D'Mars Overdrive Special

[tubenit]

Been thinking about another amp idea ............. & drew this up based on some different things I've tried that "might" work together as a great amp.

Sort of a Marshall-ish clean into sort of a Dumblish OD ......... add an active FX loop (which also works as another OD boost at some level) into a D-style LTPI.  I used this FX loop in the reverb/FX unit that I built some time ago.

I'm wondering if using the recovery on the FX loop will work well as a Master Volume. I'm thinking it might?

Just thought I'd share the idea.  CHECK for errors!

With respect, Tubenit

[TIMBO]

Hey Tubenit, Looks to be another great amp. As for the active FX, it looks similar to the boogie FX and as you can see there is three switches. In the bunch of schems in Dougs library for boogie, there was a cheat sheet that gave the sequence for the LDRs but it did not include the FX loop.I can't think what the idea is behind the three switches but i sure that the jacks can be switched out to turn the FX into a slight boost similar to your drawing.  Good luck. 

[Tone Junkie]

Looks promising Tubenit.

[tubenit]

OK, I built this today!  Nice amp. Very harmonically rich clean and OD tone.

Downside, it lost some blooming and quite a bit of the "vowel" tone that the TOS 2CF had.

Having said that, the harmonics with the tone stack following the CF are really the nicest I've been able to come up with for any amp I've built. The amp has a really super sweet musical tone.  I do like this amp for chording more. Whether on clean or OD, the individual notes really shine thru. I think playing really fast lead riffs would cut thru quite well with this amp.

I think it sounds more Marshallish on the clean & with the OD on .......... it is sort of a blend of the Marshallish and Dumblish tone.

The amp has almost about the same sustain as the TOS 2CF, but the notes don't swell and bloom the same for some reason?

The midboost on this version creates a MUCH more dramatic tone change then the TOS 2CF midboost did. The volume is more increased and the overdrive is definitely overdriven a bunch more.   Not sure why?

Because of how clean/clear the notes are ............ I think this could be a good choice for a lower wattage "Eric Johnson-ish" tone?  And if you can play riffs super clean and super fast, I think the amp would meet that need also. It is very articulate in that regard.

IF you prefer the Larry Carlton and Robben Ford tone (at low volumes), then the TOS 2CF would be a better choice.  I think the TOS 2Cf is a better blues amp also.

Just thought I'd share the results!

With respect, Tubenit

 

[Tone Junkie]

Cool,
 Man that was fast Tubenit, I like how go from thought to, ok here it is.
Bill

[tubenit]

OK,  I opened the amp back up and worked on tweaking it some more.  Nothing was needed for the clean channel.  It's really super nice with great harmonics using the Marshallish topology with the cathode follower tone stack.

I tried ALOT of different tweaks on the OD channel to add some warmth and blooming back in.  Using alligator clipped wires and small caps I tried lots of stuff and did NOT use hardly any of them.  Most attempts added grit or even oscillation.

BUT I did find two things that worked fantastic.  1) removed the network of resistors and caps between the DPDT relay and the trim pot. That opened the amp up and made it more touch sensitive and increased the gain somewhat.  2)  Changed the 1uf on the 5879 cathode to 1.68uf  that hit a really nice sweet spot for the gain.  Very smooth and rich with harmonics.

IF prior to the mods, the TOS 2CF was a 9.5 on my "blooming feature" scale where the notes sort of open up and bloom as they sustain, then I am now at a "8.5-9" on the blooming with these mods.  And I got back almost all the vowel tone now (which I like).

What has also remained is the harmonics are much richer with this amp then the TOS 2CF. It's just really musically sweet in the tone.

So it's a trade off, this has better harmonics ......... better clean tone and is more versatile.  I can get really close even to the chimey HoSo56 amp tone by dialing the tone and volume pots just right.  BUT it still has the Dumblish OD tone when I need it.

With respect, Tubenit

[tubenit]

Observation about the gain stage cathode caps prior to the cathode follower.  The cathode caps on that stage seem to really make a BIG difference in finding a sweet tone, IMO.   For example, this amp uses a 1.68uf on both the clean and OD cathode cap  (gain stage prior to CF).

