# Marshall choke issue



## blam (Feb 18, 2011)

hey guys.

I recently modded my Marshall Class 5.

most of what was done was just upgrading components and slight value changes.

the amp is equipped with a 5 watt 220ohm resistor from factory instead of a choke so I decided to swap that out with a hammond choke.

the one selected is 10H @ 192ohms and 100mA. 

when the choke is in place i get a lot of ghosting. much like a tremolo effect. a Mercry Magnetics choke has also been tried at 9H and 7H in the ~250ohm range and the same results. others have used these chokes with success and no ghosting. the ghosting is most prominent when played lightly and at amp idle. if I am digging in, the ghosting goes away and then after 5-10 seconds of idle or light playing, the ghosting returns.

when the resistor is put back in the ghosting is gone. all the filter caps have been changed out and no difference as well. different tubes have also been tried.

any ideas or suggestions on what the problem might be would be very much appreciated.

thanks.


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## keeperofthegood (Apr 30, 2008)

I suggest: Measure your current. Choke values are current dependent, 10H AT 100ma is not 10H at 20ma.


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## parkhead (Aug 14, 2009)

If your choke is more "transparent" than 220ohm resistor then you have reduced the filtering. 

Looking at the schematic, you have a 100mfd filter, then the 220ohm resistor a 47mfd filter then the screen hookup, followed by the output transformer tap. 

this is a fairly unconventional setup. Imagine you hit a note hard, the center tap will draw lots of current, supplied by the 47mfd filter, but as the current draw increases 

the 100mfd filter is supposed to help supply the current except the 100mfd filter is behind the choke so the response to big current changes is delayed. 

Also the screen is on the same node as the OT tap (although there is a little isolation) the disturbance is not isolated by the choke. 

So with this setup your choke is isolating your "disturbed power" from the fresh clean power in the 100mfd cap. 

For fun replace the 220ohm resistor with a 2k resistor and see what happens, or try a 330 ohm resistor. 

In a conventional amp the first Filter and the OT center tap Tie into the first node, the choke then feeds a second cap and the screens. 

If you look at the valve JR they use a 220ohm resistor between two stages of filtering, but this is done to stabilize the power supply and reduce noise. The center tap 

draws from the second node, but the screens draw from a third node after a more typical 2k choke resistor. 

When looking at any circuit always look at it backwards as well ... imagine a power supply "disturbance" being introduced by an earlier stage... the power supply must 

isolate these "noises" from the rest of the circuit. In otherwords always imagine how an unitended "feedback" signal is distributed through the power supply to 

understand how it really affects tone. 

This also Ilustrates how "IMPROVING" the power supply does not always improve the tone in some classic amps the underfiltered, "ghosty" or less efficient stage is the key to the 

tone. 

p


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## Wild Bill (May 3, 2006)

Someone mentioned that the 1st filter cap is 100 mf. This could be your problem! With the resistor it's generally safe to use a large value input filter cap but when you go to a choke you can get into problems. Both chokes and filter caps smooth out the ripple but inductance and capacitance work in opposite ways.

I would try replacing that 100mf with a 47 mf, 500 volt. This is more than enough capacitance with a choke. See if that helps. If it doesn't, you can always leave the 47 mf cap lying around on your workbench all charged up, waiting for fingers that should leave your stuff alone! kkjuw


Wild Bill


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## blam (Feb 18, 2011)

Thanks for the input guys. I will give that a go. The amp is currently not with me but when I get it back I'll give it a try. 

Bill, here is the schematic:
http://files.onthelam.ca/guitar/C5_Circuit_Diagram_complete.pdf


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## Wild Bill (May 3, 2006)

Blam, the schematic tells me everything! This is a Class A amp. Lose the choke!

A Class A amp has constant plate current. Chokes are great for retaining regulation under a load that swings over a large range, like the usual pushpull, Class AB1 amp circuit.

The values in the stock circuit are more than enough to ensure a clean supply. Adding a big ass choke (10H is a lot for a 50watt amp!) means that the supply will fight you on strong notes, as Parkhead had talked about.

What's more, this circuit has the entire amp fed from the 2nd filter cap, rather than the 1st as with 99.9 % of the amps out there. This means that the plate draws current through the choke instead of just the screens and the preamp tubes.

In effect, your mods have CREATED the problem! This amp has a proper design for what it is, a Class A amp. The only time I'd suggest a choke is in the usual pushpull output. For 50 watt 6L6 and EL34 amps I use a 156G Hammond choke but the plate is fed from the 1st cap, BEFORE the choke! In those cases a choke improves the sound, where the manufacturer used a resistor simply because he was cheap!

You had a good idea but not for this application.

Wild Bill


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## blam (Feb 18, 2011)

Thanks a lot for the explanation Bill.

Interestingly it was not my idea to put the choke in. There have been others who have used the same choke without issues so I thought I'd try it as well. Then a few others popped up with the same issue I am having and no one can figure it out as to why some get the tremolo effect and some dont.


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