Marshall DSL 401 - NFB???

[diddymix]

Hi Folks,

I was hoping somebody with some knowledge and experience could help me with some experimenting I would like to try out with my DSL 401 combo that I have owned for 12 years or so... I have already re-capped the amp with decent Vishay electrolytics and put in new tubes and a decent speaker and it's really improved things!. I have basic skills in replacing parts etc but no skills in understanding and reading circuits etc.. but know what I am trying to accomplish tone wise.
I like the basic clean tone of the amp and what it does as an all round workhorse... however have always found it to be a little 'polite'and/or 'flat' in its character with that all familiar marshall thing, which to me is a clean and tight low and high end... with the harmonic action and grit in the upper mid range... I'm really trying to get that action to be broader in bandwidth and more up towards the lows and top with more aggression and sensitivity to pick attack... like a more classic amps response. Almost more of a blackface bassman kind of thing

There's two thing I want to try but one is to reduce the negative feedback in the amp or even try removing it.. as I'm pretty sure this amp uses quite a bit as it has that sound and lots of clean headroom. I was hoping somebody could help me with looking at the schematic here.. I have very little skill in reading it but as far as I can see this is no global negative feedback coming from an output transformer tap.. and I cannot see any other feedback anywhere. I have a few questions if I may to any gurus out there!

1. Is there any feedback loops in the amps... in the preamp sections or power amp sections?? If so where are they located?

2 Is V3 in the amp a cathode follower?... and would I be correct in saying that this stage in effect is the application of negative feedback in this particular circuit?

3. If yes to 3... Is there any way of tweaking this stage so that the overall effect of negative feedback in the amp is considerably lower, so that a can try it out?? If possible so that the gain and master volume is unnaffected or not too much...

I really appreciate your time on this reading, and please any tips most appreciated.. it would be awesome if thhis can be experimented with by me just replacing a few resistor/cap values, or reworking signal path a bit so I can achieve this.

Many thanks and hope your having a nice Xmas time! Dave

[xtian]

There is no global NFB shown on the schematic.

V3B is a cathode follower. No, this has nothing to do with NFB. <http://www.valvewizard.co.uk/accf.html>

I'm really trying to get that action to be broader in bandwidth and more up towards the lows and top with more aggression and sensitivity to pick attack... like a more classic amps response. Almost more of a blackface bassman kind of thing

What you're asking for can be accomplished much more easily by using a clean boost pedal out front, than any circuit mods. Or a different speaker! Which "decent" speaker did you put in?

[diddymix]

Hi! Thanks for the reply. Ah Ok so no NFB.. Thats what I could see at least for global that would be from the whole output., Yeh I do use a clean boost, its good and gets me more of a juicy sound but it still has that base tone. I put in a Celestion V type.. which to me is a pretty full bodied and smooth speaker, its nice.. its kinda like a vintage 30 only without all that 'screamyness' in the mids. Its helped the amp sound more ballsy which is good.. but just wanted to try a few tricks. I also wanted to take a shot at a triode half power switch, as this could potentially get me a more raw tone perhaps, with less of that tight clean power thing that this amp seems to be all about. Is this fairly easy to accomplish??

Can I ask if you can see any NFB on V1.. I can see a small cap from the plate feeding tha cathode but I have a feeling that this isnt NFB??

[martin manning]

I also wanted to take a shot at a triode half power switch, as this could potentially get me a more raw tone perhaps, with less of that tight clean power thing that this amp seems to be all about. Is this fairly easy to accomplish??

In theory, yes, but depending on the PCB layout, it may not be. You'll need to isolate the supply end of the screen resistors for each side. Some like it, some don't. I've not tried it with EL84's, but there's no reason you can't have a go. To try it out you could lift the supply end of all the screen resistors, tie them in pairs, and switch between the screen node and the appropriate anodes.

Can I ask if you can see any NFB on V1.. I can see a small cap from the plate feeding tha cathode but I have a feeling that this isnt NFB??

No, not NFB; it's a low pass filter.

[pompeiisneaks]

If you really want to try some NFB you could steal the idea of Dumbles LNFB and just put something at the tube itself, tap into the anode and feed back to the grid:

DumbleLNFB.PNG

Here he uses 20M into a .047uF cap, into 20M back into the input. You can adjust the cap to taste on your preferred frequency response, but that's a very easy thing to do to most any amp at the socket. You'd need either a terminal strip or cover the whole thing in shrink tube and some silicone to keep it from moving, but something like that allows you to increase the clean headroom in a stage.

~Phil

[diddymix]

Thanks Pompeii, ok.. the only thing is that if you mean adding NFB then that is not what I want to try... I want to try removing it or considerably, if its there already,. my issue is that I cannot see any on the schematic.. only I may be missing something.. V1a appears to have none, but V3 being a cathode follower apparently is by nature using 100% NFB. Since the entire input signal is being fed through V3 this suggests to me that V3 in effect is having the same impact on tone as a high NFB loop, the goal and result being a higher damping factor and lower distortion.. Is there something I may be missing here?? I want to do whatever I can to get the tone back to more of an unrefined and raw sounding thing, especially when it comes to pick dynamics determining overdrive and character.

Thanks

[pompeiisneaks]

Oh sorry gotcha, so you're going for a bit more drive I'm guessing. I don't know off the top of my head what you could do, exactly, but you could try to swap some cathode resistors on V2 for something to bias those tubes a bit hotter. Right now they have 2.2k on both cathodes of V2, you could swap from 2.2 down to 1.8 or 1.5k or even as low as 820 ohm. This would increase the gain a bit but not sure what other tonal changes it would introduce. that should reduce the 'clean' of the amp.

