Dr Z EMS Touch Control - what is it?

[PharmRock]

While doing some research for a JTM45 build I am putting together with the idea of using a switchable SS vs tube rectifier, I came across this thread https://el34world.com/Forum/index.php?topic=26441.0 that discusses the Dr Z EMS amplifier, which is a JTM45-ish amp that happens to have switchable SS/tube rectification. 

Also on this amp is a "touch control", which seems (to the best I can tell) inserts a 0.047uF cap from the first filter stage/OT node to ground (see pic).  I am trying to piece together what else is going on just based on what I can see, but honestly can't tell.   And...as many of you know...I'm still in the pre-K stages of learning this stuff.  I've attached an image that highlights A) what I know is the "touch" switch and B) what I think it is switching in/out. 

Can any of you experts offer up an explanation for what this is, as well as what the effect would be on the tone (i.e. what is the "touch" referring to)?

Thanks!

[sluckey]

Post a schematic if you want more than just speculation.

[PharmRock]

Post a schematic if you want more than just speculation.

Unfortunately, I can't seem to locate one.  I'll keep looking.

[jordan86]

I doubt you’ll find a schematic. I’m learning too but I suspect the switch activates the second big filter cap that you see there, effectively doubling or halving the first filter stage capacitance. The .047 cap may just be there to prevent popping. 

Doubling the capacitance would firm up the feel or “touch”. Halving it would be a looser and more open feel. 

[Willabe]

I’m learning too but I suspect the switch activates the second big filter cap that you see there, effectively doubling or halving the first filter stage capacitance.

Those 2 big caps are in series, their only 300v for 600v total, so their not using a single cap.

[echuta13]

It could also be there as a bypass around the bigger caps (common in Hi-Fi stuff)? 

[d95err]

Looks like the two big electrolytic caps are in series and permanently in the circuit. They do not seem to be affected by the switch.

The key to the switch function is the brown wire connected to the right side of the yellow cap. This is connected to ground when the switch is closed. When the switch is open, the cap is in series with the brown wire.

If you can find out where the brown wire goes, we may be able to figure it out. 

EDIT: hmm… on second thoughts, it just seems the switch connects the yellow cap in parallell with the filter caps. That should make very little difference. I think there is something else we can’t see here…

[John G]

Hi,
Premier Guitar mag did a review quite some time ago.
The .047uF I think bypassed the bottom ?? Filter cap ??
I will see if I can find the review.
John

[John G]

Hi,
Guitar player.


https://www.dropbox.com/s/hte66ykpa3ammuf/EMS%20BB212%20Guitar%20Player%20ReviewAUG%2017.pdf?dl=0


John

[d95err]

Hmm... having a small cap between the center tap and ground doesn't make sense to me (or does it?). I'm thinking it's a misunderstanding by the journalist. It doesn't match what I interpret from the picture either (I could be wrong of course!).

I love these types of investigations. It's a fun challenge!

[pdf64]

That review states that the touch switch just connects a 47nF cap across the HT, but it would be better to verify the circuit. There’s 2 red wires and a brown wire connected to that node, which need to be tracked and identified.

[d95err]

Theory:

The two 220u caps are the first filter node. If they have high ESR (internal resistance) caps, the response to peaks could be a bit sluggish or saggy. Switching in the 47n cap could get a faster peak response.

Found some data sheets for similar IC caps, listing EC of about 1.5 ohms (or about 3 ohms, since it's two in series). Not sure if 3 ohms would make a noticeable difference though.

Just a wild theory... feel free to shoot it down! :)

[PharmRock]

I think Dr Z is known for doing some circuit changes involving the OT primaries, such as a conjunctive (or corrective) filter as seen on his Carmen Ghia amp (borrowed from the Watkins Dominator).  But IIRC there is usually a resistor in series with the cap.  Not saying this is what we're looking at but maybe along the same lines.

The brown wire appears to begin from the rectifier switch, which is adjacent to the impedance selector.  It (together with a black and red wire in a twisted bundle) follows the back of the chassis and then along the right side and comes up under the board twisted together with a black (or maybe blue) wire by the right-most edge of the board.  Perhaps the red wire pair is the OT center tap and one of the choke wires?

[pdf64]

Theory:

The two 220u caps are the first filter node. If they have high ESR (internal resistance) caps, the response to peaks could be a bit sluggish or saggy. Switching in the 47n cap could get a faster peak response.

