Marshall DSL100HR Choke Mod, My Thoughts... (With Pictures)

Michael Inglis

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EDIT: In this post I used "the feel of a Class A amp" as an example in a way which was incorrect and at the very least misleading. This was kindly pointed out to me below with some links to the correct information regarding Class A as well as Class AB amps including clarification on Push Pull, and single ended design. Because my main intention was to convey a feeling rather than information ill keep the post as is with this note to clarify. When i say the "feel of a Class A amp" or refer to that in any way I should have actually said "any amp design which produces less sag and a more "immediate" response". Again, as it was pointed out to me just because an amp is Class A doesn't automatically mean it will have those characteristics. I hope this clears up my intended example without adding to the confusion.


Original Post:

So if you were wondering if you should add a choke to your DSL100HR or 40CR hopefully my insights and experience can make your decision easier. Id like to make this post short and sweet (which i never accomplish) so ill get this out of the way up front. Did I like the results of adding the choke and do i KNOW if it was significant enough a difference to justify the trouble? And the answer to that is certainly a yes. The difference it made is tangible and the differences im feeling and hearing can be backed up by the science not just speculation.

And i should mention im not an expert and dont claim to be, if anyone can add or amend anything i say here i welcome them to do so. That way the best possible information is passed along.

So most of us know the 2 reasons to add a choke but for those who dont. Adding a Choke means a quieter amp (the choke being an inductor will filter more AC ripple than a cap/resistor LPF can). This, while nice, isnt what drew me to wanting a choke. Using decent EHX EL34's has made my 100HR very very quiet already. So that brings me to the next reason youd want to add a choke, because it will create an electromagnetic field that can be discharged whenever your amp experiences voltage drop/sag that will keep your voltage and current steadier with less sag. Or another way of putting it would be to say a Choke makes for a stiffer power supply. And just because you like sag doesn't mean you wont like a choke. That would be an over simplification. Ive thought a lot about the best way to describe what to expect if you add a choke to an amp that in stock form uses a resistor instead of a choke and i think this is the best way ive come up with...

Most of us have played Class A and AB amps. The main difference between those amp classes is that a Class A amp ALWAYS has full power going to the power tubes (unless your driving it to break up/overdrive but thats not relevant). So the tubes are always ready, the moment you think about what your ABOUT to play the sound comes out of the speaker lol. In other words Class A amps are very very immediate and responsive. Most of us probably will never forget the first time we played a Class A amp. Its like we had been driving a Volvo our whole lives and all the sudden we were introduced to a Ferrari. Thats not to say that all that power doesnt come at a cost. Now a Class AB amp like our Marshalls idle when no input signal is applied to the tubes. So they can take time to ramp up to full power. Their attack can feel softer and less responsive in comparison to a Class A amplifier. So what im getting at is a Choke will impart some of that type of immediate Class A feel. Its the same amp, same tone but the notes under your fingers will feel fuller. I can hear a difference at High and Low volumes but at high volumes the low end is more defined and less loose.

Now im no expert, but I can say with confidence my amp feels more like a Class A amp than it did before i installed the Choke. But dont get me wrong, chokes arent for making Class AB amps feel more like Class A. Thats just the way i felt it was easiest to describe the difference. Its also important to mention that the choke and resistor have differnt resistances which effects the bias and youl have to adjust by a few mv. For example if i have my amp biased at 78mv a side with the choke then I swap to the resistor my bias wll drop down to around 75-76 a side. This will all depend on the value of the stock resistor and the resistance of the choke.

Now as for the mod. What i used was a Hammond 194G (3H, 250mA Plexi style Choke), this is considered the appropriate choke for this amp but thats not a hard rule and some guys use JCM800 style 5H chokes in their DSL's (but most of the time its the 3H). I completely desoldered the 220 Ohm 5W cement resistor and in its place soldered in two wires (22 awg cloth push back, just cause i ran out of my go to Silicone stranded wire in black and cloth push back is so nice to work with so i just used it). Then i took those wires to a heavy duty 20 amp rated on on switch. Then i wired the choke on one side of the switch and the stock resistor to the other. This way i was able to compare the differences. I ended up keeping the switch an drilling a hole in the chassis for it since i want to be able to take it back to stock every now an then for a different feel. It is also enough of a difference to make having a switch worth the trouble.

At first i soldered the choke directly to the board which worked flawlessly. I just wanted to be able to change it back to stock easily for a different feel every now and then so i added the switch. So in some pictures you wont see the switch. But to be clear i personally think the amp sound better WITH the choke. But i also like the feel of the amp without it, its more loose and has more of its own character without the choke, in other words its less stiff. But thats subjective and you'll have to decide for yourself if you like the stiffer feel. With the resistor and with the choke i love this amp. And its actually really cool to be able to have one more parameter to control to dial in the sound of the amp for the given situation. It just depends on what your going for. Because of that i recommend putting the choke on a swtich. But i always put mods on switches (like literally always). I think its important to keep the stock functionality for comparison and versatility.

I got the choke on Mouser.com and the screws (#8 32x1/2) from walmart. Youll also need a grommet for the hole the wires pass through. I had a set already that i got from AutoZone last year.

I did pull the Main Board and all the daughter boards in order to install the Choke where i wanted but if your not comfortable pulling the main board you could install the choke next to the Bias test points. With that said its not possible on the 100HR to just clip out the resistor like you could will the 100H because its surrounded by capacitors so getting your flush cutters at it would be impossible. BUT these cement resistors have just a tiny prong thats actually soldered down so im very confident you could GENTLY rock the resistor from side to side and it would brake off at the board exposing the pads that you could then solder to. I wanted to salvage the resistor and remove it "correctly" so i completely desoldered it and cleaned the solder pads before adding the two wires. Whatever method you use do it slowly and carefully and make absolutely certain the caps are discharged before doing any work in the amp. You shouldn't do this yourself unless you know how to solder and know your way around common components. Or in other words dont try to do this as your first attempt at modding anything without any knowledge of electronics etc etc etc.

