D'Mars Overdrive Special

[Geezer]

I started work on my version of the DuMars today. It's going to be the preamp (only) built into a Valve Jr headcab & chassis.

I always start with the faceplate, so I can use it as a drill guide & be sure everything I want is going fit into the available space.

Here's a pic of the completed faceplate.



The pots will have to be 16mm and the knobs will be 1/2" in order to fit everything into the VJr chassis.

I will be using a 3-button footswitch (w/LED's) to control Clean/OD1, OD2, MidBoost

OD1 & 2 are just parallel controls/presets for the OverDrive, switched by relays.

There will also be a "Clean Level" control (a Master Volume that is directly BEFORE the OD relay, and is bypassed when OD is selected....only active when the Clean channel is active)

Everything else will be EXACTLY as Tubenit has in his amp. (including any mods that may arise as I slowly progress thru my build)

G

[proaudioguy]

. add an active FX loop (which also works as another OD boost at some level)

Where did the name come from?

Will your FX loop work with a 12AT7?  Will it drop right in to an AB763?

[tubenit]

OD1 & 2 are just parallel controls/presets for the OverDrive, switched by relays.

I think that is a GREAT idea!  Especially, since the OD has both such a sweet clean OD and the vowel/blooming tone also.  I think you will like that feature ALOT.

Where did the name come from?

D'Mars OD Special      (Dumblish OD Special & Marshallish topologies)

Will your FX loop work with a 12AT7?  Will it drop right in to an AB763?

It's not my FX loop design, it is used often with Dumble clones.  I don't see why it won't work with a 12AT7?  I've used a 12AY7, 12AV7 & 12AU7 with it .......... trying various gain levels.

Drop right in???? Not exactly sure what you mean by that?   Will it work with a AB763?  I think so & don't see why not?  Will it be an easy add on?  Well, the AB763 is a complicated build already.

With respect, Tubenit

[Geezer]

Here is the schematic for my version (preamp only)....I still have to work up the layout.

Check me for any errors

The OD pot values are different (doubled) because of the parallel controls.

Also, I will have to diddle with the B+ string to get the voltages needed, but I'll deal with that "live"/after construction.

G

[proaudioguy]

Here is the schematic for my version (preamp only)....I still have to work up the layout.

Check me for any errors

The OD pot values are different (doubled) because of the parallel controls.

Also, I will have to diddle with the B+ string to get the voltages needed, but I'll deal with that "live"/after construction.

G

Could you place a DPDT relay before and after (half before and half after) each section so rather than have the drive see both pots it only saw one at a time?  WOuld that cause pops?  Perhaps the entire OD section could be duplicated (with another tube) and you could just switch between OD sections?  Too many parts?

[Geezer]

Yes, it's done that way to reduce parts count & redundancy.

If you look at the OD Master section, I DID have to use a relay section at the front/entrance, as to be able to have 2x separate tone controls.

G

[tubenit]

Geezer,

That looks good to me.  As an FYI, I am changing my trim pot to 100k.  It will have a 22k going into it.   Previously there was a 59k into a 50k trim going to 4.7k ground.

With respect, Tubenit

[Geezer]

Geezer,

 As an FYI, I am changing my trim pot to 100k.  It will have a 22k going into it.   Previously there was a 59k into a 50k trim going to 4.7k ground.

With respect, Tubenit

Let me know how/if you like the 100k after you've installed it.

G

[tubenit]

OK, it's done and I've had a chance to play it.  No oscillation issues at all with the increase. It still has the sweet musical tone & it did allow some more blooming and sustain to take place. Don't know if the 22k was necessary, but I like it so it's going to stay in.

Not necessary, but definitely a useful mod, IMO.  What made me consider this is that when I measured the ("favorite") 50k pot settings, I realized I was towards the ceiling so I thought the 100k might allow even more blooming if wanted.

IF you had dual pots for the trim switching, you could do a 50k and a 100k.  Just a thought.

With respect, Tubenit

[Geezer]

IF you had dual pots for the trim switching, you could do a 50k and a 100k.  Just a thought.

A good thought! That's the type of thing I hope to do with the "blue" OD (OD1) and the "red" OD channel (Hotter OD2)

Thx, G

[Fresh_Start]

If you want OD2 to always be hotter, could you put a resistor under the Gain and Drive pots?  IOW between the pots and ground.  Reverse for OD1 - put a resistor "above" Gain 1 and Drive 1 pots.  Depending on resistor values, there would still be overlap in possible levels but you would have different ranges for the two "channels".

