2205 2210 Mods Thread!

[ricksteruk]

Ricky you are so right! I need to look at resonance as this can boost low frequencies!

Does resonance have to do with the NFB wire? Putting a cap in there right?

I've no idea where the (purple?) NFB wire goes on my amp - the schematic I have is only for the Preamp.

Does anyone have a schematic for the PI and Power amp of the later model 2205/4210?

 

[RickyLee]

The 1MEG I used for my VPR type PPIMV was just a linear. I am thinking it was just an Alpha brand. It seems to work quite well. But it will not turn down to completely OFF. It is just a volume reduction in itself. You can still adjust your other Volume's and just use this to take the edge off the total volume. It works good if you use it to go from your full volume settings or just crank the amp to full and then use it to reduce the output from there to something sensible. It does not affect the clarity too much if used sparingly.

Your NFB wire should be coming off the impedance selector.

Marshall Schematics

http://www.drtube.com/schematics/marshall/2205pwrm.gif

You can just insert a cap in series with the NFB wire. But then it would be removing most of the NFB. You might not want that. The Resonance control just uses a 1MEG pot in parallel with a cap to adjust.

 

[ricksteruk]

Thanks very much for the power schematic link. I'd looked on Dr Tube before and got the preamp circuit from there - but must have missed the power one.

I was thinking of putting a PPIMV just like the one on the older 4210 circuit - just putting the 1meg pot from signal to ground after the coupling caps c32 and 34, then adding some extra coupling caps the other side before the power tubes. I've been looking at some "improved" circuits of this type and they were recommending much larger caps for the new ones (like 1uf) so I was going to try that - it supposedly keeps the frequency response level as you adjust the master.

If the pot is on minimum and the signal is effectively shorted to ground shouldn't there be zero output (i.e. OFF) from the power tubes? Is your PPIMV wired differently?

For the resonance, I'm going to try a few cap values till I find the one I like (starting with 10nf) as you suggest. I'll move the NFB wire to the 8 or 16 ohm lug to give more neg feedback to start with so the amp doesn't get too undamped and flubby when I reduce the bass feedback. Then I'll use a pot in parallel with the cap to find how much bass I want to add (by cutting it from the NFB!) and once I'm happy I will measure the pot value with my multimeter and replace with a quality resistor.

The parts are on order so I should be able to try it by the weekend.

 

[ricksteruk]

OK you guys must be getting sick of me by now..

But I'm wondering if anyone is good at figuring out how this V2b circuit is working as a high pass filter...

V1a (not shown above sorry!) is easy to calculate - there's just 1 resistor R15 (470k) and 1 cap C13 (0.022uF), which means its operating at f = 1 / ( 2 x pi x 470k x 0.022uF) = 15 Hz for it's 3dB point - which is ok as it's below guitar range (though hi fi buffs will tell you that it will be causing your low end up to 150Hz to be affected!!)

However, I am unsure about this V2b circuit as it's a lot more complicated. I think C15 might be acting as a "bright" circuit. What I am not sure about is if I can consider just R19 (100k) along with C14 (0.022uF) as a high pass filter (operating at 72Hz), or if and how R18 ( or R21 as well !!! ) comes into the calculations. If R18 was simply added to R19, then it would work out that f = 28Hz.

Basically I want to know because my clean channel still needs more bottom end to compete with the overdriven Boost channel.

If f = 72 Hz then that means I'm losing approx 3dB of bass from the Clean channel, and I should up the C14 to at least 0.047uF or perhaps 0.1 uF.

Perhaps when the caps I have ordered for the resonance mod arrives I can try jumping some across C14 to see if that makes any difference to the sound rather than trying to wreck my head with calculating circuits.

 

[RickyLee]

OK you guys must be getting sick of me by now..

But I'm wondering if anyone is good at figuring out how this V2b circuit is working as a high pass filter...

V1a (not shown above sorry!) is easy to calculate - there's just 1 resistor R15 (470k) and 1 cap C13 (0.022uF), which means its operating at f = 1 / ( 2 x pi x 470k x 0.022uF) = 15 Hz for it's 3dB point - which is ok as it's below guitar range (though hi fi buffs will tell you that it will be causing your low end up to 150Hz to be affected!!)

