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Marshall 2144 - questions and advice required?

 
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badfan



Joined: 06 Sep 2007
Posts: 23
Location: Nottingham, UK

PostPosted: Mon Sep 08, 2008 9:22 pm    Post subject: Marshall 2144 - questions and advice required? Reply with quote

Guys

Recently I have come across a Marshall 2144 amp (made between 1978/1979 which is different from the normal pre JCM800 Marshall master volume type amps in that it has a reverb, boost/reverb pedal function and the placement of the tone circuit is not the standard cathode follower type.

I have attached the pre-amp schematic for reference and the power amp is the same as a standard 2204 etc.

The amp that I heard sounded absolutely awesome, warm tone that had excellent clarity for the notes etc and it did not sound like a typical Marshall, it had the distortion character etc but sounded a lot warmer similar to a Fender?

Anyway the guy that has this is a pro and I have not had chance to scope or blueprint as yet in detail however I did observe a couple of things when I got to play it:

a) The OT had been replaced with a Fender type which did not look like it was big enough for 50 watts? - it has 2xEL34 loaded
b) The boost function is permanently switched on.
c) It has some Greenbacks in it which I suspect is contributing to the sound etc (sound like G12-65's)
d) The reverb sounded OK up until about no 4 then after that it sounded 'tinny' and not very usable but before then it was OK

Anyway I am looking to build a clone from scratch but before I do I have some questions

1) In terms of the pre-amp I understand most things but the boost function has me foxed? I am not to sure how this is working if you see it has a .047uf and additional resistors but there also seems to be a connection to the reverb circuit? If sopmeone could explain that would be a great help

2) There is a 6meg resistor in series just after the reverb input - what does this resistor normally do? and why so high?

3) The amp sounded great even at low volumes where as you turned the pre-amp it really thickened up without having to have it up loud i.e. 1-2 on the master etc - any pointers as to why would be great.

4) The OT is not Marshall and I am certian it is a Fender type (black with cloth covered wires / femder colours etc). As this is a Marshall with 2xEl34's then what Fender type OT's would be suitable around the 40-50 watt mark - with sufficient secondaries?

Thanks for any advice
Sean
________
Glass Bubblers


Last edited by badfan on Fri Feb 25, 2011 5:38 am; edited 1 time in total
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CaseyJones



Joined: 17 Mar 2006
Posts: 848

PostPosted: Mon Sep 08, 2008 11:57 pm    Post subject: Re: Marshall 2144 - questions and advice required? Reply with quote

At a glance:

badfan wrote:
a) The OT had been replaced with a Fender type which did not look like it was big enough for 50 watts? - it has 2xEL34 loaded


The original probably blew for one reason or another.

badfan wrote:
b) The boost function is permanently switched on.


Is the original footswitch present? It sounds like a hardwired fix for a lost or broken footswitch.

badfan wrote:
c) It has some Greenbacks in it which I suspect is contributing to the sound etc (sound like G12-65's)


Everything sounds better through Greenbacks.

badfan wrote:
d) The reverb sounded OK up until about no 4 then after that it sounded 'tinny' and not very usable but before then it was OK


Pummeling what is essentially a Fender reverb circuit with high gain means you can be sure it will get raw and splattery at higher volumes.

badfan wrote:
Anyway I am looking to build a clone from scratch but before I do I have some questions

1) In terms of the pre-amp I understand most things but the boost function has me foxed? I am not to sure how this is working if you see it has a .047uf and additional resistors but there also seems to be a connection to the reverb circuit? If sopmeone could explain that would be a great help


The reverb is switched by grounding the "wet" signal. Pretty standard. The preamp cathode network is 2.7k/68uf, pretty much standard high gain Marshall. The extra resistors go in series with that resulting in a switchable "boost/normal" situation.


badfan wrote:
2) There is a 6meg resistor in series just after the reverb input - what does this resistor normally do? and why so high?


What the resistor normally does is suck tone. No kiddin'! It's 6.8 meg, that's nearly 7 meg. I'd bet it's drifted to 7 meg by now. For the preamp signal to continue on its path it first must squeeze through that resistor.