Just a late thought,

Chip

[Geezer]

Hey Chip, Thanks for the idea.....not late at all, as this project will take me weeks to get parts together & work on as I have time.

I will try these ideas on the fly after I have the amp up & running, as needed.

Thx again, G

[Tone Junkie]

I always love when you guys get together and talk about a build the exchange of idea's is very good. There are always differant angles thrown in that I would never have thought of .
Its such a good learning environment.
 Thanks guys Bill                                                       

[Tone Junkie]

Geezer I really like your dual volume and tone control for the OD channel you can get that 3 channel feel clean, crunch and lead out of it.
Tubnit I have always liked how you have changed tubes throughout this type of amp to get differant gain levels.
I now have a serious stash of tubes to get the perfect combination for each amp, it really does make a big differance.
Bill

[tubenit]

This sound clip was done rather quickly (about an hr with setting up, recording and mixing it) & the playing is not that great and I was not familiar with the backing track?

This is the new D'Mars OD special amp.  I started off on just clean withOUT the mid boost on.  Then switched to the OD with midboost switch on.  The ONLY thing I did different was flip the DPDT from clean to OD.  Nothing else on the guitar or amp was change at all.

http://www.soundclick.com/player/single_player.cfm?songid=11824794&q=hi&newref=1

For illustration purposes, ......... if 0 is clean and "10" is the OD fully cranked ................ the OD here is around 7.  I purposely somewhat cranked the OD to illustrate the range of contrast that the amp can have.

There is a sweet spot that sounds alot like the "clean" channel except with more harmonics, sustain and blooming to it where the notes sort of open up.   On that same scale the OD sweet spot would be maybe "3.5 to 4".  So this OD tone is cranked beyond that and not as "sweet".

On this clip, the trim is around 7, the drive is around 5 and the level is slightly less than 3.  The OD sweet spot is more with the trim around 3-4,  the drive around 4 and the level around 7.  The PPIMV was around 5.5 (for recording) and I prefer it at "7" or more for the best tone. The tone seems to open up more with the PPIMV dialed 7 or  higher.

Hopefully, this will show some of the diversity of the amp.  No pedals were used. I added some EQ, very slight compression and some delay in the mix.

With respect,  Tubenit

[Tone Junkie]

I liked it Tubnit it had a real nice clean with a good OD when you kicked it in .
Bill

[Geezer]

Here's the "Final" schematic & layout I'll be working with.

Will be ordering parts this week & hopefully starting in earnest this weekend or next week.

G

[tubenit]

Looks excellent!   No layout attached.

I think with your relay switching and dual pot controls, that this will be an amazing amp with just about every tonal sound that I would ever want. Very very cool.

Great idea with dual pot controls!  I am confidant that will be a winner!

On the clean channel volume after V1a, I think any cap from 300p - 500p will work according to your taste. I was tempted to use a 390p which I also thought sounded very good.

With respect, Tubenit

[jjasilli]

Another possible option, if your B+ voltage to the preamp is high and needs to be dropped anyway:  voltage regulator.  This will drop the voltage & in the process squish AC wiggle in the B+ supply to the preamp.  Might be useful for less noise when the hi gain circuit is switched in.

[Geezer]

Looks excellent!   No layout attached.

The editable layout is on "sheet two" of the schematic attachment

G

[proaudioguy]

http://www.soundclick.com/player/single_player.cfm?songid=11824794&q=hi&newref=1

 

Sounds really good.  A little panning in there toward the end.  Be cool to hear some chords/ rhythm playing.

[Tone Junkie]

On the irf 820 your using for the cathode follower. what type of 12 volt zener are you using 1 watt a certain type I still have problems figuring out zeners .
Thanks Bill

[tubenit]

what type of 12 volt zener are you using 1 watt a certain type I still have problems figuring out zeners .

I honestly don't remember?  I think that might have been addressed in the original TOS2CF thread, but I am not sure?

With respect, Tubenit

[Geezer]

Any 12v Zener (0.5w or larger) is all that's needed.....it only comes into play as protection for the mosfet if something goes haywire. You could probably use any voltage from 10v to 15v & get the same protection(?)

