Need help with Sluckey inspired Plexi/800 with Slash OD mod , active fx & PPIMV

[Mike_J]

While waiting for resistors to adjust the power string for my Sluckey inspired Plexi/800 amp, I did some research on the Slash tone.  Bakerlite was kind enough to refer me to a post on the Metro Amp forum that is over 600 pages long  that discusses the amp he used in great deal.  Bakerlite also sent me a schematic showing the circuitry.

The reason for doing this is because I am concerned that if the existing overdrive I put in the amp works properly it will sound more like a Mesa than a Marshall.  My overdrive specials sound enough like Mesa's so I have that covered.

I prepared a new schematic that has the S.I.R. #36 mod (S.I.R. is the amp Slash used to record the Sweet Child of Mine album) as a foot switchable effect.  This would replace the overdrive circuitry I currently have in the amp.  I also made some changes to the schematic to make a few corrections.

The only thing that is not shown on the schematic is a 500pF capacitor that should be placed across the V3b 100K plate resistor according to the S.I.R. #36 schematic.  The schematic is identical to Sluckey's 800 circuit except for this cap and a slight change to the grid resistor values.  The purpose of this cap is to prevent the amp from squealing at higher gain settings or to avoid parasitic oscillation.

I would rather not place this cap across the V5a plate resistor because the 800 circuit will not be authentic with this cap on V3b.  I would rather place it across the plate resistor of V5a where it would be switched out when the overdrive setting was switched out.  Does anyone know if it would work to make that change (switching the cap from V3b to V5a)?

Thanks
Mike

[2deaf]

You can wire that DPDT switch to toggle between 3 and 4 stage while also hooking and unhooking the 500pf cap on V3b.  You're going to want that cap. in four stage mode.

[Mike_J]

You can wire that DPDT switch to toggle between 3 and 4 stage while also hooking and unhooking the 500pf cap on V3b.  You're going to want that cap. in four stage mode.

2deaf

Thank you for the reply.  The DPDT will actually be a relay so it can be switched with a footswitch.  Made a mistake on my first post.  Wanted to know if I could place the cap across the V5a plate resistor instead of the V3b resistor.

Thanks
Mike

[2deaf]

I didn't state it right.  You're going to want that cap. on V3b in four stage mode.

[Mike_J]

I didn't state it right.  You're going to want that cap. on V3b in four stage mode.

Thank you.  That is clear.  Do you have an opinion regarding placing the .1uF cap across the 10K resistor on V3b?  I left it off the schematic because I was trying to keep the 800 to spec when the OD (V5a) is switched out but if I am going to put a cap across the plate resistor I might as well add the cap across the cathode resistor while I am at it.

Thanks
Mike

[2deaf]

Do you have an opinion regarding placing the .1uF cap across the 10K resistor on V3b?

These kind of things are a matter of taste.  It changes the gain and the frequency response, so you can use it and variations to alter the sound to your liking.

Attached is one possible way to toggle.  You may want Cx in order to alter the sound some more.

[Mike_J]

Do you have an opinion regarding placing the .1uF cap across the 10K resistor on V3b?

These kind of things are a matter of taste.  It changes the gain and the frequency response, so you can use it and variations to alter the sound to your liking.

Attached is one possible way to toggle.  You may want Cx in order to alter the sound some more.

Thanks for the drawing.  I suppose I could put the caps on a separate relay and tie that relay and the OD relay together on the footswitch so when the OD is off so are the caps.  Like you said, will have to see if I like it first.

