Sluckey inspired Plexi/800 w/ dual tone stacks OD, FX & PPIMV

[Mike_J]

Attached is the layout of the Sluckey inspired Plexi/800 with dual tone stacks, OD, FX and PPIMV.  It is based on sluckey's excellent Plexi/800 layout and schematic documents.  I added separate tone stacks for each preamp as well as overdrive, an active effects loop and a PPIMV circuit to what Sluckey had.  My output stage also has four EL34 tubes whereas Sluckey uses two. 

I added the second tone stack because I want to be able to use an A/B/Y pedal to switch between the Plexi and 800 preamps.  Sluckey advises that they should not be used at the same time because it won't sound good.

I am about to start the build so I have no idea whether the overdrive as shown will work with either of the Marshall preamps.  The overdrive circuitry was taken from the Dumble 124 amp that is well documented on the Amp Garage forum.  The 124 amp had 100K plate resistors as do the Marshall's so I thought it would be a good starting point.  My guess is it will work with the Plexi channel but not so sure with the 800 channel.  As mentioned by Tone Junkie in one of my previous posts you have to be careful about  overdriving the PI.  My guess is the 800 preamp would be the more likely of the two to overdrive the PI but only time will tell what works and what doesn't.  If it doesn't work on one or both of them it can just be switched out of the circuit.

The FX circuitry is a direct copy of the Dumbleator three knob active effects circuitry.  It is well documented on many forums including the Hoffman and Amp Garage sites.  It does two things very well.  First if you have an effect that would overdrive a passive effects channel  this unit will reduce the incoming signal so the pedal is more useful.  Secondly, the 250 KA pot on the front of the amp is a tube driven recovery pot.  It does two things; first it recovers the lost gain from the signal you reduced on the front end of the circuit and two it is a very effective master volume.

The PPIMV circuitry is from the LarMar circuitry that is on many different sites.  It is a proven circuit that is liked by some and not liked by others and acts as a master volume.  I will leave a turret in the PI section so I can add two 220K resistors if I don't like it and want to remove it from the amp.  One advantage of the circuit is if you max it you have essentially the same thing you would have if you had the two 220K resistors feeding the bias circuit.

If anyone wants to make an amp like this one print the layout on an 8-1/2" X 11" piece of paper and have it printed so the chassis is 25 - 1/2" wide.  You can then use it as a template for drilling holes in a blank chassis.  If you drill on the front or back of the chassis remember this is how the amp will look from the inside so the template has to be turned over and drilled from the back.  You will need a chassis that is 25 - 1/2" wide by 8 - 1/2" wide by 3 - 1/2" deep.

Without the help of members on this forum there is no way I could have done this.  My thanks to sluckey, tubenit, jojokeo, PRR, EL34, Willabe, DummyLoad and others that made the development of this design possible.  Only time will tell how well it works but I think the concept is sound.

Thanks
Mike

[tubenit]

Here is the amp with 2nd page.

With respect, Tubenit

[jojokeo]

Holy frijole that's a lot of gain stages senior Batman! Tubenit, in your schematic drawing the FX output should be on center switch pin #5 (just at a quick glance of my laptop)? I like the dual channels' set-up. Not so much the gain stages afterwards but at least it can be bypassed if wanted. I can see a lot of adjustment tweaking lies ahead. I hope it goes well Mike! I'll check back later when I have a bigger screen & more time.

[tubenit]

in your schematic drawing the FX output should be on center switch pin #5

Good catch and you are correct!   Thanks,  Tubenit

[jojokeo]

my 2 cents & MHO: V5 really isn't needed BUT the way I see it (take it for what it's worth) and experience with something very similar - this will be an already bright and gainy amp. This being said:
The beginning input voltage divider to V5a isn't enough for all that signal w/ all controls full up. Values should start with 470k over 220k = 32% output fed into V5a and tweak if necessary. The divider feeding into V5b use 220k with the 250k gain pot. Limiting output between stages is proven successful at ~50% output. I'd also put either a 1n or 500p cap across V5a's load resistor and remove both plate-cathode caps on both stages. Lower the coupling cap coming out of V5a to .005uF and raise coupling cap out of V2b to .01uF. Remove the cathode bypass cap off of V2b and lower resistor to 1k. Lastly, I'd keep output at 50% using 100k w/ 100k pot or better 220k resistor w/ 250k pot to feed into FX loop. I realize this is likely asking/saying a lot of changes but this is what's worked for me or at least the values will get you in the ballpark.
Anyway, hope it helps. And if you build it another way first & feel something is lacking (sustain from inaudible oscillation) or too much (unmusical overdrive/distortions) and maybe just want to try something with a different approach and see how much difference it could make you can refer back to this someday?
Keo

