Technical questions regarding 50 watt JMP

[beasleybodyshop]

For fun, i am studying this schem that is attached. Beginning with the first gain stage, I am trying to understand exactly what each component in the circuit is doing. Here are my questions regarding everything up too right after V1:

What is the purpose of the 1Meg resistors off of the input jacks? Is it to prevent noise when a jack is unplugged?

On this schematic, the bright channel shows a .0022uf coupling cap. Most newer "kit" layouts I have seen show a .022uf for both bright and normal channel coupling caps. Why is this? is the .0022 considered too "thin" sounding? is this why its used on the bright channel from the stock design?

What purpose does the dual 50/50uf electrolytic cap after the first gain stage serve? is it preventing DC from the signal from being passed on to the next stage?

Sorry if all this seems like rambling, but I really want to understand the nuts and volts of this circuit.

[John_P_WI]

OK, these are the fundamentals of a tube amp, glad you are asking besides painting by numbers.

The 1M resistor at the input is referred to as a "grid leak" resistor. In this configuration, it acts as a ground reference for the tube and is mandatory. Most guitar amps have the preamp stages cathode biased, therefore the grid is now referenced to ground, while the cathode may be 1+ volts (more or less) above ground setting the bias of the tube. The 1M also sets the input impedance of stage. Many times a volume potentiometer will be used as the grid leak of following stages. Finally, grid leaks are necessary as without a reference to ground (in a channel switch circuit) the tube could try to run away - the grid leak keeps a leash on the current.

Regarding the 2n and 22n coupling caps, today, this can be attributed to tastes or economy, reducing the number of different components needing to be stocked. Remember, 45 years ago, this would have been intended to be a multi instrument amplifier. Today, the amps are generally geared towards not only an instrument, but a style or tone. As an side note many of the high gain circuits of the last 20 years can benefit and use the smaller valued caps limiting the bass keeping the sound tight and focused.

The 50u / 50u cap is simply the DC rail supply. The cap is the "can cap" or multi section cap in the power supply. As shown in the schematic, the tube plate is connected to the rail by the "plate resistor". The signal is generated by the current passing through the plate resistor and the connected cap or "coupling cap" which blocks the power supply DC allowing the ac signal to pass through.

This is the abbreviated version, but should help you understand the most basics of tube amps.

[matt h]

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[beasleybodyshop]

Thanks guys, I am trying to learn the hows and whys so I can stay away from soldering by numbers (which, up to this point, have been pretty guilty of doing so.)

I have a few more questions regarding everything up to the second gain stage:

Why are one of the Anodes of V2 connected to the grid?

I have read that if you lower the value of the mixer resistors, you would have to "scale the bypass cap". Does this mean if I use smaller resistor values, I need to use a smaller bypass cap on V2?

[John_P_WI]

Why are one of the Anodes of V2 connected to the grid?

In simplest terms, that is the "cathode follower", which provides a low impedance drive to lossy circuits, in this case the tone circuit. Other applications are effects loops. Cathode follower driven tone stacks have a more throaty tone than plate driven stacks. I like both, neither are better, both have their place.

Regarding the frequency response:

f = 1 / (2 x pi x R x C)

where C, is in farads.

This gives us a constant "slope" and at what frequency the signal passes or begins to become attenuated.

But as the formula shows, IF R becomes smaller C will need to become larger for the same f, or IF R becomes larger C will need to become smaller.

[martin manning]

Check out the Valve Wizard site http://www.valvewizard.co.uk There are a number of very good articles on the various parts of amplifier circuits.

Edit: I see the Wiz has taken his power supply book out of print due to the number of errors and typos.

[xtian]

Edit: I see the Wiz has taken his power supply book out of print due to the number of errors and typos.

Oh, no! I was just rereading that! Guess it's good I didn't understand a thing. Now I can claim all my errors are Merlin's. (Hi, Merlin!)

[beasleybodyshop]

Why are one of the Anodes of V2 connected to the grid?

In simplest terms, that is the "cathode follower", which provides a low impedance drive to lossy circuits, in this case the tone circuit. Other applications are effects loops. Cathode follower driven tone stacks have a more throaty tone than plate driven stacks. I like both, neither are better, both have their place.

Regarding the frequency response:

f = 1 / (2 x pi x R x C)

where C, is in farads.

This gives us a constant "slope" and at what frequency the signal passes or begins to become attenuated.

But as the formula shows, IF R becomes smaller C will need to become larger for the same f, or IF R becomes larger C will need to become smaller.

Ok, so R is the resistor value in ohms? And if I were trying to determine the cutoff freq. of a .022 uf coupling cap, to use the formula above I would need to convert uF to F?

What do you mean by "slope"? Are you referencing a slope on a graph?

[jelle]

The slope resistor is the 33K resistor in the tone stack. It gives an advantage to the highs by attenuating the lows and mids.

Note that the Bass models have a 56K there which attenuates the bass and mids even more, but because the gate for the treble frequencies (in the form of the treble cap) is reduced from ca. 500pF to ca. 250pF, the whole mid scoop is much bigger in the Bass model.

The tonestack in the schem you attached is the Lead tonestack. It is more van Halen sounding. The bass tonestack is a bit more fendery in the frequency distribution.

Have fun studying!!!

Jelle

[jelle]

Also, see

http://www.duncanamps.com/tsc/index.html

Change the values and see what happens. Best simple tool I have seen for this.

jelle

[Reeltarded]

slope is the rate of attenuation change relative to frequency haha

R is the resistor value, C is capacitance, f is frequency.

In your common marshall thing you will find 56k/250p or 33k/470p slope/treble combos. Google Duncan's Tone Stack Generator to get a visual representation of what slope does.

never answer a phone while replying

[beasleybodyshop]

slope is the rate of attenuation change relative to frequency haha

R is the resistor value, C is capacitance, f is frequency.

In your common marshall thing you will find 56k/250p or 33k/470p slope/treble combos. Google Duncan's Tone Stack Generator to get a visual representation of what slope does.

Is this where I need to put a 50KL pot for that slope control mod you were describing to me? put it in parallel or series with the resistor?

[Reeltarded]

Series. Adds variable up to 50k, right? the resistor mounts to one leg of the pot and then you wire the pot in where the resistor went.

The two white wires that go from lower left up to middle right on a 50 I have.

[jelle]

never answer a phone while replying

Haha!

[Reeltarded]

yep. I mean YAP lol