Thanks
Mike

[jojokeo]

Mike,

As 2deaf says, you could easily use the frequency reducing caps aka load resistor caps or as tubenit calls them "smoothing caps" on most if not all of the stages which could still yield enough high end for you to give you smoother overdriving sounding distortion rather than brittle or buzzy sounding. The stage w/ only a 10k cathode resistor helps with this also. If you bypass it, you will make the result and bias much hotter but more harsh than w/out it. With all of those stages you should try mixing and matching various stages and see where it takes you. I wouldn't get caught up in what "exactly" Slashes' amp used. I'm sure his was dialed in for exactly that particular amp and his tastes. Yours is going to be slightly different no matter how much attention to detail you adhere to. In the end it should be what YOU like. Also, sometimes due to layout, grounding, parts placements, components, etc. - your end result may dictate for you what you need for you to get things just plane working well w/out parasitics, oscillations, or other quirks? The more stages the more potential for this stuff in orders of magnitude.

[jeff]

In the schematic the switch has high voltage on one lug(tied to plate). Is that OK? Dunno why but that kinda makes me nervous.

[2deaf]

Standby switches frequently have one lug connected to the highest voltage in the amp.

[jeff]

True.
I guess what makes me nervous may be psycological.

 If someone was playing an amp with a loose power or standby switch they would think 'uh-oh this is really bad I need to get this fixed immediatly'.

 But if a mini toggle got loose that same person may think 'Oh well, it's just the bright switch' and not worry about it.
 I don't know if a loose tone switch would demand the same urgency as a loose standby switch to yer average player and that's what makes me nervous.

Nothing wrong with it, and there's no problem if it never gets loose, just be aware that tone switch has high voltage on it.
 

[2deaf]

Standby switches do make me nervous having been bit by B⁺ twice in my lifetime.

[jojokeo]

Shoot I got bit by 400v's just yesterday in fact taking final measurements I can not recall all the times over the years. It pisses me off every time in just the same way too.

[Mike_J]

In the schematic the switch has high voltage on one lug(tied to plate). Is that OK? Dunno why but that kinda makes me nervous.

Everyone has good points.  The high voltage will be across a 100K plate resistor on a 5 volt relay switch.  I haven't checked to see what voltage they can handle but I certainly need to know before I wire it.

Thanks
Mike

[jeff]

 Well if the idea is to swap in and out a 500pF cap, how's about putting 1000pF from plate to switch and 1000pF from switch to B+? That way there's no DC and when the switch is closed you have two 1000pF in series for a total of 500pF.


   1000pF    SW    1000pF
------ll------o/ o------ll-------
 

[Mike_J]

Well if the idea is to swap in and out a 500pF cap, how's about putting 1000pF from plate to switch and 1000pF from switch to B+? That way there's no DC and when the switch is closed you have two 1000pF in series for a total of 500pF.
   1000pF    SW    1000pF
------ll------o/ o------ll-------
 

Jeff

Thank you for the response but unfortunately what you are saying went way over my head.  I am posting the drawing I made for what I am trying to accomplish.  It was part of a different post I had but is relevant to what we are talking about.

If you look at the drawing are you saying that if I put a 1000pF cap from the plate to the com of the switch and another 1000pF from the n/o connection on the switch to B+ there will be no B+ on the switch.  I think I recall somewhere that two 1000pF in series is equal to a 500pF cap whereas two in parallel is equal to 2000pF.  Please correct me if I am wrong.

Thanks
Mike

[sluckey]

If you are considering using Doug's 5V DIP relays just be aware that the contacts are rated 1amp at 30vdc.

[Mike_J]

If you are considering using Doug's 5V DIP relays just be aware that the contacts are rated 1amp at 30vdc.

Sluckey

Yes, those are the relays I would use.  If I put the 1000uF caps on each side of the relay as Jeff suggested will it keep dc off the switch (relay)?  I apologize for such ignorance but it is too important to get this right.

Thanks
Mike

[jojokeo]

Mike - I think you just want to figure out what's needed and leave it in or not. I've not seen caps like this to actually be on a switch to have as an option, there's other things more important to switch and control. These are either needed/wanted or not in most all cases.

[Mike_J]

Mike - I think you just want to figure out what's needed and leave it in or not. I've not seen caps like this to actually be on a switch to have as an option, there's other things more important to switch and control. These are either needed/wanted or not in most all cases.