[Mike_J]

my 2 cents & MHO: V5 really isn't needed BUT the way I see it (take it for what it's worth) and experience with something very similar - this will be an already bright and gainy amp. This being said:
The beginning input voltage divider to V5a isn't enough for all that signal w/ all controls full up. Values should start with 470k over 220k = 32% output fed into V5a and tweak if necessary. The divider feeding into V5b use 220k with the 250k gain pot. Limiting output between stages is proven successful at ~50% output. I'd also put either a 1n or 500p cap across V5a's load resistor and remove both plate-cathode caps on both stages. Lower the coupling cap coming out of V5a to .005uF and raise coupling cap out of V2b to .01uF. Remove the cathode bypass cap off of V2b and lower resistor to 1k. Lastly, I'd keep output at 50% using 100k w/ 100k pot or better 220k resistor w/ 250k pot to feed into FX loop. I realize this is likely asking/saying a lot of changes but this is what's worked for me or at least the values will get you in the ballpark.
Anyway, hope it helps. And if you build it another way first & feel something is lacking (sustain from inaudible oscillation) or too much (unmusical overdrive/distortions) and maybe just want to try something with a different approach and see how much difference it could make you can refer back to this someday?
Keo

Jojokeo

Thank you for the response.  I didn't think I was the first person to ever think of putting an overdrive channel into a Marshall amp.  You are recommending changes to the Plexi tone stack.  My guess is if that is required for the Plexi channel then the 800 channel with the cascaded gain stages is another story. 

The values I used for V5 were a complete guess on my part based solely on using the same values on another 100K plate resistor amp with success.  Nothing at all scientific about it just a guess as a starting point.  I placed this on the site in hopes that someone like you with practical experience would recommend a more practical way of wiring the circuit if the method I proposed lacked feasibility.

I believe experience is a good teacher.  The best bad experience I find is the kind others have had and are kind enough to pass along in order to keep others from experiencing the same frustration.  As such I appreciate your help in that regard.  I have no desire to  create an unstable amp.  I am not competent enough to overcome such a thing.

My first decision is do I need an overdrive channel at all as you mentioned.  Certainly what I want to accomplish could probably be done with pedals.  My major goal is to have a Plexi and 800 in one amp.  The secondary goal is to add controls to make it useable at lower volume levels.  Thus the Dumbleator and PPIMV.

Now if I eliminate the overdrive channel it gives me the problem of having two unused triodes.  Any ideas as to what you would put in its place, if anything?  I put a lot of time figuring out how to successfully put seven preamp tubes in this amp.  However, going back to six preamp tubes would not be the end of the world because it would allow me to have room on the circuit board for the preamp electrolytics.  As it is with seven tubes I need to used 50uF/50uF @ 500V cap cans under each tone stack in order to make the spacing work.

Tubenit puts a 5879 in a lot of his amps.  Is there an application for one here.  I am just throwing it out as an idea.  I am pretty ignorant where gain stages are concerned unfortunately.  But I am trying to learn.

Thanks
Mike

[tubenit]

Tubenit puts a 5879 in a lot of his amps.  Is there an application for one here. 

Sure there is. I built a couple of amps using just the 5879 as the OD stage. It's somewhat chimey-r than the Dumblish OD, but it has a FAT tone to it.  Overdrive wise, it's sort of between 2/3 to 3/4 the amount of OD that the Dumblish OD's that I've used.

with respect, Tubenit

[jojokeo]

I didn't think I was the first person to ever think of putting an overdrive channel into a Marshall amp.

