Multiple circuit preamp/amp with relay switching

[guitardude57]

I am looking over some schemos, to combine a Dumble 124 circuit with a Plexi 86/87 circuit
in a single amp.  

So, there will be #124 with OD switching, and PAB switching, and a Plexi with a gain boost switchable, with an extra gain stage.  A switch could swap the split cathode, with the shared easy enough for 86/87 difference.

With a 4th relay, I would switch between the 2 preamps themselves.

I will probably go with a Marshall style PI arrangement, (it is close to the Bluesmaster).

There will be a tube FX loop.  I have plenty of room for the numerous novels required to do this idea.

The difference between the Dumble NFB and the Marshall NFB for the presence control, are night and day different.
That will take some work to keep the tones of the different preamps close to authentic tone.

Question:

Where is the best place to relay switch between the Dumble and Marshall circuits?  I am thinking at the FX loop entry,
and shunt the unused preamp to ground.  Sound correct?

Upping the gain switching for the Plexi close to 2204 territory and beyond shouldn't be too tough.

Each preamp will have a level control, right before the relay into FX loop, to have control over levels balance.

The amp will be a quad EL34 output, using Dumble's switch and LED to cut out 2 power tubes for reduced output
when wanted.

If this works out well, I will copy this idea into a 2 space preamp only for my rack set up.

Not trying to reinvent the wheel here... just like the qualities of these amps, and the potential flexibility.
Nice clean, and Dumble overdrive, The Plexi with a little hair for the Duane tone, and the sing-like grind with that upped, in gain.

It will be a fun board to layout.  If I can get a grip on the spaces on Doug's board layout, would not be a huge board.
And now that he has those beautiful relay boards... I'm in business.

Whatcha think?

[tubenit]

I think that would be a very fun build and do-able.  Based on how you inquired about it, my guess is you're probably knowledgeable enough to pull it off just fine.  You're right about there are some areas from the LTPI into the power amp where you'll have to make some decisions or compromises, but I think you can get a great amp out of the idea.

The idea behind the Marshall and a Dumble type circuit together is what inspired me to do the D'Mars OD Special.

With respect, Tubenit

[tubenit]

In case you don't know already .......................

There is a SCH schematic for the #124 here:
http://ampgarage.com/forum/viewtopic.php?t=17449&start=0

There is a SCH Plexi 50 schematic here:
http://www.el34world.com/Forum/index.php?topic=581.0

There is a SCH FX loop schematic here (reply #10):
http://www.el34world.com/Forum/index.php?topic=10208.0

And an SCH example of how to switch between the Marshall and Dumble channels here & an example of a another gain stage for the Marshall/hot switch (reply #3):
http://www.el34world.com/Forum/index.php?topic=571.0

And you can save entire section in SCH format as a group component.  Example would be you can save the entire Marshall preamp or the Dumble OD section or the FX section etc......

And there is a SCH Marshall Plexi 50 type amp here with a Dumblish OD and an FX  (reply #17)
http://www.el34world.com/Forum/index.php?topic=14766.0

With respect, Tubenit

[guitardude57]

Thanks my Brother!

[tubenit]

OK,  I grabbed a schematic and edited it over a mug of coffee this am.

I personally would not have a master volume on the Marshallish preamp.  The active FX works great in that regard!

I have found that I did not use the 250k return pot on my D-lator.  When I built one into my Tweed BluezMeister, I used an 82k resistor to ground and it works just fine for me.

I am not sure which approach for switching between the Marshallish and Dumblish preamps would be best? So I drew up both of them for you to ponder.

I am aware that you may use a Marshall LTPI and power amp.  Or the Dumblish LTPI or power amp .............. or some type of hybrid of those two?     So there is an editable SCH schematic attached for you to edit to whatever you wish.

When you settle on an idea,  I hope that you will post it here.

With respect, Tubenit

[tubenit]

Here is the same amp with EL34's. 

The LTPI and NFB is Marshallish.

