calculationg coupling cap values

[Shrapnel]

To Start: I've got Merlin's pre-amp book for guitar and bass, 2nd ed. and I'm only at ch 6 right now...

But so far, I've seen mention of using smaller caps to help keep blocking distortion at a minimum at worst (and tightening up the bass.) But I'm a little confused at calculating what would be a better value than say the typical  22n or maybe worse, 33n to keep the low end, yet tighten it up. I've seen some amps use 10n, and I was kinda thinking 5.6n, but I'm not sure. Is it based on the grid-leak (and series voltage divider) or on the grid stop?

Config would be the coupling cap from the previous stage into a 470k resistor, in which the other end connects to a 470k grid stop and a 470k grid leak.

I'm also trying to keep a 7-string, maybe 8-string in mind (Not so sure of 8+ string guitars as I can't see myself wanting to go THAT low... and I'm debating on getting me an inexpensive 7-string.)

Any help for me to get a better grasp?

[Lectroid]

Is this about one amp in particular, or just a general question? 

[pdf64]

You're referring to caps in a RC high pass filter circuit, right?
The C value requires the context of the relevant R value for a filter frequency to be formed.
Here's a useful calculator https://www.pad2pad.com/calculators/resistance-frequency-capacitance/

[Shrapnel]

In this instance I was thinking of calculating C1 for the desired cutoff frequency.  For the sake of giving numbers arbitrarily, we could call R4 and R5 470K and R6 330k.

Attached is the reference schematic

[shooter]

run 'em through the calculator posted.
do some reading though, I believe you can't mix those 2 resistors "as one".  AC electronics is some pretty deep math, with rules and all manner of head-scratching confusion built in, just to weed out the lazy designers 

[Shrapnel]

run 'em through the calculator posted.
do some reading though, I believe you can't mix those 2 resistors "as one".  AC electronics is some pretty deep math, with rules and all manner of head-scratching confusion built in, just to weed out the lazy designers 

The lazy designer, imho, will just run with the most common... the .022uF, 0.047uf, or .033uF.... well... not all who use those are lazy.

[Shrapnel]

Is this about one amp in particular, or just a general question? 

This is more of a general question than for a specific amp, outside of one I'm trying to design. Looking to keep the low end tight, but not too restrictive. The typical suspects, unfortunately, are not really tight if you're using a baritone or an extended range guitar.

[Merlin]

In this instance I was thinking of calculating C1 for the desired cutoff frequency.  For the sake of giving numbers arbitrarily, we could call R4 and R5 470K and R6 330k.

Attached is the reference schematic

For the coupling cap the resistance to ground is what counts, so R4+R5.

[shooter]

i'm the poster child for lazy designs, why re-invent a wheel that works just fine. steal and tweak, done in a week. 


if you have a working amp that you're "tuning", post a schematic
if this is a scratch design, find an amp you've already played through that is "close", build, buy, or borrow it AND the schematic, then we can dial it close.


mathematical, hypotheticals, seldom if ever, get you anything but what's already out in the wild.

[Willabe]

This is more of a general question than for a specific amp, outside of one I'm trying to design. Looking to keep the low end tight, but not too restrictive. The typical suspects, unfortunately, are not really tight if you're using a baritone or an extended range guitar.

Why do you think coupling caps are the answer to having an amp for extended range guitars?

[shooter]

I'm a Gardner n landscaper now so my 40 year ago school'n might be all wrong


seems like C1R4 form a series pass filter
R4R5 a voltage divider
C1R4+R5 a series shunt filter


hopefully the actual smart kids will correct me?

[AlNewman]

I thought the load of the previous stage also contribute to the reactance of the coupling cap?

So it would be a parallel load between the grid leaks of the 2nd stage, as well as the plate load of the 1st stage which create the frequency cutoff?

Basically what I like to do, without meaning to sound too stupid, is try different values and see what sounds best.  Because that's probably what I'll end up doing anyways.

[pdf64]

I'm a Gardner n landscaper now so my 40 year ago school'n might be all wrong


seems like C1R4 form a series pass filter
R4R5 a voltage divider
C1R4+R5 a series shunt filter

No, like Merlin wrote, the high pass filter is formed by C1 and (R4+R5).

