My own design - 36 watt Vox, Marshall, Mesa, Soldano-ish mash up

[Fiat_cc]

Hi all.
This is my first proper post here, although I've used the form as a guest for over a year now.  I've built a few scratch built amps now, including a Marshall 18 watt style amp I redesigned to use the Tremolo channel, with a one tube reverb, 6V6 output stage, and 5Y3 rectifier.  My latest project is more ambitious, and I am hoping some more experienced builders can look over my design to spot any potential issues, mistakes, or design flaws.
The layout is a little unusual in that it is designed to fit into a Marshall JTM30 combo chassis.  This whole concept came about when my mate blew up his JTM30, and I said that rather than spend $600 on another amp that wasn't what he wanted, I could recycle a few of the JTM parts, and build something much more fun.  If the concept works, I will redraw the layout for a more conventional arrangement and build another one.

I am using a Hammond 290MX power transformer (Vox AC30 reissue replacement), and Hammond 1760J output transformer (Fender blackface 35 watt with  4KOhm primary and 4, 8 and 16 ohm secondaries).  The power section is similar to some of the 36 watt variations people build.  The preamp is a mash up of 18 watt, Plexi, and JCM800 Marshalls, with influence from Mesa and Soldano around the cold clipper stage.  I've included switchable a NFB loop, switchable tube or SS rectifier, tone stack lift (Raw switch), foot switchable parallel or series triodes in V1 ('boost' switch) and foot switchable cold clipper stage.  I also included a 50K pot in parallel with a 100K resistor, in series with a 4.7k resistor as the cathode resistor for the cold clipper.  The theory being that it would be adjustable from around 5k (warmer than Marshall) to roughly 40k (Soldano SLO100 and Mesa Rectifier).  I've called that control Saturation for want of a better word.  There is an overdrive gain control after the cold clipper that I'm not 100% sure is good design practice, but I believe it will work.

Anyway, if any of you would care to have a look and give me some pointers, I'd appreciate that.
Cheers,
Reuben

[2deaf]

Schematic doesn't match layout.  I hate that.

[Fiat_cc]

Schematic doesn't match layout.  I hate that.

That's kinda, not really helpful.
It should match.  What is different? I've been working on both late at night after work, so may have made an error, or updated one and not the other, but they should match.

I should also say these are both drafts as I wanted to seek some advice on whether the whole concept is sound, then I can tidy up a bit

[2deaf]

That's kinda, not really helpful.

Sorry, man.  I have spent the majority of my adult life reviewing documents with glaring contradictions from page to page.  The folks that produced those documents never seemed to find it helpful, either, when I would call them up. 

The plate is connected to the grid on V1A.  There is an 820R resistor loading down the power supply.

 

[DummyLoad]

SW1 is going to POP - LOUD. SW2 same, but not as loud?


--pete

[Fiat_cc]

SW1 is going to POP - LOUD. SW2 same, but not as loud?


--pete

I did wonder that.  I stole the SW1 circuit from another amp (don't recall the make and model now), so I was hoping, being tried and tested, that it may not, but I fear you are correct.  Not sure about SW2, since it just grounds the grid of the cold clipper stage when it switches.  I stole that switching circuit straight off a Dumble ODS, and I assumed that Mr Dumble wouldn't tolerate a loud pop when switching the overdrive in.  Any suggestions for taming the pop issues?

[tubenit]

I agree with Pete (DL) that SW1 will pop.  I think you'll probably be OK with SW2?  However, I would add a resistor to the V2-2 grid.  Maybe 33k - 68k value?   I've used relay switching dumblish inspired style in numerous builds with no problems with popping.

The other thing that you might need to experiment with are C6 and C9 (which you currently have as .02)?  Those values may need to be lowered some such as C6 being closer to .01 and C9 maybe .0047 to .0068 range?  (IF you look at Mesa Boogie, Dumbles, Cornford and Bad Cat type amps that have 4 or more gain stages, they often will use smaller value caps in the overdrive section)

VR7 is sort of a "crossline master volume" which is my absolute least favorite master volume of any I have tried.  I strongly dislike it and have always found it too buzzy sounding to me and lacking smoothness .  I would be inclined to use a PPIMV instead. 

