Anyone familiar with the S.I.R. #36 and #39 circutry?

[Mike_J]

I am in another holding period waiting on parts to complete the Sluckey inspired Plexi/800 build which provides the time to plan ahead to the Sluckey inspired JTM45/800 stereo amp build.

In the Plexi/800 build I am trying a Dumble overdrive channel as an experiment.  I wanted to do something different for the JTM45/800 build and happened across the story of the S.I.R. #39 amp.  S.I.R. is an amp rental company in Los Angeles that rented amps out. 

Apparently they had a Super Lead with tremolo that their tech, named Tim Caswell, converted to a tone monster.  He converted the tremolo tube to an extra gain stage.  It was their most requested amp. 

Slash stole it from S.I.R when he couldn't get them to sell the amp to him and toured with it for a while.  It was apparently used in the studio to record the album that contained the song Sweet Child of Mine and other songs.  S.I.R. eventually got it back by taking it off the stage when they were performing in L.A. during a break in the middle of the day.

Just the story deserves a look at this.

Has anyone looked at this circuitry and could they shed some light on it for me?  Could it be foot switchable with a relay?  It sounds like something that this amp has to have.

Thanks
Mike

[bakerlite]

This one is Discussed in emmense detail on metroamp 600 page thread.
Pm me email for a schematic of #36 amp

[shooter]

http://www.tonymckenzie.com/afd100_history.htm
no "exact details" but I guess good reading

[Mike_J]

This one is Discussed in emmense detail on metroamp 600 page thread.
Pm me email for a schematic of #36 amp

Thanks so much for the schematic.  I am very enthused by what I see.  It is basically just an 800 with a single triode gain stage between the preamp and tone stack.  Waste of one triode but I think I am a grownup now and can probably live with that.  Can definitely use a relay and ground the triode out of the signal if I want the standard 800 amp or switch it in for the Slash setting.

I am thinking of a new PT for the Sluckey inspired JTM45/800/36 stereo build.  It would allow me to completely separate both amp circuits, still only using one PT and giving the JTM45 a GZ34 and its own HT and screen supply.  The 800 would have a solid state supply and use the 800 HT and screen supply.

I want to use KT66s for the JTM45 side.  Does anyone have a voltage preference for the JTM45 and / or an opinion on tube rectification versus solid state rectification for that side?

Thanks
Mike

[DummyLoad]

This one is Discussed in emmense detail on metroamp 600 page thread.
Pm me email for a schematic of #36 amp

Thanks so much for the schematic.  I am very enthused by what I see.  It is basically just an 800 with a single triode gain stage between the preamp and tone stack.  Waste of one triode but I think I am a grownup now and can probably live with that.  Can definitely use a relay and ground the triode out of the signal if I want the standard 800 amp or switch it in for the Slash setting.

I am thinking of a new PT for the Sluckey inspired JTM45/800/36 stereo build.  It would allow me to completely separate both amp circuits, still only using one PT and giving the JTM45 a GZ34 and its own HT and screen supply.  The 800 would have a solid state supply and use the 800 HT and screen supply.

I want to use KT66s for the JTM45 side.  Does anyone have a voltage preference for the JTM45 and / or an opinion on tube rectification versus solid state rectification for that side?

Thanks
Mike

wasn't a waste of a triode in original plan. the unused triode in the trem osc. bottle was simply used as a gain stage. add the trem to your design. ;-)


--pete

[Mike_J]

This one is Discussed in emmense detail on metroamp 600 page thread.
Pm me email for a schematic of #36 amp

Thanks so much for the schematic.  I am very enthused by what I see.  It is basically just an 800 with a single triode gain stage between the preamp and tone stack.  Waste of one triode but I think I am a grownup now and can probably live with that.  Can definitely use a relay and ground the triode out of the signal if I want the standard 800 amp or switch it in for the Slash setting.