What it's supposed to do: It's a mixer to isolate dry signal (preamp less reverb) and wet signal (signal post reverb circuit). It's for all practical purposes a Fender reverb circuit. Unless the surf's up many musicians prefer the "normal" channel on Fenders because it doesn't have that resistor between you, your power amp and so forth.

badfan wrote:
3) The amp sounded great even at low volumes where as you turned the pre-amp it really thickened up without having to have it up loud i.e. 1-2 on the master etc - any pointers as to why would be great.


It's a high gain preamp. Someone did a little work to the amp, it has probably been tweaked for a bad-ass hair band tone with the master down. The tone is then all preamp and not much power amp. Typical master volume fare. There was a lot of good music played on pointy guitars by chaps in spandex trousers. At least I thought so at the time! Laughing

badfan wrote:
4) The OT is not Marshall and I am certian it is a Fender type (black with cloth covered wires / femder colours etc). As this is a Marshall with 2xEl34's then what Fender type OT's would be suitable around the 40-50 watt mark - with sufficient secondaries?


Ooh... where did I hear something about this? (he's talking about Weber transformers)

muchxs wrote:
The WO18343. Listed as 4k into 2/4/8 ohms that works out to 8k into 16 ohms. At $58.00 (current retail) it's cheap and available.

The WOT45HHR. 50 watt Marshall High Headroom by any other name. 6800 ohms into 4/8/16/32 ohms. $58.00 current retail.

By mixing and matching speaker taps you have 3.4k, 4k, 6.8k and 8k primaries readily available between these transformers.


Wire for a 3.4k or 4k primary for Marshall.
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badfan



Joined: 06 Sep 2007
Posts: 23
Location: Nottingham, UK

PostPosted: Tue Sep 09, 2008 8:10 am    Post subject: Re: Marshall 2144 - questions and advice required? Reply with quote

badfan wrote:
b) The boost function is permanently switched on.


Is the original footswitch present? It sounds like a hardwired fix for a lost or broken footswitch.

badfan wrote:
1) In terms of the pre-amp I understand most things but the boost function has me foxed? I am not to sure how this is working if you see it has a .047uf and additional resistors but there also seems to be a connection to the reverb circuit? If sopmeone could explain that would be a great help


The reverb is switched by grounding the "wet" signal. Pretty standard. The preamp cathode network is 2.7k/68uf, pretty much standard high gain Marshall. The extra resistors go in series with that resulting in a switchable "boost/normal" situation.

Hi Casey,

Thanks very much for taking the time & the replies. To confirm the boost function is permanently on which sounds like it is either hard wired that way or by deafault it is on (it alos has the original foot switch but the guy does not use it)

Does the extra cap play a role in changing the gain / tone for the first stage as well as the resistors? I will probably put these on a seperate toggle switch but I am still unclear on how to (correctly) wire them and what the cap is doing to the tone?

Thanks
Sean
________
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Last edited by badfan on Fri Feb 25, 2011 5:38 am; edited 1 time in total
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CaseyJones



Joined: 17 Mar 2006
Posts: 848

PostPosted: Tue Sep 09, 2008 1:47 pm    Post subject: Re: Marshall 2144 - questions and advice required? Reply with quote

badfan wrote:
Does the extra cap play a role in changing the gain / tone for the first stage as well as the resistors? I will probably put these on a seperate toggle switch but I am still unclear on how to (correctly) wire them and what the cap is doing to the tone?


I eyeballed what's goin' on in that area. The default circuit with the footswitch unplugged places two RC networks in series between ground and the first preamp stage cathode. You have your usual Marshall master volume 2.7k/.68uf cathode network in series with another 15k/.47uf network. A switching jack is used for the footswitch so with the footswitch unplugged that's all there is to it.

With the footswitch plugged in you have the option of throwing a 47k resistor in series with the .47uf capacitor. With the footswitch unplugged the cap has a direct path to ground so the 47k resistor is effectively not part of the circuit. So: High gain has the capacitor directly to ground, low gain makes the .47uf take the long way to ground. What's happening is that the .47uf is an effective cathode bypass cap in with the switch in the high gain position. Low gain virtually lifts the bypass cap. That's a standard preamp stage high gain / low gain switching scheme, take the bypass cap out of circuit for low gain and add the bypass cap for high gain. There's not much to it, just follow the path to the ground.