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/1N4742A/?qs=sGAEpiMZZMstCHp3EWKGl8HsDsCfnj2su%2f%2fN%2fRFKLus%3d

Rat Shack might even have something you could use (if you're not already placing a Mouser or Newark order)

G

[Tone Junkie]

I had a mouser order I needed to place any way.
Thanks Bill

[tubenit]

Guys,

Any interest in me drawing up a Hoffman style layout board for this D'Mars Overdrive Special ???  I spoke to Doug about it and my impression is that he is open to stocking the boards  IF there is a reasonable amount of interest.

Please give me some feedback on this idea.  If there are even a few interested, I would be happy to draw up the layout.

With respect, Tubenit

[John]

At some point I'd certainly want to build it. Right now we are getting ready to remodel the kitchen, which will keep me busy for the next year I'm sure. Plus, my skills are not quite up to all those switches and relays just yet  But I'm sure if I'm interested there would be lots of others wanting to give it a try as well.
Just a side  note to other "sort of" newbies like myself, I've learned that it is far better to start with simple and work your way up to bells and whistles. At least for myself - I'm pretty sure other guys have picked up the finer points much easier and faster than I have.

[CraigB]

You guys continue to amaze me with your fantastic sounding designs!  I love the clean sounds and overdrive and the way in which you demstrate the touch sensitivity Jeff - VERY tasty.

I've got a heap of 6AU6 pentodes, do you think that would work as a sub for the pentode you guys used?  I didn't have much luck with those (5879 if memory serves me).  Every one of them went microphonic on me.

Thanks - Craig

[tubenit]

I've got a heap of 6AU6 pentodes, do you think that would work as a sub for the pentode you guys used

The 5879 sounds better to me , ............. having said that yes the 6AU6 would work fine, I think?  When I had a dual 5879 & 6AU6 V1 combination (on a previous amp similar to the HoSo56),  I compared each by removing the other.  They were not all that far apart in my opinon.  I think one of Geezer's HoSo56's may have that combination?   Also what worked was the 5654 but read the specs because the maximum voltage is more limited.  It was a good sounding tube to me.

I would not hestitate building it with the 6AU6 if you prefer that.

With respect, Tubenit

[proaudioguy]

Would this work with ust regular 12AX7s?  Would you just need more tube sections?

[jojokeo]

I like the sound of a 6ak5 and wonder if it would be less prone to microphonics and take more abuse in general than any of the "regular" pentodes since it's a high altitude/ high g-force/ high vibration resistant aviation tube??? I have only used it in smaller powered chassis-only amps.

[Geezer]

I like the sound of a 6ak5 and wonder if it would be less prone to microphonics and take more abuse in general than any of the "regular" pentodes since it's a high altitude/ high g-force/ high vibration resistant aviation tube??? I have only used it in smaller powered chassis-only amps.

6AK5 is same as 5654 (which is the military version). I use them (5654) as V1 in some of my amps....fantastic tube & NEVER had one go microphonic.

G

[jojokeo]

  oops, I shoud've remembered that but it's been a year since I last looked at one of the data sheets I have. I even bought some of the 5654s too.

[tubenit]

Can someone please look over this Hoffman style layout board to see if it matches the schematic correctly.  I think it does, but I want to make sure before Doug makes some boards.

Once you've reviewed it, just post it's OK or whatever correction may be needed.  THANKS!

With respect, Tubenit

[jojokeo]

In just a quick initial look: why a dedicated terminal strip just for the protection diode on the mosfet when it could be easily soldered directly? On the other hand, if you do install the terminal strip and have an open lug then you may as well use is it, such as for the 33k grid resistor soldered closely to the input's tube socket and helping with the connections for the shielding wiring also.
just my layout $.02 worth. Thanks for taking the time to put it together like that for everyone.

[proaudioguy]

In just a quick initial look: why a dedicated terminal strip just for the protection diode on the mosfet when it could be easily soldered directly? On the other hand, if you do install the terminal strip and have an open lug then you may as well use is it, such as for the 33k grid resistor soldered closely to the input's tube socket and helping with the connections for the shielding wiring also.
just my layout $.02 worth. Thanks for taking the time to put it together like that for everyone.

I have a question on this in general, not regarding this amp in particular.  Is there a benefit to mounting the 1Meg and 33K right at the tube, and using shielded cable from the jack to that point?  If yes, would you also want the input jack to be ground isolated from the chassis?  This would essentially be like extending the guitar cable all the way to the tube, right?  Is there something gained by doing that as opposed to the way Fender did it back in the 60s?

[jojokeo]

If the run is very short then not much if anything is gained by shielding but the longer the run, then yes shielding is always best for low level high impedance signals. This is the most sensitive area that gets amplified the most and anything on the signal here gets amplified on down the chain. The 1M's placement is not important nor which end you ground on one of the side's of the shield's connection.

