Thoughts on my modded Marshall 2203 build?

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#102
Rikaro said:
Don't forget that more PI gain means more available presence boost.
Click to expand...
I finally got round to doing some work on converting this to the marshall PI as seen in the 1959T schematic you quoted earlier.

There are a few things that I have not completed yet, but I fired it up to try out and as it stands, the gain appears worse.
Im putting this down to too much negative feedback, as Im yet to swap out the 4.7K feedback resistor with a 27K as per the schematic.
I still have the 82K grid leak resistors installed too, which I hope the 150K will increase the gain.
Your right about the presence boost however, interestingly enough this definitely has far more presence boost, especially when I crank it right up.
There are a couple of differences in this schematic that differ from the 2003 too, firstly the negative feedback is connected to the 16 ohm tap, whereas i have it connected to 8ohms on the OT like the 2203.
The other difference I see is the presence control on the 2203 alters the resistance between the capacitor to ground, whereas this 1959T model always sees 5K to ground, but the wiper of the pot takes the capacitor to ground, will this have any bearing? Im considering modifying this configuration too.
Are any of these likely to make any impact?

I am going away for most of next week, so hope to finish it off in the weekend.
 
#106
I just got round to looking at this today and put 27K in place and the gain is much higher.
Just about to make some more measurements and will check the gain now, I am pretty sure I will be now getting 100 clean watts out of this, i still have not swapped out the grid leaks yet, am going to make them 150K, which should even provide more gain.
Looking at my notes, i have got -38V bias which is allowing each tube to draw 50mA while idle, does this sound about right for this?
 
#107
nzoomed said:
The Marshall's never rally had much effective presence boost to speak of either.
How ive got this set up is giving me a somewhat noticeable presence boost that's also clearly visible on the scope, certainly much more than a marshall, but at the same time it appears to cut off completely when dialed to zero.
Perhaps I should just back off the NFB a bit more and see how that goes.
I dont want to mess around with the phase inverter just yet, but if I can get some appreciable gain from reducing the NFB at the expense of presence boost, I would consider it.
I can also get close to 100W of fairly clean power on the high sensitivity if I adjust the volume controls appropriately too.

Something else I think might need attention is the resistor from the base of the LTP to ground.

I originally had only 100 ohms here, which turns out is what the fender amps used, however I was getting major crossover distortion and low power, only about 30W max.
I raised it to 4.7K, but would increasing this resistor give me more gain?
It looks like this goes hand in hand with the NFB and acts as a voltage divider?
Right now it's 4.7K on each side.
Click to expand...


Which OT secondary tap is the feedback attached?

The NFB resistor varies between the models and power ratings some were 100K some were lower. That 4k7 sits in parallel with the NFB resistor.

There's also a non-DC-crackly mod that places cap in the way of the DC so only the AC is affected by the presence pot to ground. I'm assuming you've not done this.
 
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#108
It's attached to the 8ohm tap, but some Marshall's connect to the 16ohms.
I've been experimenting with different values and attached a pot again, mostly due to my concern about input sensitivity, I was told that you should expect a 10mV signal to drive a marshall to 100w clean, but I feel thats unrealistic, I think somewhere between 75-100k is giving me over 100w with a signal around 40mV.
I don't know anything about that mod you are referring to, might be worth exploring.
 
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