However, I am unsure about this V2b circuit as it's a lot more complicated. I think C15 might be acting as a "bright" circuit. What I am not sure about is if I can consider just R19 (100k) along with C14 (0.022uF) as a high pass filter (operating at 72Hz), or if and how R18 ( or R21 as well !!! ) comes into the calculations. If R18 was simply added to R19, then it would work out that f = 28Hz.

Basically I want to know because my clean channel still needs more bottom end to compete with the overdriven Boost channel.

If f = 72 Hz then that means I'm losing approx 3dB of bass from the Clean channel, and I should up the C14 to at least 0.047uF or perhaps 0.1 uF.

Perhaps when the caps I have ordered for the resonance mod arrives I can try jumping some across C14 to see if that makes any difference to the sound rather than trying to wreck my head with calculating circuits.

Let me know how that works out if you try a bigger coupling cap for C14. Fender and Mesa use .047uF in some preamp circuits as well.

I have my Normal channel sounding badass running a 12AY7 in V2 and I put some big cathode bypass caps on V2A & V2B.

 

[AldermaticAmps]

Let me know how that works out if you try a bigger coupling cap for C14. Fender and Mesa use .047uF in some preamp circuits as well.

I have my Normal channel sounding badass running a 12AY7 in V2 and I put some big cathode bypass caps on V2A & V2B.

Throughout Marshall's history, they have almost always used 22nf coupling caps. Fenders tended to use 47nf caps on their blackface amps, .1uf caps were often used on 50's amps and often on PI's.

I really wouldn't fret too much over the value of a coupling cap. If I were going to mod something on an amp, that would probably be the last thing I would mod.

 

[supershifter2]

guitar center in northridge california has a JCM800 4210 for sale for $899.00 phone 818-993-0286 ask for Jessica, Sean, or Julian and tell them Mudpie told you about the amp. if they aint there ask for anyone but dont mention Mudpie. heres a foto of it.

 

[ricksteruk]

Let me know how that works out if you try a bigger coupling cap for C14. Fender and Mesa use .047uF in some preamp circuits as well.

I have my Normal channel sounding badass running a 12AY7 in V2 and I put some big cathode bypass caps on V2A & V2B.

I'll give it a go and let you know!

About those cathode bypass caps. Would you say they actually give more bass emphasis or just increase gain?

I know that a bypass cap makes a high pass filter with the cathode resistor so you can use it to cut bass below a certain frequency - you basically get 6dB more boost above that frequency compared to below it. (I modded my C2 on V1a to move this 6dB bass cut from 125Hz stock down to 12.5Hz)

But if you use a big cap it just boosts everything by 6dB - so doesn't affect tone in that sense of boosting or cutting directly like that... But what about the overall character of a big cap fully bypassed stage as opposed to no cap at all? I'd heard that an unbypassed stage has less distortion and perhaps a more clinical sound, while the bypassed one is warmer... does the cap affect how the valve works to make it more bass heavy?

It'll be pretty easy to temporarily add a cap to V2 to see what the tonal effect is - I'll have to try when I crack the amp open next.

 

[ricksteruk]

Throughout Marshall's history, they have almost always used 22nf coupling caps. Fenders tended to use 47nf caps on their blackface amps, .1uf caps were often used on 50's amps and often on PI's.

I really wouldn't fret too much over the value of a coupling cap. If I were going to mod something on an amp, that would probably be the last thing I would mod.

Yeah I'm looking at other avenues first - I've adjusted some cathode bypass caps and am looking into the resonance and the NFB wire.

But it's just to me there is something a bit off with this clean channel circuit. Maybe Marshall just bunged in 22nfs everywhere as they usually do, which was not the best idea for the clean channel. I certainly would like it to sound as Fender ish as possible, so maybe 47nf of 100nfs coupling caps are the final piece in the jigsaw.

 

[Alabama Thunderpussy]

Throughout Marshall's history, they have almost always used 22nf coupling caps. Fenders tended to use 47nf caps on their blackface amps, .1uf caps were often used on 50's amps and often on PI's.

I really wouldn't fret too much over the value of a coupling cap. If I were going to mod something on an amp, that would probably be the last thing I would mod.

