Plexi - 800 with OD & FX

[Mike_J]

I have created a hand drawn layout of sluckey's Plexi/800 plans and have added a Dumble Overdrive Special 124 overdrive section, an onboard three knob Dumblelator and a PPIMV to it.  I will also attach a hi-low switch as I had the PT made with B+ of around 400 volts and 475 volts but the layout does not include the switch.  It would go next to the standby switch.

I am also considering the addition of potentiometers to make the slope resistor and feedback resistors variable.
You may notice that I have not included a power source for the relay or foot pedal.  I haven't decided whether I will use the 5 volt Hoffman method or the 12 volt Dumble method I used in the past.  I think I have all the parts for the 12 volt system but I am missing the 4700 uF electrolytic capacitor Doug uses.

I haven't had any luck getting ExpressSCH or any other software to work as well as I would like which is the reason for the hand drawn layout.  My apologies for the less than ideal presentation.  However, my main purpose right now is to post it on this sight in hopes that more knowledgeable eyes than mine would take a look at it and see if there are any obvious errors in the layout.

Any suggestions you may have would be very much appreciated.  I plan on building the amp and pairing it with the Dumble Steel String Singer clone I made.  Should make for a pretty good rig. 

My chassis should arrive next week.  It will be 25-1/2" X 8-1/2" X 2-1/2".  I made the layout to size to make sure all the components will fit.

Thanks
Mike

[Mike_J]

Unfortunately, it looks like the resolution may not work.  I started out with an image of 126 megs.  By the time I got it down to a size acceptable to the forum the resolution is bad.  Sorry, live and learn.

[Mike_J]

There ain't 126M of information in that drawing. Which program are you using?

I went to Office Max and had the girl in the print shop department scan my 25" X 14" drawing into their machine.  It looks like it had a huge numbers of colors so I am sure the size and the number of colors both contributed to the size.  It was saved as a pdf file.  She then made it smaller and reduced the clarity significantly and saved it as a jpg file which I could then put on the forum.  Unfortunately it is not clear enough to be of any use.

I suppose one possibility is to copy this at 8-1/2" X 11" in black and white and then scan it and see where I stand from a file size perspective.  The problem then is it becomes very difficult for the reader to interpret it when seeing it for the first time.  I think the answer is to draw a schematic and post it on the forum.  It would be black and white and much easier to follow than a black and white layout diagram.  It will take me a few days but when I am done it will probably be easier for you to follow.

Thanks
Mike

[tubenit]

Here is a JCM800 version of the D'Mars.   You can edit it from here IF you want to?   Note the layout does not have the OD drive pot there & instead has a fixed resistor value, but you can insert the OD drive pot very easily into the layout.

To edit something on SCH ............... all you do is simply click on it and then edit. You click on, copy/cut & paste. You draw lines, circles and boxes.  It's that simple.   I just drew this schematic for you in about 15 min of effort.

With respect, Tubenit

[Mike_J]

I have included jpeg schematics of the overdrive and active effects circuits I have included in the sluckey inspired Plexi/800 amp with overdrive, active fx and PPIMV. 
The preamps and tone stack in my amp are identical to sluckey's drawing except that I have eliminated the master volume.  The wiper of the treble pot goes directly into the overdrive circuitry.  If someone wanted to add the master volume I would suggest putting it in the amp as a trimmer.

I have not included the LarMar PPIMV that I intend to use in the amp because it is so well documented.  My power supply is identical to the Marshall 100 watt power supply that Ken developed.  I am not concerned about these sections working.  I am confident they will.  My major concern is making sure the overdrive and fx circuits will work between sluckey's design and the Hoffman Marshall 100 watt power supply.  Any suggestions or comments would be very much appreciated.

[Mike_J]

Mike,

I am very much looking forward to you building this amp and hearing a review!  I think it will be a GREAT amp with great tone! 

I realize that you are trying to use more of the Dumblish style layout & in light of that, the SCH format is not very good for drawing it.  Visio (like Sluckey uses) is much better.

Having said that,  let's see if we can work together to at least post an editable SCH file of the schematic?  I'll PM you regarding this.

With respect, Tubenit.

Jeff,

Thank you for the comments.  I am hoping this amp will provide a very broad spectrum of Marshall overdrive tones from 60s rock to shred.  I am also hoping that some of the tricks I learned when wiring my Dumble amps will also work on this amp to make it stable in spite of the overdrive.  I guess time will tell.  Also hopefully having both a pre-PI master volume (fx master) and a post-PI master volume will allow me to go from good tone from bedroom levels to large venues.