I plan on making a label with Clear Gloss Vinyl Decal paper later today for the toggle switch. Speaking of, i swore off waterslide paper after using Vinyl decal paper for my last pedal build. Its a bit off topic but ill add a picture of that last pedal for anyone who hates waterslide paper as much as me and wants an easier, more reliable and better looking alternative.

Marshall DSL100HR Choke Mod 1.jpg

DSL100HR Choke Mounting Screw Locations.jpg

DSL100HR Choke Leads Soldered To Board.jpg

DSL100HR choke mod 2.jpg

Choke Resistor Switch Underside One.jpg

Choke Resistor Switch Underside Two.jpg

Choke Resistor Switch Underside Three.jpg

Half Inch #8 Machine Screws For Choke Install .jpg

Grommets.jpg

Chasm Reverb From Top.jpg
 
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gregr

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Michael Inglis

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Awesome!

I think you might revisit Class A vs AB. The responsiveness you’re talking about has to do with negative feedback, not the tube configuration. Also, the Class A operation in a Vox is still push-pull just like Class AB, but the tubes are biased hotter.

https://www.aikenamps.com/index.php/the-last-word-on-class-a

https://mesaboogie.zendesk.com/hc/e...Class-A-and-how-does-it-differ-from-Class-AB-

Thanks for the reply and links, youre probably absolutely correct. I debated using that example without going into further detail. And since it wasn't necessary to make my point about the difference between the way Class A and Class AB FEEL to play I probably should have either left that part out or taken the time to explain it correctly in more detail. Knowing myself the way I do im sure ill end up going over that part and rewording it and correcting any misinformation while clarifying my intended example. Again, thanks for pointing that out! Checking the links now.
 

gregr

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That’s the thing though, the difference in the feel of a Vox and an amp that employs negative feedback is the negative feedback, not because it’s Class A push-pull. This is just a side note. I hope it doesn’t detract from your mod.

You did a great job showing just how easy it can be!

PS: I like that pic of your reverb pedal.
 
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Michael Inglis

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That’s the thing though, the difference in the feel of a Vox and an amp that employs negative feedback is the negative feedback, not because it’s Class A push-pull. This is just a side note. I hope it doesn’t detract from your mod.

You did a great job showing just how easy it can be!

PS: I like that pic of your reverb pedal.

Didnt detract in the least bit, im happy to have corrections and clarifications wherever they are necessary. And had it detracted from the post that wouldn't be on you but me for not taking the time to make sure what i was using as my example was factually accurate. If id just keep things short and sweet keeping only the necessary information that wouldn't likely be an issue anymore lol. Thanks again brother!

Oh and as for the pic of the Reverb i just built; i couldn't help but mention the Vinyl Decal paper. I hate waterslides and only intend to use them on headstocks from now on. I was thrilled when I tried the vinyl decal paper for the first time so i wanted to spread to word lol! I actually just clear coated the label for the choke/resistor switch a couple hours ago so it should go on tomorrow.
 
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Pete Farrington

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Adding a Choke means a quieter amp
I dunno about that? Perhaps there may be isolated examples of it, but in general, I think most types of noise aren't affected by a choke being in play. Could you explain, demonstrate or provide evidence of it?

it will create an electromagnetic field that can be discharged whenever your amp experiences voltage drop/sag that will keep your voltage and current steadier with less sag. Or another way of putting it would be to say a Choke makes for a stiffer power supply
I dunno about the electromagnetic field thing. Rather the field supports the choke's inductance, which, when passing HT current with ripple at twice mains frequency, creates an inductive reactance, which is beneficial as it adds an extra step of filtering to the downstream HT nodes.
The downstream HT becomes stiffer (ie a lower degree of variation in voltage as the HT current draw varies) mainly due to the choke's resistance (80ohms) being lower than the resistor previously linking those nodes (220ohms?). The choke's inductive reactance does also help a bit with that because indepentand of the lower resistance, the reduced ripple (ie VAC riding on the VDC) will tend to increase the average VDC, ie maintain it closer to its Vpeak level.

I can say with confidence my amp feels more like a Class A amp than it did before i installed the Choke. But dont get me wrong, chokes arent for making Class AB amps feel more like Class A. Thats just the way i felt it was easiest to describe the difference. Its also important to mention that the choke and resistor have differnt resistances which effects the bias and youl have to adjust by a few mv. For example if i have my amp biased at 78mv a side with the choke then I swap to the resistor my bias wll drop down to around 75-76 a side. This will all depend on the value of the stock resistor and the resistance of the choke.
The choke should help the dynamic operating point to stay closer to the idle level, so the dynamic compression caused by HT sag will be reduced. So yes, the amp's dynamic behavior (eg its 'feel') may be perceived to have changed in the manner described. But to experience that, you must be cranking the amp reasonably hard, pushing a high power output from it?

Another potential benefit of a choke is that the reduced HT ripple at the screen grid node will help, at high power outputs, to reduce intermodulation distortion IM (signal and ripple sum and difference frequencies) which if excessive, can create a garbled, warbly ghosting effect to the signal.
In moderation though, a bit of IM might help to thicken up an amp's tone, with amp's which have totally smooth regulated supplies often being described as somewhat anemic and boring (I paraphrase).
 
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