Tube Question

[Hsb]

Ive noticed lately my Jubilee wasnt sounding quite right, kinda harsh and lacked gain. I pulled the back of it off and gave the pre-amp tubes a tap with a pencil, the tube furthest from the power switch was ringing through the speaker, ok cool, maybe I have a bad tube. I swapped a couple of the tubes around in there, same thing, third tube is still ringing. I try brand new tube I had laying around, still that one location is ringing.

Is that tube socket in need of some replacing? The ringing got louder if I turned the amp up and quieter if I turned the amp down. It seems to only happen in that one tube socket. Shouldnt all the tubes be quiet when tapping on them?

[jeremy]

v1 is usually the first preamp stage and gets amplified through the rest. when you say third tube... which one is closest to the input jack?

id check the wiring to see if anything needs to be touched up before i replace the socket

[Hsb]

The 3rd tube is closest to the input jack, had the back of the amp facing me.

I tried 3 different tubes, new and used, in that one socket with the same results. The other two were fine.

I even swapped the tube closest to the input jack to the socket closest to the transformer and the noise didnt move, it stayed with the socket closest to the input jack.

It really should go in anyways, it has the original 36 yr old caps in it. If I recall, last time it was in the shop, repair guy said one of the caps was starting to bulge every so slightly

[beaubrummels]

Well, V1 (which you are calling 3rd tube) amplifies audio signal by a factor of 100. The other 12AX7 tubes are processing the signal but don't manifest microphonics like V1. Could be just all your 12AX7 tubes are microphonic. Are your tubes long plates (like tall plates in the glass that almost go from top to bottom)? Or are they short plates, where the gray plates are only the middle 1/3 the height of the tube glass? What brand / model number?

Bad caps might have to do with the bad sound, but they wouldn't have to do with tube microphonics IME.

[Hsb]

They Groove Tube 12AX7s in all three slots. I also tried a Mullard and Sovtek in there.

I could try the GTs in my lil H&K to see what happens.

I know the caps wont affect the tubes being microphonic, making more the statement, if it has to go in for the tube socket, Id have the caps done as well or maybe should just have the caps done anyways

[beaubrummels]

The key usually is if they are long plate or short plate. Long plates are notoriously microphonic. Small plates can be microphonic but it is more rare.

[Hsb]

Ill have to take a look at them to determine if they are long or short plate. I appreciate everyone's assist so far.

[marcello252]

short plates could be resonating too if damaged, but unless the amp had a really big shock it's rare

[Hsb]

Got poking around in my amp tonite and figured out some of the issue, Ive had Groove Tubes in the amp for years, both preamp and power tubes.

Last year I ordered a new set of tubes, wanted to try something different so I got Mullard EL34s and a Sovtek 12AX7LPS, a Mullard 12AX7 and a Tong Sol to finish out the tubes.

At some point I mixed a GT EL34 and a Mullard one and that seems to have caused the issue.

Currently I have the 2 GT EL34s, the Sovtek in V1 and the other tubes are GTs.

Im going to leave the Sovtek for a bit but I think I prefer the GTs in V1.

Now that Ive gotten that issue resolved, prob should take it back in to be rebiased and new caps put in while its there.

Last time it was in the shop it was biased for the GTs, seems to def be stronger than it was before I swapped the tube.

Thanks for assist fellas.