I find 1uf too thin.   I find 2.2uf to have too much grind and not enough sweetness.  I tried a 2uf also, but it lost some sweetness compared to the 1.68uf for some reason?

In other words, this seems to be a very sensitive area for component values. Very small changes make some significant differences in tone, IMO. 

With respect, Tubenit

[Geezer]

Very nice, T!

I have a redundant amp that would be fairly easy to convert.
I haven't built anything in quite a while,so I think this would be an easy way to get back in the swing!

Thanks for you work & sharing!

G

[tubenit]

This is proving to be a really sweet amp!  The pot controls are definitely more sensitive and responsive.  This is especially true for the tone stack.  I can mid scoop it or add a significant amount of mids into the tone. Both bass and treble really dramatically change the tones.  I can get almost a Marshallish harmonics and crunch but that is a cleaner tone that I more normally think of for a Twin Reverb.  (I realize that is a terrible analogy, but it's kind of hard to describe)

The most interesting aspect of this amp is that I can dial the OD controls to keep the amp relatively clean but the OD stuff adds lots of harmonics and increases the sustain even though the tone is still clean and clear.  In other words, (I can dial the OD controls where)  it goes from clean to clean with more harmonics and sustain without much of an overdriven tone. 

At the same time, if I crank the mids up & engage the midboost, then I can get the blooming effect and more of the vowel tone. However, this is not as smooth of an OD vowel tone as the TOS was. It has a little more grind to it.

I think it might work to keep the OD in the circuit all the time and use the midboost for the lead.  I think I'd almost like to have a relay switch that engages the mid boost and increases the mid pot setting at the same time & dropping some higher frequencies to boot.  I may ponder that and see if I can draw up an idea to make that happen?

I don't know if I like this amp as well as the TOS, but it sure seems to have a unique tone to it. I definitely think the way this is voiced would cut thru a band mix quite well?

The amp takes pedals remarkably well also.   More later ...................

with respect, Tubenit

[jojokeo]

You aren't wasting any time on this one & I'm liking the way this one seems to be shaping up T. It has attributes that I'd like to have the way things are going right now, and I'm liking the pentode location too. What tube are you using/liking in the pi stage, 12aT or aU?

[tubenit]

So far, I have only tried a 12AX7 in the LTPI, but I plan to try a 12AT7.  To be honest, I have been surprised that the clean tone is so clean using a 12AX7 instead of a 12AT7.

I like the clean (no changes needed, IMO). I like the OD set on cleanish tone  with harmonics and sustain. I need to work on the vowel/blooming tone to get it smoother. I've written down 6 different ideas to try & will report the results back probably this coming wkend.

I mentioned it in an earlier post, but another surprise with the amp is how well it articulates the notes playing quickly. Just like a compression pedal seems to allow more definition of notes in fast riffs ......... this does the same but without the compressed feel.

With respect, Tubenit

[jojokeo]

Again, all very nice to see and hear back on with your results. Your efforts are much appreciated and anticipated as always. You have a good sense of things to try and a good way of interpreting what you're feeling and hearing and putting it into descriptions.
I don't think you'll have too much difference btwn the ax & at in the pi but there will be some ofcourse.

Lately I'm very much into qualities regarding sustain, note definition, harmoics, and sweet musicallity of tone and transparency which accompanies these characteristics. Then a little hair mixed into the notes blending a bit of pentode magic in there sounds real good if wanted.

If I may make a suggestion? I've built several amps biasing multiple stages at the same time btwn values typcially associated with Fender & Marshall. Fender's definitely give more overall gain, vol, & low end with everything else being the same. As you've seen in your sweet spot w/ the cathode/bypass caps - maybe try using more of Fender's in a few places and see if this may help open things up in a few areas too? I'm really curious what effect it may have - hopefully for the better in helping you get where you're trying to go?

Almost forgot - try taking a B1K pot wired across that 680r and instead use a slightly higher cap, maybe to 2uF - 5uF from wiper to ground. Then you'll have a variable gain/variable bypass cap going. I always like do this on my pentodes. You'll find different settings for different volumes making it very practical.

