PSU/PT question

[daz]

I have this amp that was built as a marshall style but i have been using 6V6 in it lately because JJ's can handle the voltage and sound good. Theres 440v on the plates. But if i wanted to build another amp like this with the same preamp but use a PT made to run 6V6 with whatever proper voltage is, 350v or whatever, How do you do that and still be able to get the same sound out of the preamp? What I mean is, with a PT like that you now have much less voltage so the preamp is going to be much lower too, and that changes the feel and sound quite a bit. Are there any tricks to building a amp for lower power tubes like that without having to sacrifice preamp voltage. Will using 4.7k droppers instead of 10k get it there? i suppose theres a formula....here we go again. (zero math skills)

[soundguruman]

yes, just reduce the 10K until the voltage comes back up.
The amount of drop depends on current, how the preamp is built and biased, what tubes you are using...

In the absence of formula, "substitution" method is perfectly acceptable.
You don't need to be a math genius to substitute a resistor, and measure the final voltage, with your volt meter.

What you DO need, is a kit of resistors, to substitute with.

FYI the voltage does not need to be exactly the same as the schematic.
Ballpark is just dandy.

[nashvillebill]

I don't claim to be a power supply expert but perhaps I can offer a little bit of explanation. And yes, some math is involved.

Okay, so you have 440 volts on the plates of your power tubes now. You're going to run 350 volts on the plates on your power tubes.

Coming out of the power supply, you have a transformer, then rectification to DC along with a smoothing capacitor. At that particular point in the DC power supply, you have what's commonly called B+. This voltage goes to the center tap of the output transformer's primary, where it drops a little due to the transformer's reactance, and yields your plate voltage of 440 volts (or after you change the PT, to be 350).

In other words, B+ is going to be a few volts higher than the 440, maybe around 450 volts. Now, the preamp tubes obviously don't get anywhere near that voltage. The B+ voltage is dropped through dropping resistors to get to the desired voltage.

The question is: how do we size that dropping resistor? Since I don't know the particulars of your amp, let me use a schematic from an amp I've just recapped (a GE7700 stereo amp). It has 400 volts B+. This is dropped to 350 volts by a dropping resistor, which is 3600 ohms. Now here's where the math comes in. The voltage drop was 400 minus 350, which equals 50 volts dropped. The voltage drop across a resistor is given by a formula known as Ohm's Law, and it states: V=IR. The voltage drop (V) across a resistor is equal to the current in amps flowing through that resistor (I) multiplied times the resistor's resistance in ohms (R). We can use that formula to figure out how much current drop was across my circuit's 3600 ohm resistor. V=I times R... so 50=I times 3600. We solve for I by dividing 50 by 3600...that equals 0.014 amps. So 0.014 amps was flowing through the resistor. Remember that number, it will be the same when we change our plate voltage.

Okay, suppose I change my B+ down to 360 volts. I want 350 volts on the other side of the resistor. That means I want a voltage drop of (360-350) or 10 volts. I can solve for the new resistor by using the same formula V=IR and plug in the numbers we know. V will be 10; I will be the 0.014 we just calculated. We don't know R. So we now have 10=0.014 times R. We divide the 10 by the 0.014 and we get R.....714 ohms. Round it off to the next closest common size (715 ohms).

For your amp, you can measure the voltage drop across the power supply's dropping resistors and calculate the current being drawn by the preamp, then when you change the power supply's voltage, calculate the new dropping resistor that will let you wind up at the same voltage drawing the same current.

[potatofarmer]

Oop, nashville bill beat me to it and wrote it up nicer!

The only thing I'd like to add is to keep decoupling the same or similar, make sure R1C1 is roughly the same as R2C2. So if you go from a 10k resistor to a 4k7, increase the filter resistor from 22 uF to 47 uF.

[daz]

Well, let me ask this...do you think it's possible to preserve the same preamp voltages just by lowering the 10k droppers which are at 3 nodes when the PT is putting out 100v less than the old one? Like i said, i'm seriously handicapped when it comes to math so the answer is not gonna happen In my mind.

[g1]

If you've decided the preamp voltage is your primary concern, then design your power supply around that, not around what kind of power tubes you are using.
Then it is easy to adjust the power tube plate supply to be whatever you want it to be.
I'm guessing the PT you will end up with will be somewhere between what you have now and one that give voltages typically used in 6V6 amps.

[soundguruman]

"Well, let me ask this...do you think it's possible to preserve the same preamp voltages just by lowering the 10k droppers which are at 3 nodes when the PT is putting out 100v less than the old one? Like i said, i'm seriously handicapped when it comes to math so the answer is not gonna happen In my mind."

It's still possible.
Increase the filter capacitors also, to maintain more isolation/ stiffness between stages.

Stop being afraid. Experiment to learn. Be Bold.
It's a guitar amp, not a moon rocket.

[nashvillebill]

...
It's a guitar amp, not a moon rocket.

What if it's a Gibson Apollo??

[soundguruman]

What if it's a Gibson Apollo??

You got me there, bro. I acquiesce.

[daz]

Well, normally i'm not afraid to experiment but this one requires spending $ on a new PT so i wanted answers before i potentially throw away any $.