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Old 18th July 2013, 03:59 PM   #1
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Default 6L6GC Tube Orientation

Hey, so I'm building a small combo amplifier and was wondering if there was a "best practice" for power tube orientation. By orientation I mean like what direction should the key be pointing (12:00, 3:00, etc)? I have never seen the key at 12:00 and was curious if there was a reason for this (minimizing vibration from the speaker, etc). I'm only using one single ended 6L6GC and I've looked through all my datasheets but couldn't come up with anything regarding this issue, possibly because putting a power tube in the same cabinet as the speaker is probably the last thing a tube manufacturer would want for their tubes...just trying to get a better understanding of tubes. Thanks.
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Old 18th July 2013, 04:43 PM   #2
kevinkr is offline kevinkr  United States
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Moved to Instruments and Amps. All musical amplifier threads should be started here per forum policy. Please see sub header at top of each forum for details.
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Old 18th July 2013, 04:46 PM   #3
kevinkr is offline kevinkr  United States
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Are you planning to hang the tube upside down in which case the orientation does not matter or are you planning on mounting it horizontally in which case aligning the pin so that the long faces of the cathode and grids are vertical would be best. (The orientation of modern tubes in the socket can be somewhat inconsistent so getting close with most bogies might be as good as you can do.)
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Old 18th July 2013, 04:48 PM   #4
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Quote:
Originally Posted by howe0168 View Post
Hey, so I'm building a small combo amplifier and was wondering if there was a "best practice" for power tube orientation. By orientation I mean like what direction should the key be pointing (12:00, 3:00, etc)?
In your context the phyiscal orientation of the tube has no affect whatsoever. The only exception I can think of would be for ease of wiring underneath. Meaning to have, perhaps, the grid terminal facing a certain way for a closer connection.
And hay is for horses.
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Old 18th July 2013, 04:56 PM   #5
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Ahh, thanks...I was actually planning on putting the chassis at the bottom of the cabinet so the tubes would be vertical to try and prolong life (yes, even though the life will be shortened by already being in a combo). I'm trying to reduce the possibility of power tube vibration as much as possible. A recent prototype seemed to vibrate the power tube pretty easily despite not being too close to the speaker. I had the key "pointing" to the baffle and was wondering if that was helping the problem since almost all combo amplifiers have the tube sockets at an angle. I've tried dampeners, retaining springs, and putting foam between the chassis and cabinet but nothing really solved it. Which was kind of strange because if I push directly down on the power tube it will go away. Just doesn't make sense to me since higher wattage, bigger speaker combos hanging the tubes upside down don't seem to be as susceptible to this issue (Twin Reverb for example). Wondering if I'm missing something.
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Old 18th July 2013, 05:15 PM   #6
Enzo is offline Enzo  United States
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Power tubes in a guitar amp will wear our - lose their tone - before they vibrate apart, in most cases.

How you aim the tubes won't matter in your situation. Combo amps angle the tubes because the socket fits better. Different brands can have the key at different angles, but to an amp maker, it can be important the mounting flanges not interfere with each other or with nearby other things. Since point to point wiring is almost never used, it is now less important that a pin be closer to a component. Wires can go anywhere, and pc board traces can be routed as needed.

If you have tubes vibrating, it is more than how they sit. The vibration is everywhere in the cabinet, but not all parts are free to move. For instance, the chassis metal can flex, allowing a transformer mass to jiggle a bit. Or just the flat surface can give. A Fender Twin Reverb chassis is a VERY solid piece. COntrast that with the Peavey Classic 30, a very popular amp, but the way the chassis was made it was free to vibrate with the speaker. The little EL84s in there would shake like a Shakira video.

If you mount your chassis on the bottom, how resonant is your bottom panel? How heavy the material? Is the chassis centered or more alone one side seam? If you press down on the chassis top, does it give? DO you have tube retainers installed?