No, but definitely not needed as there's plenty of gain on tap already even without tube's V5 & V6. 4 extra gain stages (3 in reality) on top of the already proven Plexi & 800 preamps is straying from the original design of Marshall feel & tones. If you're striving to replicate the original tone & feel of those two amps then adding V5 & V6 takes you away from that (but why I also said V5 can at least be bypassed). The Plexi side will be "cleaner" and the 800 is plenty to be "meaner". With the FX loop alone you have three gain/vol controls there that are configured to give even more gain than without it (V6b specifically). The way it is now, you're dumping most all of your original signal (97.4%!!!) you built up going into V6a only to bring it back up again with V6b! All to have an "active" effects loop. Do you really need this? This comes with consequences and will color your tone maybe not in a good way that you want? The devil's in the details of how you set up this tube and hopefully you see how important V6 is to your entire design? Like a traffic cop, it alone will make or break what you're looking for that you "worked so hard" to create with the Plexi & 800 stages AND with the new addition of introducing V5 into the mix.

You are recommending changes to the Plexi tone stack.

No, I'm not mentioning anything regarding this. Only comments on V5. You'll find that if you build the amp with V5, that it'll very likely need special tweaking to get things to be sound and behave properly. The only sure way to know is to build the amp and deal with the design once it's fired up and you play it. Every stage in a high gain design doesn't need bypass caps for example. It easily becomes too bright & brittle sounding where you'll want a smoother type of effect in certain places. But see, now the original Plexi & 800 feel & "flavor" becomes lost. So why I said "at least you can bypass V5", V6 you cannot and why you must be careful in setting those stages up (but again you'll know more when the amp is up & running).

The secondary goal is to add controls to make it useable at lower volume levels.  Thus the Dumbleator and PPIMV.

Now if I eliminate the overdrive channel it gives me the problem of having two unused triodes.  Any ideas as to what you would put in its place, if anything?  I put a lot of time figuring out how to successfully put seven preamp tubes in this amp.  However, going back to six preamp tubes would not be the end of the world because it would allow me to have room on the circuit board for the preamp electrolytics.

More preamp tubes is NOT always better. Having a goal of being useable at lower volume levels is reasonable but in an amp with 4 EL34s is a lot of wasted power. If it were me: I'd drop both V5 & V6 and consider the reality of if I actually needed a full 100watts or not. 40 to 50 watts is more than plenty to play anywhere in this day and age so I'd also eliminate V8 & V11. But if I also wanted to try to achieve a more Dumble type of overdrive then I'd keep the addition of V5 on a switch so that it can at least be bypassed but still get V6 out of the way/out of the equation and use a passive effects loop instead.
These are just my thoughts Mike, it's your baby to do as you please. I'm just trying to throw out a bit of perspective is all.

[Mike_J]

The way it is now, you're dumping most all of your original signal (97.4%!!!) you built up going into V6a only to bring it back up again with V6b! All to have an "active" effects loop. Do you really need this? This comes with consequences and will color your tone maybe not in a good way that you want? The devil's in the details of how you set up this tube and hopefully you see how important V6 is to your entire design? Like a traffic cop, it alone will make or break what you're looking for that you "worked so hard" to create with the Plexi & 800 stages AND with the new addition of introducing V5 into the mix.
[/quote]
No, I'm not mentioning anything regarding this. Only comments on V5. You'll find that if you build the amp with V5, that it'll very likely need special tweaking to get things to be sound and behave properly. The only sure way to know is to build the amp and deal with the design once it's fired up and you play it. Every stage in a high gain design doesn't need bypass caps for example. It easily becomes too bright & brittle sounding where you'll want a smoother type of effect in certain places. But see, now the original Plexi & 800 feel & "flavor" becomes lost. So why I said "at least you can bypass V5", V6 you cannot and why you must be careful in setting those stages up (but again you'll know more when the amp is up & running).
[/quote]
If it were me[/u]: I'd drop both V5 & V6 and consider the reality of if I actually needed a full 100watts or not. 40 to 50 watts is more than plenty to play anywhere in this day and age.
[/quote]



Jojokeo

As my primary goal is to replicate the Plexi and 800 amps I do not want to do anything that will eliminate 97.4% of my primary objective.  I thought the amp was designed so V5 and V6 can each be removed from the signal path.

You mentioned that V6 could not be bypassed but I put a switch in front of the circuit.   The sole reason I added that switch was so I could remove the active effects circuitry from the Marshall preamps if I wanted to keep the Marshall preamp signal intact.  Would you look again and see if your statement that V6 cannot be bypassed is correct?
 