~Phil

[diddymix]

Ok great thanks! I've been reading about this kind of thing... I would imagine that this would change the sound a bit as well as adding gain, the cathode resistors.. I'm definately going to try a smaller cathode bypass cap, as I've read that it can liven up the top nicely and trim some low end.. I think that that also adds gain but is frequency dependant. I may try both but not by too much. Thanks again

[Lynxtrap]

First, how do you want to use the amp? Preamp overdrive with the Master volume low, or master turned up loud for power amp overdrive?

There's no lack of gain in these IMO, and lowering cathode resistors may or may not achieve what you desire.

To open up the highs you could try and remove the snubber caps around the plate resistors. That would be C69, 70 and 71.

Be careful and make sure there's no voltage in the circuit if you're going to poke around in it.

[diddymix]

Hi thanks for the reply. I mostly want to use it on the clean channel, with master not too loud perhaps halfway.. and the preamp up about halfway to 9 oclock so pretty clean. Its not gain I'm looking for. I'm looking to get more of a raw and dirty/thicker tone, where the clipping behaviour reacts differently. At the moment to me it has that clean headroom push pull thing where it transitions into clipping later and more abrubtly. I dont mind sacrificing clean headroom but want to get that transition earlier and with more low order harmonics, and sensitive to pick attack. Where its clean as a whistle with gentle attack all the way to lots of dirt when you pick hard. This to me happens more with single ended amps and with no negative feedback.. where the tone is and signal path is very simple, but I love the feel and very thick, pure and dimensional tone ,and punch that you get from them. I realise at some point its not worthwhile to rebuild a whole circuit to get to that point...but I really like this amp as a workhorse and just want to try some simple changes.. The triode mode could really help... but can you see any application of negative feedback on clean channel? Is there any way I can tweak values around V3 cathode follower to have less 'feedback' effect or compression on the tone, I cant help but feel it has a negative effect on tone...
Ah thanks and ok about the snubbers! I can try but could this be a bad idea in case of oscillation?.

[martin manning]

... but can you see any application of negative feedback on clean channel? Is there any way I can tweak values around V3 cathode follower to have less 'feedback' effect or compression on the tone, I cant help but feel it has a negative effect on tone...

Cathode followers (i.e. V3b) have slightly less than unity gain by nature (because of their 100% local negative feedback), but are generally thought to make a desirable contribution of second-order harmonic distortion. V3b is also functioning as a low impedance drive for the tone stack that follows it.

You could give this a try (below), clipped from here: http://www.valvewizard.co.uk/dccf.html This will increase the gain of V3a and deliver a 4dB boost into the PI. You could play with the value of the cap to trim the amount of low frequency boost (effectively making it more or less of a treble boost).

[Lynxtrap]

The boring answer would of course be that this amp wasn't really designed for the style you are describing
Anyway, a couple of thoughts.

Have you explored the lead channel with its two gain controls, like keeping the gain knob low and turning up the channel volume?

Have you tried diming the master? If that gets you closer it might be worth investing in an attenuator to cook the output tubes while keeping the volume at a reasonable level (and those tubes are litterary cooking in these amps).

Modifying the second stage into a so called cold clipper like in the JCM800 amps might be worth trying, it's an easy mod.

If Marshall is your thing I wouldn't worry too much about the cathode follower. There's one in almost every Marshall amp you have ever heard.

Will it oscillate without the snubbers? Probably not, at least not in the clean channel.

[diddymix]

Hey! That's awesome thanks.. I'll try removing that 100pf snubber on V1, could get me back some upper sparkle. Yeh I know its one of those situations isnt it.. I am probably going to get an attenuator as you say... in fact this amp cranked up (4 x EL84's) sounds very impressive with careful attention to rest of chain. Yeh I think to be honest that I'm not going to worry about the cathode follower.. although I am curious about something. Can it be tweaked so there is less of that feedback effect ?. (as Ive read elsewhere the stage is in effect 100% local feedback).... I notice on V3 the first plate feeding triode b grid... is this that feedback loop?? Or is the feedback effect an internal part of the configuration?..
, I'm also curious about the phase inverter. Apparently some welcome bite and grind can be had back in the tone by taking out that plate to plate snubber too.. do you think this may be safe from oscillation too. Ive read somewhere that it's 50/50 chance... I certainly have no idea, and of measuring it if it does happen...
Many thanks

[martin manning]

I notice on V3 the first plate feeding triode b grid... is this that feedback loop?? Or is the feedback effect an internal part of the configuration?

The NFB is an intrinsic property of the CF.

[diddymix]

Hi Martin and thank you! That's kinda what I thought.. No big deal. Actually I've had a few ideas and thanks trhat link is great.. theres also something at the bottom of the page where it shows if you remove the cathode bypass feeding the follower V3b you can clean up the overdrive behaviour reducing fizz (this is definately something I want to remove in this amp, theres too much gain and it becomes pure fizz on OD channel) giving a more natural OD. I think I will also try just using a lower gain tube for V3 like a 12At7 or 5751... this may help me get less of the CF effect on the OD tone, and perhaps a more simple pure kind of distortion. Just one last question if I may... Would it be in any way possible to come straight from the clean channel tonestack, directly to the phase inverter bypassing V3 altogether?? Or would there be some kind of a gain or impedance mismatch/propblem... I was thinking of just trying that and re routing the trace with a wire... just V1 into PI..
Thanks