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Topic: Tube Works preamp factory mod? |
Cliff Swanson
From:
Raleigh, NC
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Posted 3 May 2000 7:36 am |
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The Tube Works folks advertise a factory mod that reportedly turns their preamp into an "incredible" preamp for PSG. Anyone use one of these units wiht the mod? What does the mod accomplish for PSG?
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Robert Parent
From:
Gillette, WY
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Posted 3 May 2000 10:54 am
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| I am not sure what the factory modes are but changing the 12AX7 tubes to a 12AT7 or a 12AU7 made it work well for me. What the tube change did was lower that gain structure to compensate for the high output pickups on most PSG's. BTW, I thought they are a nice preamp. |
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Steve Feldman
From:
Central MA USA
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Posted 3 May 2000 11:21 am
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Well, I use one of these units with the mod, but I've never used one without the mod! So I have no basis for comparison. Apparently, the mod is essentially designed to increase the gain level at which the thing will start to break up. It is a WONDERFUL sounding preamp, IMO. Best one I ever tried.
Steve |
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Mark Herrick
From:
Bakersfield, CA
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Posted 3 May 2000 11:37 am
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Robert,
Your comment regarding changing the 12AX7 pre-amp tubes got me thinking. Would that work on a Mesa Boogie MKIIb combo amp?
It has 5 pre-amp tubes as follows:
#1: 12AX7 - Input stages and tone controls.
#2: 12AX7 - A = Output amplifier; B = Cathode follower.
#3: 12AX7 - A = Lead drive amp; B = Reverb return amp.
#4: 12AT7 - Reverb driver.
#5: 12AX7 - Phase inverter / Output driver
The 4 power tubes are 6L6’s. If the pre-amp tubes are changed, does this necessitate any bias adjustments for the power tubes? (I don't know much about all this so I hope my ignorance isn't extremely apparent!)
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Rich Paton
From:
Santa Maria, CA.,
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Posted 6 May 2000 4:34 pm
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Preamp tube changes should not affect the
~quiescent~ bias values. However the very high gain nature of the overdrive stages demands the best tubes you can find. NOS U.S. made (RCA, Sylvania, GE) have worked fine for me since 1981. 12AX7's vs. 7025 should sound "warmer". I got theat from Red Rhodes and personally concur (my results)
If that amp is set up for "Fetrons", as is the IIC, make sure the switch is set for 12AX7 not fetron.
Fetron equipped models:
I modded a IIC when brand new to get a cleaner sound, by removing the current source/limiting diodes (TCR 5304 on V1a, TCR 5302 on V1b) intended for the fetron. The first & second stage cathode circuits of the preamp (V1A & V1B) should each have a 1500 ohm, 1/2 watt resistor paralled with a 22uF, 25V high quality cap. The V1A & V1B plate resistors shold be 100K vs. 82K.
There is also an input signal limiting network consisting of a 1K resistor in series with a pair of back to back 5.2 V zeners.
Mesa quit using this design about as fast as
they had incorporated it.
The input limiter is in the circuit as follows: Input jack, 10K series resistor, limiting network to ground, .01uf cap in series with the 10 K resistor and V1A control grid, with a 1meg resistor from grid to ground. I have the schematics if anyone needs them. |
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Robert Parent
From:
Gillette, WY
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Posted 6 May 2000 5:47 pm
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Hi Mark,
The replacement 12AT7 and 12AU7 tubes that I listed have less gain than the 12AX7. The parts are pin for pin compatible so my GUESS is that as long as they are being used as amplifiers then swapping them should not be a problem. I am not familiar with the Mesa preamp so I have no experience with them.
Changing the preamp tubes should have no effect on the bias of the power tubes. In general, bias refers to where in the operating range the tube is working.
It's been awhile sense I have been messing with tubes but I can dig up more info if you are interested. If I did not answer your question, please ask again or drop me an email.[This message was edited by Robert Parent on 06 May 2000 at 06:52 PM.] |
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