Again, I want this amp to pair up with my Steel String Singer clone.  The SSS handles cleans at any volume extremely well.  This amp will be for the overdrive sounds.

Thanks
Mike

[Mike_J]

I should probably mention where I got the OD and FX circuits from.  The OD circuit comes directly from Dumble's 124 amp.  The reason I used it is that it is what the Dumble cloners call a low-plate amp meaning it uses 100k plate resistors in the preamp instead higher resistances used in some of the other amps.  The Marshall Plexi and 800 amps also used 100K plate resistors.  Additionally, many people believe highly in the 345K trimmer pot as being magical for creating good overdrive tone.  Well I don't know about that but I did pick up two 345K pots on the internet some time ago.  I used one for my 124 amp and intend to use the other one in this amp.

The FX circuit is just a three knob Dumbleator.  It has a switch which can completely bypass the circuit as well as the Master volume and a bright switch on the front of the amp.  The rest of the circuit can be accessed in the back of the amp.

Thanks
Mike

[Mike_J]

Mike,

You didn't answer my question about the power amp?  I don't know what LTPI, etc........... that you are using?  Respectfully, you are not
providing me enough info and clarity.  I still don't know who Ken is or what you are referring to?  I used what I am guessing to be the schematic that you referred to as Sluckey's????   Can you confirm this as being correct also?

To please simplify matters ............................. 

How about opening this GIF file and use PAINT to edit it and repost it wherever corrections are needed.  Then I will redo the SCH and repost it to match yours.

There is a page 2 on the ExpressSCH file that indicates the B+ rail etc......

BTW, this only took about 25 min at the most for me to do because I simply grabbed an old schematic and began editing. 

With respect, Tubenit

Jeff

Very impressive that you can get so close in such a short period of time.  I replied to your PM basically asking how large of a file I can email to you.  Sorry it took so long to get back to you.  My wife was using my laptop today to watch Korean dramas of all things.  One thing I have learned is not to get in the way of her and her Korean dramas.

I will also give more clarification regarding the Hoffman power amp section.  It has a voltage doubler circuit consistent with the original Marshall 1959 and 2203/2204 schematic as I recall.  I will need the voltage doubler circuitry because I had my PT made with voltages in line with that of the original Plexi transformer.  These voltages are one-half the voltage you would find in a Fender amp.  Sluckey's schematic shows the voltages like a Fender amp has.  It could certainly be done that way but again I already have the PT made.

The voltage double circuitry essentially just requires that the HT CT gets attached to the mid-point of the first two 100uF electrolytics in the power supply.  I will try to refer to it in the Hoffman projects section so you can see what I mean.  You will also see a voltage doubler in the OD circuit schematic.  What it does is take the 6.3 volt tap and raises it so it can be used to energize the 12 volt relay.

I look forward to having one-half of the skill you have with Express SCH.

Thanks
Mike

[sluckey]

Jeff, regarding Hoffman's power supply drawing...

The bias supply will not work. You must use a special cap coupled bias circuit to tap into the HT winding when using a bridge rectifier for B+ (see the JCM-900 series amps). Hoffman's 100W Plexi circuit uses a PT with a separate dedicated bias winding.

[Mike_J]

Jeff, regarding Hoffman's power supply drawing...

The bias supply will not work. You must use a special cap coupled bias circuit to tap into the HT winding when using a bridge rectifier for B+ (see the JCM-900 series amps). Hoffman's 100W Plexi circuit uses a PT with a separate dedicated bias winding.

Sluckey

The power transformer I had made for this project has the following secondary's :

173-145-0-145-173 @ 450 mA with M-6 steel
100 volt bias tap
6.3V CT @ 8A
6.3V CT @ 1A

Can't I run the B+ through the HT tap using a 1000 volt bridge rectifier as shown in the Hoffman plans and the 100 volt bias winding to the 47K resistor you have as the entrance into your bias circuit?

Thanks
Mike

[DummyLoad]

suggested fixes, please see attached.


--pete

[DummyLoad]

you may have some issues with that layout: please see attached image. the parts in the orange rectangle cover the parts in the red rectangle. there may be height issues and parts touching and even if they aren't, ease of servicing is surely compromised. suggest moving some assemblies up into the green rectangle. there are no indications that stand out where the transformers mount.


--pete

[DummyLoad]

Thanks Pete!   OK,  edited to reflect Pete's suggestions.