Found some data sheets for similar IC caps, listing EC of about 1.5 ohms (or about 3 ohms, since it's two in series). Not sure if 3 ohms would make a noticeable difference though.

Just a wild theory... feel free to shoot it down! :)

Kudos on checking the ESR spec!
I’m not sure it’s a go’er though, eg otherwise 1ohm cathode current sensing resistors would have a rep as tone suckers.
It may be notable that the review didn’t seem to mention the touch switch.

[SILVERGUN]

From the manual:
"Attenuates the transient response and pick attack. Set to down position to attenuate attack or for bright single coils. Set to up position for fast transient response and cut"


TRANSIENT RESPONSE Electronics Assignment Help and Homework Help - TRANSIENT RESPONSE Project Help (electronicsassignments.com)

[d95err]

Theory:

The two 220u caps are the first filter node. If they have high ESR (internal resistance) caps, the response to peaks could be a bit sluggish or saggy. Switching in the 47n cap could get a faster peak response.

Found some data sheets for similar IC caps, listing EC of about 1.5 ohms (or about 3 ohms, since it's two in series). Not sure if 3 ohms would make a noticeable difference though.

Just a wild theory... feel free to shoot it down! :)

Kudos on checking the ESR spec!
I’m not sure it’s a go’er though, eg otherwise 1ohm cathode current sensing resistors would have a rep as tone suckers.
It may be notable that the review didn’t seem to mention the touch switch.

I did check ESR for a few other cap brands and they seemed to have somewhat lower ESR (like 0.5 ohms) for similar caps. Still seems unlikely that it would make a significant difference though.

[jordan86]

So it’s like a dull cap on the first B+ node, similar in concept to what you would see on V1 on a Friedman or higher gain marshall style amp? Usually a 500pf from plate to ground.

[Willabe]

So it’s like a dull cap on the first B+ node, similar in concept to what you would see on V1 on a Friedman or higher gain marshall style amp? Usually a 500pf from plate to ground.

Electrolytic caps distort on the frequency extreme ends and are slow in response on the high frequencies compared to film caps.

So they put a small film cap in parallel with the lytic filter cap to clean up the highs and speed up the high end response.

Hi-Fi guys do this. You used to be able to only get so large a value film cap, now you can get large value film caps, but their very large in size too. Any where from, say, 0.47uF  to 1uF should work. The bigger the better within reason. Some put it across the 1st B+ filter cap, others put it across the last (preamp tube) filter cap.

[John G]

Hi,
Looks as if is across the first PSU node to grnd when switched.
Mesa Boogie do this, I always thought it was to squelch any arching when activating the STBY switch ??
While the sluthing hat is on, can anyone tell me why the green cap across V2 cathode resistor(820 ohm) is smaller than its brother across V1 2k7 cathode resistor, it was reported as both being 0.68uF.
Different voltage ? smaller value ??....maybe.
May be I need to get a full time job !!
John

[d95err]

Hi,
Looks as if is across the first PSU node to grnd when switched.
Mesa Boogie do this, I always thought it was to squelch any arching when activating the STBY switch ??
While the sluthing hat is on, can anyone tell me why the green cap across V2 cathode resistor(820 ohm) is smaller than its brother across V1 2k7 cathode resistor, it was reported as both being 0.68uF.
Different voltage ? smaller value ??....maybe.
May be I need to get a full time job !!
John

Most likely, one of the caps are not 0.68uF. Or, the good doctor simply picked two different types of caps from the box.

[SILVERGUN]

I tried this concept late last night, with a .047uf cap across the 100uf electrolytic in my current bench tweaker SE EL34. I also had a rotary switch with a bunch of values that I was able to switch through, but they were .01 and lower. I went back to the .047 because that seemed to sound the best of the bunch. I also tried a .1uf but veered back to the .047.

Disclaimer: It was after my band practice and I had some ear fatigue but I did seem to prefer the response with the cap in there. I've got a couple other builds laying around that I can experiment with and my initial report is that there is something here for us tweakers to tweak.

This specific SE build would get a little "ratty" at higher volumes and the cap seemed to help.
Nothing scientific here but it will prompt me to take the scope and signal gen. to it this weekend.

It is a simple-enough "try it and see" mod. Just be super-careful because we are talking about 400+ volts.
Thanks for bringing this up PharmRock. 

[shooter]

take the scope and signal gen. to it this weekend.

