DSL 401 buzzy sound - help needed!

[dannyart]

Hi all
Bought a DSL 401 recently for use in the house... got it used, mint, for a great price!

I like the stock sound and versatility of this amp but always noticed a slight buzzy sound on the OD channels. I have improved the tone after doing some of the more popular mods.... fitted a 5yr old G12H speaker, changed the power tubes to JJs, swapped a couple of the preamp tubes for a Brimar and a Mullard (which sound amazing), and also snipped the bright cap (C100) on the OD channel to warm it up a little - all of these mods have worked great and have given a huge improvement to the overall tone and crunch of the amp.

However, I'm still left with the faint buzzy/fizzy noise on the OD channels - and its starting to really bug me! It is more noticable when notes decay, it remains for a few seconds and sounds a bit like a fly buzzing against a window!!

Everything else is fine with the amp (as mentioned above), and the OT is working fine. Also I tend to crank it quite loud, even in the house, so its nothing to do with the volume being too low... and it doesnt to it on the clean channel, even when the clean gain is on 10.

I'm sure its just a few components in the OD part of the circuit (V1B & V2A) that need changing/removing. This whole section seems to be a bit over-engineered. Has anyone made any improvements to this at all? ....any ideas???

Cheers!

 

[raya]

Hello, I have a DSL401 also, and I notice the same buzz quality on the OD channels. I have tried most of the same things that you have tried, except the cap snip, but no luck getting rid of the buzz. The one thing I did that made a slight difference was to raise the bias voltage a bit, and that seemed to tighten up the OD a little, but not kill the annoying buzz on the decay of notes on the OD channel. I hope someone has a remedy for this before I dump this amp. You shouldn’t have to work this hard to make an amp sound good, but maybe I just got a bad one.
Ray

 

[dannyart]

Yes, I feel the same, Ray, it would be a huge shame to have to get rid of it because in every other way - tone, crunch, size, watts, versatility, and price - this amp is exactly what I'm after.

Hope someone can suggest a way to fix that annoying buzz, if indeed there is a way to fix it?....

 

[dannyart]

I've just spent the morning examining and comparing the DSL401 schem and the DSL50 schem (I used to have a DSL50 and that didnt buzz at all, I only sold it cause I didnt have space for the cab.)

Anyway, despite a lot of people saying they are totally different, they are actually not as poles apart as they say... there are obviously major switching differences and the 50 only has one shared eq, but the actual components and signal paths through the gain stages are quite similar - especially before and after V1B and V2A.

Not sure what conclusions I can draw from this yet... maybe I could remove/change a few components to make the 401 more like the 50 and this might cure the buzz? ....let me know if any of you more experienced guys think this might be a load of bollocks - and in fact I may just have a crappy 401!

 

[dannyart]

Any ideas anyone??.....

 

[JohnH]

Ive also wrestled with the tone on my DSL401, to a point where it is sounding great, but I have not modded the main circuit. My changes for tone are, new V30 speaker, extra lower cab with V30, playing with eq in the loop, and a beam blocker on each speaker. You might try that, its a 5" diameter disc in front of the speaker, can be anything. I built mine in, but a beer mat hung in front does the job as a test. I'm suggesting it because it does smooth out the high buzz, though may or may not help with your issue.

My Mk2 blocker is the same 5" disc, with a 2 1/2" hole in it. That diffuses the high treble by diffraction without reducing it so much

John

 

[dannyart]

Thanks John, yeah, I've heard of using a beam blocker but I think the fizzy noise will still be there - but just a bit more muffled. I might try a makeshift one and see what it sounds like.

I think the fizzy noise (from my experience, and from what you are all saying) is an inherent 'sound' of this amp and it cant be got rid of without a complete reworking of the OD gain stages. Too much hassle to go through to get it to sound right IMO.

I think I'm gonna trade it for the 2266C Vintage Modern combo. I've heard some great things about it and I think it might suit what I'm after better than this 401. They've got one at my local shop so I'm gonna go crank it up and see what I think ...and hopefully it wont have that bloody awful fizzy noise!!

 

[TwinACStacks]

Actually No. I have stickied the "Joey Mod thread" for the DSL heads at the top of the Workbench section, but I'm sure some of it can be applied to the 401 as well, (also TSL). Where I'm seeing the most fizz reducing mod is changing the Value of R30 on the Mainboard. But keep in mind it also reduces Gain in ALL Channels. Let me check the schematics to see if R 30 is the same for the 401 And I will get back to you with some suggestions.