Jojokeo

It is my impression that these caps become necessary when the extra gain stage is added.  I don't know what they would do with the 800 as Sluckey drew it.  I guess I could find out by placing the caps in the Sluckey inspired Plexi/800 amp and see if I like the sound.  My intention was to leave the 800 section intact which is why my idea for the switch came about.

Thanks
Mike

[2deaf]

If you are considering using Doug's 5V DIP relays just be aware that the contacts are rated 1amp at 30vdc.

That's way more than you need for the AC signal crossing the contacts, but how much DC voltage does it take to make it break down and short through the housing or to another pin?

[2deaf]

Well if the idea is to swap in and out a 500pF cap, how's about putting 1000pF from plate to switch and 1000pF from switch to B+? That way there's no DC and when the switch is closed you have two 1000pF in series for a total of 500pF.

That's a pretty good idea.  If you do this, you should consider hooking the cap.'s together with a resistor because dangling cap.'s in a high gain amp can easily cause problems.

[Mike_J]

If you are considering using Doug's 5V DIP relays just be aware that the contacts are rated 1amp at 30vdc.

That's way more than you need for the AC signal crossing the contacts, but how much DC voltage does it take to make it break down and short through the housing or to another pin?

According to Sluckey 30vdc.  There would be close to 300vdc going to that tube from the power string.  I know caps are supposed to block DC because I have tested coupling caps for DC and they are usually below 100mVdc.  I don't know if a silver mica would do the same thing but I would expect it would.  However, I would not take the chance of using them without getting a thumbs up from someone like Sluckey or HBP or another person with considerable more knowledge of the subject than I have.

Thanks
Mike

[Mike_J]

Well if the idea is to swap in and out a 500pF cap, how's about putting 1000pF from plate to switch and 1000pF from switch to B+? That way there's no DC and when the switch is closed you have two 1000pF in series for a total of 500pF.

That's a pretty good idea.  If you do this, you should consider hooking the cap.'s together with a resistor because dangling cap.'s in a high gain amp can easily cause problems.

I am pretty sure I can find a way to make them stable and not dangly.  Jeff's idea, if I understand it properly, is to place a capacitor on each side of the switch to keep DC voltage off the switch.  It sounds like a very good solution to me.  How would you incorporate the resistors into the circuit?

Thanks
Mike

[2deaf]

I didn't mean physically dangling, I meant open ended.  Sorry about my awkward style.  I would put a resistor between the two cap.'s and then short the resistor with the switch.

[Mike_J]

I didn't mean physically dangling, I meant open ended.  Sorry about my awkward style.  I would put a resistor between the two cap.'s and then short the resistor with the switch.

2deaf

I have attached version 2 of the Two Relays - One Switch schematic.  It contains Jeff's idea of two 1000uF caps on each side of the relay.  I also planned on using shielded two conductor wire because we are in high gain territory here.

If it wouldn't be too much of an imposition would you mind drawing in the resistor(s) you are speaking of?

Thanks
Mike

[jeff]

If your first idea was to have one end of the 500pF connected to B+ and the other connected to the switch
B+----ll---o/o-----------------plate
remove this wire ^ like so.
B+----ll---o/o--x          x----plate
replace with cap like so.
B+----ll---o/o-----ll----------plate
On your schematic just replace the line between the switch and plate with a cap symbol..

[2deaf]

I have attached what I meant.  Maybe it isn't any big deal to have one end of a cap. open, but I never do it.

[PRR]

.

[2deaf]

He has a 33K resistor penciled-in in front of the grid, so the cap. would probably 22pf or maybe even less.  Could also go to the cathode.  The 10K on the cathode throws me a little bit, but that cap would probably be close to 470pf.   

[jeff]

 I agree w/ 2deaf's schem.
I don't want to get too sidetracked, but what prog do you use to make such nice looking schematics?

