60 and 50 cycle hum

[Em7]

Tube amps are notorious for 60 (North America) and 50 (most of the world) cycle hum. The primary source of this type of hum is AC filaments. Many amps avoid this problem by powering the tube filaments with DC. Anyone who has encountered a blackface or silverface amp with its original tubes noticed that the amp had 7025s installed, especially the silverfaces. The 7025 is a 12AX7 with a twist, pun intended. The 7025 was a Fender STR (special tube request) tube. The STR in this case was for a 12AX7 with reduced hum when the filament was powered by AC. The 7025 has a spiral wound filament. If you have amp that has 60 or 50 cycle buzz (e.g., the buzzy sound one gets when playing a non-humbucking guitar under fluorescent lights), it is worth looking for genuine 7025s. They do not need to be NOS. Pulls are fine as long as they are not microphonic. Alternative commonly available current production tubes with spiral wound filaments are the 12AX7EH and 12AX7LPS (only in heads). However, old production 7025s are better than current production tubes. As I mentioned earlier, pulls (used) are fine as long as they are not microphonic. Preamp tubes last forever.

 

[RickP]

I’d alway heard techs say a 7025 was “the same as a 12AX7.” I didn’t know there was a construction difference within the tube. Good thing to keep in mind!

 

[Em7]

As far as operating parameters go, a 7025 is identical to a 12AX7. Where the two differ is that the 7025 has a spiral-wound filament. That being said, many 12AX7s that were made in the 60s and 70s are actually relabeled 7025s. The same kind of thing happened to the 6BQ5 (American designator for the EL84). American tube companies produced a super EL84 called the 7189 (there is also the 7189A, but it has a slightly different pin-out). The EL84 has a maximum plate-to-cathode voltage of 300VDC. The 7189 has a maximum plate-to-cathode voltage of 400VDC. By the 70s, most tubes labeled 6BQ5 were actually relabeled 7189s, which is why the Mesa Studio 22 eats EL84-spec tubes. The only current production EL84-type tube that stands a chance in a Studio 22 is the 6P14P-EV, which is sold under a couple of labels. The most common label is Sovtek EL84M. The 6P14P-EV is the Soviet military version of the 6P14P.

 

[CandidPicker]

I'm intrigued by this knowledge. How did you learn about this? Have you been reading about, or studying tube circuitry?

 

[RickP]

I know tube construction can affect sound. Prior to my change to using Fractal Audio gear in 2011, I had been using tubes made in St Petersburg, Russia and sold under the name Svetlana. They were very reliable, sounded great, and were very consistent tube to tube. I didn’t buy tubes for years, due to gigging on the digital gear. A few years ago, while retubing an amp at home, I found the “Svetlana” tubes were being made elsewhere, the construction had changed, and so had the sound and consistency. After some checking, I found the SED plant that made the original Svetlanas quit making smaller tubes, so one more source of great tubes had disappeared. I stocked up on the SED/Winged C tubes I could find for my amps and they have, predictably, become harder to find and more expensive when you do find a NOS set.

Construction inside the glass does matter!

 

[Deleted member 5962]

I'm intrigued by this knowledge. How did you learn about this? Have you been reading about, or studying tube circuitry?

You can find info about tubes in many places on the interwebs. There is a reason I keep telling people who want to lower pre-amp gain to try a 5751 and rather than an AT7 or AY7. But that is just an example. A lot of people think they are all just interchangeable. And to some degree they are, but they aren't the same. But this is a DEEP subject.

 

[Em7]

I'm intrigued by this knowledge. How did you learn about this? Have you been reading about, or studying tube circuitry?

I have been playing with electricity and electronics since I was in elementary school and working on tube technology since high school (late seventies). My first tube repair was a silverface Champ on which I carbonized the power tube socket. My grandfather also gave me an EICO HF-81 integrated tube-based stereo amp and an EICO HFT-90 tube-based tuner that my father built in the late 50s (my father was an electronics technician from 1951 until 1994). However, I did not get really serious about tube technology until the tube renaissance after the Berlin Wall fell opening up the former Soviet tube plants to the west in the nineties. There were no kits when I designed and built my early tube guitar amps. Everyone was scratch building. I have read a metric ton of tube data sheets over the years.

 

[tiboy]

Thanks Em7 for sharing your knowledge. I recently received some 7025’s to replace 12AX7’s in a hissing/humming/feed backing amp. Hopefully I’ll be able to report good news soon.

 

[mad monk]

I've done so much tube swapping in the preamp section to hear a difference. It's mainly on the absence of noise side, but useful.
I wish I understood the technical aspect. When I can find Tungsrams with white lettering, I buy them. Yellow lettering 12ax7s don't seem (to me) to be as quiet or durable. There are lots of mislabeled tubes. I've found Tungsrams labeled as Amperex. Generic information, I know. Any tube can be worn/bad.