ampeg B200R tube bad... Help!

csm672 · #1 09-10-2011, 01:09 PM

My tube has almost died in my amp. It has a 12AU7 in there now. Could I replace this with a 12AX7 or would that mess something up? I am strapped for cash and can't afford a new tube for a couple weeks, but have the 12AX7 already. Could I use this until I can get the right tube? I just don't want to screw something up with the amp by using the wrong tube. Thanks for any help!

**B-string** · #2 09-10-2011, 01:22 PM

You could but it will sound different, 12AX7 is higher gain with less current than a 12AU7. You will probably need to change your settings to make it usable. Are you sure it's the 12AU7 thats bad?

Windreaper · #3 09-10-2011, 01:23 PM

Hard to say anything definitive without knowing the circuit. It is unlikely that anything bad would happen but the amp might not sound too good with that tube.

nashvillebill · #4 09-10-2011, 01:48 PM

Preamp tubes can last for decades, how are you sure your tube has almost died?

Don't randomly sub a 12AX7 in there, as has been pointed out.

csm672 · #5 09-10-2011, 02:12 PM

Yeah, I hear that pre-amp tubes last forever. But this is the second one I will replace in this amp and the 3rd it has had in it. I know it's the tube because it's acting the same way it did last time it went out. Mine seem to last about 4 years in this amp. Maybe I have a problem with the amp that's causing the tube to go out after 4 years instead of 40. I don't know, I'm not a tube expert or really an electronics in general expert. And I'm money light right now, so that's why I'm wondering if this other tube will work til I get a right one. Bstring, it is the only tube in the amp. Just a tube in the pre-amp, everything else is solid state. It's an Ampeg B200R 15" combo.

nashvillebill · #6 09-10-2011, 02:46 PM

Try re-seating the tube, perhaps it's just making a poor contact in its socket. You might even try a little contact cleaner (Deoxit or similar) on the socket. Clean the tube's pins gently with steel wool if they look oxidised. You may even want to disassemble the amp and re-flow the solder joints on the socket, you may have a cold or cracked solder joint.

I've got a 50 year old amp here with the original tubes in it, and a 60 year old signal generator with the original tubes in it, so it's not just hearsay that preamp tubes can last a loooong time. 3 preamp tubes in a few years would be way too many, I suspect other problems.

nashvillebill · #7 09-10-2011, 02:49 PM

Incidentally, the 12AU7 and 12AX7 actually have two sections in them; they are like 2 single tubes in a common base. If your amp is using both sections, and you sub a 12AX7 for the 12AU7, each section will have something like 40% more gain than originally called for. Multiply that together and the total gain will be twice what it was originally designed for. I would not randomly sub a 12AX7 for the 12AU7.

**B-string** · #8 09-10-2011, 03:01 PM

Well finding schematics for Ampegs is tough for some models since LOUD Tech took over. I found the user's manual and it is the first gain stage and probably a cathode fallower to the SS sections. If you have a passive bass plug into the active input. Distortion maybe present as the 12AX7 is not as well suited to low impedance loads. Try it, but get the combo looked at as it should NOT burn through 12AU7s like that.

csm672 · #9 09-10-2011, 03:36 PM

Well, I think I will just skip the 12AX and wait untill I have the cash for a new 12AU. I pulled the chassis and tube out and it doesn't appear to have dirty contacts anywhere (It may just be the CLEANEST thing in my house!) But I sprayed contact cleaner on the socket and pins and buttoned it back up. No luck. Still has the same hum and has almost no output. I have to turn the gain all the way up to even hear it. Even when I really dig in hard I can't get the overdrive light to come on. I have 5 kids and am still trying to recover from putting them all back into school, so I was hoping I could stick the 12AX7 in there for the time being. I'll just play my SWR back up for a couple more weeks. Last time I went through this same thing, everyone said the tube shouldn't go out that fast, but after emailing Ampeg about it they said they should last 1-3 years. I just figured that for this amp design that was about average. Maybe I should take it in and have it looked at. Thanks everyone!

Mastermold · #10 09-10-2011, 04:16 PM

I love the B200R, had mine fore a few years now and use it as a daily practice amp and anytime I need just a small combo for a gig. Hope you can figure out what the issue is with yours.

JimmyM · #11 09-10-2011, 04:29 PM

Ya, newer tubes don't always last like the old ones, and if you're cranking the gain, it could also wear them out faster. I know everyone says to get JJ's these days, but I've had the best luck with Sovteks for lasting and not such good luck with JJ's.

Windreaper · #12 09-10-2011, 04:37 PM

Those, or give the re-issue Tung-Sols a try someday, if they're available over there.

**B-string** · #13 09-10-2011, 04:46 PM

NOS 12AU7s should still be easy to find.

csm672 · #14 09-10-2011, 05:53 PM

Yes, I love this amp! It just has my sound in it. The SWR I have is a nice amp, but I just constantly fiddle with it trying to dial ME in, and it never is really there. It does sound nice just not spot on. But anyway, I have always put Groove Tubes in this amp 'cause that was in it from the factory. Jimmy, do the Sovteks change the sound or do they just last longer? (I LOVE the sound of my amp.) Do you think the NOS tubes would be better, and if so what brand should I look for? I'm thinking about trying out the new PortaFlex amps from Ampeg just to get away from all this nonsense. Maybe they will sound as good as this B200 and I won't have to deal with the tube! Thanks everyone!

JimmyM · #15 09-10-2011, 06:13 PM

Honestly, I could never tell the difference in tone with any of them. I guess there are differences because people tell me they hear them, but when I've swapped out tubes, they're so slightly different that I can't really pick up on it. But I use NOS when I can because they're much more reliable than current tubes. RCA, Sylvania, GE, JAN Philips...all make really nice 12AU7's that don't cost too much and sound like tubes should. There are expensive tubes you can get as well, but I just use the basic $20-25 pre tubes and they always serve me well.

As for the PF heads, I love them. Not sure which you'd prefer, but the B200r is an iron transformer amp, whereas the PF heads are micros, and micros sound more tubey to my ears. YMMV.

fenderhutz · #16 09-10-2011, 06:25 PM

Is this a US or Vietnam model? Might be something other than a tube.

csm672 · #17 09-10-2011, 06:34 PM

Well I'll definetly lokk for NOS in a couple weeks when I rebound from school starting up. I will have to hear the new Ampegs to know for sure. I played through an original B15 when I bought the B200R but just couldn't afford it at the time. LOVED the sounds coming out of that amp, though! If I only had an extra $1000 bucks lying around I'd be playing that instead! So I'll jam the SWR until I can get my NOS tube and then go try out the new Ampegs next month. Thanks for all the help everybody!