[Mike_J]

I have attached what I meant.  Maybe it isn't any big deal to have one end of a cap. open, but I never do it.

2deaf

In as simple terms as possible, what does the 1M resistor do.  Sorry for my ignorance.

Thanks
Mike

[Mike_J]

If your first idea was to have one end of the 500pF connected to B+ and the other connected to the switch
B+----ll---o/o-----------------plate
remove this wire ^ like so.
B+----ll---o/o--x          x----plate
replace with cap like so.
B+----ll---o/o-----ll----------plate
On your schematic just replace the line between the switch and plate with a cap symbol..

Jeff

In reply 25 I somehow forgot to attach the drawing last night.  Would you check it to see if it is okay.

Thanks
Mike

[sluckey]

Mike, did you ever see this? It's the Staalhoof mod into an 18 watt power amp.

     http://home.comcast.net/~seluckey/Aro_Project.pdf

[2deaf]

what prog do you use to make such nice looking schematics?

It's a free, downloadable CAD called Jschem.

[Mike_J]

Mike, did you ever see this? It's the Staalhoof mod into an 18 watt power amp.

     http://home.comcast.net/~seluckey/Aro_Project.pdf

Sluckey

No, I never did see that but thanks for sharing.  Has two additional gain stages it looks like and uses 100pF caps across the plates of the overdrive circuit and its cathode resistors are 10K and bypassed.  So it does what I am trying to do but it uses two triodes.  My design is wasting a perfectly good triode because I am not using it.  I am kind of feeling ashamed of myself seeing this if I am reading it correctly. 

Do you see what I am trying to do here?  First off, I have an extra chassis that I already made a cabinet for and a 100 watt power transformer that will be in the way if I don't put it in an amp.  I like the first amp I built from your design.  Just tried the active effects with my TC G Sharp and it worked well so everything seems to be working.  On this amp I want to continue with the same basic theme but instead of just having two tone stacks I want to also add two output sections so I can run two sets of tubes, KT66s and EL34s in stereo.  This also should get us around the problem of the two preamps being out of phase.  I want to use the EL34s with your 800 circuit.  Whatever overdrive circuit works best needs to be foot switchable or I lose the 800 amp.

Thank you for you assistance, it is invaluable.

Thanks
Mike

[Mike_J]

what prog do you use to make such nice looking schematics?

It's a free, downloadable CAD called Jschem.

I thought he was asking me about the quality of my drawing.  If so, same cut and paste technique taught to me by my kindergarten teacher.

Thanks
Mike

[2deaf]

In as simple terms as possible, what does the 1M resistor do.

You're dealing with four-stage preamps, so you're probably aware of the positive feedback problems with the improper placement of wires and components.  Open ended components seem to make this problem even worse.  Sometimes keeping the circuit complete reduces the click when you switch, other times it doesn't seem to matter.  It probably depends on what is causing the click.

[Mike_J]

In as simple terms as possible, what does the 1M resistor do.

You're dealing with four-stage preamps, so you're probably aware of the positive feedback problems with the improper placement of wires and components.  Open ended components seem to make this problem even worse.  Sometimes keeping the circuit complete reduces the click when you switch, other times it doesn't seem to matter.  It probably depends on what is causing the click.

2 deaf

Thanks for the easy to understand answer.  The 10K resistor is un-bypassed in an 800 if my memory is working properly.  The S.I.R. mod adds one triode of gain but requires these caps be placed on the 800 V2b plate and cathode resistors to tame the gain when the extra triode (gain stage) is added.  I want the 800 circuitry to be correct as it is a classis amp in its own right.  By making the changes to the circuit suggested by you and Jeff I should be able to accomplish what I was trying to do.

Thanks
Mike

[Mike_J]

I have attached what I meant.  Maybe it isn't any big deal to have one end of a cap. open, but I never do it.

2deaf

Would you look at the attached layout and verify that I have the resistor in the correct place.

