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| | |-+  Too Much gain from 18w EF86 TMB - Advice
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Author Topic: Too Much gain from 18w EF86 TMB - Advice  (Read 5818 times)
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« on: December 21, 2007, 03:20:49 AM »

Does anyone know of a way to reduce the amount of gain in the 18W TMB EF86 amp. I recently got one from Nik, and as awesome as the amp is, it has a lot more pre-amp gain than I had anticipated. I read on the forum that one could replace the 12ax7's with 5751's to reduce the pre gain.

I'm basically trying to make the amp a little more versatile, and to get a cleaner, less compressed sound out of it. I'll be using the amp mostly for studio recording, (I'm a recording engineer/producer) and so I don't need much volume out of the amp, but I do need a little more control over the gain. Will changing the pre-amp tubes make the amp a little brighter on the TBM side?

Could someone shed some light on which tube is which in the layout.
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« Reply #1 on: December 21, 2007, 09:03:41 AM »

If you don't want to be doing surgery on your amp, swapping V3 to a 12AT7 might help reduce the gain on the TMB channel.  This would be the preamp spot closest to the power tubes.

If you want to reduce the gain and are OK with doing a few mods, you can start changing cathode resistor values on the TMB channel.  The higher the resistor, the lower the gain.  You could try a 1K or a 1K5 as a cathode resistor, which would help to lower the gain.

Visualize whirled peas.
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« Reply #2 on: December 22, 2007, 11:24:36 PM »

A 5751 in V1 works to remove some buzzy, fuzzy, harsh gain from high gain amps. One such amp is a 5150. A 5751 would probably do the trick for you. They have just slightly less gain than a 12ax7. Dont remember EXACTLY but an ax7 has a gain factor of 100. A 5751 is like 80 or 90 where IIRC, a 12AT7 is down at 70%. If you have one handy, DEFINATELY, give it a shot.
Another easy way to reduce some gain is to SLIGHTLY back-off the guitar's volume pot.
« Reply #3 on: June 06, 2008, 05:42:06 AM »

My complaint about the 36er and the 18 was that I couldn't get any tops out of either of them, I tried the 5751, although it lowered the gain, it was not a cure for my delema. I tried numerous mods to the 36er, when I got it going well in the tops it would squeal, when I put a cap here or a cap there, the squealing would be gone but so were the tops. I think the JTM-45 is a way better proposition for versitility, a brillient amp, out of 6 Marshall clones that I built, for me it was the only one that sounded great stock. Just my taste of course. I know this post is old but I thought I might as well stir the pot.
« Last Edit: June 06, 2008, 08:08:25 PM by Lefty » Logged
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