[tubenit]

try taking a B1K pot wired across that 680r and instead use a slightly higher cap, maybe to 2uF - 5uF from wiper to ground. Then you'll have a variable gain/variable bypass cap going. I always like do this on my pentodes

IF I am understanding you, this is a variation similar to what KOC did on his Soma 18w amp with his "treble pot" which actually increased gain as well as adjusted treble.  Not a bad idea. I may try that.  I used this same idea on my very 1st amp design the Carolina Blues Special yrs ago.

With respect, Tubenit

[jojokeo]

I find it works really well and that whatever pentode (EF86, 5879, 6AK5, etc.) I use it on, the amp benefits from it depending on overall volume, gain, and your mood of the day. You version is basically the same as how I draw it up below.

[tubenit]

OK,  I had a few hrs to work on the amp this morning.  Initial impression now after about 30 min of playing is I have it realllllly close to what I am wanting.  I need to take some time and play it with the digital delay pedal in the passive FX loop. I find the digital delay will expose any high end hash or grind that I don't like.

Once again,  I have found the right smoothing cap combination to increase blooming & not dampen it. I am still of the opinion that at times oscillation can stifle the notes sustain and blooming & that removing the oscillation with the right smoothing cap can increase the sustain and blooming.

I have been successful at continuing to improve the sustain and blooming. Still now quite as good as the original TOS or TOS 2CF but pretty close to it  (maybe 93+% there).  This definitely has much better harmonics, much better note definition and articulation & is waaayy better for playing fast cleanly done riffs. Closer to Mark Knopler or Eric Johnson style "tone" than someone like Gary Moore or Robin Trower or even Robben Ford.

I am really liking playing this amp!  Very fun to play.

I will give a further "test" sometime in the next few days and repost my final thoughts.

With respect, Tubenit

I found the 68k in the tone stack to NOT be the ideal combination even though I was happy with it ......... UNTIL I tried the more Marshally 34k  resistor there.  Tightened up the bass notes in a good way.   I also removed some paralleled coupling caps in the OD gain stages and added more smoothing caps & some of the architechture (back into the circuit) that goes into the trim pot.

I also tried jojokeo's suggestion of using more towards Fender values on cathode caps. I know I don't care for 22uf cathode caps, but I did try larger values.    So, I tried 10uf on V1a and really liked it alot on the clean but not as much when the OD was engaged.  I tried 6.8uf on the V2a (1st OD gain stage) & again it sounded excellent but I prefered the somewhat "softer & smoother" OD of the 4.4uf cathode cap.

[jojokeo]

T, just to confirm, are you finding certain cap values being associated with more/less subtle oscillations affecting the sustain which would mean these are frequency dependant? If so, I'm curious which one(s) seem to be affecting this and being the most sensitive to the issue? Thanks for trying the other values - helps to confirm or validate a few things.

[tubenit]

are you finding certain cap values being associated with more/less subtle oscillations affecting the sustain which would mean these are frequency dependant?

Yes, that is my conclusion.

Kind of hard to describe, but it would be like the notes in the very high end have kind of a "hash" (using Geezer's term). It's almost like it has some harmonics that are a slight tad off key or something on certain notes.  I am also thinking the upper mid's can have oscillation at times also? Maybe Geezer can voice his observations about that also? 

So whenever this type of subtle  oscillation is stopped, it audible seems to turn the note into a purer more musical sustain that is smoother.

When I first started building & quasi designing amps, I tried the traditional "snubbing caps" across the LTPI plates and also from plate to cathode on power tubes.  Both of those dramatically muffled the tone like a blanket over a speaker. I still view those specific approachs as tone killers, IMO.

I think that negative experience left me (for a long time) viewing smoothing caps as a deterrant to good tone that should only be used as a last resort "bandaid" to a unresolvable problem. Now on the D-inspired amps, I view it as a positive tool that is intregal to shape the tone that I am looking for.  Done right, it seems to open up the amp making it more musical, more articulate and with smoother sustain and tone.  Anyway, that's my opinion.