While I want a Marshall clone amp when I am finished with the build I think it would be advantageous to have active effects circuitry in it.  My experience with the circuit has been very good.  It can add to the tone and makes a good master volume.  I realize that it would color the Marshall input and it would no longer be a Marshall when the circuit is engaged but as long as I can switch back to the Marshall amp I am okay with that.

I will try to make the changes to the overdrive circuitry you have suggested just to see if I can get a sound I like out of it.  If no then I will switch it out of the circuit and never use it.  I have already sent the faceplate design to my faceplate man and I don't want to change the design now but of course I want to change the circuitry to make it useable.  It is my hope this amp will be a learning platform for me and hopefully others on the forum.

My perception of a Plexi is 100 watts of pure power.  It is the amp I wanted as a kid but could never afford.  This is an emotional decision not a logical one.

Thanks for your excellent advice.
Mike

[Mike_J]

Could someone please tell me how to get those nice little quotes in the nice boxes so I don't have to quote the whole post.

Thanks
Mike

[Mike_J]

Tubenit puts a 5879 in a lot of his amps.  Is there an application for one here. 

Sure there is. I built a couple of amps using just the 5879 as the OD stage. It's somewhat chimey-r than the Dumblish OD, but it has a FAT tone to it.  Overdrive wise, it's sort of between 2/3 to 3/4 the amount of OD that the Dumblish OD's that I've used.

with respect, Tubenit

Tubenit

This may be where this amp heads down the road.  If not my next build will probably be the AC30 with the separate drive channel with a 5879 in it instead of the microphonic tube.

What type of socket does the 5879 use and do you think the spacing for it would be the same as for a 12A_7 tube?

Thanks
Mike

[tubenit]

I thought the amp was designed so V5 and V6 can each be removed from the signal path.

They are.

Tubenit

[Willabe]

What type of socket does the 5879 use and do you think the spacing for it would be the same as for a 12A_7 tube?

Same as a 12A _ 7, 9 pin.

And yes same spacing should be fine.


                  Brad     

[Mike_J]

Tubenit puts a 5879 in a lot of his amps.  Is there an application for one here. 

Sure there is. I built a couple of amps using just the 5879 as the OD stage. It's somewhat chimey-r than the Dumblish OD, but it has a FAT tone to it.  Overdrive wise, it's sort of between 2/3 to 3/4 the amount of OD that the Dumblish OD's that I've used.

with respect, Tubenit

Tubenit

I have spent a few more minutes looking at the schematic with the 5879 in it.  Thank you for doing that for me.  I am going to try to configure the layout for that section so I can go either way without having to remove the board to make the change from a 12a_7 to the  5879, especially where the mosfet is concerned.  Could you explain to me what the 1 meg pot with the .0047 resistor to ground does?  I need to pull the data sheet on the 5879 tube and get more familiar with the pins.

Also, I just noticed the master volume pot is excluded from the 800 section.  Did you think it was a bad idea or was its exclusion just something we missed earlier?
Since I will be playing one preamp or the other its inclusion would seem to be a good idea to me since I will be using the 800 side for overdrive and the Plexi side for clean most of the time.  Without the master volume I am concerned that I might have a hard time balancing the volumes as the normal volume control will probably be higher on the front end of the 800 than the volume controls will be set for the Plexi side.

Thanks
Mike

[jojokeo]

Mike,
Sorry I missed that V6 switch from one of the previous schematics while replying. I was recalling V5 & V6 from memory while replying earlier. Also, when you say that V6 can act as a Master Vol, yes it can but then you're also installing a PPIMV which is your true MV control. You may want to stick to your first gut instinct on the 12AX7 for V5 over the 5879/EF86 as I think you might just be happier with the result & effect. Plus you use any of the 12a_7s to play around with it if you want to. I mostly use pentodes as the first tube in the preamp, not later in the circuit but again, my opinion. There's a learning curve with pentodes & you already have a lot going on with your current amp idea to introduce yet one more thing to wrap your head around. Again, just trying to help-

[tubenit]

Jojokeo made a good point regarding 5879 vs.  and  12A_7 where you can change tubes to vary the gain.

I did not intentionally omit a master volume with the JCM800.  Just an oversight, I guess?

With respect, Tubenit

[Mike_J]

Jojokeo made a good point regarding 5879 vs.  and  12A_7 where you can change tubes to vary the gain.