[EL34]

Can someone please look over this Hoffman style layout board to see if it matches the schematic correctly.  I think it does, but I want to make sure before Doug makes some boards.

If I get time today, I will try and match up the layout to the schematic

[tubenit]

If I get time today, I will try and match up the layout to the schematic

If that works out with your schedule, that'd be great!  Thanks for offering.

With respect, Tubenit

[tubenit]

OK,  I had a brief break in my schedule and compared the schematic to the layout again (using a highlighter this time).

I am not finding any errors in the layout design meaning that the drilled turret board should match the schematic just fine.

I did find a few small errors in mismatched component values which I corrected. The slope resistor on the layout says 33k and the schematic says 34k (paralleled 68k's).  Just use a 33k there.  

Once Doug or someone else gives this a final review, I'll give Doug a thumbs up to make layout boards for anyone that wants one.

I think the plates & coupling caps of the LTPI could be drawn differently and perhaps save space ...........BUT I thought someone might not want to use the PPIMV master volume?  So this way, the layout board will work well for either.

I think I am getting the best amp tone of any amp I've built from this one. Very fun amp to play.

With respect, Tubenit

[EL34]

I was working on it at the same time as you
I did not see your diagrams above so maybe some of the items I have noted have been fixed

All my notes are in red on the two diagrams in my post
print both diagrams out and lay them side by side to match up red areas

Some values I changed because they are more common or may be typos?
Example: people ask me if I stock a .02 cause they don't think a .022 is the same
5uf to 4.7uf - v1a
.02 to .022
.002 to .0022
34k to 33k - by mosfet
.0042 to .0047 - v2b
251p to 250p - v4
4.4uf to 4.7uf - v2a
.00125 to .001 - v2

notes on power supply caps - other values that can be used
20/450v caps -  22/500v or 20/500v
40/450v cap - 1 x 40/500v or 2 x 22/500v or 2 x 20/500v in parallel
16/450v cap - 16/475
10/450v cap - 10/500v

explaination on some of the red areas below:
several pots need to be labeled linear or audio - see red areas
some values on schematic don't match layout like volume pot - 500ka pot or 1ma?
120p cap missing on master vol on layout
I added labels - X and B and tube labels V1, V2, etc on layout
phase inverter layout showing 100k - schematic shows 82k
layout showing 220k resistors at lug columns 29 and 30 - not on schematic
columns 29 and 31 on layout show going to input grids on power tubes - schematic shows they should come from pot wipers on mv dual PPIMV pot?
master vol mising 120p on layout
trim pot is showing 4.7k to ground on layout but missing from schematic
330k resistor on column 30 on layout goes to power supply rail on schematic - goes to column 20 v2b pin on layout
power supply 250 ohm should probably be a 10 watt
power tube cathode cap - 22uf 50v is more common than 20uf/100v?
150k on v2b shows pin 2 on layout but pin 1 on schematic
390p missing between pin 3 and 8 on tube socket of V2 - 5879

let me know if I have all that correct or I am looking at it wrong

I can spruce up the lug layout once everything is corrected
I would add a couple lugs and get rid of the terminal strip near the mosfet

Schematic and layout reposted on 3rd page.

[tubenit]

THANKS !!!!!!!

Some corrections that you've noted were done in the repost. BUT I will look over your comments and make additional ones that are still needed.

Some values I changed because they are more common or may be typos?
Example: people ask me if I stock a .02 cause they don't think a .022 is the same
5uf to 4.7uf - v1a
.02 to .022
.002 to .0022
34k to 33k - by mosfet
.0042 to .0047 - v2b
251p to 250p - v4
4.4uf to 4.7uf - v2a

Yeah, I agree with changing those.  Some of the odd values like the .00125 were the results of experiments and paralleling caps.  In this case it was a .001 & a 250p.   Same thing with the 4.4uf.  I paralleled two 2.2ufs where a 4.7uf would be just fine there. The 34k slope resistor was another experiment of paralleling two 68k's where a 33k would work perfectly.

I like the idea of adding two turrets for the zener/mosfet stuff. That's a better idea.  

I'll have to go over the rest of the comments after work today.

I appreciate your help!  Overall, it's looking like the actual board is laid out OK with the additions of the zener/mosfet turrets.