I probably would want to add a node E of filtering for V1.

Lastly, I don't care for snubbing caps like C15/47p which I have found to be like placing a thin blanket across the speaker.  Instead I prefer the "enhance cap"  (use search) which is essentially something like a 120p to 250p range cap across the LTPI entrance plate resistor  R41.  It seems to smooth the highs without eliminating them in my experience.  It's something I do with all my LTPI builds.  I'd probably try a 220p there if using EL84's.

With respect, Tubenit

[2deaf]

There is no ground reference for the grid on V1B when fed from C5.

VR1 increases the load on V1B as you turn it down.  This will change the frequency response by cutting more bass as you turn it down -- the opposite of what you would want in a high gain amp.

VR2 has DC on it.

VR3 increases the load on V2A as you turn it up resulting in more distortion and less volume.  When maxed, there will be no signal at all.

SW3 might pop, but it might be negligible being that far along in the signal path.  Putting a high value resistor across the switch should help if there is a pop.

[Ed_Chambley]

Other than a couple of issues, I like the idea.  Should be a nasty good sound.  Love a 4 tube output.


I agree with not using a crossline MV.  Easy enough to try and see tho.


and the smoothing cap is a great suggestion especially for EL84's.  Sometimes referred to as Hash in the highs.  I just call it harsh.


Careful of that 2 Deaf dude, he likes that psychedelic cowboy sh*t, but seems to know his sh*t, however I never speak directly to him.

[2deaf]

. . . however I never speak directly to him.

Ha!  I knew I was on the "Don't talk to list".  At least I'm not on the "Don't listen to list", although I am probably on the "Don't admit it list".

[Ed_Chambley]

. . . however I never speak directly to him.

Ha!  I knew I was on the "Don't talk to list".  At least I'm not on the "Don't listen to list", although I am probably on the "Don't admit it list".

You now I wrote that because of your "don't talk to comment."  You know we don't have this type of list, but if you want to start one you sir can be at the top!


I appreciate you comments and knowledge and honesty.

[jojokeo]

This is an ambitious amp build for a relative newbie. I also see a number of issues but there should be more detail on the layout and lead dress portion as well. This could make or break the outcome alone despite all the initial errors being corrected. Sorry, not enough time to document and correct things that may or may not translate to actually being utilized? And I certainly don't want to be part of that list if at all possible?  (2deaf you're always good in my book - +1 on Ed's comment also)

[Fiat_cc]

That's kinda, not really helpful.

Sorry, man.  I have spent the majority of my adult life reviewing documents with glaring contradictions from page to page.  The folks that produced those documents never seemed to find it helpful, either, when I would call them up. 

The plate is connected to the grid on V1A.  There is an 820R resistor loading down the power supply.

Sorry, I didn't mean to come across as rude.  Due to various life/health problems (none of which are anybody here's worry) I can be a little short on patience.  I appreciate any help I can get, but I'm here to learn, so don't tell me there are issues, then not tell me what they are.

I will find the two items you mention and rectify them.  I think the plate connceted to grid is on the layout, and was simply an error of drawing the wire to the wrong pin.  Which 820R are you talking about?

[Fiat_cc]

I agree with Pete (DL) that SW1 will pop.  I think you'll probably be OK with SW2?  However, I would add a resistor to the V2-2 grid.  Maybe 33k - 68k value?   I've used relay switching dumblish inspired style in numerous builds with no problems with popping.

Is there a way to avoid the popping on SW1, or should I do away with the parallel/cascade idea, and just go for a simpler single triode/cascaded triode boost switch (essentially the same configuration as SW2)?  It seems a pity to have a wasted triode was my thinking originally, but I don't want an unusable feature.