Tube sockets are mechanical. There is a housing made of an insulator, then there are the female pins that grip the male pins on the tube base. Ceramic sockets are popular, but many of them are loose. The pins sit in the holes in the housing, but are free to move a considerable amount. The individual pins may grip the tube pins just fine, but the tube is still free to shake around due to the "slop" in the holes. other sockets have the pins molded in place and allow little or no shaking. Are your sockets the loose type? Mechanically loose, not electrically loose?
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Old 18th July 2013, 06:20 PM   #7
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Wow, thanks so much for the very detailed explanation - all makes total sense, I really appreciate it.

So I'm using an aluminum hammond chassis, which is 0.040" thick - I would think if you pushed down in the center it would give a little bit. The 6L6 is located in the middle of the chassis about 1.5" back. The cabinet itself is made out of 5/8" half-blind dovetailed pine and is 10" deep (around 8" or so of interior depth). The chassis itself is mounted on the bottom, centered and as far away from the speaker as possible. The baffle is 1/4" thick. I'm using belton tube sockets but there is definitely some "rockability" of the tubes within the socket even though brand new. I initially had those spring retainers installed but the vibration issue came up while they were installed so I stopped using them.

I have tried pushing down on everything around the power tube as well as all the chassis and other components but nothing stopped the vibration other than pushing directly down on top of the tube, which is strange because I figured that's exactly what the spring retainer was meant to do. A couple other things to note:

-The vibration only happens ~130Hz, which seems to be the resonant frequency of the cab itself (when I input a sine wave I can feel the walls vibrating as well as the chassis). I've been told a little vibration is actually good for tone so up until now I haven't increased the wood thickness. Yet.
-The vibration didn't always happen and it takes a little while after being turned on and played for it to start vibrating. I figured some heat expansion was going on inside the tube, which is what was inhibiting the vibration.

So yeah, anything jumping out as obvious issues? Think I just need to increase chassis and/or wood thickness? I think your assessment of vibration between the tube socket is correct; are the ceramic tube sockets better for this? I assumed the Micalex ones better since more expensive and looked nice...just like a nice label on a bottle of wine I guess.
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Old 18th July 2013, 06:49 PM   #8
kevinkr is offline kevinkr  United States
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Next go around check out Dirty Dawg amps for custom chassis.. Probably cheaper than Hammond in most cases and can be ordered in thicknesses of up to 80 thousandths..

Amp Chassis

All of my hifi power amps and psu are built on chassis custom built by DD. Very stiff. I'm a very pleased customer, no relation.
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Old 18th July 2013, 08:28 PM   #9
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Cool, thanks for the tip.
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Old 18th July 2013, 08:48 PM   #10
Enzo is offline Enzo  United States
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Spring retainers are to keep the tube from working its way loose from the socket. Helps prevent tubes falling out when hung upside down like many guitar amps. Spring retainer do very little to prevent vibration.

Try just grasping the power tube firmly but don't push down. Use a rag or glove if it is too hot. Any help? Or maybe put a little twisting torque on the tube to hold the pins against the side walls of the socket holes.

Cabinet vibrations may contribute to the tone, but that is a separate matter from shaking your tubes.

I like ceramic sockets, but I think the molded kind are better at preventing this issue.

Consider this, in your car, if you go over a bump, everything shakes, the whole car moves as one. But look at the antenna sticking up. Even though it is firmly screwed solid to the car at the base, the whole top of it swings wildly after the bump. The antenna whip action amplified the motion in a way. Maybe ther is a better word for it than amplify, but hopefully you see my point. In an amp chassis, a resistor if tight flat against the board and doesn't move much relative to the board. But the tubes stick up like your car antenna, and so vibration allows them to shake more than the chassis as a whole.


If you are building combos that are either clones or HIGHLY resemble something commercial, consider a premade chassis. They come in nice plated steel with all socket, jack, and control holes punched and labeled. Very sturdy and ready to go.
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