Thanks
Mike

[jojokeo]

In as simple terms as possible, what does the 1M resistor do.

You're dealing with four-stage preamps, so you're probably aware of the positive feedback problems with the improper placement of wires and components.  Open ended components seem to make this problem even worse.  Sometimes keeping the circuit complete reduces the click when you switch, other times it doesn't seem to matter.  It probably depends on what is causing the click.

2 deaf

Thanks for the easy to understand answer.  The 10K resistor is un-bypassed in an 800 if my memory is working properly.  The S.I.R. mod adds one triode of gain but requires these caps be placed on the 800 V2b plate and cathode resistors to tame the gain when the extra triode (gain stage) is added.  I want the 800 circuitry to be correct as it is a classis amp in its own right.  By making the changes to the circuit suggested by you and Jeff I should be able to accomplish what I was trying to do.

Thanks
Mike

Mike - what 2deaf is saying is that he's alerting you about the troubles & issues with oscillations &/or parasitics in mulit-gain sections amps - same as what I & others have been saying but he's also anticipating the "pop" when switching caps in a similar fashion. This is similar to what occurs when switching a bypass cap on a cathode resistor (if not using one) or in effects pedals it's called a "pull down resistor" to elimintate the "pop" turning them on and off. The load resistor cap is only ONE way to deal with oscillations/brittle response. Caps from plate to cathode, grid to ground, even plate to grid w/ high resistance & cap (aka sm. feedback loop), all accomplish the same things in dealing with the issue AND take the brittle/buzzy edge off of high gain stages cascaded.

Don't get hung up on only one way to do something or because it was done on a specific amplifier. Don't worry - you'll get where & what you want in the end and that's all that matters. You won't know EXACLTY what you will want (and need?) until you get the circuit up and running. No matter how much planning & worrying won't help you and only delay your project further. Build it first, play it, and try things out. THEN start using the techniques to see what's needed and where. Even the values are subject to change because your amp will have different components, layout, lead dress, grounding etc. which all play a significant role in amps like this.

My suggestion is to simply put the load resistor cap on the extra gain stage itself - permanently (and maybe another before or after it if wanted or needed?). The extra gain stage is where you're going to want it anyway and it'll already have it to do it's job - no switching necessary. So again, those load caps are either used for taking off high end, providing a "smoother" high gain, or in many cases used as "remedies" or "corrective measures" for parasitic oscillations do to sub-par circuit issues as described earlier. These are rarely "designed in" using exact values or in exact places because you never know what value or where you're going to need them until you start the testing, debugging, & voicing stages.

[2deaf]

About that relay.
If this device conforms to FCC part 68 (and you are going to have a hard time selling it in the U.S. if it doesn't) it will withstand a surge voltage of 1500V.  If it withstands a surge of 1500V, it should withstand a constant voltage of one-half that (750V).

[Mike_J]

Jojokeo

I agree to what you are saying but you have to start somewhere and I am still learning.

My first impression was exactly the same as yours.  Why not just put the cap across the last gain stage plate resistor.  Someone said it should be on the stage before it.  I think I will try what you are suggesting as it is the additional gain stage that creates the instability thereby requiring the cap across the plate resistor.

I have had no problems yet with the Sluckey inspired Plexi/800 amps overdrive using my Strat at high volumes.  I am actually fairly satisfied with the sound I am getting.  A friend is bringing his Les Paul over later this week and we will see what happens with hotter pickups.  The advantage of the overdrive in the amp is the overdrive circuit has a high level of adjustability in it.

Thanks
Mike

[Mike_J]

About that relay.
If this device conforms to FCC part 68 (and you are going to have a hard time selling it in the U.S. if it doesn't) it will withstand a surge voltage of 1500V.  If it withstands a surge of 1500V, it should withstand a constant voltage of one-half that (750V).

