Buzz, but only with JJs in CF position?!!

[Gaz]

I noticed an annoying little buzz coming at the CF position of a 5F6A. The buzz would completely go away when the CF's driving stage was grounded.

The amp would still buzz with no guitar and the gain controls turned all the way off. The amp has a "humdinger" (or "hum balance") pot across the heaters, and I noticed that I could null out the buzz completely! But...

For any given setting on the gain controls, the humdinger had to have a different setting. WTF?!

Now, good thing for me, I had another amp that was almost the same without the buzz, and the only significant thing I could find between the two was the tubes - one had all JJ 12AX7s, and other had a Tung-Sol reissue, Mullard re-issue, and NOS RCA black plate.

Sure enough, I switched the tubes and buzz went completely away. And sure enough, the buzz came back in the other amp! After some more tube swapping I narrowed it down to just the CF position. Every single JJ would buzz, while any other tube would be quiet as a mouse.

[surfsup]

Are your heaters elevated to a dc voltage? Just curious....

[Gaz]

Yeah. And I tried it both ways just for kicks. Removing it and grounding the wiper of the humdinger produced a low hum.

I guess I didn't really say in my original post, but I tried just about everything before I thought to swap the tube in that position. I really think it's something inherent to the JJ design in this amp, and not either/or. I really wish I could explain it though. Definitely something to do with the heaters.

[Cliff Schecht]

Are the heater elements spiral-wound? If not then that tube is probably not well suited for cathode follower duties.

[Gaz]

Yes, the JJ is a spiral filament, however the tubes that were quiet (Tung Sol reissue and Mullard reissue) were also spiral filament. Not sure about the RCA black plate. Seems to be specific to the JJ. I have tried a bunch of JJs too.

[surfsup]

I elevated my heaters on both amps i have with CFs to about 70vd off b+ and there was less hiss/crackle afterwards (at least to me it seemed like it) but i don't have enough amps built under my belt to be an authority on whether that was the reason - but it was the only thing i changed...so i don't know. Your observation is interesting. When i get home in a couple weeks i'll try to remember to swap some CF tubes. Unfortunately i've been on the road for two months solid.

[Milkmansound]

I have had terrible luck with all JJ tubes in all positions lately. I think they must have put out a bad batch or two. Had to send quite a few back.

Someone here said to stick to the gold plated JJ's because they take the best of the tubes that they test and send them for plating but I have not tried any yet

[Gaz]

Interesting about the JJs. The one's I have are Ruby branded from Doug's Tubes and from Eurotubes, and a couple untested ones from CE. They behave exactly the same, and I have a bunch to try, so I don't think it's a batch thing.

I have a feeling that they wouldn't do this in every design either, otherwise I'd be able to find something else about it online, which I haven't yet.

It is a very subtle sharp buzz, and I don't think many would notice it, as it's quieter pickup buzz, but it's super annoying because it's still present with the guitar volume OFF. Wish there was another way to make it go away - tried shields, shielding heaters, lead dress, multiple humdingers, etc. Only thing I haven't done is DC heater, which I'm sure would fix it.

@ Surfsup, I'd like to hear if your amp's any noisier with a JJ in the CF position - thanks for trying that. And sorry to hear you've been on the road so long, but I bet you've had plenty of time to come up with some cool ideas for new amps

[Cliff Schecht]

So what do you guys who use modern tubes put in your amps? I'm especially interested in what the guys who build "production" amps (or at least build amps to sell to customers) use with the amps they sell.

[Gaz]

Well, I used to use JJ

Seems like more and more people are using Chinese tubes. There must be some consistency there.

[Structo]

If you use JJ tubes I would buy them from Bob at www.Eurotubes.com

That's all he sells and he screens them.

[NickC]

If you use JJ tubes I would buy them from Bob at www.Eurotubes.com

That's all he sells and he screens them.

+1

I get mine from Bob/ Eurotubes. Excellent vendor! Really top notch!

[Gaz]

Guys, already said some of the JJs were from Eurotubes. Nothing to do with the vendor, it's the tube itself.

[Structo]

Sorry about that, missed it.

I read the word is out that the Tung Sol RI 12AX7 doesn't like CF duty so maybe the JJ is the same way.

VhkMax usually around 100v but some tubes don't like that much.

[Gaz]

Just wanted to post an update. Still no buzz with any but the JJs. Settled on a Penta branded Chinese tube. Best tone too besides NOS.