The other thing that you might need to experiment with are C6 and C9 (which you currently have as .02)?  Those values may need to be lowered some such as C6 being closer to .01 and C9 maybe .0047 to .0068 range?  (IF you look at Mesa Boogie, Dumbles, Cornford and Bad Cat type amps that have 4 or more gain stages, they often will use smaller value caps in the overdrive section)

Is that just to attenuate the bass a little more? I'm trying to build something that gives the player versatility, from bluesy drive to  60's/70's classic/psych rock crunch, to a ore modern high gain sound, but I don't want a one trick pony, that only does that modern high gain sound.  I'm certainly open to advice here though, and if you think it will help, without losing too much versatility I'll try different values here.

VR7 is sort of a "crossline master volume" which is my absolute least favorite master volume of any I have tried.  I strongly dislike it and have always found it too buzzy sounding to me and lacking smoothness .  I would be inclined to use a PPIMV instead. 

I've heard a few people say this, but I have this style MV in my 18 watters, and I was surprised at how well it worked.  I figured since this was essentially the same output section, just doubled, that it would work well here too.  By PPIMV you mean the LAMAR double gang pot style MV?

I probably would want to add a node E of filtering for V1.
I ummed and ahhh'd about that too.  In the end I stuck with 4 because of space constraints in this first chassis, and (this first one at least) is a bit of a budget build so I was trying to fit it on to the circuit board I already have cut.  If you think there will be an appreciable benefit, I can add this, and try and fit it on the 14" board.

Lastly, I don't care for snubbing caps like C15/47p which I have found to be like placing a thin blanket across the speaker.  Instead I prefer the "enhance cap"  (use search) which is essentially something like a 120p to 250p range cap across the LTPI entrance plate resistor  R41.  It seems to smooth the highs without eliminating them in my experience.  It's something I do with all my LTPI builds.  I'd probably try a 220p there if using EL84's.

I lifted that straight from the Marshall JCM800 (I think...) schematic.  I will look up the 'enhance cap' you mention, and perhaps try both out and see which I like.

With respect, Tubenit

Thanks for all your help and suggestions.

[Fiat_cc]

There is no ground reference for the grid on V1B when fed from C5.

Good spot.  Thankyou.  Another 470K to ground off the grid side of C5 would fix that yes?

VR1 increases the load on V1B as you turn it down.  This will change the frequency response by cutting more bass as you turn it down -- the opposite of what you would want in a high gain amp.
This is the preamp gain control from the Marshall 18 watt circuit.  I will have a look in other preamps to see how they do it.  Would you suggest a particular way?

VR2 has DC on it.

Yes, I thought that was the case.  I've been trying to figure out a way to make this control work (varying the cathode resistance to get differing bias on the cold clipper).  I've seen it done as a simple 3 position switch, but I wanted to make it constantly variable.  Is there a better way to do this, or is it simply not possible?  I assume the issue being scratchy pot adjustment (i.e. old Bassman Presence control etc...)?

VR3 increases the load on V2A as you turn it up resulting in more distortion and less volume.  When maxed, there will be no signal at all.
Perhaps I have this wired in reverse?  The idea being a preamp gain control...  Do I put something like a 100K resistor (just grabbing a random value) in series between the pot and ground, so it never reaches zero volume, or is it not really an issue?

SW3 might pop, but it might be negligible being that far along in the signal path.  Putting a high value resistor across the switch should help if there is a pop.

OK cool.  This one is not on a foot switch (I intend SW1 and SW2 to be relays and foot switchable). I imagine SW3 would be a set and forget type control on a toggle switch on the rear panel of the amp.  Still good to know I may be able to help with th pop.  Are you thinking 220K type value or higher?

[Fiat_cc]

Other than a couple of issues, I like the idea.  Should be a nasty good sound.  Love a 4 tube output.


I agree with not using a crossline MV.  Easy enough to try and see tho.


and the smoothing cap is a great suggestion especially for EL84's.  Sometimes referred to as Hash in the highs.  I just call it harsh.


Careful of that 2 Deaf dude, he likes that psychedelic cowboy sh*t, but seems to know his sh*t, however I never speak directly to him.

Thanks. I think it's a cool concept, and I like trying different things to see what I have learned and what I still need to learn.

