, they are usually priced accordingly. (A 'C' designator usually denotes a Chinese-built tube.) JJ (nee Tesla) is usually pretty good, but they had some quality control problems a few years back. Sovtek and Svetlana are quite good for the money, I've never had any problems with either of them.

I'm sure that there will always be "horror" stories about Brand X tubes, or petty diatribes about how Brand Y is the best tube of all, but in the end, it's your money - keep your emotions in check, and shop wisely, you won't be sorry.

And don't be afraid to experiment! If you have to purchase more than one tube, do so, and craigslist the ones you don't want to keep. (But do keep at least one spare, you know how Mr. Murphy loves to party!)

HTH


sumgai