Marshall 2203/2204: resistor on presence pot?

cardinal

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5,318
Hi all:

What is the impact of changing the resistor circled in red from 4k7 to 10k?

On the 2204 head, this value is 4k7. On the 4010 and 4104 combo, it's 10k.

Working on a 4104 and wondering if I should change it to 4k7, but I don't know what impact that would have?

thanks!
 
Last edited:

cardinal

Member
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5,318
What do you mean by 4k7?
4700 ohms. Color code is yellow violet red. That's the value on the 2204 head.

The 4010 and 4104 combos (nearly identical to the 2204 head) use a 10,000 ohm resistor here. Brown black orange color code.

Wonder what difference that causes?
 

pdf64

Member
Messages
7,765
On the 2204 head, this value is 4k7. On the 4010 and 4104 combo, it's 10k.

Working on a 4104 and wondering if I should change it to 4k7, but I don't know what impact that would have?
Going from 10k to 4k7 will increase the power amp gain, reduce the range of control of the presence (ie less change from min to max setting), and reduce speaker damping.
The differences may seem pretty small, unless you put it on a switch and AB the change, it may seem fairly negligible.

Note that @Jeff Gehring was answering the first query, which was phrased the other way around
What is the impact of changing the resistor circled in red from 4k7 to 10k?
 

HotBluePlates

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11,302
What is the impact of changing the resistor circled in red from 4k7 to 10k?
The circled resistor is marked "R23." The horizontal line from it runs over to "R22" (100kΩ), which itself runs over to the 4Ω secondary tap of the output transformer.

Changing any of those 3 items (R22, R23, transformer tap) changes the overall negative feedback applied.​
When you play the amp, there is signal voltage present at the output transformer tap. The 4Ω tap has the smallest signal, the 8Ω tap is √2-times bigger, and the 16Ω tap is 2x bigger.​
R22 and R23 form a voltage divider to sample a portion of the signal voltage present at the OT secondary, and feed it back at the phase inverter. Changing either R22 or R23 changes what % of transformer voltage is sampled for feedback.​

Often, folks look at a different-model or different-era Marshall, and just see a different resistor value. Just as often, they fail to notice which OT secondary tap the feedback was sampled from, or that there are 2 resistors working together to determine the amount of feedback.
 

cardinal

Member
Messages
5,318
The circled resistor is marked "R23." The horizontal line from it runs over to "R22" (100kΩ), which itself runs over to the 4Ω secondary tap of the output transformer.

Changing any of those 3 items (R22, R23, transformer tap) changes the overall negative feedback applied.​
When you play the amp, there is signal voltage present at the output transformer tap. The 4Ω tap has the smallest signal, the 8Ω tap is √2-times bigger, and the 16Ω tap is 2x bigger.​
R22 and R23 form a voltage divider to sample a portion of the signal voltage present at the OT secondary, and feed it back at the phase inverter. Changing either R22 or R23 changes what % of transformer voltage is sampled for feedback.​

Often, folks look at a different-model or different-era Marshall, and just see a different resistor value. Just as often, they fail to notice which OT secondary tap the feedback was sampled from, or that there are 2 resistors working together to determine the amount of feedback.
Thanks. From the schematics I saw, the 2204 and 4010/4104 power amp circuits are the same except for R23, including the same R22 and negative feedback tap.
 




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