I am thinking of a new PT for the Sluckey inspired JTM45/800/36 stereo build.  It would allow me to completely separate both amp circuits, still only using one PT and giving the JTM45 a GZ34 and its own HT and screen supply.  The 800 would have a solid state supply and use the 800 HT and screen supply.

I want to use KT66s for the JTM45 side.  Does anyone have a voltage preference for the JTM45 and / or an opinion on tube rectification versus solid state rectification for that side?

Thanks
Mike

wasn't a waste of a triode in original plan. the unused triode in the trem osc. bottle was simply used as a gain stage. add the trem to your design. ;-)


--pete

Thanks for the information.  Didn't know that.  Do you know if the Marshall tremolo was very good?

Thanks
Mike

[DummyLoad]

This one is Discussed in emmense detail on metroamp 600 page thread.
Pm me email for a schematic of #36 amp

Thanks so much for the schematic.  I am very enthused by what I see.  It is basically just an 800 with a single triode gain stage between the preamp and tone stack.  Waste of one triode but I think I am a grownup now and can probably live with that.  Can definitely use a relay and ground the triode out of the signal if I want the standard 800 amp or switch it in for the Slash setting.

I am thinking of a new PT for the Sluckey inspired JTM45/800/36 stereo build.  It would allow me to completely separate both amp circuits, still only using one PT and giving the JTM45 a GZ34 and its own HT and screen supply.  The 800 would have a solid state supply and use the 800 HT and screen supply.

I want to use KT66s for the JTM45 side.  Does anyone have a voltage preference for the JTM45 and / or an opinion on tube rectification versus solid state rectification for that side?

Thanks
Mike

wasn't a waste of a triode in original plan. the unused triode in the trem osc. bottle was simply used as a gain stage. add the trem to your design. ;-)


--pete

Thanks for the information.  Didn't know that.  Do you know if the Marshall tremolo was very good?

Thanks
Mike

i do not. may be some samples on youtube...


--pete

[darkbluemurder]

There is a thread on this forum on this topic where Tim Caswell himself posted.


I had this circuit in my SLP RI at one time. Gives you various shades of crunchy tones. It sounded very good for hard rock rhythm guitar, not as much gain as you would expect from 4 gain stages, and nowhere near an SLO overdrive channel.


Cheers Stephan 

[Mike_J]

There is a thread on this forum on this topic where Tim Caswell himself posted.


I had this circuit in my SLP RI at one time. Gives you various shades of crunchy tones. It sounded very good for hard rock rhythm guitar, not as much gain as you would expect from 4 gain stages, and nowhere near an SLO overdrive channel.


Cheers Stephan

Stephan

Thank you for the post.  Would you scrap the overdrive idea and just rely on pedals?  Can't you get the same crunch with pedals?
Many minds greater than mine were trying to get me to do that earlier but I refused to listen choosing to learn the hard way I fear.

Thanks
Mike

[MFowler]

I built Stephan's DBM #39 and it is my favorite hot rod Marshall.  Love the gain it has and you can kick in 2 switches to add more boost.


One and only change I made since the photos were taken was I changed to shielded wire on the effects loop.

[Mike_J]

I built Stephan's DBM #39 and it is my favorite hot rod Marshall.  Love the gain it has and you can kick in 2 switches to add more boost.


One and only change I made since the photos were taken was I changed to shielded wire on the effects loop.

Thank you for your reply.  Are you using just one triode and is the circuitry on the far right side of your board?  Any access to a schematic or layout drawing?  I would like to try this as an option.

Thanks
Mike

[MFowler]

I used Stephan's schematic DBM#39

[Mike_J]

I used Stephan's schematic DBM#39

I am looking at the Darkbluemurder's #39 schematic and comparing it to my 800 circuitry as it is currently wired.  I am quite happy with the tone I am getting from the 800 but would like to add some additional gain as an option.  As I am envisioning it now I could do it one of two ways.  First, my amp now is a Plexi and an 800.  I like the 800 quite a bit more but that  could be because the pull pot is not working on the Plexi side and I can't get it as bright as I want it.  I think the vendors all set their rejects aside until old Mike J calls with an order and they send all those rejects my way because they don't think I am already confused enough.  By the way another pull pot is on its way.  Sorry for getting sidetracked.

