Tube swap causing buzz

[steve]

Hello All,

I tried to make my late 70's 2 hole Hiwatt sound a little cleaner by swapping the 12AX7 with a 12AT7 in V4 (PI). I verified the correct spot as V4 seems to be in the middle of the 3 tubes close together. I installed a CV4024 12AT7 and voila, alot of BUZZ coming through the speaker. If I strummed, I could still hear the guitar playing through as well, but the buzz makes it unusable. Put the 12AX7 back, everything was ok. Tried the 12AT7 in V1, no buzz. Put the 12AT7 back in V4, all BUZZ again.

At first I thought I received a bad tube, but after trying it in the V1 spot and sounding ok, I think there may be another reason for the buzz.

Has anyone heard this before? Can a bad 12AT7 sound good in V1 but have a BUZZ in V4?

Thanks,
Steve

[mhuss]

It might. The PI position has a very high heater-to-cathode voltage not seen elsewhere in the amp. I would try another ECC81/12AT7, as there is nothing about that tube that would make it intrinsically buzz more than an ECC83/12AX7.

--mark

[steve]

It got a little more interesting. The CV4024 worked fine in my other non-Hiwatt amp in every spot - no buzz whatsoever, so the tube appears ok.

In the Hiwatt PI position:
1. I tried a different CV4024 and a Jan Philips 12AT7 and both caused the buzz.
2. The existing 12AX7 and a 12AU7 I had lying around did not create any buzz.

The buzz sound is like a really bad ground.
If I find what is up, I'll post. If you have any idea's what to look for, I'd appreciate. I received the amp pretty well stock except for the Canadian mod which I reversed; there doesn't seem to be any other wiring change. I'll start checking for wiring issues, cold solder joints, tube socket issues, incorrect voltage levels from the transformers...

hi ho, hi ho

[steve]

Success

There was a full set of the original Mullard 12AX7's when I purchased the amp. I swapped out V3 with a different 12AX7 and the problem is solved. I guess putting a 12AT7 in V4 just exposed some defect in the original V3 12AX7 that couldn't be heard otherwise.

Gremlins

[Chris9723]

You gotta love it when you can find Gremlins on your own!! Just keep them away from water!

[GigawattCustom121]

Success

There was a full set of the original Mullard 12AX7's when I purchased the amp. I swapped out V3 with a different 12AX7 and the problem is solved. I guess putting a 12AT7 in V4 just exposed some defect in the original V3 12AX7 that couldn't be heard otherwise.

Gremlins

Good catch, man.

[steve]

You know, I always read how good and reliable the original Mullard 12AX7's are. I don't think I ever read a bad thing about them, but it seems even they have a limited lifespan.

[mhuss]

Preamp tubes last almost forever.

--mark