D'Mars OD Special (Hoffman)

[tubenit]

This schematic and layout are designed for the Hoffman board using Doug's part.  This is a NICE amp & I am enjoying the tone I'm getting quite ALOT.

I'd use Doug's 022848 OT (35w) or 018343 OT  (40w).    You can use his Bassman PT 022798  with a 5V4 rectifier for about 420V on the 6L6/5881 plates.  (You can then also switch out to a 5Y3GT if you want to use 6V6's.)

I'd simply increase the B+ dropping resistor to maybe 10k - 22k  between nodes B & C. You'll have to experiment. Main thing is to simply match up the preamp, OD and LTPI voltages.  (note voltages posted in previous post)

I honestly don't think having higher volts on the power tubes will change the tone significantly.  The tone is far more in the preamp and OD sections on these D-inspired amps, IMO.

Where you see values of .00125 just parallel a .001 & 250p.   1.68uf just use a 1uf & .68uf.  You can use a 100p & 120p for 220p or just use a 250p.  You get the idea here, hopefully

Bill of Materials attached. NOTE:  the BOM does not reflect Geezer's dual OD control pots  & it does not reflect any relay switching parts at all.  So if you want those, you will need to add that to the list.

Edit EL34:
You can use these two documents for my turret boards for assembly tips
https://el34world.com/Hoffman/instructions.htm

https://el34world.com/Hoffman/files/CommonHookups.pdf

[tubenit]

EL34 designed the mosfet drain wire to power rail to run under the amp. On the 1uf from NFB loop tail,  it will go at an angle to the buss ground.

EDIT 8/25/12:  This populated board is incomplete and needs a mosfet, different B+ resistors, .033 post LTPI coupling caps and           220R/1w resistor going into the mosfet.  However, it does show how well the components fit.  About a 2.5 hr wiring for me.  Doug did a great job laying out this board for component spacing. It is an easy board to wire up.

Layout turret board is 10" x  3 & 1/8"

NOTE:  The V2-1 resistor is 180k like on the SCH schematic & layout.  The cap guide layout shows 100k where it should be 180k

[EL34]

I added several parts to stock this week that are used on this project

I have a post here with the list of new parts I stock
http://www.el34world.com/Forum/index.php?topic=14415.new#new

[EL34]

Ok, Looks like Jeff has given the thumbs up to my board layout
I added the Pre-Drilled D'mars board to my board building page on my web store

You can add all the D'Mars parts I stock to your shopping cart at once
Look at the Public parts list page on my shopping cart for the D'Mars project

I still am waiting for a few backordered parts like 680p caps
I will add those parts to the public part list as soon as they are in stock

Here's a link to my board building web store page
http://www.hoffmanamps.com/MyStore/catalog/parts12.htm

Jeff,
PM me with the details on that 12v diode next to the mosfet
I need to stock those and I forgot what we talked about in a previous PM

[EL34]

The last few DMars parts that I did not stock finally came in
I created a public parts list for all the DMars parts that I stock
You can see that parts list on this page
http://www.hoffmanamps.com/MyStore/catalog/PartsListIndex.htm

I now stock
680P ceramic caps
12 volt zener diodes - Note to Jeff - this diode is missing from your BOM

The only parts I don't have is a chassis and 5879 tubes
My supplier does not stock that tube

A couple notes on the BOM 3 watt resistors
1.5K 3 watt - Use a 1K and a 470 Ohm 3 watt to make 1.47K
3.3K 3 watt - Use a 1K and a 2.2K 3 watt to make 3.2K

I made these 3 watt resistor changes on the Dmars parts list on the public parts list page

[Attila]

Ok Gents

So the time has come for me to take on the mighty D'Mars ODS build. I like Geesers approach of  having two channel controls sets.

Problem is I cant tell from Tubenits file naming convention what the final version of this build is on this page..... please help me out here

One crazy question is there any sense in adding a reverb circuit for clean playing, or could one dial that in on the FX loop stage  ?

Peace

Attila       

[tubenit]

No, mods have been made since the 12/'12 version.

Here is my current schematic.  

I love this amp!  IF I had to do it over, I would have added an active FX loop.  I personally would probably skip the tube reverb and just put reverb in the FX loop.

With respect, Tubenit

EDIT   10-13  Put the negative feedback on a switch.  I decided I like the amp better without negative feedback. Sounds somewhat fuller to me in tone. But in some settings, the NFB would be quite useful IMO.

