Active fx for sluckey inspired Plexi/800 amp

[Mike_J]

I am in the process of planning my Marshall Plexi/800 build.  It will be based on sluckey's layout and schematic with a Dumble inspired overdrive channel, an active fx, PPIMV and probably a seven way switch to choose resistor size for the negative feedback circuit.  My chassis will be 25-1/2" X 8-1/2" X 2-1/2".

I am trying to design it in pieces and then try to put them all together in the end.  The piece I put together today is an active fx unit based on the Dumbleator 3-knobber with a switch to bypass it if I want to get closer to the original Plexi circuit.  Since I will have a PPIMV and the DLator as master volume controls I am probably going to eliminate the 800 based master volume and enter the fx from the output of the overdrive relay.

I apologize for the handwritten layout I haven't seemed to quite enter the 21st century yet.  I tried to make it as neat as I could and I hope nobody will have a problem understanding it.  What I am hoping is that people on this site that know a lot more than I do will look at layout and see if they see any problems with it.  It would go between the overdrive circuit and the PI circuit.

Thanks
Mike

[shortfuse]

Mike
Read this thread [size=78%]http://el34world.com/Forum/index.php?topic=17203.msg172038#msg172038[/size]  and look at tubenit's and geezers past threads on this. 
I am in the process of building / doing the same thing I think.
I am going to build a d'lator and try it out in my 800 and plexi and then possibly incorporate it into the Sluckeys dual 50.  And install controls on the front?
Look forward to following your progress as well.


Steve

[Mike_J]

Mike
Read this thread [size=78%]http://el34world.com/Forum/index.php?topic=17203.msg172038#msg172038[/size]  and look at tubenit's and geezers past threads on this. 
I am in the process of building / doing the same thing I think.
I am going to build a d'lator and try it out in my 800 and plexi and then possibly incorporate it into the Sluckeys dual 50.  And install controls on the front?
Look forward to following your progress as well.


Steve

I have revised the layout I posted above so that you don't have to turn your head sideways to read it.

Steve

I made a Dumbleator for the three Dumble builds I have constructed.  They are each 100 watt amps and I can play them at bedroom level or at large venue level and I think they sound good.  My experience with the construction of those amps has led me to not fear a four EL34 powered amp.  That is how the original Superleads were made and that is how I am going to make this one.  Part of the reason for doing so also is I always wanted a Superlead Marshall back in the early 70s but couldn't afford one at the time.  I guess it is a bucket list type of thing.

The schematic I drew is just for an onboard three-knobber Dumbleator which is comparable to the stand alone Dumbleator I made for my Dumble amps.  The difference is it has a much smaller footprint than the 1-unit rack Dumbleator I made and would be powered off the main power transformer.  Tubenit had a good idea in one of the posts you referred to when he suggested putting the return pot on the front of the amp since it will be used as a master volume.  Otherwise all of the pots, jacks and switches would be placed on the back of the amp.  I would use the smaller sized pots and the mini switches to accomplish the small footprint.

I would still like someone's comment on the bypass switch in the bottom right hand corner.  Does anyone see a reason this wouldn't work to take the fx loop out of the signal path.

Thanks
Mike

[shortfuse]

100 watter very cool I had a 1974 JMP super lead that i sold a year ago and regretted it ever since.  Dont know why I did not use it that much I just loved the sound of that head.

Hearing that you too have used this and like the results with 100w amp may be just enough to push me over the edge and say F it and just build it with it on board.

Look at Tubenit's link in my post and down load the schedules I believe he has it designed with by pass cant confirm until this evening when I get home dont have that software on my work computer.  Also look in this link http://el34world.com/Forum/index.php?topic=10208.msg93844#msg93844

[Mike_J]

There is now a version 3 as I came to the realization that I don't need in and out inputs if I am already in the amp.

[Mike_J]

Tubenit placed a bypass switch in the middle of the circuit in the series / parallel circuit.  It is wired like the switch in my layout except that my switch is placed at the input of the fx loop and switches it out in it's entirety.  The reason I am having trouble with this is my experience with overdrive channels is when they are not in use they are grounded out.  The method I have used here does not ground out signal to this tube the signal just bypasses it.  I don't know that the signal needs to be grounded out.

I would appreciate any direction in that regard.  Tubenit has a on and off switch for his D'Mars ODS & FX schematic that grounds the overdrive channel but it looks like the loop is always on.  I have not found a schematic that specifically addresses the issue of turning off an onboard fx loop.

Thanks
Mike

[sluckey]

Your circuit would be much easier to confirm if it were drawn in standard schematic format.

[tubenit]

Since I always have delay or reverb on,  I have not had a bypass switch for the FX on any of the 4 amps I have built with on board FX.   Even if I did not have delay/reverb on,  I still want the FX in the circuit as my master volume.


I do install a DPDT bypass on external D'Lators

With respect, Tubenit

[Mike_J]

Since I always have delay or reverb on,  I have not had a bypass switch for the FX on any of the 4 amps I have built with on board FX.   Even if I did not have delay/reverb on,  I still want the FX in the circuit as my master volume.


I do install a DPDT bypass on external D'Lators

With respect, Tubenit

Thank you for your response Tubenit

My goal with the switch is to make it possible to have a Marshall clone amp.  It will also have something that an original Plexi or 800 never had which is an overdrive channel and an active fx loop as well as a PPIMV.  What I am trying to accomplish is the ability to take the OD and FX out of the circuit.  I can then max the PPIMV and I have an original Plexi and 800 dimed which probably makes no sense at all because it would be so loud that it would be useless in almost any venue I would ever play in.  But I am not so sure that I wouldn't like the Plexi through the PPIMV without any coloring from the fx loop.  Therefore, I would like to have the ability to bypass the fx.  That way I would have a Marshall Plexi and an 800 in one package.  Something that I have wanted for 40 years.

I am pretty confident now that the circuit will work.  I know what voltages I need to have on the plates and cathodes of the fx tube so some adjustments may be necessary there but that is normal tweaking.  I am just not sure whether I need to ground out the fx tube or can just bypass it like it is shown on my layout.

Thanks
Mike

[Mike_J]

Your circuit would be much easier to confirm if it were drawn in standard schematic format.

I have attached a schematic of the fx loop.  Hope this makes it easier to analyze.

Thanks
Mike

[Mike_J]

Attached is a revised layout with the on/off switch and master volume on the front of the amp if I am not mistaken the circuit was taken from an original Dumbleator posted on the Amp Garage site.  I have not constructed this yet so I am not 100% positive it will work (although my DLator was constructed based on the same circuitry (minus the transformer and associated power caps) and it works very well.  I don't see why this shouldn't work also.  If you wanted to eliminate the on/off switch then go directly into the .047 cap feeding pin 2 and it should work.

I am planning on putting the power caps on the same power supply board that will contain the main caps and the caps for the screens.  I will probably branch off the screen caps.  All the other caps I plan on putting on the preamp board like sluckey's layout shows.

I am sure the same thing has been posted on this site before but I couldn't find it in a format that I was looking for (which is the three knob version).

Thanks
Mike

[tubenit]

I think what you're doing will work.

Tubenit

[Mike_J]

I think what you're doing will work.

Tubenit

Thank you for the reply Tubenit.

I am going to use the same overdrive circuit values I used for my ODS #124 amp with good results.  It has a 345.5K trimmer which quite a few people seem to like.  Someone was selling 350k pots on the internet and they were nice enough to measure out two of them that had 345K values for me.  I used one of them on 124 with good results and hope to get good results here too.

Thanks
Mike