With respect, Tubenit

[jojokeo]

I think you're experiencing "normal" shelving effects from the partially bypassing techniques determined by cap values used? So it seems you're right on with your thinking based on your findings and testing. With layout, lead dress, and other things out of the equation, the solution might just be corrected possibly being the way a single stage is being bypassed? Could be another way for using the "variable bypassing cap" as suggested earlier as a troubleshooting aide? Wire up one or two and simply clip them in after removing the originals. You can also see these affects in Merlin's biasing program and sub values of various components to see the affects there too which could help with the process. I don't know if you've used this before or not?

I agee with your last paragraphs and have re-assessed my prior neg tone ideas regarding smoothing or snubbing caps if done carefully. I originally resisted those ideas and techniques as you may remember some of the comments made in the past? I guess it just took me a little longer to warm up to things? :)

The schematic does have a lot of not only plate load caps but also plate to cathode caps, is it necessary to use both at the same time? I also see bright caps on large value gain/vol controls and also many voltage dividers mixed in. Have you thought about using split load resistors instead to help lessen the need voltage dividers and then bright caps on some of those? Also maybe try lowering vol/gain pot values to 100k, 250k, or even 500k, then the bright caps wouldn't be needed? This would also place more of a load on prior stage thereby reducing gain to help eliminate the need for voltage dividers here also. This alone could easily eliminate that gain-induced high end hash?

Lastly, are the osciallations coming only when the OD channel is in use?

Just throwing something our there to maybe also try? I had an amp that had an oscillation issue on a MV control when it had a bright cap on it, and went away w/out it but the amp NEEDED it since it wasn't ran wide open very often. The osciallation was most noticeable on a setting of "7 to 8". I ended up using a 47k in series w/ a 470p and this got rid of it and restored the brightness that was needed. Crazy but worked.

[tubenit]

Lots to ponder there! Thanks for your thoughts.

Yes, prior to the last tweaks, it was only on the OD that I was really  concerned about.

Just to clarify,  I am not perceiving any loss of tone or frequencies or sustain , etc...... with these caps. In fact, even the higher frequencies seem to still be there but simply withOUT the hash.  It just makes a  richer and more musical tone.  I don't hear it muting or lessening any tone that I am wanting to be there.

I have tried lowering some of the gain pots and have actually done so already. The clean volume is now 500k (from 1M).  The OD trim is 50k  (from 250k).  However, when I tried lowering the Mstr Vol, OD level or OD drive ............ it took away from what I was wanting.

Regarding using both plate and plate to cathode caps...........       I originally was using larger value caps across plates only, but found if I split those up that I could get by with somewhat lesser values and have a more articulate tone.   For example:  a  .001 across a plate resistor would be swapped out and instead I could use a 390p (plate to cathode) & then maybe a 250p or 390p across that plate resistor and get a clearer tone.

The enhance cap across the LTPI plate resistor has been a HUGE player in smoothing the amp.  It also helped allow lower cap values further up in the preamp and OD.  I am finding in these higher gain amps  (TBM, TOS, SoLow Watt) that the enhance cap is really important to the tone being smooth.  Again, I can not hear any loss of high frequencies but simply a smoother high.

Some of the use of the mulitple  caps on the OD is partially due to the 5879 tube which is fat but very chimey, IMO.  I like smooth more than chime.  Just a personal preference.  I think without the 5879 that fewer smoothing caps would be needed such as with a 12A_7 tube alone for the OD.

The next thing I am going to try is lowering the 220k into the trim,  maybe lowering the 150k into the drive and 150k into the level  (all in the OD section).  I am thinking that probably only lowering the 220k into the trim will get me what I am wanting which is to increase the blooming effect if possible.

Anyhow,  I hope these ramblings and thinking may be useful to others as they tweak and design their own amps?

with respect, Tubenit

[jazbo8]

Observation about the gain stage cathode caps prior to the cathode follower.  The cathode caps on that stage seem to really make a BIG difference in finding a sweet tone, IMO.   For example, this amp uses a 1.68uf on both the clean and OD cathode cap  (gain stage prior to CF).

I find 1uf too thin.   I find 2.2uf to have too much grind and not enough sweetness.  I tried a 2uf also, but it lost some sweetness compared to the 1.68uf for some reason?