The high gain / low gain setting can be selected by placing a SPST switch between the junction of C3/R5 and the ground. Closed is high gain, open is low gain. That's what the footswitch does.

Read carefully, I've said the same thing about 20 different ways.

This isn't high gain / low gain in a modern cascaded preamp stage sense. The preamps are cascaded by selecting inputs. The high gain input places the tube stages in series, the low gain input doesn't use the first preamp stage. The low gain input goes directly to the second triode stage. If there's no low gain input available it may have been bypassed at some point. The first preamp stage isn't especially high gain compared to a Trainwreck or a Dumble. It's roughly the classic Marshall "one wire mod" with a gain reduction on the first preamp stage so the distortion tone doesn't sound buzzy and thin.

The reverb switch is a SPST switch that grounds the wet signal before the reverb recovery stage.
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badfan



Joined: 06 Sep 2007
Posts: 23
Location: Nottingham, UK

PostPosted: Tue Sep 09, 2008 3:16 pm    Post subject: Reply with quote

Hi Casey,

Thanks again for the comments really helpful. Thankfully I am up to speed with amp topology and understanding cascaded vs parellel and the Marshall low/high gain approach for this type of amp.

What i was struggling with was the series approach for the cathode connections i.e. why and what impact this has? effectively this chages the operating point for the valve? and my swithing knowledge is not that great (yet)

Now i understand (I think) the default is 2.7k/.68uf cathode network in series with another 15k/.47uf network and the switch eithers brings R5 in or out for low / high (learning all the time)

I now have to ask another obvious question if the cathode network sets are in series then why not combine to reduce the number of components and just switch R5 as required?

And will having these sets contribute to the voicing of the amp considerably or not?

Cheers
Sean
(I owe you beer if ever we are in the same vacinity...)
________
Website host


Last edited by badfan on Fri Feb 25, 2011 5:38 am; edited 1 time in total
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CaseyJones



Joined: 17 Mar 2006
Posts: 848

PostPosted: Tue Sep 09, 2008 3:39 pm    Post subject: Reply with quote

badfan wrote:
Now i understand (I think) the default is 2.7k/.68uf cathode network in series with another 15k/.47uf network and the switch eithers brings R5 in or out for low / high (learning all the time)

I now have to ask another obvious question if the cathode network sets are in series then why not combine to reduce the number of components and just switch R5 as required?


My instinct would be to eliminate R5. The simplified circuit would either have a ground path for C3 or not.

The network without C3 would be 2.7k/.68uf tied to a 15k to the ground. Remember: The classic Marshall bright channel network is 2.7k/.68k between cathode and ground. More resistance between that network and ground results in less gain. For this application Marshall chose 15k, you can fiddle with that value for more or less gain. Zero to 15k would be likely choices.

It's popular to wire a big ol' rotary switch in there, then you could have half a dozen possibilities available. If you want to go crazy with it change V1a and V1b cathode networks. Your choices are limited though unless you use voltage dividers between stages to manage gain or plug into V1b only. Use 1.5k/4.7uf or 1/5k/25uf, plug into V1b (ignore V1a!) and this thing will think it's a Fender. Marshall used the values they used to keep the amp semi-sweet, too much gain in the wrong place sounds raw.

For a mental exercise compare say a Soldano Atomic 16 (really a Marshall master volume preamp w/ low power amp), an Express and an old JTM45/5F6 schematic and your amp. You can see the evolution from early '60s Marshall to yours and variations on a theme. Yours is an odd duck in that it's a master volume Marshall pretending to be a Super Reverb or something.
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Bob-I



Joined: 02 Feb 2006
Posts: 3674
Location: Hillsborough NJ

PostPosted: Tue Sep 09, 2008 4:02 pm    Post subject: Re: Marshall 2144 - questions and advice required? Reply with quote

badfan wrote:
1) In terms of the pre-amp I understand most things but the boost function has me foxed? I am not to sure how this is working if you see it has a .047uf and additional resistors but there also seems to be a connection to the reverb circuit? If sopmeone could explain that would be a great help


It's pretty simple actually. The boost lowers Rk for more gain, and adds some additional bypass Ck.