I'm sure 2Deaf is fine, and a helpful individual, I just lack patience at the moment for various reasons, and if someone says there's a problem, I want to know what the problem is.  That's why I'm here asking questions. 

[Fiat_cc]

This is an ambitious amp build for a relative newbie. I also see a number of issues but there should be more detail on the layout and lead dress portion as well. This could make or break the outcome alone despite all the initial errors being corrected. Sorry, not enough time to document and correct things that may or may not translate to actually being utilized? And I certainly don't want to be part of that list if at all possible?  (2deaf you're always good in my book - +1 on Ed's comment also)

I like being ambitious and pushing myself.  I find it's the best way to find the gaps in my knowledge, and crystalise the stuff that I have learned.  I liked redesigning the 18 watter as a single channel trem/reverb with 6V6 tubes.  No new territory there perhaps, but drawing my own schematic and layout (albeit very much based on existing ideas) was a great learning curve.  This is taking it to the next level for me, and trying to better my understanding of biasing and coupling stages, and learning about higher gain preamps.

I like to think I have a pretty good understanding of lead dress, and trouble shooting a tremolo issue in my first 18 watter taught me a lot about interaction and good lead dress concepts.  My last amp is very quiet (pics attached for anyone who is vaguely interested).  I'm always keen to learn more though, and it is obviously more critical in higher gain circuits. The layout drawing program is very limited, in that I can only have 4 points per wire (that includes the beginning and the end), so the layout diagram is messier than I would like the chassis to actually be.

I am definitely interested to hear more of your idaes.  I already have nearly all the parts to build this, and I intend it to be finished in the next 2 weeks, so I would suggest that anything you can see that needs corrected, or suggestions you have for improvement will more than likely be utilised.

[sluckey]

This is the preamp gain control from the Marshall 18 watt circuit.  I will have a look in other preamps to see how they do it.  Would you suggest a particular way?

Not quite. Take a look at the Marshall 18 watt schematic again. There is no jumper between the wiper and high side of the volume pot. Remove that jumper from your schematic to fix.

[Fiat_cc]

This is the preamp gain control from the Marshall 18 watt circuit.  I will have a look in other preamps to see how they do it.  Would you suggest a particular way?

Not quite. Take a look at the Marshall 18 watt schematic again. There is no jumper between the wiper and high side of the volume pot. Remove that jumper from your schematic to fix.

Ah, I see that now.  That isn't supposed to be there. The way it's supposed to work is that that pot is out of the circuit when the cold clipper stage is engaged (which I now realise means I need a 470k grid reference resistor on the grid side of SW2 yes?).  I think the software has put a junction there when it is supposed to cross the wiper with no connection, if that makes sense?

[2deaf]

You now I wrote that because of your "don't talk to comment."  You know we don't have this type of list, but if you want to start one you sir can be at the top!

Yeah, I know.  I was just running with the continuing joke.

[2deaf]

There is no ground reference for the grid on V1B when fed from C5.
Good spot.  Thankyou.  Another 470K to ground off the grid side of C5 would fix that yes?
Yes.

VR2 has DC on it.
Yes, I thought that was the case.  I've been trying to figure out a way to make this control work (varying the cathode resistance to get differing bias on the cold clipper).  I've seen it done as a simple 3 position switch, but I wanted to make it constantly variable.  Is there a better way to do this, or is it simply not possible?  I assume the issue being scratchy pot adjustment (i.e. old Bassman Presence control etc...)?
I'm sure there are ways that would use the pot to control something else that is actually modulating the DC, but I can't think of anything just off the top of my head.  I don't know about you, but scratchy pot adjustment on a presence control never bothered me.  I rarely adjusted those and never adjusted one on the fly.   

VR3 increases the load on V2A as you turn it up resulting in more distortion and less volume.  When maxed, there will be no signal at all.
Perhaps I have this wired in reverse?  The idea being a preamp gain control...  Do I put something like a 100K resistor (just grabbing a random value) in series between the pot and ground, so it never reaches zero volume, or is it not really an issue?
I think wiring up a standard volume control here would be a good idea, but I'm not sure what you are trying to achieve in addition to the level control.