 


if you can't see it on a scope, you can't hear it with your ears. 
sometimes though what a scope misses a spectrum analyzer finds, especially when looking at harmonics
the $2000 scope I have can be had for about $300 now, has an FFT mode which comes in handy for harmonics

[Willabe]

I tried this concept late last night, with a .047uf cap across the 100uf electrolytic in my current bench tweaker SE EL34. I also had a rotary switch with a bunch of values that I was able to switch through, but they were .01 and lower. I went back to the .047 because that seemed to sound the best of the bunch. I also tried a .1uf but veered back to the .047.

I've read (KOC) where if you where to use all film caps for B+ filter caps in the PSU, in stead of lytics, you would hear a difference in the amps distortion, "something would be missing".

So, maybe that's why you liked the smaller value cap? Hi-fi guys use bigger than that, but then it's hi-fi.

I think it's jjasilli who puts the film bypass cap across the last B+ filter cap, to effect the 1st preamp gain stage.

[jordan86]

While the sluthing hat is on, can anyone tell me why the green cap across V2 cathode resistor(820 ohm) is smaller than its brother across V1 2k7 cathode resistor, it was reported as both being 0.68uF.

I can’t see that part of the circuit, but One may be electrolytic and the other a film cap. I know Suhr does this on their SL68. Supposedly, a film bypass cap has a different tone but to get them in that value requires a huge (size) cap.

[John G]

Hi,
Full gut shot.
John

[PharmRock]

my initial report is that there is something here for us tweakers to tweak.
------
Nothing scientific here but it will prompt me to take the scope and signal gen. to it this weekend.

Thanks for doing the preliminary test SG...looking forward to hearing some more details  after you get a chance to look into it some more. 

I have my Plexi50 on the bench right now.  While I don't have a scope or a device such as you have to cycle through different cap values, I'll try to hook up a 0.047uf cap as shown on the EMS, as well as any other caps in that neighborhood that I have lying around and report any changes in the tone that I find. Hopefully I can get to it this weekend. 

If I decide to put this on a switch (ala "touch" switch)...just to verify...the switch must go between cap and ground, correct?  There should be no DC on the switch in this configuration.

[SILVERGUN]

If I decide to put this on a switch (ala "touch" switch)...just to verify...the switch must go between cap and ground, correct?  There should be no DC on the switch in this configuration.

Correct.
Not that it MUST go there, but it's a wayyy better place for it.


I wouldn't call it so much of a TONE tweak, but more like a TEXTURE tweak (if that makes any sense)
It seems to tighten things up and take away some harshness. At least that was my first impression.
I'm looking forward to hearing your thoughts and getting more time to dive into it.

[PharmRock]

Browsing JTM45 schematics and happened upon this one from Mojotone.  They have the 0.047uf cap in their schematic.  See attached.

[sluckey]

Several of the Fender Reissue amps use a .047.

     https://el34world.com/charts/Schematics/files/Fender/Fender_59_bassman_manual.pdf

     https://el34world.com/charts/Schematics/files/Fender/Fender_57_bandmaster_reissue.pdf

[PharmRock]

Alright...so today I wired in an ON-OFF-ON mini toggle in my 50 watt Plexi.  Both caps were coming off the first 50+50uf filter stage, ran each cap to separate poles on the switch, with a ground wire in the center pole on the switch.
 I tried a 0.047uf cap in one position and a 0.022 in the other.  I had it pretty cranked and played some chords and single note stuff in different switch positions.  Could hardly tell anything was happening.  If anything, the 0.047uf cap maybe made things a little "smoother" if that means anything.  I would let the chords ring out and change the toggle position from no cap to either the 0.022 or 0.047 and really didn't notice much. Honestly I think if I was blinded to the switch position I wouldn't notice any difference at all.  Maybe my ears aren't trained for picking this stuff up, but I can't seem to justify having it on a switch.  Worthwhile experiment, and learned something along the way. 

[Willabe]

You have to use a larger value cap. Those values are too small to hear a difference. 

Try 0.47 to 1uf up to 10uF. They have to be film caps.

[PharmRock]

You have to use a larger value cap. Those values are too small to hear a difference. 

Try 0.47 to 1uf up to 10uF. They have to be film caps.

I was scrounging the parts drawer but its a little bare at the moment...I didn't have anything in those ranges that were rated for 450+VDC.  I'll keep the switch there for now, as I have some other tweaking to do anyways.  In the meantime I'll get a variety of caps ordered and once they arrive I'll "plug and play" with a few.   

[sluckey]

You have to use a larger value cap. Those values are too small to hear a difference. 

Try 0.47 to 1uf up to 10uF. They have to be film caps.