With respect, Tubenit

TN, this is probably moot. M_J posted a custom wind with a 100V bias tap - conventional fender type bias supply will work. i don't know why he specified 100V for bias as it's typically a 50V tap. perhaps he has plans to use 6550/KT88 in triode or UL mode?


--pete

[sluckey]

Can't I run the B+ through the HT tap using a 1000 volt bridge rectifier as shown in the Hoffman plans and the 100 volt bias winding to the 47K resistor you have as the entrance into your bias circuit?

That depends. If you have a DEDICATED 100V bias winding (two leads) then just wire it exactly like Hoffman's 100W Plexi. He has a note about his special PT.

But.... If your 100V tap is actually just a tap on the HT winding, you cannot use that circuit.

I believe Pete's circuit will not work either. You must use this circuit if you use the HT winding to feed the bias circuit and use a bridge. The cap and 56K resistor are necessary. This is the exact circuit, exact component values used by the JCM and many Ampeg amps. See attached...

[Mike_J]

Can't I run the B+ through the HT tap using a 1000 volt bridge rectifier as shown in the Hoffman plans and the 100 volt bias winding to the 47K resistor you have as the entrance into your bias circuit?

That depends. If you have a DEDICATED 100V bias winding (two leads) then just wire it exactly like Hoffman's 100W Plexi. He has a note about his special PT.

But.... If your 100V tap is actually just a tap on the HT winding, you cannot use that circuit.

I believe Pete's circuit will not work either. You must use this circuit if you use the HT winding to feed the bias circuit and use a bridge. The cap and 56K resistor are necessary. This is the exact circuit, exact component values used by the JCM and many Ampeg amps. See attached...

The PT I ordered has a 0 - 100 (two taps) windings.  I have attached the Marshall 1959 (Plexi 100 watt) schematic.  It shows why I wired my layout the way I did.

[Mike_J]

Jeff

The Marshall schematic for the 100 watt Plexi and 800 amps call for plate voltages of 205 and 220 volts respectively on the PI tube.  The voltages for the 50 watt versions are about 20 volts less.  The plate voltages required for the fx circuit are higher than that and each triode requires a different voltage.  So what I am saying is I think there should be a new branch starting at the screen node.  The fx plates should be fed a from an F&T 33uF/33uF @ 450 volt resistor and the OD circuit plates should be fed from a 20 uF @ 450 volt cap.  The suggested power resistor between the 33uF caps is 100K - 3W then a 10K - 3W to the 20uF cap.

Thanks
Mike

[Mike_J]

Thanks Pete!   OK,  edited to reflect Pete's suggestions.

With respect, Tubenit

TN, this is probably moot. M_J posted a custom wind with a 100V bias tap - conventional fender type bias supply will work. i don't know why he specified 100V for bias as it's typically a 50V tap. perhaps he has plans to use 6550/KT88 in triode or UL mode?


--pete

You are probably correct about what you are saying.  I considered using a 50 volt tap and wiring it like a fender AB763 bias supply but the specification sheets I have seen for 100 watt Marshall PTs show taps in the 100 volt range.  For example, if you go to the Magnetic Components, Inc. site and look at their specs for their 100 watt PT it shows 98 volts.  I have seen them as low as 90 volts but never as low as 50 volts for a 100 watt Plexi.

Thanks
Mike

[Mike_J]

I have finally figured out how to post the layout drawing for this project that can be read on the forum.  Again, I apologize for the lack of artistry contained therein.  I never could stay in the lines with my crayons when I was in kindergarten and not much has changed in the fifty plus years since then. 

The only other change I am considering, presuming no errors can be identified in the layout, is either the addition of a seventh preamp tube in order to split the tone stacks between the two amp choices.  Another option would be to stay at six preamp tubes and use tubenit's mosfet cathode follower idea. 

Does anyone know if there are any mini potentiometers that are any good.  Pots would need to be stacked in order to fit.  The normal size CTS pots wouldn't fit if I was going to stay with my usual 1-1/2" high chassis.  I guess I could have the chassis made taller.  Have to decide what to do soon because my metal man plans on making my chassis next week so if I want it deeper I will have to tell him during the next few days.

Thanks
Mike

[sluckey]

If you have a DEDICATED 100V bias winding (two leads) then just wire it exactly like Hoffman's 100W Plexi.

The PT I ordered has a 0 - 100 (two taps) windings.

You're good to go. That is a dedicated bias winding. We're saying the same thing.