In other words, this seems to be a very sensitive area for component values. Very small changes make some significant differences in tone, IMO. 

With respect, Tubenit

I think it's the corner frequency interacting with the resonance of the speaker (~80-120Hz)... What speaker(s) are you using? Does anyone else have some good hypothesis why such small change in the bypass cap could have such dramatic effect? Does this happen with the other 'OD amps as well?

Jaz

[tubenit]

Same dynamic whether using a Cannabis Rex or Red, White & Blues 12" speaker. No differences with speaker combination.


With the OD engaged,  I am now getting the sweetest most musical tone that I've gotten from any amp design I've tried.  I do get some of the blooming still with guitar vol on "8" and the amp clean channel vol on "6.5". 

However, it is still not blooming as much as the TOS or TOS 2CF. Gonna work on the resistor value going into the trim OD pot and will repost what happens.

Neither my playing or amp sounds like Larry Carlton.  Having said that,  the amp does have a sweet musical tone like I hear from his guitar/amp when he was with FourPlay.  It's a clearer/cleaner OD tone that is very musical.

With respect, Tubenit

[jojokeo]

Great to hear the feedback T & that you seem close. For some these steps are painstakingly dreadful and frustrating, but for others - esepecially you, this is where all the fun and rewards are derived. Yes? ;)

I like the thinking in splitting the difference btwn the plate load and plate-cathode caps and maintaining the tone. I haven't tried the ltpi smoothing cap and have my Tweed-ish amp that needs it to be tried on. When I go from parallel to cascading V1, it could use more of the smoothness you're describing. I don't play it in that mode as much as I'd like because of this and a tweed amp should have a smoother drive to it anyway. Too bad the tweed chassis is so dang small and cumbersome to do much of these things, but I'll find a way to make room it's not to bad to add a cap or two or three...lol.

BTW, yes some of the pot changes will lower your gain and in places you may not actually want. It's just another way to manipulate gain, response, and bandwidth of a stage.

[proaudioguy]

What is the transistor and diode for?

[tubenit]

Do a search on the Mosfet Cathode follower

with respect Tubenit

[tubenit]

HOORAY !!!!!!!!!!

Amp OD channel sounds FANTASTIC !!!!!    Still got the clean harmonically rich OD tones   AND  now have more of the "blooming effect" and vowel tone.   I can even get the blooming tone without the mid switch on & with the clean volume only on "5".

Incredibly sweet musical tones from this.  I still need to try it with the digital delay (because that will show up any "hash" in the higher frequencies).  

Having said that  (short of the delay test),  this appears to be a HUGE success for me getting a tone on both the clean and OD that I loved.  Typically, I favor one more than the other ........ but both of these are really a sweet musical tone.

I tried about 6 different more tweaks and undid all of them but two (which are indicated in red).  Changing the resistor going into the OD drive from 100k lowering it to 47k  adds some great ZZ Topish  grind complete with that sqwack (sp?) tone that they get.  However, it loses a dab of sweetness so I left it at 100k.

The amp now has the "wow" factor that I've been tweaking to try to get.  I'll post the revised layout later today or tomorrow.  I'll also post some sound clips demo-ing the tone hopefully in the next wk or two.

I'd still give the nod to the original TOS for blues.   BUT this is a sweeter more musical tone and is far more versatile in tones that you can dial it.

I am using a 12AY7 in V1 for the clean.   And a 5751 in the V2 position.

with respect, Tubenit

EDIT: schematic and layout reposted on the 3rd page

[jojokeo]

Sweet! Actually, the changes you've made aren't all that much from the original design build in your first post to the last one. Three preamp tubes, a mosfet, and three power section tubes (w/out the send/return). Are you using anything special for the iron? I may already have everything I need to get going on this one, it's just a matter of time & when now.

[Geezer]

Excellent news! Updated schematic saved.

I have a nice, large blank chassis here, so I think I'll just start from fresh with a new build rather than re-work the old amp. But that will have to wait for the Fall (too much grass to mow & other work to do right now).

Thanks for all your work on this T!

G

[tubenit]

OK,   I posted the layout with the schematic.  IF there is a discrepency ............ go with the schematic which is correct.

A couple of thoughts ............................