But isn't the cap in the Dr Z is a .047µF? Sure looks like it to me.

[PharmRock]

You have to use a larger value cap. Those values are too small to hear a difference. 

Try 0.47 to 1uf up to 10uF. They have to be film caps.

But isn't the cap in the Dr Z is a .047µF? Sure looks like it to me.

Yes...as well as the Mojo JTM45 schematic and the Bassman reissue.  Could be that the 0.047uf is more evident in these circuits vs the 1987 circuit?

[sluckey]

I never though the .047s in those amps had anything to do with tone. Always figured it was to suppress high freq. noise spikes from the power line or rectifier diode switching. Kinda like the .047 "death cap".

[Willabe]

But isn't the cap in the Dr Z is a .047µF? Sure looks like it to me.

True, but that's also in that amp build, might make a difference because of the PSU in that amp and demands on it. In a diffenert amp, Dr. might have used a different value for that cap, not a 1 size fit's all. Just guessing. 

Not trying to compare apples to apples, he doesn't have the same amp. But trying to see if he can hear any difference by using a larger value. Experiment.

Might just be a feel factor thing, could be very subtle. Like upping/lowering, doubling/halving the filter caps uF value. 

[mresistor]

This is totally unrelated to the discussion at hand, but what the heck is the significance of the kinky bends in the wires? Is it just the "artists" little quirk to identify his work?

[Willabe]

This is totally unrelated to the discussion at hand, but what the heck is the significance of the kinky bends in the wires? Is it just the "artists" little quirk to identify his work?

Good question.          

Some old Gibson amps they did that to the C and R leads, so they didn't run ||, less signal cross talk, parasitic, injection.

[sluckey]

The kinks are for strain relief. I knew one tech that put little curly pig tails on ecaps for strain relief...

http://www.timeelect.com/jimsuper-33.jpg

[brewdude]

In my earliest builds I employed a 100nF cap across the first filter—mostly because it was common to the Valve Jr at its many modified incarnations.  Hey, we all start somewhere.


The last many builds I have abandoned this technique.


I was inspired to see if it made a difference on my latest build.  It’s a single channel no tremolo amp inspired by the 6G3 but employs a 6K6GT PA.  I’ve been fine tuning it trying to carve away some of the mud.


I clipped in a .1uF across the reservoir cap and it made a noticeable difference in clarity reducing some of the hairy fuzz. It made much less of a difference (virtually none) when place across the preamp’s filter cap. 


The .047uF was not really effective enough to use it in this amp, but .1uF definitely made a difference.

[SILVERGUN]

Alright...so today I wired in an ON-OFF-ON mini toggle in my 50 watt Plexi.  Both caps were coming off the first 50+50uf filter stage, ran each cap to separate poles on the switch, with a ground wire in the center pole on the switch.
 I tried a 0.047uf cap in one position and a 0.022 in the other.  I had it pretty cranked and played some chords and single note stuff in different switch positions.  Could hardly tell anything was happening.  If anything, the 0.047uf cap maybe made things a little "smoother" if that means anything.  I would let the chords ring out and change the toggle position from no cap to either the 0.022 or 0.047 and really didn't notice much. Honestly I think if I was blinded to the switch position I wouldn't notice any difference at all.  Maybe my ears aren't trained for picking this stuff up, but I can't seem to justify having it on a switch.  Worthwhile experiment, and learned something along the way.

Nothing happens when you switch it while you are holding a chord because the initial transient surge is over.
Because it's not a TONE tweak it is difficult to pick up and identify, but from my experience there is a difference and I could see there being a value in either having the cap or not, for some guys. It makes sense to me that Dr. Z called it TOUCH.
I agree that it doesn't really justify being on a switch, but I've seen dumber switches.


I was only able to pickup a very slight blip on my scope and still didn't have enough time to give it it's due. I'm still not done with it.

[pdf64]

Several of the Fender Reissue amps use a .047.

It may be significant that they’re on the rectifier side of a hot switching standby. And so by acting as a mini reservoir cap, may be taking some of the current surge away from the rectifier when standby is flipped to  play mode.
So perhaps intended as a ‘be nice to rectifier’ cap, rather than a ‘fast transient response’ cap.

… Kinda like the .047 "death cap".