With respect, Tubenit

[EL34]

actually, maybe don't need two more lugs, depends on how the mosfet is mounted
probably can just jumper the diode across the two mosfet lugs?

a few lugs may be changed like the 220k and 330k resistors noted on the layout

I'll wait for you to go over it all and then I can have a look at the turret layout

[EL34]

I took your .sch layout from reply #89 and reworked a few things

I attached the .sch and .gif files
The image file has these areas in red

I didn't try and make the .sch layout real pretty, just moved things around quickly

whole mosfet area was redone.
Hard to tell what rows all those parts go on without doing a paper template layout with the real parts to see if they fit

The 220 ohm and 33k resistors can just jumper in the air to the lugs
330k was moved
added 120p and 390p caps
a few values changed like .02 to .022, 251p to 250p, etc
eliminated columns 4, 29 and 30

dual gang pot goes to input grids on power tubes. I may have G1 and G2 reversed

[Willabe]

actually, maybe don't need two more lugs, depends on how the mosfet is mounted
probably can just jumper the diode across the two mosfet lugs?

The closer the diode is to the mosfet the better for it's stability, it can make a difference, as per KOC.


                        Brad       

[tubenit]

OK, I made some more changes from Doug's.  I added the values for the pots except the dual gang because I don't rememeber what that is?

I changed some cap values to the ones Doug carries where it would not make any significant difference, IMO.  

Some of the paralleled ones like 1.68uf (1uf & .68uf) can be changed to 2.2uf.
The .00125 is one that I would keep.  Parallel a .001 with a 250p.  I think you could substitute anything between 250p - 500p on the volume pot.


The 330k resistor hook up on Doug's needed to be changed back to be correct with the 5879 tube. Doug's wiring wouldn't have worked (I don't think?)

I still lean towards punching holes for the rows 29 & 30 so people can eliminate the PPIMV IF they want to. Whoever buys the board could decide whether to install turrets in those holes or not?

I will take one more look at it later tonight for any further corrections that are needed.  AND I will match the schematic now to the layout & repost it.
'
THANKS for your hard work on this Doug!    With respect,  Tubenit

[EL34]

Joe,
I only do it the way I know works and that is how it is done on all my layout diagrams. I don't know anything about how all those other guys do things. Some of them are way too anal for me personally.

The closer the diode is to the mosfet the better for it's stability, it can make a difference, as per KOC

The way I drew it is that the diode is right next to the mosfet. see the image down below

The 330k resistor hook up on Doug's needed to be changed back to be correct with the 5879 tube. Doug's wiring wouldn't have worked (I don't think?)

I still lean towards punching holes for the rows 29 & 30 so people can eliminate the PPIMV IF they want to. Whoever buys the board could decide whether to install turrets in those holes or not?

Yeah, I looked again at the 330K section again and I had it wrong. It needs to go to input grid 7 which is what I missed.

I was not sure what the 220K resistors were for, but that makes sense if you are trying to make it so you can have the PPIMV or not. Better to have some extra lugs than not.

This is how I would do the 330K section instaed of having the 330k jumping lugs at an angle

[tubenit]

OK,  I think we're good at this point!  

Doug,  thank you for all your help on this!  I like how you did that 330k wiring & changed to that. Makes more sense.

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.  (volts posted in this SCH thread:    http://www.el34world.com/Forum/index.php?topic=10776.msg134943#msg134943)

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.  You can use 250R/10w to 330R/10w on the power tube's cathodes.

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.

I will go in later (today or tomorrow) and "punch out" the layout board with Doug's layout board design program.

I think this amp is worth building. Best most versatile amp tone I've gotten of any of the designs I've tried.

With respect, Tubenit

EDIT:  schematic and layout reposted on 3rd page.

[tubenit]

I listen to alot of Larry Carlton playing with FourPlay where he uses a less overdriven tone that is fairly smooth. I really like his tone that he brings to FourPlay.

So I wanted to demo that this amp has a very CLEAR overdrive tone. And that it has versatile tone. 

The clear OD almost sounds like the clean channel but it's warmer, has more sustain and better harmonics.  The clean channel is more percussive, IMO.

Clear overdrive tone with light picking 0- 1:17

Clear overdrive with heavier picking 1:18 -----

Clean channel only  1:30-------

Fat OD  2:05    (note some blooming around 2:09)

Fat OD with delay boosted  2:32

return to Clear OD til end  2:54

http://soundclick.com/share.cfm?id=11842172

I am really enjoying the tone that I am getting with this amp!

With respect,  Tubenit

[jjasilli]