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Aftermarket 12ax7s causing buzz in AC15HW1 V1 position

Tritone

Member
Messages
1,117
I recently replaced the three preamp tubes in my AC15HW1 with Tung-Sol 12ax7s. On the normal channel everything is fine, but there's an extremely loud buzz on the top boost channel. From swapping tubes a half dozen times or so, it seems like the problem only occurs when there's a Tung-Sol in what I assume is the V1 position (the position farthest to the left, away from the power tubes). Has anyone else encountered issues like this? Is my only option to avoid that brand for this position?
 

Tritone

Member
Messages
1,117
Update: I actually switched out the Tung-Sols for some "Preferred Series" 12ax7s from Thetubestore.com I had been using in another amp, and the massive buzz is still there. It disappears when I swap out V1 for one of the original Ruby tubes. What's going on? :-/
 
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TimmyP

Member
Messages
2,488
I believe it is the heater current being coupled into the tube. JCM900s are very particular like your amp - get 10 tubes that are fine as V1 in a Twin, and 6 of them will hum as V1 in a JCM900. However sometimes if you put in a bunch of hummers and then tweak the hum balance pot, the thing quiets down.
 

Tritone

Member
Messages
1,117
I believe it is the heater current being coupled into the tube. JCM900s are very particular like your amp - get 10 tubes that are fine as V1 in a Twin, and 6 of them will hum as V1 in a JCM900. However sometimes if you put in a bunch of hummers and then tweak the hum balance pot, the thing quiets down.
So what is the solution? Simply keep buying and testing tubes until one works? Yikes. I wonder if this is common with this model amp or if I just got unlucky.
 
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826
I did some comparisons for V1 in a Marshall JTM45 clone and found that an Electro-Harmonix 12ax7 had the lowest buzz and microphonics. However, that might have just been a lucky sample. I know EH and Tungsol are just different brands from New Sensor and are made in Russia, but whether there is any physical difference between them, other than the printing on the glass, I don’t know.

I noted that the EH had a spiral filament and wondered if this accounted for the absence of buzz.

EDIT: Just had a close look at Tungsol 12ax7 and EH 12ax7. The plates look similar, but the micas are very different.
 
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sg~guy

Member
Messages
681
I did some comparisons for V1 in a Marshall JTM45 clone and found that an Electro-Harmonix 12ax7 had the lowest buzz and microphonics. However, that might have just been a lucky sample. I know EH and Tungsol are just different brands from New Sensor and are made in Russia, but whether there is any physical difference between them, other than the printing on the glass, I don’t know.

I noted that the EH had a spiral filament and wondered if this accounted for the absence of buzz.

EDIT: Just had a close look at Tungsol 12ax7 and EH 12ax7. The plates look similar, but the micas are very different.

-GOLD LION & ELECTRO HARMONIX are the only cp pre's I use-
 

Tritone

Member
Messages
1,117
So I took it to a shop, they said the problem was two pins soldered together on the tube socket. I don't know if the solder connection was bad, or if they weren't supposed to be soldered together in the first place. Very strange that the problem didn't manifest with the original tubes! I'm picking it up tomorrow.
 

Avatar Tech

Member
Messages
455
So I took it to a shop, they said the problem was two pins soldered together on the tube socket. I don't know if the solder connection was bad, or if they weren't supposed to be soldered together in the first place. Very strange that the problem didn't manifest with the original tubes! I'm picking it up tomorrow.
Hmmm....
Certain tubes from certain factories actually differ in the size of the glass envelope. I'm not positive but I think there is some variance in the pins as well. At any rate, there are some socket/tube combinations that just don't work. Most commonly people will buy new preamp tubes only to discover that the glass envelope has too big of a diameter to fit in a socket with a shield base.
There are 2 pins that are very commonly tied together on the socket for balanced AC heater operation. I would think that a solder joint problem with the socket would be present regardless of tube brand. If it's fixed, though... Problem solved :)
 

Tritone

Member
Messages
1,117
Talked to the tech when I went in, the problem was some solder was connecting two pins that shouldn't have been connected. My guess is that the original tubes' pins were just angled or positioned in a way that pushed the physical connection apart.
 

stahlhart

Silver Supporting Member
Messages
2,665
I'd love to know which two pins it was. Assuming a cathode to a grid.

As a fellow owner, I'm just curious -- how do you handle the spring-loaded retainers when changing any of the tubes besides V1?
 

Tritone

Member
Messages
1,117
I'd love to know which two pins it was. Assuming a cathode to a grid.

As a fellow owner, I'm just curious -- how do you handle the spring-loaded retainers when changing any of the tubes besides V1?
It was pins 5 and 6. As for the retainers, I use two hands, one hand on either side of the top, with a rag between me and the metal so I don't dig into my skin too deep, and just pull directly down until it can clear the tube. They're tough!
 

sixstring531

Silver Supporting Member
Messages
3,223
The new Tung Sol 12ax7s are the only new production tubes to give me the same exact symptoms you describe in V1. So, I never use them.
 

sixstring531

Silver Supporting Member
Messages
3,223
^May be true in your case, but the new Tung Sols were, and are, the only new production tubes to give me excessive noise in V1.

I'm glad you got it worked out!
 
Messages
6,028
So, the heater was contacting the pin 6 plate.....very strange.

Wow. Reviewing the schematic -- it looks like ground for the heaters is not tied to chassis ground, thankfully. Seems like it would have at least blown the HT fuse otherwise.
Aren't the two legs of the heater filament referenced to ground with 100ohm resistors...a virtual center tap? We can see in the video in post #5 that the heater filament wiring is twisted to bring it to pins 4/5 and pin 9....normal operation, ime.
How there was not a more significant problem than just a hum is a wonder. Was this a mistake that caused an errant and improper reference above ground for the heaters??

At 12 seconds in to the video, we can see a good shot of that tube. Pin 6 is bent in an odd way...different from the next preamp tube as we can see.
Fwiw, check out the dressing for the heater filament feed to pin 9. I was reading yesterday on The Valve Wizard's site that this wire for pin 9 should pass above the tube and directly across that circle from pins 4/5. He states that looping that wire for pin 9 around the tube socket can lead to hum/noise problems.
 




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