Bear in mind that the death cap’s purpose was to keep the chassis as close to ground potential as possible (ie the neutral wire), without passing too much current if it was switched to connect to the live wire. Not to suppress switching noise.
With polarised mains connectors, its on off on switch wouldn’t have been beneficial / necessary, with it switched to connect to the live wire, only a masochist would want to experience a few mA of mains current.
Of course these days, the live wire carries most of the hash noise, but it would be erroneous to conflate the death cap with a X type mains noise suppression cap.

[sluckey]

but it would be erroneous to conflate the death cap with a X type mains noise suppression cap.

I wasn't conflating it with a class X cap. I was comparing to a class Y cap. Back in the two wire unpolarized day they worked very well to reduce some line noise.

[pdf64]

but it would be erroneous to conflate the death cap with a X type mains noise suppression cap.

I wasn't conflating it with a class X cap. I was comparing to a class Y cap. Back in the two wire unpolarized day they worked very well to reduce some line noise.

Sorry, I’ve no experience of ungrounded, unpolarised mains, so was hypothesising 

But in the absence of an actual ground connection, I don’t see how a class Y (ie live to ground) cap could be described as such? I suppose the chassis becomes a notional ‘ground’ in that scenario.

Or how flipping the ground switch so that the chassis was pulled up to mains live potential might reduce noise?

I thought that noise would automatically be lowest when the chassis was AC connected to ground (ie via the neutral earthing rod), rather than to AC live?

I can envisage that scenario would change with a grounded mains electrical installation and 3 wire power cable and grounded chassis. ie that the lowest noise setting of the ground switch would probably be as you describe, with the death cap connected between ground (the chassis) and the live wire.

[jordan86]

Pardon my slowness but how is this not like a dull or smoothing cap on V1 plate? It’s essentially a very low capacitance non-lytic capacitor passing B+ to ground. Usually in Marshall styles it’s a 200-500pf silver mica. This is just a film placed off the first node instead of the last (preamp)??? 

[SILVERGUN]

Pardon my slowness but how is this not like a dull or smoothing cap on V1 plate? It’s essentially a very low capacitance non-lytic capacitor passing B+ to ground. Usually in Marshall styles it’s a 200-500pf silver mica. This is just a film placed off the first node instead of the last (preamp)??? 

A dull or smoothing cap on V1 does not pass B+ to ground. Neither does the cap in question.
B+ is DC.

[pdf64]

Pardon my slowness but how is this not like a dull or smoothing cap on V1 plate? It’s essentially a very low capacitance non-lytic capacitor passing B+ to ground. Usually in Marshall styles it’s a 200-500pf silver mica. This is just a film placed off the first node instead of the last (preamp)??? 

Have you got your head around HT nodes being at AC 0V?
That’s what the HT decoupling caps do, ie ensure a very low impedance between the HT node and 0V.

[SILVERGUN]

Pardon my slowness but how is this not like a dull or smoothing cap on V1 plate? It’s essentially a very low capacitance non-lytic capacitor passing B+ to ground. Usually in Marshall styles it’s a 200-500pf silver mica. This is just a film placed off the first node instead of the last (preamp)??? 

Have you got your head around HT nodes being at AC 0V?
That’s what the HT decoupling caps do, ie ensure a very low impedance between the HT node and 0V.

So Jordan, by the time we are downstream at the filtered node for V1 we have 0V AC.
But, coming straight out of the rectifier we have some AC ripple voltage.
The cap you mention just shunts some highs from the AC signal to ground whereas this cap in question shunts AC ripple to ground.
That's what makes it thread worthy. A closer look at why the mighty Dr. Z thought this was worth a front panel switch.

[Willabe]

So Jordan, by the time we are downstream at the filtered node for V1 we have 0V AC.

Cure ball. From Aiken amps web site;

"I also tend to put the preamp filter caps in the circuit where they are used ("local" bypassing). For example, the first preamp tube's filter cap will be physically placed in the area of the first preamp tube, with the ground and B+ connections made right to the bottom of the cathode resistor/cap and to the top of the preamp tube resistor. If I decide to instead group all the filter caps together, I'll be sure to also locally decouple each filter cap node with a smaller capacitor, typically a 0.1uF/400V) directly from the top B+ node of that stage to the ground node at the bottom of the cathode resistor. You'd be surprised how many brand new filter caps have very high reactances at frequencies within the range of a distorted guitar. Sometimes you can take a "bad" filter cap and bypass it with a 0.1uF cap and it will sound fine. It never hurts to have good high-frequency decoupling at all nodes."

[SILVERGUN]

Sounds like Mr. Aiken agrees with us.
Maybe we need a switch with a cap on each stage.