70s Ampeg SVT preamp tubes

gearHed289 · June 26, 2013, 05:56:07 PM

Can someone tell me qty. and type? I can't believe I can't find this on a Google search, but I'm just hitting dead ends and getting pi$$ed off. LOL! Trying to avoid pulling mine out of its flight case and taking all those weird headed screws off the back to see for myself. Thanks!

Psycho Bass Guy · June 26, 2013, 07:35:23 PM

The preamp tubes in an SVT in the top ought to outlast YOU. Why do you want to know? 12DW7's do have problems with microphony, but that's usually an issue from the get-go. Old preamp tubes don't generally become microphonic, especially since the SVT has a shock-mounted chassis. I'll be happy to answer your question, but I may save you a lot of time, trouble and money by asking you the things I am.

gearHed289 · June 26, 2013, 09:15:38 PM

Cool, thanks. It just seems to be getting a little harsh sounding, like I'm overdriving the input, so I assumed it was pre tubes. It's been well maintained, but I haven't changed any tubes in years.

Psycho Bass Guy · June 27, 2013, 06:26:44 AM

It probably needs a bias adjustment for the power section. As the power tubes age, they emit less easily and require more input current drive to reach full power. This makes the amp seem more harsh and less punchy. Since bias is an emission limiter, you just back it off and get back the lost gain. It's perfectly normal and your power tubes are probably fine; they just "settled in."

gearHed289 · June 27, 2013, 08:28:21 AM

Makes sense. I'm hoping I can get my amp guy to do a quick, on the spot bias for me. Love the guy, but I know if I leave it there, it will be in the black hole indefinitely, and I'm smack in the middle of festival season! Should have planned ahead I guess... Thanks for the advice man!

Psycho Bass Guy · June 27, 2013, 06:27:34 PM

I can do it by ear with my playing since I'm so used to how the amp is supposed to react. If you feel like you want to give that a try, PM me and I'll give you some instructions. So long as you are extremely careful, cautious, and observant, there should be no danger to you or the amp, but it's not a procedure to be undertaken lightly.

bobyoung · July 06, 2013, 04:09:36 PM

Buy a cheap digital multimeter for 10 or 20 bucks, its very easy to do as long as you can read the multimeter. All you do is pop off the front grill and it is self explanatory.

Psycho Bass Guy · July 07, 2013, 02:06:49 PM

The biasing instructions on the amp are just general voltage specs for 70's high quality power tubes. With most modern tubes, it will be too much, and the amp will be too cold.