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Re: Ampeg: 12AU7 to 12AX7 Possible?

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2/8/1998 3:31 PM

Re: Ampeg: 12AU7 to 12AX7 Possible?

Jeff,

The application is a Ampeg Vt-22 amp. The guy who fixed my amp changed the tube from a 12AU7 to a 12AX7 what would the amp sound like with a 12ax7 because it makes a very loud popping/hum noise and the speaker pops in and out. I was wondering if this could be caused by that change. I am suspicious of the preamp section and I am getting new tubes for it I can't seem to find 6k11 and a 12Dw7 but i'm still looking. Could you give me a cross now?,......
Thanks for you help.
Jeff

2/8/1998 8:14 PM
jon	you can get those tubes from antique electronic supply (www.tubesand more.com)

2/9/1998 8:49 AM

The 12AU7 is used as the driver tube in this amp. I wouldn't run a 12AX7 here; use either a 12AU7 or 12AT7. The plate load resistors are 47K (unless your "tech" changed them) which is an ok match for those two higher current tubes.

The 12DW7 in this amp can be replaced with a 12AX7. The gain will be increased slightly, but if you make this substitution, stick with the 12AU7 driver or try a 12AT7 to see which tone you prefer. You can still get 12DW7's (or 7247) but since it wasn't popular there weren't many mfd. so the price seems high.

I guess you want this amp to stay clean sounding, since you changed over to KT88's. There is a 7027 being manufactured now, and in this amp it's safe to run sovtek 5881/6L6WGC's if you increase the screen grid resistors to 1k or 1.5kohm. Either of these options would have preserved the original sound for guitar. If you're using this for a bass amp, the KT88 or 6550 conversion is a good one.

2/10/1998 2:04 PM

Man, i cant believe these problems you are having are exactly what ive gone through already.
im gonna tell you now,
that speaker popping you are having(probabably with the amp cranked)
i think its microphonic output tubes,
i had that same shit in my v4b head, and i eventually suspected that.
sure enough one was microphonic, and loud playing with the head mounted on the cab triggered off this big thumping that would be triggered off by a hard slam on the strings.
i took the head off the cab and solved the problem.
but in your case its a combo so you might have to swap in newer tubes.
try and plug the head into a separate cab and see if the problem dissapears.

2/10/1998 2:20 PM

Hey Jon,

Yeah,....It does pop but not nessasary at high volumes,....also there is alot of hum and shit. When i pull the preamp tubes everything is fine. no hum, no noise. I noticed that a 12ax7 isn't lighting up. I am going to get some new preamp tubes this week. The 6K11 seems to be the hardest one to find. The Kt-88's are brand new, they are military issue. I buy my tubes from a military surplus outlet. They have been tested. So it leads me to wonder if the preamp tubes are at fault. after all when i pull them the amp becomes stable.
Thanks,
Jeff

2/11/1998 8:32 AM
Jeff Lacio	6K11/6Q11 at Antique Electronic Supply = $7.20 (1997 Catalog) 602-820-5411 (Phoenix, AZ). Buy three and be done with this!!!

2/12/1998 2:50 PM

Do you have a schematic for this amp? They're available on the net if you need one.

Back in your original post you stated that the speaker was fluttering and you have hum. This is indicative of bad power supply decoupling filters, causing this "motorboating". Could be something in the filter can ground connection & standby switch part of the power supply circuit.

Regarding the results you get plugging in or pulling the driver tube, what happens if you pull all the preamp tubes except the driver. Is the hum still there, but discontinues when you pull this tube? If so, have the coupling caps been replaced from pins 1 & 6 of the driver to the power tube grids?

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