If I still like the 800 better than the Plexi, I could make the 800 my clean channel and #39 as the overdriven channel.  The reason I wanted to build this amp in the first place is to supplement my Steel String Singer clone which handles cleans much better than average.  I want this amp to do the dirty side.

If you wouldn't mind answering a few questions for me related to the #39 schematic I would very much appreciate it.  Differences I see between the way my 800 is currently wired and the #39 are as follows:
   - V1a has no 100pF cap from the plate to the cathode.  I guess if you can get away with not using it then it is a good thing.
   - R15, the 1MA pot has no 1000pF across the pot.  I kind of like the bright cap and I have it on a pull out pot so it can be left out of the circuit so I guess mine can be the same if I don't pull the pot out.
   - R16, I am unsure as to the purpose of the 10K resistors feeding the grids of the next tube.  Could you please explain its purpose to me?
   - C5, on my amp it is 470pF and on #39 it is 1000pF which would let more mids through if I am correct.  R25 to ground on my amp is 470K and on #39 it is 390K.  Could you explain why you are lowering the resistance to ground?
   - S2,my amp just has a single 10K resistor.  If you place another 10K resistor across the first one you get a resistance of 5K.  You said the switches added more boost so I am guessing that is what must happen when you lower your cathode resistor from 10K to 5K, correct?
   -V2b is where you are getting your additional gain from if I am not mistaken.  Switching in the .68uF cap then must add more boost, correct?
   -Tone stack.  Using a .047uF bass cap instead of a .022uF.  Should increase the bass response a little.  I have used the .047uF bass cap before with favorable results so I think I understand this change.  The addition of C12, a 1000pF cap from the wiper of the mid pot to ground I am afraid I don't understand.  Could you explain to me what it does?

Another thing I would probably consider is putting V2b on a relay where I could bypass it.  Idea would be having a lower gain 800 and then an overdrive channel on top of it.  If I did that would you suggest changing R10 to a pot and feeding R11 from the wiper of the pot?  What I am thinking is the overdrive channel, having quite a bit more gain, may be a lot louder and the pot would balance the two.  Might ruin the quality of the design but I know in order to make that idea feasible I need to be able to make the more overdriven channel a little louder but not a magnitude of order louder.

Hope I am not asking too many questions but I am really curious about this and still trying to learn.

Thanks
Mike

[MFowler]

You have to be a member of Ampgarage to see the posted photos of the Caswell #39 gut shots: http://ampgarage.com/forum/viewtopic.php?t=21704&highlight=sir


10k are grid blockers and not always necessary but do make for more stable amp.  http://www.valvewizard.co.uk/gridstopper.html


One boost is 10k parallel with another 10k on the cathode of V2a (switch #2)
The other boost switches in a .68uf bypass cap V2b. (switch #1)


The treble peakers 1000pf/470k voltage divider can be to trebly for some people but no problem for me.  I also added resonance with the presence circuit so I can add back in the low end.


The resistor off the mid pot wiper can be other values change to taste.


The .047uf cap just gives darker flavor compared to .022uf.

[2deaf]

- V1a has no 100pF cap from the plate to the cathode.  I guess if you can get away with not using it then it is a good thing.

This cap. rolls off the highs just like a cap. in parallel with the plate resistor or from plate to grid.  If you don't roll off the highs on the first stage of a high gain pre-amp, you get a "crackle" that you cannot get rid of in later stages.  This effect is peculiar to the first stage only.  Not everybody acknowledges the existence of this crackle, but those people also don't acknowledge a whole spectrum of effects that irritate me to no end.

I only use the plate-to-cathode strategy for the reverb driver when I want to limit the high frequencies going to the tank.  For the first stage I use a 10pf to 22pf cap. from plate to grid with a 33K to 47K grid resistor.  The idea here is to roll off just enough to stop the crackle without jeopardizing the frequency response you want.  Every amp seems to require its own combination. 