[DummyLoad]

I believe Pete's circuit will not work either.     






sorry, i knew that - i just inserted the caps in the editor without thinking.


on the sweep tube amp i built i used a divider there with a rather large 220nF cap. the secondaries in that application were 230VAC CT though.


--pete

[Mike_J]

you may have some issues with that layout: please see attached image. the parts in the orange rectangle cover the parts in the red rectangle. there may be height issues and parts touching and even if they aren't, ease of servicing is surely compromised. suggest moving some assemblies up into the green rectangle. there are no indications that stand out where the transformers mount.


--pete

Your comments concerning spacing are valid.  I drill my own chassis.  I usually drill the holes for the tubes first.  I will check with some of my other amps before I start drilling to make sure there won't be a spacing problem.

The transformers are all stand up style and therefore the wires will enter through grommets.  I reposted the layout later in the post and hopefully you can see there were the transformer wires enter the chassis.

Thanks
Mike

[DummyLoad]

16MM pots:


click me!


--pete

[tubenit]

Does anyone know if there are any mini potentiometers that are any good.  Pots would need to be stacked in order to fit.  The normal size CTS pots wouldn't fit if I was going to stay with my usual 1-1/2" high chassis.  I guess I could have the chassis made taller.  Have to decide what to do soon because my metal man plans on making my chassis next week so if I want it deeper I will have to tell him during the next few days

I think a  1.5" tall chassis would be VERY problematic for me.  I'd want 2.5"-3" on a build like this with so many high gain aspects to it.

I use mini-pots on my builds with NO issues.

With respect, Tubenit

[sluckey]

I'm looking over your layout drawing and still chewing on the bias circuit. It appears that you are picking off the negative bias voltage from the wiper of the bias pot? The bias should come from the junction of that 15K, 47K, and 10µF cap.

I can find one of the bias wires from the PT, but where is the other one that should be connected to ground?

[tubenit]

Gentlemen,

I'll offer this latest edits ...........................    PLEASE feel free to use SCH and repost any edits.   OR ......... use "PAINT" to edit a GIF file.

Thanks, Tubenit

[Mike_J]

Does anyone know if there are any mini potentiometers that are any good.  Pots would need to be stacked in order to fit.  The normal size CTS pots wouldn't fit if I was going to stay with my usual 1-1/2" high chassis.  I guess I could have the chassis made taller.  Have to decide what to do soon because my metal man plans on making my chassis next week so if I want it deeper I will have to tell him during the next few days

I think a  1.5" tall chassis would be VERY problematic for me.  I'd want 2.5"-3" on a build like this with so many high gain aspects to it.

I use mini-pots on my builds with NO issues.

With respect, Tubenit

With a 3" deep chassis I could use 25mm pots.   The 25 mm pots are about 1-3/16" high which would provide about 5/8" for wire routing.  I looked at the list of Mouser 16mm pots (thanks DummyLoad) and didn't see any 1 meg pots which would be critical to accomplishing the reduced size.

I am going to put together a possible faceplate stacking the pots.  Would need to make sure that it is possible to eliminate cross talk that might be created by having so much packed in a small area.  The volume pots would have to be stacked above the tone pots.  However, I think it would be advantageous for both preamps to have their own tone stack.

Does anyone have any experience with this type of an issue that they could weigh in on?

Thanks
Mike

[Mike_J]

I'm looking over your layout drawing and still chewing on the bias circuit. It appears that you are picking off the negative bias voltage from the wiper of the bias pot? The bias should come from the junction of that 15K, 47K, and 10µF cap.

I can find one of the bias wires from the PT, but where is the other one that should be connected to ground?

sluckey

I modified the layout to reflect the proper Marshall method of wiring the bias (I hope).  I also included the second wire in the bias supply connected to ground.

Thanks
Mike

[Mike_J]

Gentlemen,

I'll offer this latest edits ...........................    PLEASE feel free to use SCH and repost any edits.   OR ......... use "PAINT" to edit a GIF file.

Thanks, Tubenit

Jeff

Thank you so much for your help with this.  I could not do it without your help.

Thanks
Mike

[Mike_J]

Here is a very rudimentary drawing of the Plexi/800 amp idea with separate tone stacks.  I drew it this way because it appeared aesthetically pleasing to me.  The biggest concern I would have is avoiding cross talk.  Does anyone have any opinions one way or another concerning whether they think this could be accomplished?  I would also add the OD and FX circuits as well as the LarMar PPIMV.