I did not intentionally omit a master volume with the JCM800.  Just an oversight, I guess?

With respect, Tubenit

Tubenit

In addition to the volume balancing explanation that I mentioned in the post above each preamp side needs to have balance in order to look right.  The Plexi has a normal volume and a bright volume and the 800 has only a normal volume.  It is six jacks and pots on one side and only five on the other without the master volume balancing out the aesthetics of the faceplate.  Seems minor I know but I always thought the slant cab on the Marshall amps was a significant reason for their success because a Marshall stack has always had a good appearance.  They also worked with Peter Townsend and others to increase the power of their amps early on but other companies were doing that too.  I am probably way more hung up on these things than I should be but I think looks has always mattered at least as much as content.  Just the way humans are in my opinion.

Another thing I noticed is the 800 preamp's tube 4a cathode should only have an 820R resistor without the .68 cap.  The '68 Plexi had the 820R resistor / .68 cap but not the 800.  When I make the amp the .68 cap will be omitted from the 800 side.  When there was only one tone stack sluckey had to make a choice of which tone stack to replicate but since we are splitting the tone stacks we can be more accurate to the original amps.

Sorry for the long winded reply and as always my appreciation for all the help you have given me.  I will use the 5879 on my next amp.

Thanks
Mike

[Mike_J]

Mike,
Sorry I missed that V6 switch from one of the previous schematics while replying. I was recalling V5 & V6 from memory while replying earlier. Also, when you say that V6 can act as a Master Vol, yes it can but then you're also installing a PPIMV which is your true MV control. You may want to stick to your first gut instinct on the 12AX7 for V5 over the 5879/EF86 as I think you might just be happier with the result & effect. Plus you use any of the 12a_7s to play around with it if you want to. I mostly use pentodes as the first tube in the preamp, not later in the circuit but again, my opinion. There's a learning curve with pentodes & you already have a lot going on with your current amp idea to introduce yet one more thing to wrap your head around. Again, just trying to help-

Jojokeo

Thanks again for your input.  Your involvement in this project is immensely helpful.  When I mess up I am very confident you will know why I messed up and be able to help me get back on track.  I very much appreciate your experience and willingness to share it.

Thanks
Mike

[Mike_J]

What type of socket does the 5879 use and do you think the spacing for it would be the same as for a 12A_7 tube?

Same as a 12A _ 7, 9 pin.

And yes same spacing should be fine.


                  Brad   

Brad

Thank you for clearing that up for me.  I will use it on my next build.  An AC30 with a separate 5879 tone stack and maybe some other things.

Thanks
Mike

[Mike_J]

No, I'm not mentioning anything regarding this. Only comments on V5.

Jojokeo

I think your comment above conflicts with your statement in reply #4.  Did you mean 5b instead of 2b in the post.  I am trying to absorb the information in your replies and I can't reconcile the two comments.

Thanks
Mike

[jojokeo]

Did you mean 5b instead of 2b in the post.

I can't see exactly what quotes you refer to but on V5 you'd want the smaller coupling cap to be between stages (output of V5a) usually between .0022uF to .0047uF and a larger coupling cap going out of the 2nd triode (V5b), say between .01uF to .022uF.

[tubenit]

you'd want the smaller coupling cap to be between stages (output of V5a) usually between .0022uF to .0047uF and a larger coupling cap going out of the 2nd triode (V5b), say between .01uF to .022uF.

To the best of my knowledge, all the Dumblish overdrives have the larger value cap .01 after the first gain stage of the OD and the smaller value cap usually .0047 after the 2nd OD gain stage.

Just an FYI. 

With respect, Tubenit

[SILVERGUN]

Mike J.,
This thing is right up my alley, and I'm just standing back and watching....

You are in great hands with these guys and I'm looking forward to the end result.

I'll add one little thing while I'm here and just say that I've had some good results with using a 12AT7 in the PI when I've got this much gain in the preamp. Just a straight substitution, with the existing values in tact.

Brings some note definition back and seems to 'feel' like it pushes the output stage 'easier'.
Just something I believe is worth trying before you button it up.

Good luck and have fun with this!
 

[jojokeo]

To the best of my knowledge, all the Dumblish overdrives have the larger value cap .01 after the first gain stage of the OD and the smaller value cap usually .0047 after the 2nd OD gain stage.