[Attila]

thank you so much

[Glennjeff]

OK, time to take the plunge. Just put in an order with Hoffman Amps for the rest of the parts I need.

Tubenit, see that you are still working on this 5879 2 channel topology that started with the Humble TOS.  I've always liked "The Look" of it on paper, soon I'll get a chance to hear it.

Notice that a lot of the development effort has gone into the tone stack - RC based BMT's are mathematically a bit ordinary, hence the age old dilemma with tone stacks.
(Tone Stack Calculator here http://www.duncanamps.com/tsc/)

Once I get mine going I'll play around with RC based Bass / Treble with a couple of bands of LCR midrange, bolt some more copper and iron onto the chassis. Going to run DC +/- 6V regulated heaters in the preamp section also. Also will be using 4 EL84 18/36 power amp section plus the active effects loop, the more valves the better.

[tubenit]

Tubenit, see that you are still working on this 5879 2 channel topology that started with the Humble TOS

I did NOT use the "pseudo dual channel OD"   And I am OK and happy with just clean to OD with boost switches.

EL84's break up MUCH easier than 5881's.

In light of that,  I have listed some suggestions.  

1)  First thing I'd do is change everything that I am showing from the LTPI thru power tubes.  Especially the coupling cap into the LTPI
     needs to be .01 or even .0047.  The coupling caps after the LTPI prior to the power tubes should probably be lowered to .01 or .02?

2)  IF you are still getting too much gain in the OD mode, then I'd change the OD values that I am showing.

3)  IF you are still getting too much gain in the clean mode,  first thing I'd do is change the coupling cap into the FX loop

4)  IF after all that,  you're still getting too much gain change the volume pot from 500k to 250k and the slope resistor to 68k

I will be watching this build with great interest.  I hope you'll post ongoing pics and info on the Tweaks section.  I am going to get a thread called D'Mars ODS EL84 started and you can have that be your thread and add to it.    This would be a great project to get additional info from Geezer and others on the forum so you'll be pleased with the result.

With respect, Tubenit

EDIT:  I am presuming that you will be building your own board?   IF so, here is an example of how to incorporate an onboard FX using a Hoffman style layout board.
http://www.el34world.com/Forum/index.php?topic=15406.0

[moonbird]

Tubenit -


How feasable would it be to adapt this design to use 6AQ5 output tubes? Would the "tips" be the same as for the EL-84? Could you diagram the power supply as well? What PT B+ voltages should I be looking for? FYI - I am using the Hoffman board.  Thanks much!!

[tubenit]

I would not use the EL84 "tips".  I would build it like the original with 6AQ5's. 

You'll want no more than 305v on the 6AQ5's  (I think).   So, in order to do that and have the other preamp, OD, FX and LTPI voltages correct would be a true challenge.   

Only answer I can think of would be to use Dana Hall's VVR and VVR the power tubes and LTPI section.  You'd probably want to have the VVR pot "set" so it did not turn the voltages too high.

You can look at ARCHIVES for VVR information.   You'll have to figure it out yourself just like I would have to do for you.

Respectfully, Tubenit

[moonbird]

Tubenit --


Thanks kindly for the guidance. Sounds like the 6AQ5 might be tough. Seem to remember the Geezer building a DMars preamp to get around this problem with 6AQ5s, but I'll be darned it I can even find it now.


Probably best to just stick with 5881. I noticed that your version of the DMars does not have a Master Vol while the Hoffman version does. Do you think a VVR would work better than a MV? Thanks again for the responses.

[tubenit]

I would go with a PPIMV or an active FX like a D'Lator instead of VVR if using 5881's.


VVR would be a better choice for 6AQ5

[moonbird]

Tubenit -


Great thanks - never thought of an active FX acting that way but makes perfect sense. Now doesn't your version of the BluesMeister have a D'lator FX? Can it be added to a Hoffman board-based D'Mars amp? I already have the board done. Do you just P2P the D'lator? Is there a layout around for that? Thanks loads!!

[tubenit]

I think you'd need to build an external  D'Lator if you're using a Hoffman D'Mars board with 5879 OD section.

[moonbird]

Figgered - thanks!

[jazbo8]

Perhaps it already been covered before... but are the layout files (.sch) drawn to scale? i.e., can they be used to make pre-drilled boards? Or do you still need to use Doug's turret board program to generate the drill patterns?

[tubenit]

I'd use Doug's program.

This was what I previously used.

http://el34world.com/Forum/index.php?topic=14802.0

[jazbo8]

That's what I thought...


