ON TWISTING HEATER FILAMENTS LEADS

I take a pair of 18g solid core wire and put them in the chuck of my battery

operated hand drill and slowly spin them up. IT GETS VERY TIGHT!!

Use two different colors and makes sure you set in your mind which color

ALWAYS goes to the same pin number on same tubes.

Like yellow to all the 12AX7's pin #4 and #5 and blue on pins 9.

That way later you will always be able to go to any amp you built and run the

filaments on DC. The polarity will always be correct!

The way I do the filaments is like this.

The transformer has two (or three if there is a center tap filament winding)

6.3v wires that come out of one end of the transformer. I run those to a
small

barrier strip wiht 5 or 6 tabs on it. Don't bother with this for the 5V

rectifier tube winding if you do this on a later project.

If you feel you will not use a DC filament supply then use a green wire for

this.

If you think you might like to try a DC supplied filament sometime, then use
a

red wire and a black wire......that way you can make sure all the tubes have

positive voltage on the same pin numbers in a multiple tube preamp. Not this

one.

Take about 24" of 18g wire and fold it in half. Or two 12" pieces, one red ,

one black. Then take the end that has the folded loop in it or just the ends

of the red and black wires and insert it (them) into the chuck jaws of a
slow

speed drill motor. Tighten the chuck down on the folded end(s) of the
wire(s).

Then hold the open ends in one hand and the drill in the other and pull the

wire away from the motor to keep the tension (now it looks like there is two

wires coming out of the chuck of the drill). Now turn the motor on slow and
it

will starting winding the wires nice and even and VERY tight. Keep the
tension

on the wires and it will twist them real fast.

Take the twisted wires out of the chuck and cut the closed loop part in half

and you half a pair of tightly twisted filament wires. The red and black
ones are obvious.

This forces the alternating 6.3v voltage from the transformer to buck itself

and every time the 60Hz reverses itself, it it met with a small collapsing

magnetic field that you introduced by making this a bifillar wound coil!

This will acutally reduce the AC filament induced hum considerably VS. not

doing anything at all. And it is free!

You wire both the 6V6 and the 12AX7 filaments in parallel with wires like

this.  Then push the filament wires way down low to the bottom of the chassis (as

you look at it from the big bottom opening) and tuck the wires into the folded

back corner as much as possible. it doesn't matter if the wires seem too

long as long as they are wound pretty tight and they are laying right on the

chassis every where except when you bend them up to the sockets.

Now when you get done you bring the last trwisted set of wires up to that

small barrier strip youinstalled and connect them to the same place as the

wires that came from the transformer did. This also gives you a nice place to

insert the 100ohm AC balancing resistors to chassis ground. I make that

connection also the same spot as the center tap from the hi voltage taps.

This will now be the centeral star ground connection point so run a nice short

wire right from there to a screw in the chasis itself. Everything that is

grounded will now be referenced to that one spot on the chassis..... the center of

the star ground, if you will.

Remember to keep the grid leads up high and away from the strong AC or

pulsing DC signals and keep the AC filaments and the plate DC wires as low and deep

in the chassis as possible.


ON SIMILATING THE EFFECT OF A TUBE RECTIFIER WITH A SS RECTIFIER

The way to use the SS diodes and still get a little compression sound from
the power supply is to insert a small resistance in the B+ lead from the SS
diodes to the first filter cap. Usually about 150-500Ohms is all that is needed to
simulate the drop under load....

Voltages on the plate of the 6V6 and the 12AX7

It sounds great just about anyway you assemble it as long as there is 290

to 340 volts on the plate of the 6V6 and 160 volts on the 12AX7.

EX:

150 Ohm resistor with 50ma being pulled through at idle in your amp.

Ohms law states R x I = V... 150 x .050a = 7.5V So it takes 7.5V to

push/pull 50ma of current through a 150 ohms resistor.

Now what if you really cranked up the amp and were playing medium loud?

150 Ohms x 150ma = 22.5 volts. 22.5V drop through the resistor that the amp

can't ever use because it is not available to the tubes now.

Now you are soloing and have some very loud passsages and dynamics.

Peaks of current are approaching 5 to 6 times the idle current.

150 Ohms x 300ma = 45V 45V drop.

Now that is enough to change the gain and tone in all the tube sections.

When you start dropping the plate voltage on the entire string of tubes, not

only does the gain drop but the tone does also because of an interesting

effect of high voltage (or lack of) on the tube.

The impedance of the tube drops with the voltage and when the impedance

changes so does the frequency response curve. The amp will start to sound

more and more dark or brown sounding. Plus the amp will distort easier.

But the instant the high volume level ceases, the voltage jumps back up and

the amp instantly cleans up and gets brighter and more cheerfull.

Cool effect!

So some players love that sound.

But it is more evident in a small under powered amp then, say, a 50 watt

amp.  That is because you simply can not play loud enough most of the time to get

the 100 watt power supply to sag out in a 50 watt amp. They are just too

darn loud!

Now in a 20-30 watt amp you can probably do it when playing those loud

sections.  In a 5-10 practice amp, you can do it almost at loud room volume.

