Mesa Boogie Mark I

Stompboxes circuits published in magazines, books or on DIY electronics websites.

Mesa Boogie Mark I

Postby POTL » 18 Aug 2018, 21:53

Hi
This is certainly not stompboks, but I'm sure that they will help me here)
Since there are no schemes in good quality on the Internet, I attached the lonestar scheme, as the closest analogue.
I'm interested in one question, before the last stage of amplification is the RC filter 2M2 // 20p, I'm confused that the resistance of the resistor is very large, and the capacitance of the capacitor is very small.
As far as I understand, there will be a large loss of volume and also this filter will let pass mostly only high frequencies.
Maybe I do not understand the circuit and when the reverb does not work, the signal somehow bypasses this filter.
Similar filters I've seen in fender schemes, please tell me what its purpose is.

And one more question.
The Drive Control is similar to a typical volume control, but why it is connected to the reverse.
usually the central leg is at the output of volume control, and here the central leg is the input.
here the central leg is at the volume control input.
What is the point of this connection?

Image
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Re: Mesa Boogie Mark I

Postby Manfred » 19 Aug 2018, 08:56

I'm interested in one question, before the last stage of amplification is the RC filter 2M2 // 20p, I'm confused that the resistance of the resistor is very large, and the capacitance of the capacitor is very small.

Have a look at the entire circuit, the reverb stage comes out with a 470 Kiloohms resistor.
If the FET J175 ist switched on the resistance is 680 Kiloohms in its off state ~ 1 Megohm.
The 470 Kiloohms, the 2.2 Megohms resistors and the switchable resictance forms a summation circuit,
to summing the try and reverb signal.
The large value of 2.2 Megohms is required to determines the right signal mixing ratio.
I guess the 20pF capacitor serves to prevent oscillations.

The Drive Control is similar to a typical volume control, but why it is connected to the reverse.

The drawing shows the divided potentiometer resistances, I hope it makes it clear.

Drive potemtiometer reverse mode.JPG
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POTL (19 Aug 2018, 14:33)
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Re: Mesa Boogie Mark I

Postby Optical » 19 Aug 2018, 11:48

The 2M // 20p components are a characteristic of every Mesa Mark, it's part of their unique tone.

The drive pot is configured like that so the gain responses also changes the frequency response. Low gain, tight bass; high gain, loose bass.

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POTL (19 Aug 2018, 14:33)
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Re: Mesa Boogie Mark I

Postby phatt » 20 Aug 2018, 12:45

It's a High Z mixer circuit, With low Z circuits it would suffer great loss.
That 2Meg mix resistor does loose a bit of volume but not a major problem for hi Z circuit.
There is some treble loss hence the small cap to make up for the hi freq loss.
Impedance (Z Ohms) is a rather tricky little mongrel as it's resistance of the AC signal which is frequency dependent where as resistance is just simple DCR.

Study of classic tone circuits will help,
Research some low Z circuits with a similar tone control setups and you may find some look just like the Lone star tone *Except* they use much *lower* value pots and much *larger* value capacitors but the tone curves are very close to the same response.
The rule of thumb for conversion from high to low Z tone circuits is X10 lower resistors and pots and x10 higher value capacitors.
Hope it Helps, Phil.

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POTL (20 Aug 2018, 14:11)
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Re: Mesa Boogie Mark I

Postby POTL » 20 Aug 2018, 14:13

phatt wrote:It's a High Z mixer circuit, With low Z circuits it would suffer great loss.
That 2Meg mix resistor does loose a bit of volume but not a major problem for hi Z circuit.
There is some treble loss hence the small cap to make up for the hi freq loss.
Impedance (Z Ohms) is a rather tricky little mongrel as it's resistance of the AC signal which is frequency dependent where as resistance is just simple DCR.

Study of classic tone circuits will help,
Research some low Z circuits with a similar tone control setups and you may find some look just like the Lone star tone *Except* they use much *lower* value pots and much *larger* value capacitors but the tone curves are very close to the same response.
The rule of thumb for conversion from high to low Z tone circuits is X10 lower resistors and pots and x10 higher value capacitors.
Hope it Helps, Phil.



Thank you
I really did not think about reducing the resistor and increasing the capacitor.
I'm building a Jfet copy and I was confused by this filter, I changed the values ​​of the components (by turning on the resistor 1M, which I will replace with 100K) and look at the results =)
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