1)  I think someone could use a 100k trim pot instead of the 50k pot with 59k resistor going into it?

2)  I think a really cool mod to go from a sweet musical harmonic tone to a Billy G crunch, grind and squawk tone would be to have a  
     paralleled 100k resistor on a spdt switch to switch from 100k to 50k resistance into the OD drive pot

OR

3)  Maybe use a 100k pot hooked up as a variable resistor to drive pot.  That way you could dial  in between harmonics and    
     grind/crunch/squawk.   This would be sort of like the original TOS "fat" switch that is a tone shaper.

I am looking forward to hearing someone's review of this amp.  I am very stoked about the tone I am getting!

I should have mentioned that I have a 100uf/100v cathode cap hooked up to a spdt on the 5881's cathodes.  I can alternate between 20uf which has a softer more compressed vowel tone  to a tighter bass tone with this switch.  It's on the back panel of the amp.

With respect,  Tubenit

Schematic and layout on the 3rd page

[tubenit]

IF I were building this from scratch again ...........................

I'd be tempted to add a dumblish-ator on board and use the recovery pot on the front panel as a Master Volume.  The Send and Return pots would be on the back panel.

I keep staring at my chassis and how it's laid out trying to figure out if I can squeeze this in somehow??  Unfortunately, I'd have to probably build an entire new board.

With respect, Tubenit

EDIT:  Geezer, here is the idea (SCH) that I pm'd you about regarding a D'Mars preamp & OD with FX on board.  You could use 16mm Alpha mini-pots for the FX controls .......OR mount the send and return on the back panel and use the Recovery on the front panel as a Master Volume.      

[jojokeo]

I was wondering if you might try one more thing??? I've always preferred using the 470r / 10k pi value set-up when driving 6L6s/5881s for a better driving signal to the power tubes. Or have you tried these before and settled with the values of what you're using now?

[sluckey]

Which power supply node feeds the 5879?

[tubenit]

Which power supply node feeds the 5879?

     Sorry about that. Got left off the schematic but is on the layout.  Node D

I was wondering if you might try one more thing??? I've always preferred using the 470r / 10k pi value set-up when driving 6L6s/5881s for a better driving signal to the power tubes. Or have you tried these before and settled with the values of what you're using now?

Haven't tried it.  Just adopted the Dumblish values & the amp sounds really good to me with that.

with respect, Tubenit

[tubenit]

Tried the amp with the digital delay and an hr plus playing til my fingers (not just finger tips) hurt!   

This amp (IMO), has an amazingly beautiful musical tone that is so clear and articulate.  I am VERY VERY pleased with how this turned out.  I am kinda afraid/hesitant  to try anything else in the way of a tweak. I don't know what else I could possibly do to get a better tone other then to actually learn how to play my guitar.

The clean is truly clean and rich sounding. Very resonant tone.   The cleanish "OD tone" is very harmonically rich and musical. I get some slight blooming even on the cleanish OD now. It just has a very nice clear tone & you could switch from lead to rhythm and still hear every note about as well as if you were actually on the clean channel.

When I switch the midswitch on .............. the notes will blossom reasonably well and sort of open up when you sustain them.

The amp plays note very clearly even when playing faster riffs.  I especially like this feature even though I am not much of a fast player like Geezer would be.

The amp is the most sensitive to changing the tone from when you pick lightly or really dig in. Lots of tone available from the playing style & the amp lets it come thru.  (In contrast, I've played some amps that however you played the amp sounded pretty much the same).

To give you more of an idea on the blooming and harmonics ............   these are the settings I've been playing on this afternoon & please note that nothing is "cranked" and I am still getting the sustain and sweetness.

Volume 5.5
Treble 4.5
Bass  7  (bass tone is very clear and not muddy or gritty at all)
Mid  4
Clean Master volume  9.5   ( I don't think the amp really needs a master volume on the clean unless one is really wanting to play quietly)

Trim  6.5
OD drive  4
OD  level  3.5
PPIMV  6.5

With those settings,  all I am doing is changing the guitar volume, guitar pickup or flipping the mid switch.  That's it!!!
 

Again, sweetest most musical amp tone I've gotten out of any amp.

