LA2 Mod: 6AQ5 > 6V6GT

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CJ

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Jun 3, 2004
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The wiring on the 6AQ5 inside the LA2 is a real pain. Those 7 pin sockets are really tight. Especially those big green heater wires.

The 6AQ5 is nothing more than a 6V6 in a smaller bottle. The pinouts are different, but electrically, they are very identical. So why not splice in a beefy octal in place of the wimbly wambly 7 pin?

That's what I did. Here are a few shots, before and after.

This is how the point to point wiring looks on your typical 6AQ5:

http://vacuumbrain.com/The_Lab/TA/LA2/la_mod2.jpg

And this is how it looks with the 6V6
Tons O Room:

http://vacuumbrain.com/The_Lab/TA/LA2/la_mod3.jpg

See what I mean?

So if you are starting one of these bad boys, do your self a favor and go for the 6V6GT/octal setup.

http://vacuumbrain.com/The_Lab/TA/LA2/la_mod1.jpg

Pass the Lazertran, will ya?

http://vacuumbrain.com/The_Lab/TA/LA2/la_mod4.jpg
 
If you want a "strange" version, a 7C5 would be another option...

Best regards,

Mikkel C. Simonsen
 
> All parts are the same?

6V6 is the the same as the 6AQ5, except the 6AQ5 can't run as hot due to its smaller bottle. They are exactly interchangeable up to the power limit.

> how's it sound?

Aside from being the "same" tube working the same way: the LA2's 6AQ5/6V6 is the grunt to flash the light on the opto. It is not in the audio path. It carries audio but gross distortion makes no difference to the main audio path.
 
If you convert to octal, you can fool around with other tubes.

Be the first one on your block to drive your EL panel with a vintage KT66!
Watch those filament amps. Other candidates include 6K6, 6Y6, 6W6, 6L6, etc, but thats just being weird.

Note custom T4 that dosen't care how the socket is mounted. It will always be square. Or round. Or whatever.

Thats neon inside the box. Not a fire. I hope.

la2_kt66.jpg
 
While I had the LA2 out of the rack, I fooled around with the line amp. I tried all the mods I had for the 6BH7. The Jensen mod, a mod by Jhalibe, etc. I found that the Jensen mod did not make that much difference. The Jhalibe mod helped to soften distortion at onset, but unfortunately eliminateing the negative feedback made it so the amp had a lot more gain, which meant you had to turn the gain knob down even further to avoid distortion. Eliminating the negative feedback allowed the amp to gently round off the tops of the triangle wave, instead of lopping it off like a sword. But once you got past a certain point, you got the flat top haircut on the wave.

No matter what you do to the 6BH7, if those first two stages mess up the wave, there is nothing you can do down the line to fix it.

I did find something that really helped besides changing the first tube to a 12AY7. This involved changing the 220 k plates resistors to 100k. Gain was reduced as this mod affecected two gain stages, not just one, so the effect gets multiplied. Also, distortion became more symetrical. With the 220 k's, the top half gets lopped off way before the bottom half. This was due to the reduction of plate current from the 220 k resistors causing the bias to be too low, which caused the tube to saturate. With the 100 k's, the bias went up, which put the tube on a better place on the load line.

220 k's are great for those early Fender circuits where you want maximum boogie woogie, especially carbon comp 2 nd orders, but with something in the mic path, 100 k 1 watt metal resistors are the way to go, at least according to my ears. This is where you want to install them. Replace R9 and R13 with 100 k 1 watt. Now I could turn the gain knob up past 1 without clipping.

la2_100k.jpg
 
CJ,

I'm investigating reducing gain of the LA2As, based on your statement, "distortion became more symetrical" says to me, less odd order harmonics. Is this accurate? Did you eliminate some of the characteristic "LA2A sound" with this mod?

thanks
 

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