The power amp came from a D- #183 schematic. The bias is from the D-183 also.

With respect, Tubenit

[guitardude57]

Tubenit,
You get more done with your morning coffee, than most folks!
Nice job!  I like it.  Now if I could wrap my head around the component spacing
on Doug's turret board program, a layout is in order.  Not having done anything with
Doug's layout style, (they do make sense) I look at the holes that you can mark for the drilling
and get confused by distance and spacing.

I do have some material to drill an eyelet board, but I think the turret set up
would be more appropriate.

The version with the guitar input direct to relay 1 looks a little better for more accurate tone, circuit to circuit.

The only reason I would place a pot for level at the end of the Marshall preamp before the relay,
would be for level matching with the Dumble preamp.

What does the hotswitch mod give as far as gain?  Looks interesting.

Thanks for your time on this.

Mike

[tubenit]

1)  I think the master volume on the Marshallish channel is reasonable

2)  The hotswitch thing is Hoffman's idea.  I prefer a gain pot instead of resistor to ground.  I'd try a 250k or 500k pot there. It gives
      a substantial amount of gain, IMO.

3)  I agree with you that the relay switching after the input jack and prior to the first triode of each channel would allow one to do a
     better job of attempting to preserve the original tones.

4)  I might be inclined to experiment with a hybrid LTPI using values halfway between the Marshall and 124. Perhaps that might preserve
     a little more of each tone?

5)  A hoffman style layout board would be a good choice for this amp design.  It would be compact and I think quiet. They make sense.
     Having said that,  I think you could get away with a paralleled turret style also like Vox or Hiwatt.

Something to look at to help visualize how to draw the layout board up would be the D'Mars OD Special hoffman style layout here:
http://www.el34world.com/Forum/index.php?topic=14302.0

And if you note .......... reply #3 on that same thread has a Hoffman style layout for the D'Lator.

Here is a Hoffman style layout board for the Plexi 50:
http://www.el34world.com/Hoffman/images/Plexi50Old.gif

And how to add the "hotswitch" to a Hoffman Plexi board:
http://www.el34world.com/projects/hotswitch.htm

I love seeing innovation taking place on this forum.  THANKS for sharing your idea here!

With respect, Tubenit

[Geezer]

I love seeing innovation taking place on this forum

.

Me too....

I may be tempted to try my hand at one of these!

G

[tubenit]

OK,  I found my generic "Hoffman style turret board" blank.

Remember, this is editable & is in the SCH format.  This is what I use to draw up my first draft on a Hoffman style board.

With respect, Tubenit

[guitardude57]

I laid out the turrets with Doug's board program for the amp and the rack preamp.
Somewhat time consuming, but not too horrible.
I am ordering the amp board first, to make sure it is good.
I'm excited!

[tubenit]

Uh oh!!

I put the gain pot for the hotswitch on the plate side by mistake. That would've been redundant anyway because it would be in series with the volume pot.

It should be in front of the grid for that triode.

My apologies.  Geezer noted this (sharp man that he is)  

With respect, Tubenit

[guitardude57]

I also noticed something with the PAB switch too.
I have seen them lift the mid or the bass.  It is on the bass wiper to nowhere...

[tubenit]

Good catch.

Also corrected a couple of things on the SCH schematic.

With respect, Tubenit

[Geezer]

Here's my version of this Dual preamp....Marshall JCM800 / Dumble 124 preamps, with the option of running them in parallel, if so desired.

I went with the JCM800 (cascaded V1 version,  here>>> http://www.el34world.com/charts/Schematics/files/marshall/jcm800_lead_mstvol_50w_2204.pdf ) for the Marshall section.
The "HotSwitch" adds the "cold biased" triode, along with another triode in order to keep phasing correct when in parallel mode. The V5b triode will need to be tweaked after the circuit is up & running in order to get the best tone, trying to acheive lowest addition of gain and stay close to the original JCM800 gain structure and tone.

On the Dumble side, I added a "clean level" to better match volume output of the Dumble clean and OD sections.