BTW I would not get the Combo, They have a Bad tube rattling problem they, (Marshall), can't resolve. Get the Head and a 2X12 Cab if you want portability.

TWIN

 

[Armann]

I think I'm gonna trade it for the 2266C Vintage Modern combo.

I did the exact same thing, sold my 401, got the 2266C and sold it because of tube rattle, now I have the 2266 head and I'm a happy camper.
The 2266C used to be plagued with tube rattle problems, I'm on the VM forum and there where a lot of threads about it, about 2-3 years ago. Try before you buy and crank it and see what happens.

 

[TwinACStacks]

I'm back. Before You try this, Check with one of the resident Techs like Wilder to make sure I have Identified the correct resistor to be playing with!!!!! According to the schematic the resistor to be looking at is R137 1meg Value, near V2 on the Mainboard. Using a Pair of test leads you can attach one to each leg of this resistor and run a series of resistors clipped across the other ends, (paralleled), to sample a bunch of different values. I would start with a 910K this will change the resistance from 1 Meg to about 475K. The amp must be plugged in and speaker attached to do this. Then You can plug in your guitar and test the Various resistances I wouldn't try any lower than 220K TOTAL Resistance as you probably won't have any Crunch channel distortion by this value.

The formula for figuring out the resistance is:

R1XR2 Divided by R1+R2 Use the 1meg (1000k) for R1 and the value of the resistor you want to try for R2. I would shoot for between 475 and 820 total resistance, I'm guessing. Once You have found the value you like you can solder it across the legs of R137. This will make it easier to remove if you decide to change it, You won't have to pull the Mainboard!!!

Oh, and be CAREFUL working on a live amp, it can kill you.

TWIN

 

[raya]

Thanks Twin, that sounds like an easy thing to try. If I get some time this weekend, I'll give it a shot and report back.
Ray

 

[TwinACStacks]

Do that Ray, I'd be curious to know as that same resistor in a DSL100 is 470K stock value and I played for a bit with it at 320K, then I went to 235K for a while, I finally settled on 190K and it JUST KILLS....

TWIN

 

[dannyart]

Well, I went to my local shop, played the VM 2266C and WOW! I was hooked after about 2 minutes! What a fantastic amp... for me its everything I've been looking for and combines the two Marshall sounds I love the most - a Bluesbreaker/Plexi and hotrodded JCM800! It obviously isnt exactly like those two amps but its pretty damn close. And I love the simple one channel layout, still versatile though because of the switchable dynamic range.

Anyway, I traded the DSL401 for the VM - no regrets, the DSL was just too fussy for me, and as you had stated Ray, one shouldnt have to work that hard to get a great sound out of an amp.

Thanks for your input guys... hope you manage to get your DSL401's sorted.

Cheers!

 

[Armann]

Sweet congrats

 

[wangchung]

I'm back. Before You try this, Check with one of the resident Techs like Wilder to make sure I have Identified the correct resistor to be playing with!!!!! According to the schematic the resistor to be looking at is R137 1meg Value, near V2 on the Mainboard. Using a Pair of test leads you can attach one to each leg of this resistor and run a series of resistors clipped across the other ends, (paralleled), to sample a bunch of different values. I would start with a 910K this will change the resistance from 1 Meg to about 475K. The amp must be plugged in and speaker attached to do this. Then You can plug in your guitar and test the Various resistances I wouldn't try any lower than 220K TOTAL Resistance as you probably won't have any Crunch channel distortion by this value.

The formula for figuring out the resistance is:

R1XR2 Divided by R1+R2 Use the 1meg (1000k) for R1 and the value of the resistor you want to try for R2. I would shoot for between 475 and 820 total resistance, I'm guessing. Once You have found the value you like you can solder it across the legs of R137. This will make it easier to remove if you decide to change it, You won't have to pull the Mainboard!!!

Oh, and be CAREFUL working on a live amp, it can kill you.

TWIN

Hey Twinstacks;
I know this is a 7 year old thread, but the search for effective
tweaks by the layman is perpetual. Was there ever anything
further posted about R137? Did anyone try modding the values?

 

[ampmadscientist]

Hi all
Bought a DSL 401 recently for use in the house... got it used, mint, for a great price!

I like the stock sound and versatility of this amp but always noticed a slight buzzy sound on the OD channels. I have improved the tone after doing some of the more popular mods.... fitted a 5yr old G12H speaker, changed the power tubes to JJs, swapped a couple of the preamp tubes for a Brimar and a Mullard (which sound amazing), and also snipped the bright cap (C100) on the OD channel to warm it up a little - all of these mods have worked great and have given a huge improvement to the overall tone and crunch of the amp.