I am happy happy happy!    With respect, Tubenit

[bnwitt]

I wish it was a lower wattage amp.

[tubenit]

I wish it was a lower wattage amp

I can't tell if you're kidding or not?  

It's cathode biased with 5881's and it's only 23 watts.  

At home, I play it at a volume not much different than the Princeton Reverb I had. I can get it close to Deluxe Reverb volume.

With respect, Tubenit

[jojokeo]

T - I'm assuming you're doing all the tweaking w/out the effects loop engaged? How much does the send/return circuit affect things when in use? What I mean is that if it's engaged, you can get extra gain going on there too affecting either channel right?

[tubenit]

I'm assuming you're doing all the tweaking w/out the effects loop engaged?

It currently just has a passive effects loop. I tweak it without anything in the loop, .......... then try it with digital delay & tweak again if needed. The only thing I typically use in the passive loop is usually a MXR analog delay. I use the Boss digital delay to listen for high end hash.

I am thinking about if I can install an active effects loop mostly to use the recovery pot as a master volume. Not sure I have the room?

with respect, Tubenit

[Jack_Hester]

I think that I asked you this, about one of your previous builds (TOS, I think).  But, are you using the MOSFET just for the economics of not having to add another tube?  Or, lack of real estate to add the extra tube?  I'm mostly curious as to the sound comparison, MOSFET vs. tube. 

Either way, great build.  You are very articulate with your descriptions of how each change makes a difference in sounds.  Real pleasure to read about your build(s).

Jack

[bnwitt]

I wish it was a lower wattage amp

I can't tell if you're kidding or not?  

It's cathode biased with 5881's and it's only 23 watts.  

At home, I play it at a volume not much different than the Princeton Reverb I had. I can get it close to Deluxe Reverb volume.

With respect, Tubenit

I guess I didn't look at the schematic long enough.  I missed the cathode bias.  Might be a good build for me then.

[thelonious]

...are you using the MOSFET just for the economics of not having to add another tube?  Or, lack of real estate to add the extra tube?  I'm mostly curious as to the sound comparison, MOSFET vs. tube.

I'm sure tubenit can answer this more completely than I can, as it is his build, but Keen has a great article in which he describes some positions where MOSFETs might be preferable. In the cathode follower section, he says, "the MOSFET will still be linear when the preceeding plate has bottomed out; a typical 12AX7 triode section just can't "saturate" in normal operation to much less than 50V. When the driver tube is saturated, the MOSFET follower is still happily linear." If linear is what you want in that position, a MOSFET is a great way to get it.

[Jack_Hester]

I'm sure tubenit can answer this more completely than I can, as it is his build, but Keen has a great article in which he describes some positions where MOSFETs might be preferable. In the cathode follower section, he says, "the MOSFET will still be linear when the preceeding plate has bottomed out; a typical 12AX7 triode section just can't "saturate" in normal operation to much less than 50V. When the driver tube is saturated, the MOSFET follower is still happily linear." If linear is what you want in that position, a MOSFET is a great way to get it.

Thanks for the link!  Got it bookmarked.

Jack

[proaudioguy]

It would probably help if I knew what a cathode follower was.  Gots to gets me one of dose books....

[sluckey]

It would probably help if I knew what a cathode follower was.  Gots to gets me one of dose books....

Google is your friend.

But basically, a cathode follower is a tube circuit that has it's signal input on the grid and it's signal output is taken from the cathode. The voltage gain is always less than 1 ( typically .8 to .9), but it is capable of driving a relatively low impedance heavy load (as compared to a plate driven load) such as as TMB tonestack or a 100 foot 75Ωz video cable. The output signal is always in phase with the input signal, IE, no phase inversion. It's a mostly uninteresting circuit that can add some very interesting tones to a guitar amp.

[Jack_Hester]

It would probably help if I knew what a cathode follower was.  Gots to gets me one of dose books....