SW3 might pop, but it might be negligible being that far along in the signal path.  Putting a high value resistor across the switch should help if there is a pop.
OK cool.  This one is not on a foot switch (I intend SW1 and SW2 to be relays and foot switchable). I imagine SW3 would be a set and forget type control on a toggle switch on the rear panel of the amp.  Still good to know I may be able to help with th pop.  Are you thinking 220K type value or higher?
I was thinking more like 1M, which should have about the same effect on the tone stack as infinite resistance.

[2deaf]

Which 820R are you talking about?

R40

[Fiat_cc]

Which 820R are you talking about?

R40

Thank you. That should be a 220K bleed resistor. I must’ve cut and pasted the resistor symbol but not changed the value.

[Fiat_cc]

There is no ground reference for the grid on V1B when fed from C5.
Good spot.  Thankyou.  Another 470K to ground off the grid side of C5 would fix that yes?
Yes.
Excellent

VR2 has DC on it.
Yes, I thought that was the case.  I've been trying to figure out a way to make this control work (varying the cathode resistance to get differing bias on the cold clipper).  I've seen it done as a simple 3 position switch, but I wanted to make it constantly variable.  Is there a better way to do this, or is it simply not possible?  I assume the issue being scratchy pot adjustment (i.e. old Bassman Presence control etc...)?
I'm sure there are ways that would use the pot to control something else that is actually modulating the DC, but I can't think of anything just off the top of my head.  I don't know about you, but scratchy pot adjustment on a presence control never bothered me.  I rarely adjusted those and never adjusted one on the fly.   
OK, besides scratchy, there's no ill effects?  I'll run with it for this prototype and see whether or not the control is worth keeping.  If it does what I want it to, I'll investigate ways to do it without the pot being directly in the chain.

VR3 increases the load on V2A as you turn it up resulting in more distortion and less volume.  When maxed, there will be no signal at all.
Perhaps I have this wired in reverse?  The idea being a preamp gain control...  Do I put something like a 100K resistor (just grabbing a random value) in series between the pot and ground, so it never reaches zero volume, or is it not really an issue?
I think wiring up a standard volume control here would be a good idea, but I'm not sure what you are trying to achieve in addition to the level control.
All it is meant to do is control how hard the cold clipper hits the next gain stage, and to allow you to balance between the clean sound and the overdrive sound.  I'll redraw schematic and layout with a more traditional volume control.

SW3 might pop, but it might be negligible being that far along in the signal path.  Putting a high value resistor across the switch should help if there is a pop.
OK cool.  This one is not on a foot switch (I intend SW1 and SW2 to be relays and foot switchable). I imagine SW3 would be a set and forget type control on a toggle switch on the rear panel of the amp.  Still good to know I may be able to help with th pop.  Are you thinking 220K type value or higher?
I was thinking more like 1M, which should have about the same effect on the tone stack as infinite resistance.
I realised after my last reply that you were talking about the tone stack lift switch and not the NFB switch, but wither way, thanks for the suggestion. I'll try it out and see if it works.

[DummyLoad]

suggestions: see attached... DC VR2 is not an issue if you use a 2W cc PEC pot. it may still be "scratchy" sounding when adjusted. i'd scrap SW1 scheme: cascade or bypass V1b.


--pete

[jojokeo]

When cascading the first two stages there's going to be a lot of signal voltage. There is a Gain control but not any grid resistor going into V2a, this would be a good spot for one. Also there's no voltage divider going into this third stage. As the signal goes into the 4th stage at V2b again there's no voltage divider and only a bypassed grid resistor set-up. But, there is at least another Gain control however this control will never get close to even a half way setting the way the current set-up is configured unless the first Gain control is set well down or V1 is utilizing only it's single gain stage option. There's going to be a lot of voltage divider & grid resistor tweaking adjustments needed once things are up and running if the controls' full sweeps are going to be maximized through all settings. There's still no anti-pop resistors in place on the switches for the caps/grids but at least things are workable for a starting point.

