OT- 12ax7/12au7 swap

[jazzyfingers04]

I know this is ot, but this is where I come for techie advice... so please forgive me.

I just blew 4 12ax7s in my amp (they were dying anyways) but after I have everything checked out to make sure its ok, I was wondering what effect a 12au7 would have on volume?

Correct me if I am wrong, but the 12au7 has a mu of 17, whereas the 12ax7 is 100. In this case, the mu is rated on db boost (IIRC)... So would swapping out 12ax7s for 12au7s cause a drop in preamp vol, leading to having to work the power tubes harder for the same vol output? IE: less preamp vol means that I can push the ptubes harder to acchieve more poweramp distort at similar volumes?

[Lonestarjohnny]

You should'nt hear much of a volume drop normally speaking, but on my tube driven pedal's swapping out to the 12au7 can make a pedal or an amp sound more smoother, and the O/D sound more natural, some amps have to much gain anyway for my taste, if you choose say a mullard ecc 82 or 83 you loose some gain but the tone will darken and have a thicker, warmer tone. I love these for thier smooth compression for blues and they can make some of your pedal's sound less cutting and not so Boomy.
JD

[AL]

I just swapped out pre-amp tubes in 2 of my amps. I'm not sure about 12AU7's but there was a noticeable volume difference between the Groove Tube 12AX7's and other brands. The GT's were noticeably louder. I ended up using the new Mullards in my pre-amp sections on both amps.

Just wondering - you blew all 4 tubes!!???  :shock: Ouch !! How did all of them go at the same time?

AL

[Jim Jones]

How did you blow up preamp tubes?  That makes no sense to me...  They get weaker over time but I've never heard of a catastrophic failure of a preamp tube before...

12AU7's would be bad swaps for 12AX7's.  The pinout is the same so you won't hurt anything, but you'll have no gain (in comparison) and they'll be misbiased to boot.

Jim

[jazzyfingers04]

Well, I was cranking my amp (traynor ygl3 mkIII combo). and I had the preamp set at around 7.5 and the power amp was opened up... I was getting great poweramp distortion, and the amp was just screamin. Then I kicked on a trebble boost for a solo and the amp goes microphonic sounding.  :roll:  So I kick off the pedal and about 2 seconds later the sound stops comin out of the amp   :oops:
Well, I shut 'er down. and waited for a while. Then I turned it back on in standby and looked at the tubes. the v1,2,4,6 sockets wouldn't even light up. So I looked at the schematic, and that is all of the preamp tubes for gain.  
So what I was hoping was the using 12au7s in the future would allow me to bypass the preamp sounding distortion and allow me to crank the poweramp for some excellent crunch.

[AL]

I don't know much about amps and I'm not familiar with your Traynor but ... could you please explain what you mean by I had the preamp set at around 7.5 and the power amp was opened up. Am I missing something? Are the channels on the amp labeled as this or do you mean you had the Master Volume cranked and the Volume at 7.5? :? And, if ALL of your tubes aren't lighting up your problem may be somewhere else. Again, I'm no expert but it seems odd to me that all of the tubes would go at the same time.

AL

[Lonestarjohnny]

I would pull the preamp tube's that are blown and check the voltages on all pin's, make sure your getting 6.3 V. A.C. on pin's 4,5 to pin 9, sound's like you might have a short between your heater voltage and a higher D.C voltage, unless you just had very old, weak tube's that were all ready to blow and kickin your pedals on put a strain on them that blew them up. be hard to tell with out seeing the voltage's inside your preamp, and do be careful if you do this yourself, High Voltage is Dangerous,
JD

[Paul Marossy]

I don't know about anyone else, but I noticed a very large drop in volume when I replaced some tubes that are normally supposed to be 12AX7 with some 12AU7s. At least this was the case in my Fender Hot Rod DeVille...

[Lonestarjohnny]

you will notice more of a drop in high gain, cha. switching amps like the Jr. but not enough to notice in a B/F fender amp, if your only swapping one tube out, I think you would also ruin the tone of your amp by switching all pre's out for lower gain tube's.
JD

[jazzyfingers04]

I had the preamp set at around 7.5 and the power amp was opened up.

My amp is MV, so the preamp is set high, around 75% of full power. The Poweramp was running full out (100%).

The trick is, of my preamp, 1 of the preamp tubes is lighting up. the V1 is the non reverb channel and that died a while ago, V2 is the reverb channel, and it was weakening... so maybe it was just bound to happen sooner or later. I can't figure out what V4,6 are... maybe you can help me... there's a schematic at: http://www1.korksoft.com/~schem/bargainbin/traynor_markiii_ygl3_all.pdf


As for testing the tubes, I am taking the amp in to a tech tomorrow. I want to be sure that it isn't something more important than a coupla blown tubes.

[bwanasonic]

In general, checking to see if tubes *light up* is a poor test for 12ax7 types. Some perfectly fine tubes will not visibly *glow* under normal lighting. 12au7 can be swapped for 12ax7 in some applications, but rarely more than one tube (usually the first preamp tube).

Hope your amp is an easy (and cheap) fix.

Kerry M

[jazzyfingers04]

Well, I did a quick tube swap with the V3 12ax7 and the V1 12ax7. I got a light up. So I am tending to think that it is the tubes...

As far as the light up test under normal lighting, my amp is in my basement with no lights around it. I have a lamp that I turned off in order to better see the tubes. I know it isn't a perfect test... but its all I have right now. Like I said I'm takin ma baby in to a doc tomorrow.

BTW, thanks to all of you for a bit of empathy, if not some sympathy.

[Lonestarjohnny]

Check your P.M.
JD

[Paul Marossy]

It seems to me that preamp tube filaments glow more with a 6.3V supply than with a 12.6V supply. Probably because more current is flowing thru them? Anyway, I have had some 12AX7 tubes that I wondered about because I couldn't really see them glowing, but they worked just fine.

[puretube]

visibility of filament-glow varies from one brand to another;

assumed a 12AX7 is correctly wired for either 6 or 12 volt,
it will light up equally with exactly 6 or 12 volt respectively.

[Paul Marossy]

Hmmm... that's interesting. I could swear that when I changed my Firefly's heater voltage from 12.6V to 6.3V, the filaments got a lot brighter.  :?

[puretube]

measured the voltage on the pins?

[jazzyfingers04]

Just an update...
   Took 'er in to the amp techs today, and it doesn't appear to be ONLY the tubes... :cry: We plugged in some new tubes, and those didn't function either. It appears to be something more serious, and so I'm sending it out to be repaired.

[Paul Marossy]

"measured the voltage on the pins?"

Yep. Measured 6.8 volts.

Sorry to hear about the amp, jazzy. Sounds like a catastrophic failure. Hopefully it will be something simple to fix and and it won't cost you an arm and a leg.

[puretube]

...then you need 13.6 volts to get the same "light factor"