Pre Amp Tubes Swap 12ax7's & No Sound

chaosMK · #1 08-24-2011, 08:29 AM

I picked up some EH 12ax7's to try out in my Bass 400, which is a little bit of a fixer upper right now. It had some unlabeled preamp tubes in it with old looking pins (look/texture of a 20 year old paper clip) and one that says "Fender" in V4 (furthest left if you are facing in front of it, furthest from the input jacks).

After a full switch, the pre section fired up but I got no sound. At best I'd get a tiny tiny whisper of a tone after the swap.

After some fiddling around, I could get a tone again with just one of these EH tubes in V3.

Any thoughts on why this batch came out so bad? Could the sockets just need some cleaning for better contact?

beans-on-toast · #2 08-24-2011, 08:37 AM

Some tubes have larger diameter pins. If your older pins were larger, the tube socket contacts may have been pushed out. If this were the case you would need to perform a procedure called re-tensioning the socket contacts. This is a fancy way of saying carefully pinch the contacts closed. This has to be done properly so that the tube socket isn't damaged.

It is also possible that the pins need cleaning with a product like DeOxit.

Make sure that you haven't bent a tube pin while inserting it.

Then again, maybe you simply have a bad tube. It is a bit of a luxury but I like keeping a good set of tubes just for testing. A gold standard that I know works.

runmikeyrun · #3 08-24-2011, 11:26 AM

I had a bad 12AX7 in my amp a few months ago. couldn't figure out why I could hear myself, but it was super quiet. Changed the tube, problem solved. Try subbing in known good tubes.

grendle · #4 08-24-2011, 11:36 AM

Bad tube or a bad socket. One of the 2

coreyfyfe · #5 08-24-2011, 12:29 PM

Quote:

Originally Posted by runmikeyrun

I had a bad 12AX7 in my amp a few months ago. couldn't figure out why I could hear myself, but it was super quiet. Changed the tube, problem solved. Try subbing in known good tubes.

Definitely a possibility. I bought a NOS 12DW7 for my SVT from a reputable seller. Popped the tube in prior to a gig, didn't have a chance to test it at home so I figured I'd test it live. No sound on channel 1. Swapped to channel 2 for the night and all was good. I emailed the place and they were 100% certain the tube was fine, must be my amp, they test all their stuff and guarantee it to be in good shape when it arrives, yadda yadda yadda. If I wanted I could mail it back and they would retest it and send it back to me with proof that it read fine on their instruments. Ok, I mailed it back to them. About 2 weeks later I had a different NOS 12DW7 on my doorstep with no other word from the seller. This one worked fine and sounded great.

Caca de Kick · #6 08-25-2011, 03:20 AM

Could just be a bad new tube. A few months ago I sent my new Titan back to Mesa to get the updates done. And before they sent it back to me they told me they went ahead and put a full set of new 12AX7's in it. Great...

The amp lasted one practice and went dead. 1 tube not lighting up caused the whole amp to not make any noise.
So in went a favorite set of JJ's!
Inspecting the new failed tube with a magnafying glass, I saw the tiny heater wire that attaches to the pin was broken. That wire is as thin as a strand of hair!

But anyway, I like doing an annual cleanup of my amps. I pull the tubes and clean the sockets with deoxit, and I check the tension of pins.

chaosMK · #7 08-25-2011, 10:19 PM

I did some fairly extensive testing with every tube in every slot, after cleaning.

One good tube worked fine in V1-4. The rest gave me a fraction of volume and one was totally dead. Bad batch for sure.

I am not sure what was in there in the first place, but the single Electro Harmonix 12ax7 brought out a little bit more highs, maybe a little more openness on the bottom. In an A/B test where I recorded it, it had a little more sensivity/gain than what was in there with breakup earlier on the B string being slapped.

grendle · #8 08-25-2011, 10:59 PM

I would stick a 5751 in v1 . Or the fist tube in the input section, hell maybe in all of them. But that's just me.

chaosMK · #9 08-25-2011, 11:17 PM

Quote:

Originally Posted by grendle

I would stick a 5751 in v1 . Or the fist tube in the input section, hell maybe in all of them. But that's just me.

I'll think about it, might work out great.

grendle · #10 08-25-2011, 11:25 PM

If you haven't hear them , get one to start and check it out. Get a nos tube though. My fav was a RCA branded Sylvania. Nothing like it.