[2deaf]

- S2,my amp just has a single 10K resistor.  If you place another 10K resistor across the first one you get a resistance of 5K.  You said the switches added more boost so I am guessing that is what must happen when you lower your cathode resistor from 10K to 5K, correct?

S2 with the two 10K's is going to have a minimal effect on gain as compared to S1 with the .68uf cap.  If you make R38 1.8K, the effect on gain would be a lot more.  10K will cause the tube to clip the negative incoming signal sooner than it would with 5K and this is probably what they are primarily aiming for.  You usually see those higher cathode values on the third stage where the signal is larger and the intent is to clip it more (e.g. Bogner, Mesa, Soldano with the later two using 39K).

[2deaf]

-V2b is where you are getting your additional gain from if I am not mistaken.  Switching in the .68uF cap then must add more boost, correct?

Adding a cathode capacitor has a huge effect on gain, but there is more to it than that with the .68uf cap.  This cap. is going to boost the higher frequencies more than the lower and you usually want less bass as you overdrive more.  Funny, Walter Trout just came on Sirius and he apparently doesn't agree. 

[2deaf]

The addition of C12, a 1000pF cap from the wiper of the mid pot to ground I am afraid I don't understand.  Could you explain to me what it does?

They're trying to make the midrange control act more like a mid control by dropping the higher frequencies to ground.  Now when you turn the mid up, only middle frequencies go up instead of mid's and high's which is more like a gain control than a mid control.  In fact, a lot of people put a large pot in the mid slot and call it "gain" or "boost" or whatever.  A large pot also allows more bass to pass through the .022uf cap. because the RC value is higher so that it is even more like gain than mid. 

See the Marshall 2205 tone stack for a similar approach to mid control.

[Mike_J]

The addition of C12, a 1000pF cap from the wiper of the mid pot to ground I am afraid I don't understand.  Could you explain to me what it does?

They're trying to make the midrange control act more like a mid control by dropping the higher frequencies to ground.  Now when you turn the mid up, only middle frequencies go up instead of mid's and high's which is more like a gain control than a mid control.  In fact, a lot of people put a large pot in the mid slot and call it "gain" or "boost" or whatever.  A large pot also allows more bass to pass through the .022uf cap. because the RC value is higher so that it is even more like gain than mid. 

See the Marshall 2205 tone stack for a similar approach to mid control.

2deaf

Great explanation.  I will try that in the future.  Nice to know how to increase mids without increasing the highs at the same time.  How large of a pot would you put in the mid slot to increase gain?

Thanks
Mike

[2deaf]

 

  How large of a pot would you put in the mid slot to increase gain?

I would start with ten times whatever the mid pot was originally and then adjust from there.

In your schematic, the tone stack is post overdrive so that increasing the gain there won't increase the gain in the four-stage pre-amp.  The larger mid pot is typically used when the tone stack is after the first or second stage.  It occurs to me, however, that a switchable mid in a post tone stack could make a cool lead boost since it can increase the master volume while also increasing the lower frequencies (if the master volume is pre PI). 

[darkbluemurder]

  Differences I see between the way my 800 is currently wired and the #39 are as follows:
   - V1a has no 100pF cap from the plate to the cathode.  I guess if you can get away with not using it then it is a good thing.
   - R15, the 1MA pot has no 1000pF across the pot.  I kind of like the bright cap and I have it on a pull out pot so it can be left out of the circuit so I guess mine can be the same if I don't pull the pot out.
   - R16, I am unsure as to the purpose of the 10K resistors feeding the grids of the next tube.  Could you please explain its purpose to me?
   - C5, on my amp it is 470pF and on #39 it is 1000pF which would let more mids through if I am correct.  R25 to ground on my amp is 470K and on #39 it is 390K.  Could you explain why you are lowering the resistance to ground?
   - S2,my amp just has a single 10K resistor.  If you place another 10K resistor across the first one you get a resistance of 5K.  You said the switches added more boost so I am guessing that is what must happen when you lower your cathode resistor from 10K to 5K, correct?
   -V2b is where you are getting your additional gain from if I am not mistaken.  Switching in the .68uF cap then must add more boost, correct?
   -Tone stack.  Using a .047uF bass cap instead of a .022uF.  Should increase the bass response a little.  I have used the .047uF bass cap before with favorable results so I think I understand this change.  The addition of C12, a 1000pF cap from the wiper of the mid pot to ground I am afraid I don't understand.  Could you explain to me what it does?