[tubenit]

It seems a pity to have a wasted triode was my thinking originally, but I don't want an unusable feature.

OK,  some ideas for consideration:

paralleled triode  (if you use this check on how to set up the plate and cathode values, I don't remember off hand?)

switchable paralleled triodes

Using a 12WD7 where you can "pan" or dial in the 12AX7 or the 12AU7 side

put in a 5879 pentode for a FAT chimey tone

With respect, Tubenit

[tubenit]

OR ................... also for consideration,  use the "extra triode" for a tube FX.  I did this with my D'Mars ODS amp and really like how it worked as an FX loop.  The cathode follower and tone stack become the send for the FX loop.

With respect, Tubenit

[Fiat_cc]

OK.
Firstly, thank you all for the ideas and expertise. I appreciate it greatly.
I've attached version 2 of the schematic.  I haven't updated the layout yet, as there were quite a few tweaks, changes and updates, so I thought I'd get some opinions first.  I've accepted the fact I'll need a longer board for this, or a separate power supply/cap board at least. Also, thanks Tubenit for some of those ideas. The guy this first prototype is for is kinda finicky, and specifically said ‘no fx loop’ but I’ll keep the idea for down the track.

SW1 is now a bypass/cascade V1B switch, rather than a parallel/cascade. I've also played around with the way each option feeds V2A and added a 'Boost' level control (VR2) for V1B. I was wondering whether to make this a 250K pot with a 220K resistor in series to ground instead of the 500K I've got there now, so that the control can't wind the signal to zero completely.  Thoughts? Maybe different values would work better?

Originally I was trying to bypass VR1 when overdrive was engaged, so that it didn't effect the overdrive level/volume, but I think that might be a folly, so I've revised that too. It's good in theory, but you end up with V2A getting the full output of V1A (with V1B bypassed), and therefore a weird situation where the 'Boost' function may actually end up being the opposite, since it does have a level control pre-V2.

I've tried to add some attenuation between stages and also grid resistors in a couple of spots, looking at the SLO 100, and Dumble ODS for ideas.

I deleted the snubber cap, and replaced it with Tubenit's 'enhance' cap.  I might experiment with both options when it comes time to build, so I can hear the difference with my own ears.

I've added a proper PPIMV, and an extra power supply filtering stage for the first preamp stage.

Hopefully I've solved more problems than I've caused, but please let me know if I've stuffed up etc.
Cheers,
Reuben

[jojokeo]

Ruben,
Don’t try to get all of the fine points figured out exactly just the overall ideas should be considered and thought of beforehand. This will allow a little turret/eyelet board room for mods &/or tweaks after the initial build is in place. There’s always final adjustments when you get everything working.

[Fiat_cc]

Ruben,
Don’t try to get all of the fine points figured out exactly just the overall ideas should be considered and thought of beforehand. This will allow a little turret/eyelet board room for mods &/or tweaks after the initial build is in place. There’s always final adjustments when you get everything working.

Thanks.
Yeah, I figured that. I just want to be well in the ball park before I commit to buying parts etc.

[Fiat_cc]

Ok, so I built it. Now for trouble shooting. I should point out that putting this amp in this chassis was a bit fool hardy! I knew it would be tricky, but this chassis is an absolute pain in the neck. Can’t even put it in a cradle. It looks roomy, but because of weird angles, and the double tier design, it’s really limiting in real estate. I’d love to build this circuit again (once bugs are ironed out) in a proper chassis.

On first power up I had terrible microphonics and squealing and all sorts of nastys. It seemed related to the pentode/triode switch, so I removed that. In the process, I discovered that one OT primary wasn’t properly soldered to the plate pin on the tube socket. Rectified that, and did some lead dress adjustment, and it improved markedly. There is still a bit of work to totally stabilise the power amp section, but it’s close.

Also, initially the tone stack wasn’t working. Turns out I’d missed the ground connection from the bass pot, that actually completes the tone stack circuit. It works now, although it doesn’t do as much as I thought it would. Still, it works.