[pdf64]

I thought the load of the previous stage also contribute to the reactance of the coupling cap?

So it would be a parallel load between the grid leaks of the 2nd stage, as well as the plate load of the 1st stage which create the frequency cutoff?
 ...

Yes, the source AKA output impedance of a typical common cathode 12AX7 stage is about 40k.
So the voltage output of the typical Fender optoisolator driver into the 50k trem intensity pot is nearly halved.
The HPF CR terms there are 0.1uF and (40k + 50k).

The additional 40k in the example in this thread makes a negligible difference to the combination of R4&5.

[Shrapnel]

This is more of a general question than for a specific amp, outside of one I'm trying to design. Looking to keep the low end tight, but not too restrictive. The typical suspects, unfortunately, are not really tight if you're using a baritone or an extended range guitar.

Why do you think coupling caps are the answer to having an amp for extended range guitars?

In this case, I'm looking to tighten up the low end response a bit, make it responsive and punchy... not muddy and flubby.

I'm thinking getting rid of more of the subsonic and maybe a portion of the low end that won't be there anyway to move it in that direction. I'm open to hear and learn other methods that can accomplish this.

[Willabe]

This is more of a general question than for a specific amp, outside of one I'm trying to design. Looking to keep the low end tight, but not too restrictive. The typical suspects, unfortunately, are not really tight if you're using a baritone or an extended range guitar.

Why do you think coupling caps are the answer to having an amp for extended range guitars?

In this case, I'm looking to tighten up the low end response a bit, make it responsive and punchy... not muddy and flubby.

I'm thinking getting rid of more of the subsonic and maybe a portion of the low end that won't be there anyway to move it in that direction. I'm open to hear and learn other methods that can accomplish this.

I think your over complicating this by wanting to calculate the caps value and bringing up subsonic's?

It seems often guys will change the preamp tubes K (cathode) to a smaller value more than changing the coupling cap. You can do both if needed.

Playing with the coupling caps, you can just tack solder them in, or you can just very carefully swap in a different value with insulated gator clip leads/cable. Just try different values, you can do the same with the K bypass caps.   

But the lower the note the more power the amp needs to reproduce it cleanly. The lower the note, the larger the sound wave that has to be reproduced, so the amp/speakers have to move/push more air. That's why bass players, and keyboard players, use amps with a lot more wattage/power than a guitar amp.

A weak(er) power supply can't keep the low end tight. So a stronger power supply, PT B+ wind that has more mA's and larger B+ filter caps will stiffen up the bottom end. Bigger PT/more B+ mA's cost more. And a solid state (SS) rectifier for the B+ will be stronger/tighter on the bottom than B+ from a tube rectifier. Because a tube rectifier has an internal resistance that will cause the dcv to drop/sag if the amp try's to draw to much current through the tube. SS rectifiers will not sag, but then the PT's B+ wind might if it can't supply enough current.   

Bigger power tubes will have/handle bass better, like KT66, 6550, KT88, KT90, KT120, and the big boy on the block, KT150. But you can't just swap in these tubes, most need more heater supply current, and the bottle may be too big to fit next to each other with the current tube sockets spacing.

A change to a 6L6GC from a 6V6 tube will tighten up the bottom some. KT66 tubes might be enough change from a 6L6GC tube. Most guitar players don't like 6550 tubes, except for metal players, they do like KT88's. Some like the KT90. KT120 and KT150 are probably way over kill for power. 

Does the amp you have, have -FB? -FB will help tighten up the bottom end because of the speaker dampening that happens when an amp's power amp has -FB. You can try increasing the -FB a little too.

And the speaker will make a huge difference. Try speakers with more wattage handling ability. Try different brands, models. The new Fane speakers are said to have great bottom end.

And a closed back/sealed cab will tighten up the bottom end. An open back cab, the paper cone kind of flaps in the wind, a closed cab helps support the cone during forward/backward excursion.

Probably need a combination of some/all of these, depending how low and how clean/tight your looking for.

I'd start with changing the K bypass caps, 1 at a time, down to 4uF, or 2.2uF, or 1uF. See how that goes/sounds.