Hi Mike,


- V1a 100pf cap is an anti-oscillation measure but not needed if your layout is good.
- my first version had a bright cap across the gain pot. Couldn't take it out fast enough but it's a matter of personal preference. Good idea to put it on a switch like you suggested.
- the 10k grid stoppers are intended to guard against blocking distortion but since the stages with the 10k Rk are not high gain they probably can be left out.
- you are correct on the treble peakers. The 390k to ground is an arbitrary value - more gain, use a 470k, less gain, use something like 220k or 270k.
- you are correct on the 10k to 5k Rk change.
- also correct on V2b.
- the 1000pf cap from the mid pot's wiper is effectively in parallel with the mid pot and bleeds frequencies above a certain breakpoint to ground. That breakpoint is lower, the higher the resistance in parallel to the cap gets. At lower mid pot settings the cap is barely audible if at all. At higher settings it bleeds some of the higher frequencies to ground, thereby counteracting the usual effect that increasing the mids also increases the highs. I first saw this in a Diezel VH-4.
EDIT: just saw 2deaf answered that already.

Hope that helps.
Cheers Stephan 

[darkbluemurder]

S2 with the two 10K's is going to have a minimal effect on gain as compared to S1 with the .68uf cap.  If you make R38 1.8K, the effect on gain would be a lot more. 

Yes, and that was exactly what I wanted. First I had the 0.68uf also on S2, and found it was simply too much.


Cheers Stephan

[Mike_J]

Hi Mike,


- V1a 100pf cap is an anti-oscillation measure but not needed if your layout is good.
- my first version had a bright cap across the gain pot. Couldn't take it out fast enough but it's a matter of personal preference. Good idea to put it on a switch like you suggested.
- the 10k grid stoppers are intended to guard against blocking distortion but since the stages with the 10k Rk are not high gain they probably can be left out.
- you are correct on the treble peakers. The 390k to ground is an arbitrary value - more gain, use a 470k, less gain, use something like 220k or 270k.
- you are correct on the 10k to 5k Rk change.
- also correct on V2b.
- the 1000pf cap from the mid pot's wiper is effectively in parallel with the mid pot and bleeds frequencies above a certain breakpoint to ground. That breakpoint is lower, the higher the resistance in parallel to the cap gets. At lower mid pot settings the cap is barely audible if at all. At higher settings it bleeds some of the higher frequencies to ground, thereby counteracting the usual effect that increasing the mids also increases the highs. I first saw this in a Diezel VH-4.
EDIT: just saw 2deaf answered that already.

Hope that helps.
Cheers Stephan

Stephan

Thank you for your response.  I am about to get back to this amp and do some more tweaking.  I will try to eliminate 100pF cap on the first gain stage as you mentioned and will add capacitance incrementally as suggested by 2deaf if required.

I have attached the schematic for the amp I am currently building.  It is based loosely on Sluckey's Plexi/800 schematic and layout drawings.  I added a second tone stack, so each preamp would have its own tone stack, an overdrive channel (which I will be eliminating to try to implement your #39 values, an active effects loop (three know Dumbleator) and PPIMV.

My goal has been to keep the Plexi values at the '68 level and use the 2203 values Sluckey used.  I finally got a replacement 1MA pot with the SPST switch in it so hopefully I can brighten up the Plexi side a little bit because it needs it.  I really like the 800 side.  I am trying to decided how best to implement the #39 circuitry into my amp.