I also have some hum, but as I work through other issues, and adjust and tidy lead dressas I go, it is slowly diminishing.

I’ve not even gotten to trying the boost or overdrive circuits yet. I want to get the basic vanilla TMB 18 watt/36 watt preamp/Power amp combo working properly first. I’ll get back to troubleshooting and testing in the morning, and I’ll post a voltage chart, and maybe some sound clips of some of the issues, which might help some of the more experienced builders make suggestions.

Thanks to all who have helped so far. I feel like there’s going to be a fair bit of troubleshooting and tweaking, but that the basic foundation is there, and I’ll get it sorted in the end.

[jojokeo]

It's ALIVE!!!  Take it slow, think things through, try to remain patient, this part can be the most frustrating but also the most rewarding. Sometimes the "real learning & understanding" phase of the process also?

[Fiat_cc]

Ok.
Taking it slow. Had a tinker this morning. Realised I’d wired relay power backwards. So, the boost circuit isn’t working yet, but I suspect a wiring error or air gap somewhere. The overdrive circuit is working, but it’s lacking grunt, and is voiced much thinner than the normal circuit. I think I may need to lessen the between stage attenuation and increase coupling cap size.

I’ll do some experimenting and trouble shooting with the boost circuit, and the overdrive circuit over the next week. If anyone has any suggestions, I’d welcome them. Otherwise, I have some ideas to try.

[jojokeo]

Sounds like you're on the right path at least to start with ideas to shore things up? It's all a matter of personal choices at this point and you being there playing and hearing it, I doubt other's input is going to mean a whole lot? Unless you get stuck on ideas and your approach getting to what you want?

[Fiat_cc]

Sounds like you're on the right path at least to start with ideas to shore things up? It's all a matter of personal choices at this point and you being there playing and hearing it, I doubt other's input is going to mean a whole lot? Unless you get stuck on ideas and your approach getting to what you want?

Thanks jokokeo. Yeah, I feel like I’m on the right path, and you’re right, it is down tooersonal preference. This is the first time I’ve ever designed an amp and voiced it from the ground up. Well, not quite, as there are lots of borrowed circuit ideas, but you know what I mean. I’m not 100% confident in this realm yet, but I’m learning a lot!!!

[shooter]

I’m not 100% confident in this realm yet

couple things that have helped me on the road to 90%;
I start with a clean schematic, as I check/change/fix I note it up on that schematic, BEFORE I go to bed, I update the file, print and continue, repeat as necessary.  my last build has 6 revisions in a plastic binder

here's the latest version as an example

[Fiat_cc]

I’m not 100% confident in this realm yet

couple things that have helped me on the road to 90%;
I start with a clean schematic, as I check/change/fix I note it up on that schematic, BEFORE I go to bed, I update the file, print and continue, repeat as necessary.  my last build has 6 revisions in a plastic binder

here's the latest version as an example

Thanks.
That’s good advice.

[Fiat_cc]

Ok, so I have the boost circuit and overdrive ‘operational’. An error in the switching circuit wiring.
Now, the boost seems to work, although there is more gain on tap than is required/stable. It doesn’t sound bad, but with both gain, and boost control cranked up it’s got too much gain and it’s unstable. I’ll have a look at how I might attenuate that a little.

The overdrive circuit is passing audio but sounds very wrong. Hard to say what exactly is going on there, as I’ve only spent 10 minutes with the amp today. Just enough to fix the switching issue. It’s distorted alright, but it’s nasty and fizzy and reacts strangely to the guitar. Perhaps too much, or not enough signal hitting the cold clipper?
I’ll have another play soon, and try to get some sound clips.

[Fiat_cc]

The boost circuit sounds pretty good really. I need to attenuate the signal input to V1b a bit, but otherwise, it’s very useable. I’m quite pleased. Tweaking to do, but it’s going to work.