Thanks
Mike

[tubenit]

Mike,

I want to toss an idea out simply to consider.  It may not be anything that you are interested in at all?

AND ................ I think you should build your amp the way you want to &  definitely support whatever approach you choose.

With so many tubes in your design,  I think it can possibly complicate the build and also possibly add to some oscillation concerns?

As I am looking at what you're wanting to do by adding another tube and another set of tone stacks ........... I am thinking you may end up with a lot of choices that you will never use????    And again, that may not be the case, perhaps you would use all of them. 

LOok at reply #3 of this thread ......................  this is an offshoot of that.

With two relays, you can get pretty darn close to a Plexi or a JCM800 design out of just two tubes.  Study the S1 relay idea.  I think it will come close to capturing the two Plexi tones?

As I understand it,  currently your design has either JCM800 ............. OR ............ Plexi channels combined.

I think what I am suggesting would use less tubes and have more tones available.

with respect, Tubenit

[DummyLoad]

  I looked at the list of Mouser 16mm pots (thanks DummyLoad) and didn't see any 1 meg pots which would be critical to accomplishing the reduced size.   

there is. see attached image and they have stock.


--pete

[DummyLoad]

try these guys. they seem to be popular with the stomp box folk.


click me!


--pete

[jojokeo]

Here is a very rudimentary drawing of the Plexi/800 amp idea with separate tone stacks.  I drew it this way because it appeared aesthetically pleasing to me.  The biggest concern I would have is avoiding cross talk.  Does anyone have any opinions one way or another concerning whether they think this could be accomplished?  I would also add the OD and FX circuits as well as the LarMar PPIMV.

That's a very ambitious amp there (was there a schematic besides Tubenits?) and to fit all of it into a 1.5" high chassis kicks it up another few difficulty notches. Why if it's not absolutely necessary is my first thought? Personally I don't go less than 2" for even the smallest amp's I've designed. As long as you make it plenty wide and long you "might" be okay but if it were me, I wouldn't do it at 1.5" & I'm also wondering about heat & ventilation? Another challenge with a small size chassis is that doesn't help deal with possible parasitic oscillation issues with too much too close together but it's all in the parts placement, circuit layout, & grounding scheme for an amp like this one. Actually for most any amp to be "successful". With so much money, time, and effort going into a project like this one I hope you have a good understanding overall? It's much different than copying or cloning an amp. On your hand-drawn layout I have a couple things that jumped out to me: Your negative "D" supply node seems to be going to a .1uF cap which goes to your Pre P1 Vol. This will short out your signal if it's what I'm seeing correctly and not the place for it. It also appears your HT center tap is drawn connecting to between your series reservoir caps (looking at the red/yellow wire)?! Ground this and your reservoir caps together keeping the wire as short as possible and away from anything sensitive using thick wire also locating it to a nearby tranny bolt? Large currents flow here. Mind your OT wiring due to high current flow also. It appears that you're placing your iron close together? Do you know about using a "listening" device trick to keep evil noise away? All your best efforts will be for naught if this is screwed up. It gets worse too when the amp is being pushed more & more, it's not just at idle conditions. It's assumed that you know about heater wiring since it's not shown? The things I'm mentioning are the usual suspects but then there are many "special techniques" and opportunities for things to go wrong. Too many to list generically. Good luck Mike.

[Mike_J]

Mike,

I want to toss an idea out simply to consider.  It may not be anything that you are interested in at all?

AND ................ I think you should build your amp the way you want to &  definitely support whatever approach you choose.

With so many tubes in your design,  I think it can possibly complicate the build and also possibly add to some oscillation concerns?

As I am looking at what you're wanting to do by adding another tube and another set of tone stacks ........... I am thinking you may end up with a lot of choices that you will never use????    And again, that may not be the case, perhaps you would use all of them. 

LOok at reply #3 of this thread ......................  this is an offshoot of that.

With two relays, you can get pretty darn close to a Plexi or a JCM800 design out of just two tubes.  Study the S1 relay idea.  I think it will come close to capturing the two Plexi tones?

As I understand it,  currently your design has either JCM800 ............. OR ............ Plexi channels combined.

I think what I am suggesting would use less tubes and have more tones available.

with respect, Tubenit

Jeff

I would use an ABY box to switch between preamps.  That could give me either the plexi sound the 800 sound or both.  Without a separate tone stack I am concerned about the ability to get the sounds I want when switching between amps as I have doubts that the same tone settings would be ideal for both amps.