Thanks,
Jaz

[toneseeker]

Was this thread ever started. I can't find it. Is there another thread that tracks the D'Mars build?

I will be watching this build with great interest.  I hope you'll post ongoing pics and info on the Tweaks section.  I am going to get a thread called D'Mars ODS EL84 started and you can have that be your thread and add to it.    This would be a great project to get additional info from Geezer and others on the forum so you'll be pleased with the result.

[tubenit]

I don't think it ever became a thread?  Reply #9 on this thread does have a GIF schematic with suggested mods to make the idea possibly work.

With respect, Tubenit

[MakerDP]

Hey Tubenit... couple questions for ya...

1) Has anyone to your knowledge figured out an EL84 or 6V6 version of this amp?

2) Why do you think this is true:

I would go with a PPIMV or an active FX like a D'Lator instead of VVR if using 5881's.


VVR would be a better choice for 6AQ5

3) Do you think there would even be a reason to do an EL84 version if I was to do a VVR or PPIMV to begin with anyways?

Thanks!

[tubenit]

1) Yes I have done two 6V6 versions.   One just used a FX level as master volume.  The other also had PPIMV in addition to FX level.

2)  I think a FX or PPIMV sounds "better" at lower volumes then the VVR.  I think the closer you come to 30 watts the more I would lean
     towards the reliability of FX or PPIMV as a master volume.

3)  EL84's in push pull can be loud, so I am not sure what your question is.  I would do a FX or PPIMV or VVR with any of the power tubes.
     Pick what you think will work best for you and do that. 

with respect, Tubenit

[MakerDP]

OK thanks for that.

Silly me, I only just now noticed that the schematic in post #1 has the PPIMV in it...

Which versions you like better... 5881/6L6 or the 6V6?

On the 5881 version as-is in the first post, would I use a 4k-primary OT?

[tubenit]

I like them all and prefer Tung-Sol Re-issue 5881's.   I think 4k would be fine.

[MakerDP]

Thanks again...


one more for now... what PT & OT did you end up using with your 5881 version?

I'm thinking Edcor XPWR163 for PT and have no idea really for the OT...

[tubenit]

I think any good PT with 275-0-275  or 300-0-300 with enough ma handling to work would be fine. That way you can use 6V6 or 5881/6L6.

As far as the OT,  I would not use the one I have again.  I would probably use Doug's Tweed Bassman OT.

With respect, Tubenit

[MakerDP]

I have a Peavey Classic 30 that I am thinking of gutting and re-purposing. The PT will put out about 335V to the power tube plates. Do you think I'd be happy with this voltage in a 6V6 build of this amp or should I do a Stout or something similar? I'd most likely get a better OT than the Peavey's.

[tubenit]

Yeah, I think 335v to a 6v6 would be just fine.  You may need to change the resistor values on the B+ rail to a somewhat lower value to
try and maintain a 150v-180v range on V1.

With respect, Tubenit

[EL34]

I have the D'mars turret boards already made and ready to ship on this page 

http://hoffmanamps.com/MyStore/catalog/TurretBoard.htm

[tubenit]

Here is a mod you can do to the tone stack to get a thicker OD tone while keeping the clean channel the same.  I have done this to my D'Mars.

I am using the mod on the top left.

With respect, Tubenit

[dwietzel]

I see that Hoffman still has the circuit boards and the BOM for the build.

But cepedals is out of stock on the chassis. Is there any other chassis options out there for those who do not have the tools (or talent) to punch their own?

[dwietzel]

Bump...Any chassis options for this build?

[Willabe]

In Doug's on line store are at least 3 chassis that might work. Hoffman stout v.2, Princeton Reverb and 18w Marshall. 

http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Chassis&ORDER_ID=802775054

[J-Lew]

Hi all,  I’m preparing for this build and can’t open the schematic files above.  Everything I try claims they are corrupted in some way,  this includes some legitimate CAD programs I use for my work, including Autodesk and PSpice. Can someone recommend a viewer or program?  Otherwise, is there any chance the schematic and voltages be post here as a PDF or image?

Any help is appreciated!

[EL34]

Not sure what program you are trying to open the .sch files with?
I am able to open all 3 of them with Jschem
https://el34world.com/Forum/index.php?topic=590.0

[sluckey]

The ".sch" files were created with a free program called ExpressSCH. Google it. Download it.

[EL34]

All that info is in the link I posted above
Jschem is a stand alone program that you do not have to install if you do not want to install new software