Thanks for pointing that out. Maybe a dyslexic moment on my part but it goes against common sense. The tube has a smallish value bypass cap for shelving away bass response but is then given the high value coupling cap in between allowing for possible blocking distortion w/out the high value grid resistors, but are inserted to help prevent this. So it seems there's opposing and/or corrective factors simultaneously at play here. Then the subsequent stage is going to increase the signal bandwidth to be cut down (maybe more than necessary?) with the smaller coupling cap. Countless designs are not built this Dumb(le) way when cascading stages.

Here's some sound advice taken from Aiken:  Blocking can (and does) occur just as bad on RC-coupled preamp stages that are overdriven. The solution is similar.  Add large value series grid resistors (100K - 470K), reduce coupling capacitor values to the minimum required for the desired low frequency response, add interstage attenuators to limit the amount of grid drive to the next tube, reduce the size of, or eliminate, the cathode bypass capacitors, as they also contribute to the blocking due to the long time constant associated with the recovery from a transient signal, or  add a diode clipper bounding circuit to prevent the grid from being driven too far. 

[jazbo8]

The cut-and-paste design approach often leaves out important details, e.g., the Dumble ODS employs LNFB for its gain stages and they are specifically tuned for the tone controls. You can throw in a "OD" stage all you want, but just don't call it a Dumble ODS...

[Mike_J]

The cut-and-paste design approach often leaves out important details, e.g., the Dumble ODS employs LNFB for its gain stages and they are specifically tuned for the tone controls. You can throw in a "OD" stage all you want, but just don't call it a Dumble ODS...

I think I explained that this overdrive section is an exact copy of the overdrive section in the Dumble Overdrive Special #124 amp.  It is just a starting point because I needed to put something there for the overdrive channel.  I have made three Dumble clones and have never used the Dumble name on any of the amps because there is only one person that should use that name and that is Dumble himself.  However, if I copy his circuitry exactly as a starting point, solely because the #124 amp used 100K plate resistors and so do the Marshall preamps I am copying, then I feel I owe it to Dumble to give him credit for it.

Apparently you are the purveyor of names and can decide what people can call their work.  It is only a name used to describe where it came from.  If you are as knowledgeable as your post purports I am sure you will be of immense value to this project when it comes to working the bugs out of the overdrive section when it comes time to do so.

I use the cut and paste approach because I have no competence in any of the software where layouts are concerned.  I prepared the layout at the full size of the chassis I will be using.  It allows me, and anyone else that cares to use it, a template for drilling holes in the chassis if it is blown up to the correct size.  It also allows me to make sure component spacing is adequate and that wires are routed in order to minimize problems when the amp is finished.  I will readily admit it is not pretty but it is functional and a good starting point for the build.

If you are technically competent to prepare a more professional looking layout then I would be happy to see your involvement in same.  Tubenit was kind enough to give his time in the preparation of the schematic for the project for which I am very appreciative.

[jazbo8]

You are getting worked up over nothing, all I said was slapping in a Dumble OD gain stage in the amp doesn't make it a "Dumble ODS", since you have cloned several Dumbles already, then you know the whole is greater than the sum of its parts. In this case, you have changed the gain structure, the tonestacks before the OD stage, what does it still have to with a Dumble? Perhaps you just wanted to give credit where it's due, but it just does seem not right after all the changes you've made... 

[Mike_J]

You are getting worked up over nothing, all I said was slapping in a Dumble OD gain stage in the amp doesn't make it a "Dumble ODS", since you have cloned several Dumbles already, then you know the whole is greater than the sum of its parts. In this case, you have changed the gain structure, the tonestacks before the OD stage, what does it still have to with a Dumble? Perhaps you just wanted to give credit where it's due, but it just does seem not right after all the changes you've made... 

I may have reacted overly harshly to your comments.  I realize that it has been very difficult to crack the code of the ODS because of Dumble's secretive nature, the goop in the amps that makes them harder to copy, etc.  Once that code is cracked like it was with the pictures of 124 then some people want to maintain the purity of something that was so hard for them to attain in the first place.  If that is your intent in the comments you made I understand where you are coming from.  Like I have already said I was just trying to give the man credit for my starting point because he is due the credit. 