Quote:
2) There is a 6meg resistor in series just after the reverb input - what does this resistor normally do? and why so high?


This is the mixer resistor. The reverbed signal goes direct into the PI where the dry signal goes through the 6M. IMHO this large of a mixer resistor kills the tone. In my own amps the reverb mixer resistor is more like 220k. It lowers the amount of reverb available while preventing signal loss.

The downside is this mixer resistor acts as local negative feedback, so adjust to taste. Too small a mixer and the reverb and overall tone degrades as you turn it up, too large and the dry tone degrades. Fender bypasses that resistor (3.3M in Fender) with a small 10pF cap to prevert loss of highs.

Quote:
3) The amp sounded great even at low volumes where as you turned the pre-amp it really thickened up without having to have it up loud i.e. 1-2 on the master etc - any pointers as to why would be great.


IMO this is a factor of the preamp design. There are 3 gain stages, bass limited by the .022uF cap to prevent mushyness, and some bypass caps on the volume and a voltage divider in r11 and r12 preventing overload on the final gain stage before the ton stack. Also the high impedance tone stack would sound different than the standard Marshall low impedance stack with a CF stage.

Personally I prefer this type of preamp. It's very TrainWreck like and has excellent harmonics and gain.

Quote:
4) The OT is not Marshall and I am certian it is a Fender type (black with cloth covered wires / femder colours etc). As this is a Marshall with 2xEl34's then what Fender type OT's would be suitable around the 40-50 watt mark - with sufficient secondaries?


I'd use a more rugged OT. Maybe Mojo?

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badfan



Joined: 06 Sep 2007
Posts: 23
Location: Nottingham, UK

PostPosted: Tue Sep 09, 2008 5:49 pm    Post subject: Reply with quote

Guys,

I think you both summed it up nicely and i'm definately sold. Decision made I am going to build one of these pre-maps, and probably mate it with a 2x6v6 output stage to start with as what as been pointed out. a lot of the 'good' sound from this amp does come from the pre-amp section and I like the idea of a morph between a Fender SR and a Marshall MV but at a lower overall volume to start with.

If it gets me in the ball park of excellent note definition (even with the pre-amp being pushed) and then when you back off you also get excellent note definition with less of the harsh Marshall bite and more of the Fender sweetness then that will do for me.

I'll let you guys know how I get on, you have also given me another thought about putting the reverb resistor as a variable type (pot maybe) so I can experiment to taste and swicth a bypass cap in/out as required (ala Fender) as well switching everything in site Smile

Thanks a lot for the tips and advice great forum.

Cheers
Sean
________
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Last edited by badfan on Fri Feb 25, 2011 5:38 am; edited 1 time in total
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Bob-I



Joined: 02 Feb 2006
Posts: 3674
Location: Hillsborough NJ

PostPosted: Wed Sep 10, 2008 3:33 am    Post subject: Reply with quote

badfan wrote:
Guys,

I think you both summed it up nicely and i'm definately sold. Decision made I am going to build one of these pre-maps, and probably mate it with a 2x6v6 output stage to start with as what as been pointed out. a lot of the 'good' sound from this amp does come from the pre-amp section and I like the idea of a morph between a Fender SR and a Marshall MV but at a lower overall volume to start with.


Sounds like a great idea. I did this with a Dumble preamp and the results are amazing. It's huge and fat and sounds great at low volumes through a 1x12. I built it into a Princeton chassis and a little tiny head cab. The best part, when you're done with the gig you just put it in your pocket.

Quote:
I'll let you guys know how I get on, you have also given me another thought about putting the reverb resistor as a variable type (pot maybe) so I can experiment to taste and swicth a bypass cap in/out as required (ala Fender) as well switching everything in site Smile


I'm not so sure about that one. The balance resistor should be fixed so you don't screw with the dry signal. Use the reverb out pot to adjust the amount of reverb.


Good luck with the project. Sounds like a good one.

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