Pre-amp tubes question...

Mark Herrick · From: Bakersfield, CA

In another thread, Robert Parent mentioned changing out the 12AX7 tubes in a Tube Works pre-amp for 12AT7 or 12AU7 tubes.

I subsequently asked about changing the pre-amp tubes in a Mesa Boogie combo amp but never got a reply. So, I will ask again:

The 5 “pre-amp” tubes in the Boogie are:
#1: 12AX7 - Input stages and tone controls.
#2: 12AX7 - “A” side = Output amplifier; “B” side = Cathode follower.
#3: 12AX7 - “A” side = Lead drive amp; “B” side = Reverb return amp.
#4: 12AT7 - Reverb driver.
#5: 12AX7 - Phase inverter / Output driver

Which of these tubes can I exchange for 12AT7 or 12AU7 tubes and what might the effect be? Would I have to make any bias adjustments anywhere? The 4 power tubes are 6L6’s.

Jerry Hedge · From: Norwood Ohio U.S.A.

Mark, the tube you would want to change is tube 1 (the 1st 12AX7). Also you might want to try a 5751 tube. It's an industrial grade tube that is just between a 12AX7 and a 12AT7 gain wise. I've used them before and they work well for steel.

Bobby Lee · From: Cloverdale, California, USA

You won't get that Carlos Santana singing lead sound if you use different tubes. The Mark IIB (that's what you have, right?) is such a sweet sounding design, I wouldn't mess with it. Just my opinion...

------------------
Bobby Lee quasar@b0b.com gigs
Sierra Session S-12 (E9), Speedy West D-10 (E9, D6),
Sierra S-8 Laptop (D13), Fender Stringmaster D-8 (D13, A6)

Mark Herrick · From: Bakersfield, CA

I went ahead and tried the 12AT7 in place of the 12AX7 in the “input stage/tone control” position. (Turns out it had a 5751 in it to begin with!)

I can't tell that much difference.

Oh well...

Len Amaral · From: Rehoboth,MA 02769

You wouldn't want to change the reverb driver tube from a 12AT7 to a 12AX7 as it would make the reverb too hot or boingy. Try experimenting with 12AT7 and 12AU7 with the other 12AX7 tubes as it will decrease the gain and produce a cleaner sound.

Robert Parent · From: Gillette, WY

Mark,

I just posted a reply in the other thread. Sorry for the delay but I have not had much internet time the past few days.

Tube #1 is the one that I would mess with if I wanted to change the gain of the preamp. I am not familiar with the Mesa but usually the first stage has the most gain.

Changing the tube will have very little effect on the tone from my experience. The change just lowers the gain giving a bit more headroom before the signal approaches the power supply level.

The issue I saw with the Blue Tube II was that the high output signal from the steel pickup when amplified would clip. Changing the tubes, lowered the internal gain in the preamp so the clipping would not occur.

Robert

[This message was edited by Robert Parent on 06 May 2000 at 07:00 PM.]

Bobby Lee · From: Cloverdale, California, USA

On Boogie amps, if you keep your first volume control low the tube won't clip. I run it at about 4. But if it's too low, you lose tone. Think of the first volume control as the "guitar to amp signal matcher". Once you've found its sweet spot, you should never have to change it. Adjust your volume with the master volume control.

------------------
Bobby Lee quasar@b0b.com gigs
Sierra Session S-12 (E9), Speedy West D-10 (E9, D6),
Sierra S-8 Laptop (D13), Fender Stringmaster D-8 (D13, A6)

Rich Paton · From: Santa Maria, CA.,

Robert, the first two stages of a Mesa (seriesII) are the first 12AX7, V1A/V1B. It is essentially identical to any basic Fender-type preamp. V1A gets the input signal directly. The output drives the "tone stack",
the output of which is amplified by V1B. The volume pot is the load on this stage.
Here the signal has two paths, the clean path, and the overdrive path. The O/D stage is simply 1/2 of another 12AX7 configured for extreme voltage gain. Depending on the setting of the Volume pot, its output ranges from a fairly clean signal to that of nearly a square wave (100% harmonic distortion). The clean and O/D path signals are then summed or added together. Another 1/2 12AX7 further amplifies the signal and mixes the "returned" reverb delayed signal with the "dry" signal. Then another 1/2 12AX7 configured to increase the signal's current, but not its voltage provides a low-impedance signal to the master volume pot, preamp-out jack, and the power amplifier section.
The overall gain of a small-signal triode voltage amplifier using a 1/2 triode such as the 12AX7 in an audio preamp stage is set much more by its cathode and plate resistor values, plate voltage, and cathode bypass capicitor value, if used, than by the actual "amplification factor" or the design transconductance of the tube itself.
If not, the slightest gain variations in a tube with aging would have the preamp's output level going all over the place. This is also why such a preamp tube can give years of satisfactory service even as its elements age and its gain drops to nearly zip.
The subject of swapping various preamp tubes (12AT7, 12AU7, 12AY7, 12AZ7, etc.)to affect tone, sustain, etc. dates back to the early, simpler designs of such amps as the Bassman and Tweed Deluxe Fenders, where the choice of
the tube type, what with fewer tubes in the circuit running at higher gains...more significantly affected the amount of signal applied to the input grids of the output tubes, which WILL greatly affect the clean/crunch tone they produce.
The first tube in the signal chain should be the quietest, least microphonic, and most stable (resistant to oscillations) that you can find.
A 12AT7 is designed to provide more current with less voltage gain than a 12AX7, hence their use in driving reverb transformers and the phase inverter/driver/output tube grids. Subbing one for the 12AX7 is simply a misapplication.
Try to scrounge up a Telefunken or Mullard 12AX7 from an old european radio or phonograph and sub with it. I think you will definitely hear an improvement that way.

Robert Parent · From: Gillette, WY

Hi Rich,

Thanks for the insite on the Mesa preamp and the other technical info relating to tubes.

I don't claim to be a tube expert as my experience is in embedded microprocessor system design. I was just passing along my first hand experience with the Blue Tube in the hopes that it would help.

I am not sure how the Blue Tube is using the 12AX7 but I know that when I replaced it with the 12Au7 or 12AT7 the peak to peak voltage of the signal decreased roughly 30% - 40% when measuring the waveforms on my scope. The clipping of the signal would occur just below the voltage level of the power supply which indicates to me that the system ran out of headroom (I believe that is the correct term).

After replacing the tubes the input gain control could be increased from a 9:00 position to about 1:00. With the tube change, the sound of the preamp greatly improved in my experience. All of the tubes I used were manufactured by RCA and were of the late 60 era.

Thanks for the info Rich.

Michael Brebes · From: Northridge CA

If your problem is that you overdrive the preamp too quickly, then you might want to do what Fender did to get a lower gain input. Input to the grid of the first preamp section has a 1 meg ohm resistor to ground. Remove that wire from the input jack and install a 68K resistor between the jack and the wire. Also install a 68K resistor from the wire, or the place where the wire comes off the circuit board, and ground. This will be identical to plugging into the "2" input, or low gain input, of a full Fender preamp design. This will cut the signal by half.

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