Then depending on what speaker you have now, I'd try different speakers. This costs a lot more to do, but it makes a HUGE difference in sound and response.     

[Shrapnel]

I think your over complicating this by wanting to calculate the caps value and bringing up subsonic's?

It seems often guys will change the preamp tubes K (cathode) to a smaller value more than changing the coupling cap. You can do both if needed.

Playing with the coupling caps, you can just tack solder them in, or you can just very carefully swap in a different value with insulated gator clip leads/cable. Just try different values, you can do the same with the K bypass caps.   

But the lower the note the more power the amp needs to reproduce it cleanly. The lower the note, the larger the sound wave that has to be reproduced, so the amp/speakers have to move/push more air. That's why bass players, and keyboard players, use amps with a lot more wattage/power than a guitar amp.

A weak(er) power supply can't keep the low end tight. So a stronger power supply, PT B+ wind that has more mA's and larger B+ filter caps will stiffen up the bottom end. Bigger PT/more B+ mA's cost more. And a solid state (SS) rectifier for the B+ will be stronger/tighter on the bottom than B+ from a tube rectifier. Because a tube rectifier has an internal resistance that will cause the dcv to drop/sag if the amp try's to draw to much current through the tube. SS rectifiers will not sag, but then the PT's B+ wind might if it can't supply enough current.   

Bigger power tubes will have/handle bass better, like KT66, 6550, KT88, KT90, KT120, and the big boy on the block, KT150. But you can't just swap in these tubes, most need more heater supply current, and the bottle may be too big to fit next to each other with the current tube sockets spacing.

A change to a 6L6GC from a 6V6 tube will tighten up the bottom some. KT66 tubes might be enough change from a 6L6GC tube. Most guitar players don't like 6550 tubes, except for metal players, they do like KT88's. Some like the KT90. KT120 and KT150 are probably way over kill for power. 

Does the amp you have, have -FB? -FB will help tighten up the bottom end because of the speaker dampening that happens when an amp's power amp has -FB. You can try increasing the -FB a little too.

And the speaker will make a huge difference. Try speakers with more wattage handling ability. Try different brands, models. The new Fane speakers are said to have great bottom end.

And a closed back/sealed cab will tighten up the bottom end. An open back cab, the paper cone kind of flaps in the wind, a closed cab helps support the cone during forward/backward excursion.

Probably need a combination of some/all of these, depending how low and how clean/tight your looking for.

I'd start with changing the K bypass caps, 1 at a time, down to 4uF, or 2.2uF, or 1uF. See how that goes/sounds.

Then depending on what speaker you have now, I'd try different speakers. This costs a lot more to do, but it makes a HUGE difference in sound and response.     

You may be right on over-complicating things. It wouldn't be hard to do.


Anyway, I was thinking 5.6n coupling caps for a cutoff around 30Hz. I only have one cathode bypass cap in the preamp section. first triode:  Rk= 2k7, Ck 680nF (may bump to a 1.8uF (33Hz) or 2.2uF (27Hz) for a little more low end there, maybe.)

I was planning on solid state rectification with this design, although I hadn't locked in exactly what output tubes. I was debating 6V6 but may go with EL34 or 6L6 since I have some not in use.  I do have a NFB loop in the output section that I've set up for a presence control and something like a resonance control in series. Also planning on fixed Bias. I haven't solidified the topology of the power supply yet.


The whole design is meant to be a head, so speakers and cabinet would be separate. Maybe running it through a Marshall MG412A cab.

[Willabe]

I think you should re-read what I wrote above again.

......although I hadn't locked in exactly what output tubes. I was debating 6V6 but may go with EL34 or 6L6 since I have some not in use.

I looked at your schematic. 

So you want to build a very hi-gain 6V6 amp that has 2 channels with - 4 preamp gain stages -  into 2 CF's (cathode follower) to drive the 2 TS's (tone stack) plus a tube FX loop and you want it to have a tight low end response, be responsive and punchy... not muddy and flubby and to play a 7, maybe an 8 string guitar through it. 

That's way too much gain for 6V6's and 6V6's are not known for good solid bottom end when pushed. As the distortion increases, the lower the note, the more problems it will have with being muddy/spongy. As a rule of thumb, in general, smaller power tubes take less to drive them into distortion. Nor would I use EL34's for a build with a tight/solid/punchy bottom end. 