My Steel String Singer clone uses six 12a_7 tubes in about the same size chassis.  It is a very quiet amp, hum and hiss wise, with no oscillation problems although it is not a high gain amp.  The chassis for this amp will be 25 - 1/2" wide which should give me adequate space for the tubes.  I think I would be more concerned about problems that would arise from stacking the pots on top of each other with two different tubes being fed from the conglomeration of the potentiometers.  I think I probably will need to do a full sized mockup and see if I can run the wires in a way that would avoid crosstalk.  My steel man will make me a second chassis for only $40 so I will probably order one 2-1/2" deep (don't know why I was thinking 1-1/2" earlier) and another about 3 - 3/4" deep.  That way I could try it both ways and see which I like best.

I like the design in the schematics you posted.  It probably makes a lot of sense to simplify the amp but like Don Quixote off to slew another windmill I am charging headlong into this project with visions of great tone ahead.

Thanks
Mike

[Mike_J]

try these guys. they seem to be popular with the stomp box folk.


click me!


--pete

Thanks so much for the Small Bear referral.  I will be placing an order today.  Don't know how I missed the 1 meg pot on the Mouser website.  Must have been getting tired at about that time.

Thanks
Mike

[Mike_J]

Here is a very rudimentary drawing of the Plexi/800 amp idea with separate tone stacks.  I drew it this way because it appeared aesthetically pleasing to me.  The biggest concern I would have is avoiding cross talk.  Does anyone have any opinions one way or another concerning whether they think this could be accomplished?  I would also add the OD and FX circuits as well as the LarMar PPIMV.

That's a very ambitious amp there (was there a schematic besides Tubenits?) and to fit all of it into a 1.5" high chassis kicks it up another few difficulty notches. Why if it's not absolutely necessary is my first thought? Personally I don't go less than 2" for even the smallest amp's I've designed. As long as you make it plenty wide and long you "might" be okay but if it were me, I wouldn't do it at 1.5" & I'm also wondering about heat & ventilation? Another challenge with a small size chassis is that doesn't help deal with possible parasitic oscillation issues with too much too close together but it's all in the parts placement, circuit layout, & grounding scheme for an amp like this one. Actually for most any amp to be "successful". With so much money, time, and effort going into a project like this one I hope you have a good understanding overall? It's much different than copying or cloning an amp. On your hand-drawn layout I have a couple things that jumped out to me: Your negative "D" supply node seems to be going to a .1uF cap which goes to your Pre P1 Vol. This will short out your signal if it's what I'm seeing correctly and not the place for it. It also appears your HT center tap is drawn connecting to between your series reservoir caps (looking at the red/yellow wire)?! Ground this and your reservoir caps together keeping the wire as short as possible and away from anything sensitive using thick wire also locating it to a nearby tranny bolt? Large currents flow here. Mind your OT wiring due to high current flow also. It appears that you're placing your iron close together? Do you know about using a "listening" device trick to keep evil noise away? All your best efforts will be for naught if this is screwed up. It gets worse too when the amp is being pushed more & more, it's not just at idle conditions. It's assumed that you know about heater wiring since it's not shown? The things I'm mentioning are the usual suspects but then there are many "special techniques" and opportunities for things to go wrong. Too many to list generically. Good luck Mike.

Jojokeo

I have no idea why I wrote 1 - 1/2".  I have had all my chassis made to a 2 - 3/8" or 2 - 1/2" depth.  This chassis will be 8 - 1/2" wide and 25 - 1/2" long.  The cabinet will be 30" wide to match the width of the speaker cabinet I will be using for it.  The top of the cabinet will have the small Marshall grill for ventilation and the tubes will be facing up.  I also had the PT made a little more beefy than is probably needed to reduce heat buildup.

Thank you for catching the D node mistake.  I moved it to its proper place.

The HT center tap is wired the way it is because it is designed as a voltage doubler.  This is consistent with the Marshall 1959 schematic.  I had my transformer HT wound at 173 - 145 - 0 - 145 - 173 volts.  Must have the voltage doubler in my circuit.

I have to admit that I am a little concerned that the overdrive channel may push the 800 over the edge but I won't know for sure until I try it.  I am more concerned with the impact of stacking pots if I decide to add a second tone stack tube.  I have never done that kind of wiring and I can see were I could get myself in trouble there.

Pretty confident in the transformer spacing.  Will do the same thing I did on my two Overdrive Special builds.  Shouldn't be a problem.

