A question about amps with no master volume

[tot_Ou_tard]

How do these kind of amps work?

Is the power tube always cooking at the same level & the volume controls the the input signal to the preamp tube (which of course changes the input to the power tube)?

Is turning the amp down & turning the level up on a boost pedal essentially the same as turning the amp up & turning down the level on the pedal?

 

[tunghaichuan]

I'll take a crack at this since no one has responded yet.

How do these kind of amps work?

Is the power tube always cooking at the same level & the volume controls the the input signal to the preamp tube (which of course changes the input to the power tube)?

For the simplest explanation take a Valve Junior or Tweed Champ: two gain stages (a 12AX7) in the preamp driving a single ended Class A output stage. The volume control in these two amps is between the two gain stages. In both cases it is a 1meg pot acting as a variable grid load resistor on the second gain stage. When the pot is turned all the way up, the resistance is maximum and allows all of the signal amplified by the first stage to pass into the grid of the second stage. Which is then amplified again and sent to the output stage. When the pot is turned all the way down, the grid is at ground potential and no signal is allowed to pass.

The grid load resistor on the power tube, be it the EL84 in the VJ or the 6V6 in the Champ, acts the same way: it creates resistance to ground on the power tube's grid which allows the signal to pass. In both of these amps this is where you'd put the master volume. For example you could wire in a 250k pot for the 220k grid load resistor on the 6V6 tube's grid in the Champ to make a master volume. Same with the VJ.

In a Class A amp (whether SE or push pull) the output section is drawing current from the power supply and dissipating power whether or not there is a signal present.

It gets more complicated with push pull amps as most of them are Class AB, and have phase inverters. In a push pull amp you can put the master volume before the phase inverter or after it.

My understanding of Class AB circuits is that the tubes idle at a lower current draw when there is no signal present, and start to draw current when a signal is applied to the power tubes (i.e., turning the preamp volume control up).

Is turning the amp down & turning the level up on a boost pedal essentially the same as turning the amp up & turning down the level on the pedal?

Based on my limited understanding, I would say yes, a booster pedal is mainly a gain stage placed before the first preamp stage. So you're adding gain with the boost and subtracting it by turning down the volume control.

tung

 

[tot_Ou_tard]

Spectacular info Steve! That was exactly what I was looking for.

Based on my limited understanding, I would say yes, a booster pedal is mainly a gain stage placed before the first preamp stage. So you're adding gain with the boost and subtracting it by turning down the volume control.

tung

From what you said before, it seems like these would act differently as the amp volume affects how much of the post pre-amp signal gets through to the power tube, while a booster affects the strength of the input signal to the preamp.

It would seem that if the amp volume is cranked & the "booster" pedal set way below unity then there would be very little preamp distorsion.

If the booster way cranked and the volume was way down, then there would be a lot of preamp distorsion.

In both cases the signal strength to the power tube would be the same, but the character of it would be different.

Am I wrong? I'll have to go to my basement lab & do some experiments.

Thanks!

 

[Plank_Spanker]

It's good to have a guru in our midst! :AOK:

 

[Tone2TheBone]

I agree. Be ready to turn on your consulting meter for future questions you're going to be inundated with Tung. ude:

 

[duhvoodooman]

It's good to have a guru in our midst! :AOK:

:AOK: :AOK: :AOK:

 

[tunghaichuan]

Spectacular info Steve! That was exactly what I was looking for.


From what you said before, it seems like these would act differently as the amp volume affects how much of the post pre-amp signal gets through to the power tube, while a booster affects the strength of the input signal to the preamp.

It would seem that if the amp volume is cranked & the "booster" pedal set way below unity then there would be very little preamp distorsion.

If the booster way cranked and the volume was way down, then there would be a lot of preamp distorsion.

In both cases the signal strength to the power tube would be the same, but the character of it would be different.

Am I wrong? I'll have to go to my basement lab & do some experiments.

Thanks!

Like I said, I have limited knowledge. But I agree with your assessment. To be sure, you'd have to calculate the voltage gain levels of the various stages.

One thing that many guitar players don't realize is that even though the theoretical gain of a 12AX7 triode is 100, a lot of the time the actual real world gain is closer to 50 due to the chosen values for plate and cathode resistors.

steve

 

[tunghaichuan]

It's good to have a guru in our midst! :AOK:

Well, I wouldn't really call myself a "guru." I know just enough to be dangerous A lot of concepts in electronics still confuse me :thwap:

I really could use some formal electronics education.

tung

 

[duhvoodooman]

I know just enough to be dangerous A lot of concepts in electronics still confuse me :thwap:

I really could use some formal electronics education.

Same here, x 10!!

 

[Bloozcat]

Well, I wouldn't really call myself a "guru." I know just enough to be dangerous A lot of concepts in electronics still confuse me :thwap:

I really could use some formal electronics education.

tung

Ain't it the truth. I know that I've got some BIG holes that need filling in...

 

[Plank_Spanker]

I'm an electronics tech by trade, but I deal mostly with radio and radar. Give me a good schematic, and I could probably ham fist my way around a guitar amp - but not the guys that really know their stuff with them.

 

[tot_Ou_tard]

Ain't it the truth. I know that I've got some BIG holes that need filling in...

Shouldn't they be there? That's how the sound gets *out* of an acoustic guitar. No need for electricity--or electrical theory.