It's your build, but, with that many gain stages, 2 channels, tube FX, 6V6's and the extended range guitars bottom end, your asking for trouble. You can/will probably run into noise, humm and oscillation problems because of;

1. So much/too much gain.
2. The amount of circuitry.
3. The layout.
4. The lead dress.
5. The grounding.

From your schematics, it's a very complex build, including 8 tubes (with SS rectifier for B+) in total and it will be a very difficult build to get problem free. It can be done, but it's really for a very experienced builder who has built a lot of amps. It's not a build being cloned from an already proven build.   

On top of all of that, it also needs to have a tight low end, be responsive and punchy... not muddy and flubby for extended range guitars. It's going to be very hard to get all that to work. Your asking a lot.

Anyway, I was thinking 5.6n coupling caps for a cutoff around 30Hz. I only have one cathode bypass cap in the preamp section. first triode:  Rk= 2k7, Ck 680nF (may bump to a 1.8uF (33Hz) or 2.2uF (27Hz) for a little more low end there, maybe.)

This is the least of your problems. Forget the Hz. your making this complicated. You already know the values to get you in the ball park, so just swap them in/out and listen to how they sound, that goes for any build.

.... first triode:  Rk= 2k7, Ck 680nF (may bump to a 1.8uF (33Hz) or 2.2uF (27Hz) for a little more low end there, maybe.)

Even if it only had a couple or gain stages, your just guessing now, let your ears decide, not the math. And with 4 preamp gain stages, with all the gain/frequency stripping between stages, you can't tell until you hear it. And all those gain/frequency stripping R's/C's will take a long time to balance them and adjust them/listen to them. No matter how much math on paper work you try to do, your still gonna spend a LOT of trial and error by ear time.     

I was planning on solid state rectification with this design,

Well that will help with the bottom end some, and would save a tube, keeping it at 8 tubes, not 9. But you can still out run the PT's B+ available current. 

I do have a NFB loop in the output section that I've set up for a presence control and something like a resonance control in series.

How are you figuring the amount of -FB?

Too much -FB can/will make the amp sound/feel sterile and cold, too little won't help enough to keep the bottom end where you want it. Including not enough speaker damping, for what you want, speaker dampening should help a lot. 

The presence control should be fine, it only removes the -FB from the highs. But the resonance type control dials out the -FB from the low end and that will defeat your wanting a tighter/solid bottom end. 

Also planning on fixed Bias. I haven't solidified the topology of the power supply yet.

The power supply on a build like this with bottom end needs will be very important. Fixed bias would work good for this type of build.

The whole design is meant to be a head, so speakers and cabinet would be separate. Maybe running it through a Marshall MG412A cab.

Celestion speakers would not be my 1st choice for a nice tight, solid bottom. I'd check into the new (re-born) Fane speakers.

[Willabe]

Almost forgot, the OT.

Guitar amps often use OT's that are under rated, and are not rated for full band width/full audio frequency, because they cost less and are not needed. They don't burn up, they just keep cutting/shaving off the far outer ends of the frequency range from reaching the speaker. It compresses the frequency spectrum.

If you look at pictures of say, a 50w guitar OT and a picture of a 50w full band width OT for a stereo, the stereo OT will probably be about 2x the size of the 50w guitar OT.   

So for a better, fuller, clearer, tighter bottom end, you don't need a full band width rated OT, but you do want to go with a larger wattage OT.

[pdf64]

5 common cathode 12AX7 gain stages in cascade - yikes!

[shooter]

 
I hear the train a come'n.....the death metal version just in time for Christmas

[Shrapnel]

Most people just borrow from other schematic builds, taking different building blocks from existing amps. (we're all guilty.) anywhere from a simplified "Plexi" or Champ to a Soldano SLO or some other gain monster if we're not afraid of that much gain. Not many dig deeper, Some just experiment without understanding the details, just that it works. I want to dig deeper, learn more, adjust my starting points. Academic? Probably, but it also mean not just trying to be a copycat with a few "let's see if I change this, will I get what I want" tweaks. (Nothing wrong with that approach.) I want to get past copy-paste-maybe tinker.