That would seem to limit the use of the relay to the signal path, or can I still use the cap across the cathode resistor, shouldn't be more than a couple of volts dc there?  Don't think there is a chance of it being able to handle plate voltages if the surge requirements are that high but I would think 2K volt ceramic caps on each side of the relay would provide protection.  Am I wrong there? 

What I learn from time to time is that some of the ideas I have will work with the use of a proper device.  However, the device does not exist in many cases or is impractical for use in a guitar amp.  Don't know until you explore the possibilities. 

There are a lot of knowledgeable people on this forum, including you.  It is nice to have a place to explore ideas even though often times the ideas don't have a practical application.

Thanks
Mike

[2deaf]

There are two different ratings I am talking about.
 The first one is how much current at a given voltage you can run across the contacts.  If you increase the voltage, you have to decrease the current in order to stay within the ratings and vice versa.  For your use, you are easily within the ratings.

The second one is how much voltage it takes to short through the insulation, pole to pole, or pole to coil.  750V constant is way more than what you are going to subject it to.

Just don't run 300V across the contacts.

[Mike_J]

There are two different ratings I am talking about.
 The first one is how much current at a given voltage you can run across the contacts.  If you increase the voltage, you have to decrease the current in order to stay within the ratings and vice versa.  For your use, you are easily within the ratings.

The second one is how much voltage it takes to short through the insulation, pole to pole, or pole to coil.  750V constant is way more than what you are going to subject it to.

Just don't run 300V across the contacts.

I am so confused.  As Sluckey wrote in an earlier post the contacts of the relay I would be using are rated for 1amp at 30vdc.  Given I have a 450mV B+ winding and most Plexi PTs are 290 mV I would think we are more than covered in the first step above.  In the second step, if 1000 pF at 2kvdc ceramic caps are used on both contacts then I wouldn't think the contacts would see 300mVdc let alone 300vdc.  Is this making any sense?

Thanks
Mike

[jojokeo]

I am so confused.  As Sluckey wrote in an earlier post the contacts of the relay I would be using are rated for 1amp at 30vdc.  Given I have a 450mV B+ winding and most Plexi PTs are 290 mV I would think we are more than covered in the first step above.  In the second step, if 1000 pF at 2kvdc ceramic caps are used on both contacts then I wouldn't think the contacts would see 300mVdc let alone 300vdc.  Is this making any sense?

Now you're confusing me,  "...have a 450 mV B+ winding and most Plexi PTs are 290 mV"

Those would be actual Volts not 1/1000ths (aka milli) Volts.
*If you were to put a cap on the B+ side of your DC voltage prior to the switch's contacts then you shouldn't have any (practically speaking of course) DC voltage past that which will be applied to your contacts. You should be safe in doing that (if you still feel you MUST do this ;) ). See for yourself; use a test lead and temporarily attach a cap to one of your amp's B+ points and make a DC voltage measurement to ground and note your reading.

[Mike_J]

I am so confused.  As Sluckey wrote in an earlier post the contacts of the relay I would be using are rated for 1amp at 30vdc.  Given I have a 450mV B+ winding and most Plexi PTs are 290 mV I would think we are more than covered in the first step above.  In the second step, if 1000 pF at 2kvdc ceramic caps are used on both contacts then I wouldn't think the contacts would see 300mVdc let alone 300vdc.  Is this making any sense?

Now you're confusing me,  "...have a 450 mV B+ winding and most Plexi PTs are 290 mV"

Those would be actual Volts not 1/1000ths (aka milli) Volts.
*If you were to put a cap on the B+ side of your DC voltage prior to the switch's contacts then you shouldn't have any (practically speaking of course) DC voltage past that which will be applied to your contacts. You should be safe in doing that (if you still feel you MUST do this ;) ). See for yourself; use a test lead and temporarily attach a cap to one of your amp's B+ points and make a DC voltage measurement to ground and note your reading.

You are correct.  I meant 450mA not 450mV.  The contacts on the relay have a 1A rating.  Sorry for the confusion. 

Thanks
Mike