That's one of the first things I mod when addressing the lowend response of a preamp. You can increase cathode bypass all day long, but if the coupling cap to the next stage is only so large, it's not going to pass frequencies below the knee set by its value in conjunction with the voltage divider at the next stage input.

That being said, 100n coupling is usually about as high as you would need to go to allow passage of all frequencies of the guitar signal, but for most people it only sounds good in clean-to-moderately-pushed tones. Even 47n is usually enough.

 

[mickeydg5]

RicksterUK

One big obstacle to a Fenderish sound in that amplifier is the first gain stage.
It is almost typical Marshall design and not Fender voicing. It feeds both channels. What have you done to it?

There is much more to a bypass capacitor of a tube's cathode circuit. It is not a high pass but actually a low pass filter with relation to the circuits and operation. The bypass capacitor does not cut bass frequency. The operation of the circuit must be understood. It does work in conjunction with the other circuits of the tube. But most of all the bypass capacitor also sets gain at frequency. The incoming signal will have most gain at the filter's resonance due to the tube's operation.

Depending on the stage design you may witness overall gain increase or notice more lower to resonance frequency gain. You cannot increase what is already maxed. AlabamaTP pointed that out.

 

[RickyLee]

RicksterUK

One big obstacle to a Fenderish sound in that amplifier is the first gain stage.
It is almost typical Marshall design and not Fender voicing. It feeds both channels. What have you done to it?

There is much more to a bypass capacitor of a tube's cathode circuit. It is not a high pass but actually a low pass filter with relation to the circuits and operation. The bypass capacitor does not cut bass frequency. The operation of the circuit must be understood. It does work in conjunction with the other circuits of the tube. But most of all the bypass capacitor also sets gain at frequency. The incoming signal will have most gain at the filter's resonance due to the tube's operation.

Depending on the stage design you may witness overall gain increase or notice more lower to resonance frequency gain. You cannot increase what is already maxed. AlabamaTP pointed that out.

This is a good point and something I forgot that I addressed in my 4210. Now I want to say up front that I do not advocate the hacking up of an old Marshall amp circuit and soldering on these old PCB's. You take the risk of having an amp that could possibly give you major problems down the road as these old PCB's are VERY FRAGILE to the solder iron heat.

On my 4210, I gave the Normal and Boost their own dedicated first gain stage. I put a Fender type gain stage on the Normal channels first gain stage as I did not want a big cathode bypass cap for the Boost V1A (bigger than the .47uF is what I mean).

 

[ricksteruk]

RicksterUK

One big obstacle to a Fenderish sound in that amplifier is the first gain stage.
It is almost typical Marshall design and not Fender voicing. It feeds both channels. What have you done to it?

There is much more to a bypass capacitor of a tube's cathode circuit. It is not a high pass but actually a low pass filter with relation to the circuits and operation. The bypass capacitor does not cut bass frequency. The operation of the circuit must be understood. It does work in conjunction with the other circuits of the tube. But most of all the bypass capacitor also sets gain at frequency. The incoming signal will have most gain at the filter's resonance due to the tube's operation.

Depending on the stage design you may witness overall gain increase or notice more lower to resonance frequency gain. You cannot increase what is already maxed. AlabamaTP pointed that out.

On my V1a I increased the bypass cap to 4.7uF. It was 0.47uF. A 2203 circuit has 0.68uF as far as I can see and also the same 2.7K cathode resistor. The 0.47 cap works out to a -3dB point of 125 Hz, and by the time you get to 80 Hz there is 6dB less bass. With the 2203's 0.68 uF the -3dB point is 87Hz which is preferable.

The 4.7uF I went for moved the -3dB point down to 12.5 Hz. There is more warmth in the amp now and I do have to dial back the bass on the boost channel a little. However the clean channel still seems wimpy.

This is why I suspect the coupling caps ( I'm trying to figure out C14 at the moment - see the diagram above if you can help). The low end does not seem to be getting through the clean channel.

 

[ricksteruk]

Check my latest post out

http://www.marshallforum.com/marshall-amps/68933-my-new-4210-jcm-800-1x12-combo-2.html#post1124220

I am very happy!