Yes, high gain does have its caveats. Dress, grounding, and layout are very important. A stiff power supply is important. A high gain preamp begs for a clean power section. removing unused or unwanted low end that could sneak in, leaves more power focused on those low end frequencies you do want.

Yes, one channel is high gain... initially based off the Soldano SLO high gain channel. The second channel is more plexi like. without driving the power tubes into saturation, it will at most tend to get fat/compressed. If I choose to go 6V6 instead of 6L6/5881/KT66 It will be easier to get the power tube overdrive IF I choose the more classic metal sound.  If not, I have headroom/untapped power with the more modern metal sounds of the high gain channel.
As to speaker choice... most modern metal choices of speaker is the Vintage30.At to my Marshall cab, Many don't care for it, as it's a collab of Marshall and Celestion, not pure Celestion. Speaker choice will always make a difference in the final voicing of ANY amp.

[pdf64]

The SLO overdrive preamp has 4 CC stages cascaded, one of which has a massive degree of cathode degeneration, I guess maybe 10dB gain.
Whereas you've got 5 stages, of which 4 have a bit of cathode degeneration, and pretty much nothing to limit the bandwidth.

How have your 3 and 4 CC cascaded stage overdrive preamps design and builds worked out?

[Willabe]

How many amps have you built successfully? From scratch or a proven clone?

Most people just borrow from other schematic builds, taking different building blocks from existing amps. (we're all guilty.)

It's not about being guilty, it's about being smart. Carpenters, plumbers, auto makers, chef's, etc, there's a lot of things that have already been figured out.

No need to reinvent the wheel. Marshall copied Fender to the point of because Marshall built a head when they flipped it right side up the input jacks were on the other side from the Bassman they copied.

gain monster if we're not afraid of that much gain.

Not afraid of, many don't play music that needs that level of distortion and/or don't like the sound of. 

Not many dig deeper, Some just experiment without understanding the details, just that it works. I want to dig deeper, learn more, adjust my starting points. Academic? Probably, but it also mean not just trying to be a copycat with a few "let's see if I change this, will I get what I want" tweaks. (Nothing wrong with that approach.) I want to get past copy-paste-maybe tinker.

That's fine, but lets put this in perspective, the schematic you've posted is a MASSIVE undertaking. That's not an amp you start with to learn, take 1 or 2, maybe 3 steps at a time. 

It's fine to want to understand how a circuit works so you can fully adjust it/tweak it. And you can't always cut and past everything. Often you have to know how to get the blocks to be able to work together.

Yes, high gain does have its caveats. Dress, grounding, and layout are very important.

Your minimizing this. An amp this large, 8 tubes, extremely high gain, 2 channel, tube FX loop, caveats of dress, grounding, and layout? 

And you've already learned how to do lead dress, grounding, and layout by successfully building amps from scratch this complicated and large? 

A stiff power supply is important. A high gain preamp begs for a clean power section.

No, I brought up the PSU (power supply) because you said your "looking to tighten up the low end response a bit, make it responsive and punchy... not muddy and flubby." So you can play a 7 string, maybe 8 string guitars, lower notes than a regular guitar. 

Removing unused or unwanted low end that could sneak in, leaves more power focused on those low end frequencies you do want.

That's not why you strip out bass with a hi gain preamp. You take it out so the tone is not muddy. Do you know what a Top Boost pedal is? Same reasoning.

Yes, one channel is high gain... initially based off the Soldano SLO high gain channel.

No, a Soldano is high gain, your trying to build an extremely hi gain preamp. You have added even 1 more gain stage than Soldano.

The second channel is more plexi like. without driving the power tubes into saturation, it will at most tend to get fat/compressed. If I choose to go 6V6 instead of 6L6/5881/KT66 It will be easier to get the power tube overdrive IF I choose the more classic metal sound.  If not, I have headroom/untapped power with the more modern metal sounds of the high gain channel.

No, that 'plexi' pre will drive 6V6's into saturation. And will probably drive 6L6's/EL34's past "fat/compressed."