Here's more info, when I posed a question last year about it:

http://www.el34world.com/Forum/index.php?topic=12213.0

Jack

[William_G]

Hi Tubenit,
I updated my 40 watt TOS 2 CF with TAD 6L6WGC to this D'Mars Overdrive Special, WOW!!! The clean channel is so beautiful sounding, I liked the TOS 2 CF clean channel, this amp has even better sounding clear, clean channel. I made one change to the tone stack, I took the jumper off between the bass and mid pots and grounded the bass pot. This gave me a better range on the bass control.
This amp sound really good with single coil or humbucker guitars on the clean channel!! The humbucker guitars have a much more overdriven sound on the overdrive channel than the single coil guitars do. Theo I really like the single coil overdrive sound better IMO.

"(1)I think someone could use a 100k trim pot instead of the 50k pot with 59k resistor going into it?"

I put the smaller resistor before the 50K trim pot, on the overdrive channel and played the amp that way for a day or so. This change softened the overdrive sound, compared to the TOS 2 CF, IMO. I than took out the resistor and 50K pot and installed just a 100KL pot. This change gave me a wider range of the input going into the overdrive channel than before. With this control alone, you can have soft to heavy overdrive.

 "(2) I think a really cool mod to go from a sweet musical harmonic tone to a Billy G crunch, grind and squawk tone would be to have a paralleled 100k resistor on a spdt switch to switch from 100k to 50k resistance into the OD drive pot."
    
I took the 100K resistor, between the first overdrive tube and the OD drive pot and put it on a switch. I tried several resistors parallel on the switch, a 33K and a 50K with the 100K. I ended up with the 100K on one leg of the switch and nothing on the other leg. This is like a boost in overdrive going from 100K to no 100K resistor. They're was not as much of a boost, in the overdrive with the 33K or the 50K resistors. I am pretty happy with the overdrive sound at this stage.
I tried one more change just to see what it would do. I changed the 250k pot to a 100KL on the OD drive control, this gave me less range in that control, but the overdrive sound is now fuller sounding IMO.
I noticed one thing the OD drive control can only go to about 6 before I get a squealing sound, it also did this with the 250K pot. This could be the 5879 tube, is bad or just the way the design works, I don't know.
This amps overdrive can now go from soft to pretty heavy overdrive, with lots of any amount of overdrive in between. This overdrive is more natural sounding than the TOS 2CF IMO.

Thanks Tubenit for all your hard work and tweaks that you have done, to make this a fantastic sounding amp!!!!!!

William_G

[tubenit]

Any chance you can post a schematic of the changes???  Maybe use one of the editable SCH ones I posted and change it to what you're doing.

I am VERY interested in visually seeing the changes.

The clean channel is so beautiful sounding, I liked the TOS 2 CF clean channel, this amp has even better sounding clean and clear channel.

THANKS of course for sharing the results!  I am glad you like the cleaner clean channel. I think it really does sound musically sweeter.

With respect, Tubenit

[Geezer]

Any chance you can post a schematic of the changes???  Maybe use one of the editable SCH ones I posted and change it to what you're doing.

I am VERY interested in visually seeing the changes.

Yes, please show a drawing of the mods you made. I'm in the process of drawing up my version & would be interested in what you've done.

Thanks to both of you.......

G

[William_G]

Any chance you can post a schematic of the changes???  Maybe use one of the editable SCH ones I posted and change it to what you're doing.

I am VERY interested in visually seeing the changes.

Yes, please show a drawing of the mods you made. I'm in the process of drawing up my version & would be interested in what you've done.

Thanks to both of you.......

G

Geezer and Tubenit, Here is a gif of the changes I made to the D'Mars amp. The master Volume was put in before any of the new changes. I built the TOS with VVR the first build than updated to LaMar and than to the Master Volume. I prefer the MV over the other two.
William_G

[tubenit]

I think the 100k trim pot is a good idea.  I would've paralleled a 100k resistor or 68k resistor with the 100k to the drive pot. OR you could switch in series two 56k resistors.     The tone stack change might be a good one to do?

Not sure about the change in the Mstr Vol after both the clean and OD, but I think the D'Lite 22 used something similar to that?

THANK you for the information and schematic!  I sure appreciate your sharing.  I will probably change my trim to a 100k.

With respect, Tubenit

[thelonious]

Would be really interested in seeing gut shot pics if you are so inclined!