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12AX7 preamplifier

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Hazmatt_(The Underdog)

Wed Mar 08 2006, 06:31AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

where your circuit 'ends' is after the decoupling capacitor. These vary in many different values depending on what kind of frequency range you wish to pass. If you want more bass and to cut off some of the highs, then .047 .056 .1 .22 could be your values. You'll have to 'adjust to taste', but if you put some turrets or leads to solder in/remove the capacitor, you can make fairly quick work of this. Just make sure you have a meter connected to B+ before you go in, I got shocked by my little supply I posted on your other thread and it REALLY hurt.

So its pretty simple, play with the caps and see where your 'tone' is at :)

Throw in a switch with a 470pF cap in series across the decoupling cap and you have a bright switch. pretty neat huh.
I took the same idea with a .1uF to ground and got, hehe....this is neat....Acoustic!
So I switched from electric to Acoustic with a cap.

Matt

Steve Conner

Wed Mar 08 2006, 11:03AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

hazmatt, you could be more specific, since an average tube amplifier has about half a dozen decoupling capacitors. I guess you mean coupling anyway, not decoupling.

Usually the preamp is taken to end at the wiper of the master volume pot. If the amp is the kind that doesn't have a master volume, then the preamp ends at the coupling capacitor that connects between the last preamp tube plate, and the phase inverter tube grid. (the phase inverter is the 12A*7 tube that drives the grids of the large power amp tubes)

Hazmatt_(The Underdog)

Wed Mar 08 2006, 06:58PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

My nomenclature gets bad if I don't use it often.

I was trying to come up with a general case for a single stage.

I've got another preamp here which is marked In and out if you look carefully.

Now, with the preamplifier, its a bit different because its a resistor coupled amplifier. These are used in Marshalls to get the higher gains and use a single tube or 'two gain stages'. Notice the high cathode resistance. This is selected for amplification but also note the very high cathode voltage at 180V. The resistor must drop this voltage at very low current, so its natraually a very high value. So then the objective of the deign of any stage is to amplify then block any D.C. at the amplifier. That means that after every plate or cathode in this case, that there is some capacitor to block the D.C. voltage present and allow the audio signal through.

please be patient and I will be able to post some examples later.

EDY19

Thu Mar 09 2006, 03:06AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
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Here are some schematics from the book I have, any preferences on which one is best? They are all really similar, so probably not much preference. Heres the pictures- I chose these because they are simple for my first tube project, and they only use a couple tubes, and i have some of the ones i need. Heres the schematics

1141873559 105 FT3332 2006 0308 214952aa

1141873559 105 FT3332 2006 0308 215008aa

1141873559 105 FT3332 2006 0308 215018aa

Hazmatt_(The Underdog)

Thu Mar 09 2006, 06:07AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

yea...same book "A Guide To.....Amplifiers"

good starter schematic. The more powerful amps are a major problem if you don't know how to bias. I built the Twin 6 times before I learned that it was correct, just wrong bias! Man that was expensive! All those chassis, effort, it was totally futile.
for larger amps 46L6 you want to be at 30-50mA per tube, depending on your tastes for AB amps.

Dr. Shark

Thu Mar 09 2006, 08:40AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Go have a look at

which is probably going to help you a great deal more than the book you have got. It is aimed at beginners and walks you though every step of amp design. They also have some very nice low-power stuff, e.g. a 12ax7 preamp with a 5W poweramp, which makes for a great practice amplifier. Go check it out!

Steve Conner

Thu Mar 09 2006, 11:36AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The Fender Champ is a good bet, since the power stage is a single 6V6, you have all the tubes needed to build it. Also, it's so simple, it makes a perfect starter project. The only hard part to get hold of would be the output transformer, but you can probably buy a single-ended OT from ax84.com. To simplify it even more (while giving tone purists the heebie-jeebies) the choke can be replaced with a big 100 ohm resistor, and the tube rectifier by a couple of high voltage diodes (1N4007 etc)

EDY19

Thu Mar 09 2006, 11:53AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Can you tell me how the diodes would be hooked up in place of the tube rectifier? Althoguh I may try to find one of the rectifier tubes. Also, for the power supply, could I use a doubler or tripler with some small caps and a big filter capacitor then big resistors to drop down the voltage to the correct rating?

Steve Conner

Thu Mar 09 2006, 12:38PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Why on earth would you multiply the voltage up just to drop it down again? :-/

How you replace the tube rectifier with diodes depends on what kind of power transformer you've got. Basically all you need for the champ is a source of about 250-300V DC at 50mA. The voltage really isn't critical for a tube amp: there is a famous story about how Eddie Van Halen would run his Marshalls off a variac and turn the voltage down until he got the "Brown sound" he wanted. He really should have kept the filaments at 6.3V though.

If you have a power transformer with a centre tap designed for a tube rectifier, you use two diodes, connecting one diode anode to each end of the winding, and both cathodes together to the + of the first filter capacitor.

If you have a transformer designed for a diode bridge, you use a diode bridge.

If you have one that's too low a voltage, then you use a full-wave doubler with two diodes.

EDY19

Thu Mar 09 2006, 08:49PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

See, thats the problem, I only have a filament transformer and an output transformer, no 115=>250-300V transformer. Thats the only reason I was going to use the doubler.

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