As to speaker choice... most modern metal choices of speaker is the Vintage30.At to my Marshall cab, Many don't care for it, as it's a collab of Marshall and Celestion, not pure Celestion. Speaker choice will always make a difference in the final voicing of ANY amp.

I only brought up Celestion's and Fanes, because again, your "looking to tighten up the low end response a bit, make it responsive and punchy... not muddy and flubby." So you can play a 7 string, maybe 8 string guitars, lower notes than a regular guitar. 

Now your saying you want an amp for classic metal and modern metal.

Sounds like you need 2 different amps, 1 for metal, 2. for 7 string and 8 string extended bottom end guitars. 

[Shrapnel]

The SLO overdrive preamp has 4 CC stages cascaded, one of which has a massive degree of cathode degeneration, I guess maybe 10dB gain.
Whereas you've got 5 stages, of which 4 have a bit of cathode degeneration, and pretty much nothing to limit the bandwidth.

How have your 3 and 4 CC cascaded stage overdrive preamps design and builds worked out?

Some of my early ones certainly didn't like pedals at any setting... wait. that was my first and only 5CC type to date.  I forgot about that. I'll have to revisit this since you reminded me as this is probably too close to that one. It sounded good on its own, and with the first gain pot turned all the way down the guitar sounded a little acoustic like. I just didn't like that it was so hot. Poor design didn't help as I was still fairly new. [that design is attached]

I don't have any problems with my 3CC and 4CC builds. My first one was a hand-wired (all terminal strip) Master Volume clone in a small PA chassis (Davis 201A). Good overdrive, handles pedals good. no noise or feedback. (used 6EU7s instead of 12AX7s as I reused its tube compliment.) My only fault on that one was no grid stoppers... also a fault of J.W. Davis & Company and the MV schematic I adapted.

[AlNewman]

The best way to learn is to make mistakes.  Not that you will make mistakes, but you're definitely creating the best situation in which to do so.

[Shrapnel]

Not many dig deeper, Some just experiment without understanding the details, just that it works. I want to dig deeper, learn more, adjust my starting points. Academic? Probably, but it also mean not just trying to be a copycat with a few "let's see if I change this, will I get what I want" tweaks. (Nothing wrong with that approach.) I want to get past copy-paste-maybe tinker.

That's fine, but lets put this in perspective, the schematic you've posted is a MASSIVE undertaking. That's not an amp you start with to learn, take 1 or 2, maybe 3 steps at a time. 

It's fine to want to understand how a circuit works so you can fully adjust it/tweak it. And you can't always cut and past everything. Often you have to know how to get the blocks to be able to work together.

Removing unused or unwanted low end that could sneak in, leaves more power focused on those low end frequencies you do want.

That's not why you strip out bass with a hi gain preamp. You take it out so the tone is not muddy. Do you know what a Top Boost pedal is? Same reasoning.

The second channel is more plexi like. without driving the power tubes into saturation, it will at most tend to get fat/compressed. If I choose to go 6V6 instead of 6L6/5881/KT66 It will be easier to get the power tube overdrive IF I choose the more classic metal sound.  If not, I have headroom/untapped power with the more modern metal sounds of the high gain channel.

No, that 'plexi' pre will drive 6V6's into saturation. And will probably drive 6L6's/EL34's past "fat/compressed."

Now your saying you want an amp for classic metal and modern metal.

Sounds like you need 2 different amps, 1 for metal, 2. for 7 string and 8 string extended bottom end guitars. 

You do make some good points here.

I am going to go back and revisit this design.... archive it as a reminder... As I stated in my last post, I was reminded of another amp I built (and the only time I went that many stages so far. I'm not looking to have it so hot that it couldn't handled pedals if I chose to use them. Better to catch it now than after I start actually building and finish the thing.

When I said the Plexi like channel will produce a fat/compressed sound, that is with a master volume added in that channel turned down enough the power amp wouldn't be overdriven. I apologize for not making it clear that a master volume was involved. Without it you are 100% correct.

The information I have gathered here has been helpful though. So... Back to the drawing board and something a little more pedal friendly and saner.

[Shrapnel]

The best way to learn is to make mistakes.  Not that you will make mistakes, but you're definitely creating the best situation in which to do so.

You're absolutely right. Make mistakes. Ask questions. Seek Solutions.