The overdrive circuit is weird. If I strum softly, I get a fairly clean sound, then as I gradually strum harder it correspondingly gets a little louder until a threshold, where suddenly it’s like a switch is flicked and it gets much louder, crunchier etc. almost sounds usable, except as soon as the input volume drops back under the threshold, the volume totally dies again. I’m sure this is an important symptom, but of what, I don’t know. Haven’t tried tube swaps yet, but it’s a brand new tube, and I doubt that’s the issue (although I guess it could be).
Any ideas what would cause that symptom? I tried adding a 1M grid resistor after the ‘Drive’ pot, but I didn’t notice a difference, although I suspect I should add one here for good practice anyway.

[Fiat_cc]

Not sure how obvious the sound problem is in the video, but this is the weird volume/distortion thing the clipper stage is doing.
Tried adding 1M grid resistor on V2B and halving the value of the 220K resistor to ground in the voltage divider before V2A. No difference.
Thoughts?

[jojokeo]

Take a look at some re-adjusted parts of your Boost Circuit. This will go a long way to getting things more where you want them:
1) when off, you have a larger cap for fuller signal getting to the CF stage for clean setting
2) when on, the lower cap value will now limit bass going into V1B (much better way to have it)
3) when on, you now have control of signal after V1A going into V1B and V1B's output is knocked down before going into either CF stage or next OD circuit (much more useable gain than before)
4) The 1M resistor on switch will eliminate popping while toggling Boost stage off/on
5) 220K grid resistor going into CF should not be necessary now?
6) eliminated unnecessary cap leaving V1B from before where the voltage divider is now drawn in
7) your issue going into the OD circuit was likely from too much signal driving the grid positive? (and/or the clipper on cathode)
8) *I'd eliminate that clipper arrangement and just leave the 4.7K resistor on V2A - you can always re-visit it later once things are working.

These changes should go a long way to making your amp much more useable now in all settings.

[Fiat_cc]

Thanks jojokeo.
I appreciate your suggestions.
I’ll have a look when I’m next at home and compare your changes to my schematic. I think you’ve got a slightly older version too. I’m currently feeding the overdrive stage from before the gain control pot. I might change that too.
When I’m at home I’ll upload the most up to date version of the schematic (possibly with your suggested changes), a voltage chart, and an update on how things are sounding.
I’ll also trace through everything in the overdrive stage to make sure I’ve not messed up anywhere. The adjustable cathode setup seems to work. I’ve measured from cathode pin to ground and I get a sweep of 4.7k to 39k, which is exactly what I wanted. I could bypass temporarily if you think it will help trouble shoot.
In your opinion, should I move the overdrive gain control to before the clipper?

[jojokeo]

In your opinion, should I move the overdrive gain control to before the clipper?

No, you already have a control before it when that relay's activated.

BTW, you should always provide the most current schematic for others that are trying to help you. Not doing so is bad forum juju. It's the quickest way to make others not want to help and have anyone give of their personal time to help YOU out. It's the least you can do...and when you do - do not mix ideas that you haven't done. It needs to be accurate.

If you have ideas or something else - make another schematic with those things. It gets too confusing mixing real world with hypothetical and speaking for myself...I just check out of the thread if things are not clear with getting reliable info from the poster.

[Fiat_cc]

Understood. I thought the most up to date schematic was on here, so it is not a case of not providing up to date info, so much as a mistake in terms of knowing which version I’d uploaded. There are only very minor differences.

[Fiat_cc]

OK.
Here is a schematic of what is actually in the chassis right at this point in time. Again, sorry for the confusion. I didn't realise I'd not uploaded the correct version.

[jojokeo]

That looks like what I corrected in the preamp areas earlier (and has drawbacks). You might want to utilize what I submitted to you to try for the reasons stated?

[Fiat_cc]

Sure, I’d be glad to try those ideas.
I figured I should at least share where I was up to, in case anything I’ve changed already effected your most recent suggestions. I’ll report back once I’ve had a chance to do some soldering.

[Fiat_cc]

No real updates to report. I’ve been too busy with out of town jobs.
I have decided to get a chassis made though, that will fit the original Marshall cabinet, but not have the stupid layout of the Marshall original. It is just too hard to get good lead dress etc with the limitations of this chassis.