[Stone]

Hi Nigel

It 's the "normal" one, following the tone stack, 1Meg log potentiometer, followed by a 220k resistor in series off the whiper.

From that point of view it shouldn't be as loud as it is.

Regards, Stone

p.s: re-checked my wiring once more and there is no error - maybe a good chance to give "power scaling" a try ...

[dukeamps]

Hi Nigel

It 's the "normal" one, following the tone stack, 1Meg log potentiometer, followed by a 220k resistor in series off the whiper.

From that point of view it shouldn't be as loud as it is.

Regards, Stone

p.s: re-checked my wiring once more and there is no error - maybe a good chance to give "power scaling" a try ...

Hi Stone, If you use the normal 1 like on the 2203/2204 layout there is no 220k in series, it goes directly to the 0.022uf cap. Here is good schem if you want to switch the bypass cap on & off, with this schem you get no clicking noise,   http://img258.imageshack.us/img258/128/3in1cath.jpg

Nigel

[dukeamps]

Hi stone, here is a layout from the metro forum of the #34 mod.

http://img39.imageshack.us/img39/3811/slash34.jpg   

The 470pf over the first 100k should be over the second 100k. The .47uf over the 1k can be a .68uf for more beef.

The 150k from the gain pot to ground just takes the gain down, you can just leave that out for more gain.


Cheers

Nigel

[Stone]

Hi Nigel

Calculating a little bit around with a stock mv preamp and that modded one (1µF across 10k resistor and 680nF across 820 (1k)) ... and it shows, that the modded version gives around 12 to 15dB more output (in practice) to the phase inverter than the stock one.

I will give the "power scale" within the phase inverter a try and see how it sounds and brings down volume; I expect it to become a little bit more treble sounding, so high frequencies need to be attenuated when using "power scale".

Hopefully I can do some videos this evening.

Regards, Dietmar

EDIT: by calculation a 10 meg resistor before the mv pot would reduce the volume to the desired amount (as it was before) ...

[dukeamps]

Hi Nigel

Calculating a little bit around with a stock mv preamp and that modded one (1µF across 10k resistor and 680nF across 820 (1k)) ... and it shows, that the modded version gives around 12 to 15dB more output (in practice) to the phase inverter than the stock one.

I will give the "power scale" within the phase inverter a try and see how it sounds and brings down volume; I expect it to become a little bit more treble sounding, so high frequencies need to be attenuated when using "power scale".

Hopefully I can do some videos this evening.

Regards, Dietmar

EDIT: by calculation a 10 meg resistor before the mv pot would reduce the volume to the desired amount (as it was before) ...

Hi stone, I was just playing mine amp today & it is a lot louder. What type of power scaling do you use? Wonder what would happen if you put a 10meg there? never tried that before. Looking forward to hear how your amp sounds.

I have also a #36 mod if you’re interested.

Cheers

Nigel

[Stone]

Hm ... not satisfied with it, but gives an impression: http://www.youtube.com/watch?v=XoK-pOUM0t8

Added the power scaling as a potentiometer to the cathodes of the PI (added to the 470 ohms resistor) and I have to say, it works better than expected - I need to adjust it, because it adds a little bit saturation to the sound, but basically it is working very well.

At least I think most of the sound is generated while driving the PI that hard ...

Still left out the bright cap on the 470k/470k voltage divider.

Regards, Stone

EDIT: what I need to add - my guitar is tuned half step down ... and the 150k on gain is not active

[dukeamps]

Hi stone, sounds real good & still got the good old Marshall flavor What's your opinion? Could you tell me a bit more about the power scaling, where did you put the poti & what size is it.

Cheers

Nigel

ps i don't use the 150k  

[Stone]

Hi Nigel

http://mirror.stones-amp-inn.de/marshallamps/jcm800_lead_mstvol_100w_2203.pdf

If you refer to that schem, I "replaced" R16 at the PI with 47k potentiometer (linear type) - as I could not lift up the circuit board I disconnected the wire from the tube socket which goes to cathodes of V3 and inserted the potentiometer. The recording was done with potentiometer set to half, so the total resistance for R16 would have been around 20k to 25k.

I think the overall sound of the amp is ok - with potentiometer set to "0" it is a real killer, but even too loud. So next time I will do a recording with a power soak in order to get the PI driven hard as before.

While simulating the "power scale" with LTSpice I can see that the high frequencies are not attenuated as they are without - so I think I can recreate most of the original (killing loud) sound by adding anode bypass caps; simulation shows that 500pF across the 100k will be a step in the right direction (have to calculate the value for 82k anode).

At least I must say I am a little bit more satisfied with the fat crunch sound I had before, but that also depends on personal taste. Maybe using another guitar with a straight DiMarzio PAF on it will give a more common example.

Regards, Stone

[Stone]

btw would be nice to see the version of your SIR #36 mod

[carlitz]

Yes, show it to all of us !!

[dukeamps]

Hi stone, thanks for the info.

Ok here is the mod

#36 mod

Stage 1 input 33k shield to ground. Cathode 2k7/0.68uf/ coupling cap 0.022uf/ 470k/500pf  1meg a pot/ 1000pf bright cap silver mica.  

Stage 2 Shield on anode. Cathode 10k/.1uf/ coupling cap 0.022uf/ 500pf over the 100k plate resistor. 470k/500pf/470k.  

Stage 3 Cathode 10k coupling cap 0.022uf 100k plate resistor. 470k/500pf/470k.  
 
Stage 4 Cathode 820r/0.68uf.  

Tone stack 33k/250pf the rest stock.

100pf fizz cap

NFB 47k/0.47uf /4ohm tap.

Stock power amp


Make sure that the shield is on anode or it will not work.
It will be just one big hiss oscillation machine

I think that’s all

Cheers

Nigel


PS try this if you only know what you are doing.

[Stone]

Hi Nigel

Ok, it seems to be a #39 without switching option(s). I have already done a #39 based conversion on basis of my 2203.

I think, I will make schems for them mods and post them.

Regards, Stone

[dukeamps]

Hi Nigel

Ok, it seems to be a #39 without switching option(s). I have already done a #39 based conversion on basis of my 2203.

I think, I will make schems for them mods and post them.

Regards, Stone

Hi stone, its more or less the same, I have found that, that circuit is very sensitive & any type of switching changes the tone. That would great if you would do a schem for them both . Oh yes all the coupling caps are 0.022uf. How did your #39 work out?

Cheers

Nigel   

[Stone]

Hi

Changed it a little bit - it is documented here in the Marshall section (have a look for "Neues Projekt SIR #39" somewhat of darkbluemurder).

A video is also uploaded here http://www.youtube.com/user/StoneAmplification#p/u/1/PqH4DfUdjyw

The initial stage is taken off a SLO100 and currently I have added something like a tube booster with the left triode. Will do some recording of the acutal setup next days.

Regards, Stone

[dukeamps]

Hi

Changed it a little bit - it is documented here in the Marshall section (have a look for "Neues Projekt SIR #39" somewhat of darkbluemurder).

A video is also uploaded here http://www.youtube.com/user/StoneAmplification#p/u/1/PqH4DfUdjyw

The initial stage is taken off a SLO100 and currently I have added something like a tube booster with the left triode. Will do some recording of the acutal setup next days.

Regards, Stone

Hi stone, sounds good, what do you think of the greenback speakers?  I was using 4x12 scumback m75 65w speakers, but found them to be too bassy. I have changed to celestion g12h30 heritage & find them to be the speaker I was always looking for.

Cheers

Nigel