I hope to start building this this weekend.

G

[guitardude57]

Looks good Geezer.  I am more of a plexi kind of guy, with a hotswitch approaching almost 800 territory.
It would be interesting to hear both preamps together.  I would expect to dump some gain somewhere
when latched together, to prevent a swirling smear of distortion.  But could be a cool effect nonetheless.

I worked up a Hoffman board, but will have to break up the preamp board and power amp section into
2 pieces... darn thing is too long, like 18 inches!  This would be the 100 watt jobby in one headbox.
The preamp only design, is somewhat shorter.

Might be a good idea to do each preamp separate from
each other to keep things smaller, and to control ground issues.  A 2 rack space, 10" deep chassis would allow
2 preamp boards and the power supply board, as well as space for all the controls to not be jammed together.
A toroid tranny could be had, to provide HT, Heaters, and relay power and not take up much space.
The tubes would be easy with an aluminum angle bracket drilled to mount the tube sockets... either laydown style
or upright.

I'm just looking for some extra funds to get mine going...

[tubenit]

Hey Geezer,

VERY much looking forward to hearing about your build!!!

NOTE a correction in how the PAB is wired in my reply # 13.  Your schematic shows my previous incorrect wiring.  

You are the innovation "go to" guy around here .............................   cool stuff.

Is this amp going to be built into the Peavy?  Just curious.

With respect, Tubenit

[Geezer]

Well, I decided to not do the parallel thing....doubt it would be usefull, & there will already be plenty of different tones available anyway.

Decided to go with (mostly) Dumble 183 topology...."High-Plate Skyliner"...for the Dumble side

I re-examined the original Dumble PAB, and did my own version without the Rock/Jazz switch. (Thx T!)

I will have the MidBoost and PAB capable of being switched together with relays, or be able to turn each one on of off from the front panel, so the BOOST footswitch can either switch in PAB, MidBoost, Or BOTH.

I made the LNFB switchable on V1B of the Dumble side, and also made it possible to switch the OD TONE control completely out of circuit if I want.

I kept the extra gain stage on the Marshall side HOT Switch......I will tweak it on the fly once the circuit is built, to get a nice, crunchy OD goin'

Updated schem attached

[DummyLoad]

In case you don't know already .......................

There is a SCH schematic for the #124 here:
http://ampgarage.com/forum/viewtopic.php?t=17449&start=0

There is a SCH Plexi 50 schematic here:
http://www.el34world.com/Forum/index.php?topic=581.0

There is a SCH FX loop schematic here (reply #10):
http://www.el34world.com/Forum/index.php?topic=10208.0

And an SCH example of how to switch between the Marshall and Dumble channels here & an example of a another gain stage for the Marshall/hot switch (reply #3):
http://www.el34world.com/Forum/index.php?topic=571.0

And you can save entire section in SCH format as a group component.  Example would be you can save the entire Marshall preamp or the Dumble OD section or the FX section etc......

And there is a SCH Marshall Plexi 50 type amp here with a Dumblish OD and an FX  (reply #17)
http://www.el34world.com/Forum/index.php?topic=14766.0

With respect, Tubenit

looks like the ampgarage is serving up malware.

--DL

[guitardude57]

Hey Geezer... how is your build going?
I am chasing parts myself.

[Geezer]

Hey Geezer... how is your build going?
I am chasing parts myself.

Well, in the interim I have built a Preamp using the 183 Dumble topology. Been tweaking it and have some GREAT tones! I'll blueprint it and update the schematic with what I ended up with....this is what I'll use on the dual preamp build for the Dumble side.

For the Marshally side, I actually found an amp that uses almost the exact topology that I was going for....the Guytron GT100. I have the schematic and am going switch out the 2nd gain stage of the "B" channel as the "hot switch".   I think  i'll be able to get a fairly wide range of tones out of this setup.

Guytron GT100 and updated schematic attached

[tubenit]

That's impressive!  Very cool.

Thanks for sharing it!

With respect, Tubenit