 

[davidf556]

Glad to see this 2210 thread going! Lots of interesting ideas for the clean channel, so far the one change I made was to run a 5751 in V2. that made it usable. I plan on making the C40 change as I do have a bit of bleed.

One issue I am working on is eliminating a hiss coming from V4A-effects return- tried a bunch of different tubes to no effect. any ideas or has anyone had this issue? I have the later 88 circut.

 

[RickyLee]

Welcome davidf556.

I have not noticed the hiss you are referring to. I will have to check that out whenever I can get my 2205 out.

I can say that a 12AY7 in V2 really helped my Normal channel be a Clean channel.

You can always bypass one stage of V2 as well. Real easy to do and undo as well if you want to go back to stock.

 

[davidf556]

Ricky, I have read through all your posts, You have tried a lot of mods! thanks for sharing all that info. After doing some more tests on the FX loop hiss issue It has come down to the fact that it just doesn't like the analog output from my G-Major 2. I don't get this issue with any other loops I have.

I need to try out a 12AY7 in V2. It works with the 5751 but when I hit it with a TS9 the things goes through the roof volume wise. The gain channel doesn't react like that. A 12AX7 is just too much in that spot.

 

[ricksteruk]

I'm thinking it would be a good idea to install the presence control on my 4210.

I can see the spot where the resistor is that I'd need to replace with a pot - but I'll have to put the pot on the back as there's no space on the combo front panel.

Ricky, do I need to use shielded wire to connect to the pot?

 

[ampmadscientist]

Does anyone want to get this thread going? Or should I say, does anyone have any info to share on mods they have done to their Split-Channel amps?

Early versions or later versions and all in between.

I have a question on something that is also common with the Jubilee amps. Has anyone bypassed/removed and put a jumper in place of the .047uF cap directly after the front input jack? And what is the purpose of this cap in the first place? Is this cap there to attenuate some low end/bass frequencies to make the first gain stage a bit tighter/less farty bass?

I know that adding a bypass cap on the last gain stage of the later versions is a good thing. I did not go that drastic though, as I only used a 6.8uF there.

The most obvious thing is the cathode bypass cap on V1A.
On my schematic, C2 is .47 uF.
It won't pass low frequencies. It's a high pass filter.
This will make the amp sound harsh in the high frequencies, because there is almost no bass to start with.
Replace that C2 with 2.2 uF or 4.7 uF or larger. This will cause the amp to finally pass bass. The + of the cap connects to the cathode. The - of the cap connects to ground. The cap should be rated 25 volts or more.

After you fix that problem, the low frequencies will be much better.
The highs and the lows will be balanced much better. The bass EQ will work much better.

AND if you want to try different values, and see the result, before modifying the amp, use THIS:
Cathode Bypass Capacitor Calculator

Now back to the input question.
The .047 cap on the input will block DC from entering the preamp. It will attenuate the low frequencies.
This CAN be bypassed. However, the cap should be replaced with resistors.
There should be a 33K - 68 K series resistor between the input jack and the first preamp tube. You don't need the capacitor.
I am replacing this .047 cap with a 33K resistor, that's all folks.

If you don't use the resistor, the input will be noisy, and distort too much (muddy sounding) from being overloaded with signal... The guitar should not be hooked directly to this tube, without a resistor in series.

Now, one more thing about high frequency harshness.
You can remove the "bright cap" C4, 100pf from the first volume control VR1.

 

[ampmadscientist]

Ricky, I have read through all your posts, You have tried a lot of mods! thanks for sharing all that info. After doing some more tests on the FX loop hiss issue It has come down to the fact that it just doesn't like the analog output from my G-Major 2. I don't get this issue with any other loops I have.

I need to try out a 12AY7 in V2. It works with the 5751 but when I hit it with a TS9 the things goes through the roof volume wise. The gain channel doesn't react like that. A 12AX7 is just too much in that spot.

Nah, the Marshall effects loop has always been screwy.
The levels are wrong for standard effects pedals. It overloads the input of the effects unit with too much signal.
You have to mod the effects loop to get a good level into the effects pedal.
The effects pedal is made for a guitar level signal. The Marshall effects loop is too much, and overdrives the input of the effects unit.
This is a problem on many Marshall amps. But, it can be corrected.