Hence, the reason a 5 watt tube amp is so popular to record with.

For a real cool distortion with all those side effects the amp has to be on

the edge of a major B+ crash.

The best way to simulate this effect is to use a 5 to 50 watt 20v-36v zener

diode in the center tap to ground lead with a 150-500 Ohm resistor.

Now you have dropped the over all plate voltage by the zener value plus when

you play real loud the resistor will start to drop even more voltage!

ON THE PLACEMENT OF THE OT AND PT

Make sure the OT is a good few inches away form the PT and the PT is mounted

way over to one side of the chassis where the power line is and the fuse....

and the OT is mounted a few good inches from the 12AX7. Spread them out and

make sure both the transformer's iron plates are not in the same plane.


MEASUREMENTS

IMPORTANT STEP

First thing is to measure the DC voltage right on the plate of the 6V6 while

your negative DVM lead is on the cathode. You want the plate voltage

relative to the cathode voltage since you are holding the cathode up from chassis

ground with a small resistance.

Then you divide the tubes dissipation rating (12-14 watts) by the relative

plate voltage.  This will tell you how much plate current you can allow for the given

plate voltage.

EX: You measure 315VDC.... you then divide 13watts by 315 and get 41ma.

Now comes the tricky part...

You have a resistor in there now so you van actually measure the voltage

drop across it to arrive at the current throught the resistor.

If you set your meter to a low voltage (under 100) you'll see tha there is

something like 17vdc to 22vdc on the cathode pin compared to the grounded

end of the cathode resistor.

This is nearly the exact voltage the grid would see only the cathode is

positive and the grid is grounded so the grid acts as if it is the same

voltage only more negative.

If you take that measurement...say 19volts... and divide it by the resistors

value in ohms. 19/470=40ma! pretty close to our guess of 41 ma at 315 volts

to bias the tube in class A at 13 watts!  With this informaion you can now 

experiment with the cathode resistance to get

the tube exactly in solid class A and keep it at the correct plate

dissipation level so it will last for many many hundreds of hours.

The red lead of the voltmeter goes to the + end of the cap that bypasses the

cathode resistor. You could also touch the red lead of the voltmeter to the

cathode pin on the socket. that still will be the same thing as long as the

black lead is clipped to the chassis ground somewhere.

When comparing the voltage on the plate you need to move the black lead to

the + part of the cathode cap and then move the red lead to the plate of the
6V6.  This way you are measuring the actual voltage potential of the anode to

cathode.  It will always be the difference between the anode voltage and the cathode

voltage in a self biased amp.  It is that voltage that you need to divide into the

tube's wattage rating when you are determining how much current in takes 

to have the tube on fully during idle or Class A. So if you measured 

330 volts on the plate and 20v on the cathode then the real anode voltage is 

310v. Now if you know that a standard 6V6 has a 12 watt rating you can 

divide the 12watts by the 310volts to determine how much curent it will have 

to draw at 310 volts to get 12 watts at idle. 12/210=39.7ma class A.

A single ended amp can only run in class A to be linear. So the only time you

could change that would be to drive the tube's grid so hard that when the

signal on it is very negative it will shut the tube down from class A to class

AB. That will sound a little distorted because the tube is now shut off for a

little bit of the cycle and you'll hear that as distortion unless you have 2

tubes in push pull where the other can fill in that dead spot. Now it can be

run class AB but with 2 tubes, the max power out can now be 4 times greater

then the single tube. That is why most amps are run push pull class AB. A

push pull class A amp will only be twice as powerfull as a single class A

tube amp. Confused?

When you measure the small voltage drop across the cathode bias resistor,

you're really measuring the voltage caused by the tube pulling current up

through the bias resistor from chassis ground. Yes, the current flows

backwards from ground to the positive part of your power supply!

That little voltage drop is the pressure it takes to push the bias current

through the cathode resistor and that is why Ohm's law can be used to

determine what that current is. You should see around 17volts to 21volts.

I tried to make this boad up as close as possible and I doubt that you can

hurt anything at this stage so don't worry. But for fine tuning the amp,
we'll

have to do this and get the tube set on the bias curve for max power in

class A.

If the tube is drawing more the 80% of its tube dissipation at idle, then it

is running close to class A now and I'm sure it is.  The sound of two 

tubes class A is twice as loud as one. Two tubes in the same

amp class AB push pull is louder and more headroom.

Two tubes AB PP with fixed bias is about 4 times louder.

If you hear a squeal when the amp is powered up

Assuming that nothing is wired wrong, the squeal you hear is not normal and

sounds like you have the output transformer primary wired backwards or the

secondary is backwards.  Either can do that. If not then, you have a 

serious capacitive coupling feedback problem from poor wire location in the 

high gain areas.

 Operation of the feedback control

Yes the feedback at max setting, (control set fully counter clockwise or

minimum resistance), or max negative feedback voltage, does reduce the gain

of the circuit quite a bit. But that also will clean up the tone quite a bit

too.  I think of it as more like a free distortion control.

With it set CCW there is the most control of distortion and fully CW has the

least control of the distortion.


OUTPUT IMPEDANCE OF THE PA

5K to 6K5