I don't know anything about the listening device trick but would sure like to.  I have a feeling I will be pushing the envelope with this build and will need all the help I can get.

Thanks
Mike 

[tubenit]

Mike,

After reading your response about using  A/B/Y switching ............  I think you're on the right track.  And the comment about possibly needing different tone stacks is valid, IMO.

I'd say move ahead with your plan and simply use a 2.5" chassis & you should be fine. 

I don't know about the JCM800 preamp into the OD, but I bet there are some "workarounds" to make it feasible. Since the D'Mars ODS has similar topology as the Plexi into the OD,  I am sure that will work out just fine for you.

Looking forward to seeing and hearing your success with the build!   With respect, Tubenit

[Mike_J]

Mike,

After reading your response about using  A/B/Y switching ............  I think you're on the right track.  And the comment about possibly needing different tone stacks is valid, IMO.

I'd say move ahead with your plan and simply use a 2.5" chassis & you should be fine. 

I don't know about the JCM800 preamp into the OD, but I bet there are some "workarounds" to make it feasible. Since the D'Mars ODS has similar topology as the Plexi into the OD,  I am sure that will work out just fine for you.

Looking forward to seeing and hearing your success with the build!   With respect, Tubenit

Jeff

If you were building the amp would you add the extra preamp tube for the added tone stack or use two of your mosfet cathode follower circuits instead?  I am going to need two triodes so an extra tube makes sense to me and I am pretty sure I can fit it in without much trouble.  I read somewhere they used to place 12ax7s about 1/2" apart without problems.  I think it was in car radios or some other type of radio.  I am pretty sure they were very careful in isolating the grid wires from the plate or heater wires.

Thanks
Mike

[sluckey]

I would use an ABY box to switch between preamps.  That could give me either the plexi sound the 800 sound or both.

Switching A or B will be fine, but switching to Both will likely be disappointing. The 2204 preamp has one more inverting stage than the 1987 preamp. This out of phase signal will cause a lot of cancellation when the two channels are mixed together. This cancellation usually results in a weak, thin sound.

[Mike_J]

I would use an ABY box to switch between preamps.  That could give me either the plexi sound the 800 sound or both.

Switching A or B will be fine, but switching to Both will likely be disappointing. The 2204 preamp has one more inverting stage than the 1987 preamp. This out of phase signal will cause a lot of cancellation when the two channels are mixed together. This cancellation usually results in a weak, thin sound.

sluckey

Thank you for the information.  The main advantage I could see out of having the two preamps is I could set the Plexi side for more of a clean signal and the 800 for an overdrive signal and have a pretty complete rig in one amp.

I did a layout mockup of the preamp section for the two tone stack preamp using mini pots and 3/4" knobs and stacking them to provide the space needed to include it,  It would require a 3" deep chassis and would use less space horizontally along the faceplate than my existing layout has. 

It gives me space for more knobs for the potentiometer for varying the slope resistor and another one to vary the feedback resistor.  Do you have any opinions regarding these mods?  I would leave space on either the faceplate or back plate for them.  I think adding anything more to what I have already done without making sure it works is asking for trouble.  But planning ahead is never a bad idea.

Thanks
Mike

[sluckey]

It gives me space for more knobs for the potentiometer for varying the slope resistor and another one to vary the feedback resistor.  Do you have any opinions regarding these mods?  I would leave space on either the faceplate or back plate for them.  I think adding anything more to what I have already done without making sure it works is asking for trouble.  But planning ahead is never a bad idea.

You're gonna need a bigger boat! 

I've never done either of those mods, but I understand the idea behind both of them. I don't have any opinions about them. These days I'm liking simple stuff. I'd have to hire a knob twister to set the amp for each tune. 

[tubenit]

I've tried a pot for the NFB and for the slope resistor.  After a couple of minutes, I removed them.  Just a personal preference.

I have built amps with too many bells and whistles and was somewhat disappointed and began to whittle stuff down to things I know
I will actually use.

IF you can fit the tubes in, maybe go with those?  And the mosfet CF's would work also.  Honestly, this is simply a decision that you will need to make. Either is fine. 

It may be useful to you to look in ARCHIVES at the thread on tube spacing: http://el34world.com/Forum/index.php?topic=17406.

With respect, Tubenit

[jojokeo]

I have built amps with too many bells and whistles and was somewhat disappointed and began to whittle stuff down to things I know I will actually use.