However, I'm still left with the faint buzzy/fizzy noise on the OD channels - and its starting to really bug me! It is more noticable when notes decay, it remains for a few seconds and sounds a bit like a fly buzzing against a window!!

Everything else is fine with the amp (as mentioned above), and the OT is working fine. Also I tend to crank it quite loud, even in the house, so its nothing to do with the volume being too low... and it doesnt to it on the clean channel, even when the clean gain is on 10.

I'm sure its just a few components in the OD part of the circuit (V1B & V2A) that need changing/removing. This whole section seems to be a bit over-engineered. Has anyone made any improvements to this at all? ....any ideas???

Cheers!

Just looking at the schematic, I would venture too much sag in the preamp high voltage power supply.

Try change C58, C66, C59, C60, C61 to 50uF instead of 10 uF.

(it might be one or more of those that does the trick)
Use alligator clip leads to clip the 50uF across the 10uF, then turn the amp on and test w/ a guitar. (watch your cap polarity)
Please do not electrocute yourself.

What happens is:
When a note is played, it drops the power supply voltage. Then the caps start to recharge, and return to a higher voltage.
This is constantly happening in the preamp...power voltage is going up/down all the time.

But when the plate voltage of the preamp tube drops below a certain level (temporarily), then starts moving back up - it produces a "cutoff".
(it's actually result of too much sag)
The resulting distortion of this cutoff: sounds like a dying screech owl in the wilderness....

But luckily,
You can make the power supply a bit stiffer---by adding some more uF.
And believe it, (try it yourself) it can make quite a difference (in the decay of notes...)

If you are looking on a scope, you will actually see in the audio: ripple
which occurs right when the note decays...
and it goes: bzzzzzzzzzzzzzzzzz

 

[wangchung]

And this sonic artifact then gets amplified by the rest of the circuit
with the rest of the signal? Surely Marshall has seen this on a scope in their factory. I'm sure it has a frequency. Are you able to discern if there is a design element later in the circuit that attempts to get rid of it or "clean it up"? I'm guessing sending it to ground or thru a filter cap or something to be cancelled in the feedback loop might be more cost effective than raising the value of the caps you've identified. (Follow the money.) I've just begun trying to understand all this stuff. The Fender AB763 "Blackface" circuit has a NFB resistor that can be jumped with a cap and "tuned" to send very high frequencies back thru the NFB loop to be cancelled. A so-called "ice pick cap". I got rid of the Deluxe Reverb RI I had before I had a chance to experiment with that tho. Thanks for taking the time to look at this.

 

[tmingle]

"Anyway, I traded the DSL401 for the VM". That's how I fixed my 401, only I got the DSL40C.

 

[wangchung]

It's good you found one you like. I've had several 40C'S. I know it's easy to find mint used ones. Finding the tone in your head is
all that matters.

 

[ampmadscientist]

And this sonic artifact then gets amplified by the rest of the circuit
with the rest of the signal? Surely Marshall has seen this on a scope in their factory. I'm sure it has a frequency. Are you able to discern if there is a design element later in the circuit that attempts to get rid of it or "clean it up"? I'm guessing sending it to ground or thru a filter cap or something to be cancelled in the feedback loop might be more cost effective than raising the value of the caps you've identified. (Follow the money.) I've just begun trying to understand all this stuff. The Fender AB763 "Blackface" circuit has a NFB resistor that can be jumped with a cap and "tuned" to send very high frequencies back thru the NFB loop to be cancelled. A so-called "ice pick cap". I got rid of the Deluxe Reverb RI I had before I had a chance to experiment with that tho. Thanks for taking the time to look at this.

You would be surprised:

Mesa Boogie & Marshall BOTH missed it at the factory. (try a Nomad, same problems)

And yes, it did take some time to figure out what was causing it...

But, what these guys try to do is:
They make a power supply with LOTS of sag, because sag sounds GOOD.

THEN: with different preamp tubes (as you know there is huge variation in preamp tubes)
You reach a point where the supply sags too much....(because of) variation in preamp tubes.

What I did:
Installed 2X 50uF in the preamp supply (instead of 10uF - 22uF)
(in the right position, by trial and error guitar testing)
This was enough to stop the artifact from occurring.

"Artifact" is the perfect word to describe this.