This amp I am building is a Marshall clone with an overdrive channel.  I am somewhat concerned that the HT caps on #124 totaled 150uF and I am looking at 50uF with this amp.  Good chance that the overdrive will introduce problems due to a lack of filtering.  Only time will tell.  If there is not enough filtering then I will remove the overdrive channel because I want a Marshall.  That requires adherence to the power supply Marshall used.  I am having two chassis made in anticipation that I will be rebuilding the amp.  This amp is an experiment to learn from.  I have already learned quite a bit from it.

[jazbo8]

No worries, perhaps I have been reading way too many threads over at TAG where they take the Dumble circuits very seriously, no doubt some of that rub off on me... Anyway, since you have already done a few Dumbles, you know they are not really Marshall-y, so if you want Marshall, you already got that in spades with the Plexi and JCM800 preamps - the die has already been casted as they say. Good luck with your build, and don't mind my occassional rambling...

[jojokeo]

I am somewhat concerned that the HT caps on #124 totaled 150uF and I am looking at 50uF with this amp.  Good chance that the overdrive will introduce problems due to a lack of filtering.  Only time will tell.  If there is not enough filtering then I will remove the overdrive channel because I want a Marshall.  That requires adherence to the power supply Marshall used.

I think you may not notice any affect to the amp until or unless you're really cranking this amp up to more than half way? That's when you'll find the power supply will start to be taxed and suffer more voltage dropping than it otherwise would. Will you not have enough room or ability to simply parallel another cap if wanted? It could be a radial or axial type simply put across the cap can, if you find you even need or want it?

[Mike_J]

I am somewhat concerned that the HT caps on #124 totaled 150uF and I am looking at 50uF with this amp.  Good chance that the overdrive will introduce problems due to a lack of filtering.  Only time will tell.  If there is not enough filtering then I will remove the overdrive channel because I want a Marshall.  That requires adherence to the power supply Marshall used.

I think you may not notice any affect to the amp until or unless you're really cranking this amp up to more than half way? That's when you'll find the power supply will start to be taxed and suffer more voltage dropping than it otherwise would. Will you not have enough room or ability to simply parallel another cap if wanted? It could be a radial or axial type simply put across the cap can, if you find you even need or want it?

Jojokeo

Thanks for the reply.  That is good news.  I expect to use the overdrive channel for bedroom or low levels mostly anyway.  I agree with you that these amps have a lot of gain at high volumes.  I just don't get the opportunity to play at those volumes much be it a 50 watt version or a 100 watt version.

Thanks
Mike

[Mike_J]

No worries, perhaps I have been reading way too many threads over at TAG where they take the Dumble circuits very seriously, no doubt some of that rub off on me... Anyway, since you have already done a few Dumbles, you know they are not really Marshall-y, so if you want Marshall, you already got that in spades with the Plexi and JCM800 preamps - the die has already been casted as they say. Good luck with your build, and don't mind my occassional rambling...

Jazbo8

My apologies for coming across somewhat harshly in my earlier post.  Thank you for the kind words.

Mike

[Mike_J]

I am doing the final review of this amp with my schematic and yellow highlighter and have decided that the layout for the tone stacks are not optimal as currently laid out on the layout shown at the beginning of this post.

As mentioned somewhere in the post I switched the direction of the circuit board so the input jacks ended up on the right side.  By doing it this way it seems to make sense to wire the tone stack the way I am showing it in the attached drawing.  This will result in shorter leads and eliminates the crossing of any of the wires.

Could I get someone to confirm for me that it is electronically the same?

Thanks
Mike


 

[tubenit]

I am not exactly sure what you're saying your attempting to do.  Having said that, you drawing looks just fine to me.

I've done all kinds of different TMB tone stack layouts and have never found the layout or order to make any difference at all either positively or negatively with noise or tone.

I'm guessing that it is not that big of an issue? 

With respect, Tubenit

[Mike_J]

I am not exactly sure what you're saying your attempting to do.  Having said that, you drawing looks just fine to me.

I've done all kinds of different TMB tone stack layouts and have never found the layout or order to make any difference at all either positively or negatively with noise or tone.

I'm guessing that it is not that big of an issue? 

With respect, Tubenit

Just thought making the wires shorter would be advantageous not sure why.

Thanks
Mike