+1  I've found during gigs/shows I rarely use the extra gain stage channels (power tubes driving is where it's at) and it's mostly used while practicing or fooling around at home under volume restriction conditions.

[Mike_J]

I've tried a pot for the NFB and for the slope resistor.  After a couple of minutes, I removed them.  Just a personal preference.

I have built amps with too many bells and whistles and was somewhat disappointed and began to whittle stuff down to things I know
I will actually use.

IF you can fit the tubes in, maybe go with those?  And the mosfet CF's would work also.  Honestly, this is simply a decision that you will need to make. Either is fine. 

It may be useful to you to look in ARCHIVES at the thread on tube spacing: http://el34world.com/Forum/index.php?topic=17406.

With respect, Tubenit

Jeff

I have decided to go with the extra tube.  The tube spacing I will use will be at least 1-1/8" between preamp tubes and 2 - 1/2" center to center for the EL34s.  I am not going to add any more to this than just the extra tone stack.

I want to adjust the layout by removing the preamp and tone stack electrolytics from the circuit board and install cap cans.  I would like to install the cans as close as possible to preamp circuitry as there is a lot of space there and the wiring would be shorter.  Do you know what the rules are concerning the use of cap cans near preamp wires?

Thanks
Mike

edit... fixed quote... sluckey

[sluckey]

I would like to install the cans as close as possible to preamp circuitry as there is a lot of space there and the wiring would be shorter.

That's a good plan.

Do you know what the rules are concerning the use of cap cans near preamp wires?

I would just follow a typical Marshall layout. Here's an example...

[Mike_J]

I have built amps with too many bells and whistles and was somewhat disappointed and began to whittle stuff down to things I know I will actually use.

+1  I've found during gigs/shows I rarely use the extra gain stage channels (power tubes driving is where it's at) and it's mostly used while practicing or fooling around at home under volume restriction conditions.

Jojokeo

Thank you for the response.

Unfortunately, health problems make it unlikely that I will be playing a lot of gigs.  Playing out will probably be limited to playing in the church band when I am feeling up to it.  So the most use it will get is at home.

Also, thanks for the Marshall picture.  It looks like they placed one of the cap cans right in the middle of the preamp section.

Thanks
Mike

[Mike_J]

I would like to install the cans as close as possible to preamp circuitry as there is a lot of space there and the wiring would be shorter.

That's a good plan.

Do you know what the rules are concerning the use of cap cans near preamp wires?

I would just follow a typical Marshall layout. Here's an example...

Sluckey

It looks like the cap can is right under the circuit board where the tone stack wiring is placed.  Do you think that placement would inject noise into the preamp circuitry and if so, do you think that a metal cover over the end of the cap can would reduce the interference?

Thanks
Mike

[sluckey]

It looks like the cap can is right under the circuit board where the tone stack wiring is placed.  Do you think that placement would inject noise into the preamp circuitry...

no

Just out of curiosity, why do you want to take the filter caps off the board?

[Willabe]

Jazbo8, did you build that?

Great work!


            Brad     

[shortfuse]

Mike
Very interesting concept.  I always wondered about two complete amps (800 and Plexi) in one chassis where you could use either or both at the same time like my old Sunn Beta Lead but that was a SS amp.  I think LC built an amp with 2 complete amps in one chassis switchable to one the other or both at the same time.  I remember getting him the switch for it a few years back.  Maybe he will see this and comment but I have not seen him post here much anymore.
I am still working on my Dual 50 I got side tracked a month.  I got all my parts and will build my dalator in the next week or so to test out and make my final decisions on how I will proceed.  I just installed a PPIMV on a 100w plexi and was not overly impressed so I am hoping the dalator will do the trick for the global volume control and keep the marshall style MV as Sluckey has in his layout.
Anyway I am going to keep tabs on this progression I really like your ideas.  I am not seasoned enough to make a lot of comments but one thing I will say is everyone is telling you in a very nonchalant way trying to be polite, to use the larger chassis.  I went against the grain and recommendations of some very knowledgeable folks on this board on 2 builds in the past and created lots of headaches for myself.  One I had to dismantle and put into a larger chassis to correct the ability of being able to tune in Tokyo from Florida.  With as much as you want going on and the costs you are going to incur use a normal size large Marshall box 29"L, put yourself a 27"L X 2.5 to 3" H chassis in it you will be much happier with the results and it will be a much more enjoyable build form a construction stand point.  Just my $.02  Good luck and post lots of pics.