My initial thought was to throw a few more relays in it and switch values in and out on a footswitch.  Also, did you ever give thought to putting R25 on your schematic on say a 250K resistor with a 345K resistor in series versus the fixed 390K resistor?  I think you mentioned that would give the amp quite a bit of variability.  Don't know if that is good or bad.

Right now I have a mixer circuit that mixes the Plexi preamp and the 800 preamp and sends the output to the overdrive circuitry which is from a Dumble ODS I constructed and squeals at high volumes.  Would you change the overdrive circuit (V2b on you schematic) so it only comes off the 800 preamp or would you leave it so that either channel could be boosted by it?  I do not play both channels at the same time as they are out of phase with each other.  Use an A/B/Y box and switch back and forth between preamps.

In reference to the relays, your schematic shows a 1000pF cap for C5 whereas I have a 500pF cap on my 800.  Otherwise, with the exception of the 10K grid resistors the 800 circuitry I have should be useable with the exception of R38 which I would place on relays and call them a boost.  C9 would be on a relay and would be part of the overdrive system (your V2b) that I would be able to switch in and out using the same idea the amp already has.  (Ground out signal and bypass your V2b (would be my 5a) and send the signal to the active fx circuitry or run it through you circuitry).

Thanks
Mike

[Mike_J]

Attached is a layout incorporating Darkbluemurder's #39 into the 800 circuit via footswitch.  It also has the two boosts on toggle switches.  All the changes are reflected in red.  If anyone sees an error I would appreciate their input.  There is more shielded wiring than I would prefer.  Don't know if any of it can be eliminated.

I am thinking I will do one thing at a time instead of my usual shotgun approach.  Probably will start by removing the 100pF cap off V3 and see how that works.  Then I was thinking I would try the 1000pF cap from the wiper of the mid pot to ground and see how I like that.  Changing the 820R resistor to a 1K on V4a shouldn't have that large of an effect I wouldn't think.  Then I will rewire V5 to the #39 specs.  Last I will check the 10K and .68 switches probably by wiring them into the circuit one at a time to see if I like the effect before I place them on switches.
 
There sure is a big fuss about this mod so I expect it will probably be better than average.  I left the 10K grid resistor off V3.  If needed it can be added relatively easily.

One thing I didn't talk about earlier is the change to the presence circuitry.  I have a .1uF cap from the wiper of a 5K pot to ground and the #39 schematic shows a .68uF cap from the wiper of a 25K pot to ground.  My pot output (my word, not technically correct) goes straight to ground whereas the #39 schematic has a 4.7K resistor between the pot and ground. Could anyone explain what impact the differences would have?   

The #39 schematic shows .1uF caps in the PI circuit which would allow more bass through from my experiences.  I am going to try it both ways and see which I prefer.

My thanks for everyone's input.  Very informative.

Thanks
Mike

[Mike_J]

Decided to wire the extra triode on a relay so it can be switched in and out of the circuit.  Also wired the 10K cathode resistor on a switch and the .68 cathode cap on a switch.  Huge increase in gain and volume when the overdrive is switched in especially when the .68 cap is switched on.  May not be possible to keep it on a relay for that reason.  Amp has a Plexi channel and an 800 channel.  Wasn't a big difference between their gains before this mod.  Definitely is now when the extra triode is added.

The biggest problem so far is the amp goes from quiet to having a lot of white noise when the overdrive (extra triode) is kicked in.  Does anyone have a solution to reducing the white noise?  Very satisfied with the tone otherwise.

Haven't installed the 1000pF cap on the mid pot or removed the 100pF cap across the input triode from the plate to the grid.  Decided to start with installation of the additional triode first.

Any suggestions regarding reducing the white noise would be very much appreciated.  If I can't get rid of the white noise the advantage of the relay is I can switch the extra triode off and eliminate the white noise.  The Plexi and 800 channels are switch via an A/B/Y box which is working well.

Thanks
Mike