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Thread: Safe to put V1a coupling capacitor on a switch for variety?

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    Safe to put V1a coupling capacitor on a switch for variety?

    I'm building another prototype amp at the moment and want to make lots of things switchable just to explore options. I'm basing the circuit around a JCM 800 single channel. One option I'd like to have is to make the coupling cap following V1a switchable between 3 different caps; .001, .0022, and .022. Is it safe to just rout those to the front panel and put them on a 3 way selector? There's roughly 260 volts passing through it.

    Thanks for thoughts and ideas.
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    Quote Originally Posted by Mr. Bill View Post
    I'd like to have is to make the coupling cap following V1a switchable between 3 different caps; .001, .0022, and .022. Is it safe to just rout those to the front panel and put them on a 3 way selector? There's roughly 260 volts passing through it.

    Thanks for thoughts and ideas.
    You could put the 0.022u followed by a 10M resistor to ground on the circuit board then run a wire from there to the switch with the 0.001u and 0.0022u mounted on the switch. That way all the switching is at ground and there shouldn't be any pops when switching.
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    Supporting Member Chuck H's Avatar
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    What Dave suggests will work fine. You get .002u (rather than .0022u) and .00095u (rather than.001u) and that should be close enough. More complicated (but more accurate) would be to decouple the circuit from DC with something like a .47u film cap and follow with the switching for the .022u, .0022u, .001u caps. You'll still need to reference the caps to 0V with a resistor to avoid popping. Another option would be to series the whole arrangement. A .022u cap followed by a .0022u cap (paralleled with a 220p cap) followed by another .0022u cap. Then you tap from the junctions. And you STILL need the 0V reference resistor at the end of the string. This would give you .022u, .00218u (pretty damn close!), .00109u (also pretty damn close).

    Lost of ways to approach this without DC on the switch
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    Thanks for the help guys. While I've come pretty far tinkering with these circuits, and can throw together superleads and 800's like legos at this point, I think your suggestions are a little over my head. Being a coupler cap, it has no direct reference to ground but feeds the next stage. If I tapped each one to a 1M to ground, wouldn't I end up creating a voltage divider with the 470k feeding the gain pot and drastically alter the gain feeding the next stage? I'll attach a picture of what I mean. Maybe I'm just not getting it.coupler-v1a.png
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    I was thinking you could do it like this.

    vj-1.png

    I've done it like this to switch in octaves. It gives roughly 10n, 4n7, 2n2, 1n, 470p

    vj-2.png
    Last edited by Dave H; 01-06-2018 at 02:01 PM.
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    Supporting Member Chuck H's Avatar
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    Plus, a rotary switch such as might be used in diagram two would allow the use of a standard knob that matches the others, or not (maybe a pointer type knob) for a nice visual appeal
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    When switching caps in an audio circuit you need to ensure that there is no potential difference between the caps, otherwise this difference will cause a pop when switching. The way I do this is to use 10M Ohm equalizing resistors between the lugs of the switch.

    Edit: Take a look at the EF86 channel of a Matchless DC30
    Last edited by Mick Bailey; 01-06-2018 at 03:27 PM.
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    Supporting Member Chuck H's Avatar
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    Positively! (pun intended)

    But IMHE when actually building or modding things can get crowded very quickly. I've burnt up more than one switch over soldering clunky. crowded lugs. Sometimes it's easier to simply make sure there is a 0VDC reference resistor at either end of the caps as Dave has done. I haven't had a popping problem doing it this way.
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    Supporting Member Chuck H's Avatar
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    Here's the arrangement I was talking about. It allows for the stock tone one position, but skips the 470k/500p circuit for the other positions. I don't think keeping that in the circuit with the smaller coupling caps would be to anyone's liking. You will need an on/on/on toggle (can be hard to find) or a rotary switch for this.
    Attached Thumbnails Attached Thumbnails swpos1.png  
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    Ah! I see now. I was planning to also follow that switch with another bypass switch that would take the 470k/ 500p out of circuit then put two different bright cap values on a push pull 1M pot as well. This would be used in conjunction with another switchable cap or two in the NFB loop so I could experiment with varying degrees of thinning out the front end and beefing up the NFB to compensate. In theory, I'd also like to add switchable smoothing caps on the plate resistors of the first two stages, but that might add noise if I'm not careful.

    I have worried about the idea that juicing up the front end too much might also require grid stoppers on following stages for stability, and making those switchable wouldn't make too much sense to me.

    I'm guessing the switchable cap in the feedback loop could be put on a direct switch without the 0 volt reference, correc
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    Quote Originally Posted by Mr. Bill View Post
    Ah! I see now. I was planning to also follow that switch with another bypass switch that would take the 470k/ 500p out of circuit then put two different bright cap values on a push pull 1M pot as well. This would be used in conjunction with another switchable cap or two in the NFB loop so I could experiment with varying degrees of thinning out the front end and beefing up the NFB to compensate. In theory, I'd also like to add switchable smoothing caps on the plate resistors of the first two stages, but that might add noise if I'm not careful.

    I have worried about the idea that juicing up the front end too much might also require grid stoppers on following stages for stability, and making those switchable wouldn't make too much sense to me.

    I'm guessing the switchable cap in the feedback loop could be put on a direct switch without the 0 volt reference, correc
    I like your switch stuff idea, I'm into that too. I don't like the sound of switching plate resistor bypass caps, you're putting plate high voltage on a switch you touch, not good.

    Also grid resistors like to be right on the socket lugs as close as possible to the tube. By bringing the circuit out to the faceplate you may have instability problems.

    There are a lot of techs here and I'm not one of them, but I believe these are valid cautions.
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    Quote Originally Posted by Chuck H View Post
    But IMHE when actually building or modding things can get crowded very quickly.
    Yes, that's why I only used one 10M resistor. It was difficult enough fitting xicon caps on a little rotary switch. I've made three now and they work well enough without popping.
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    Supporting Member Chuck H's Avatar
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    Changing coupling caps and bright caps doesn't beef up the gain any. You will get a little more gain in the low mids (and a little less in the bass) by bypassing the 470k/500p circuit and switching to the smaller coupling caps. A smaller bright cap on the volume control will actually reduce gain in the upper mids at volume pot settings below max. If by the cap in the NFB loop that you mean the presence cap, that cap does isolate signal from DC. That's why the old style Marshall circuit can have a scratchy presence pot. So you do need to arrange or design that circuit specifically to avoid switch noises. Increasing that cap value will increase mid gain and distortion in the power amp. Decreasing the value would reduce gain and distortion in the mids. Adding "smoothing caps" to the plate resistors will reduce gain in the treble. Nothing you propose so far will increase gain and cause blocking distortion. In fact most of what you propose will actually decrease gain in one frequency or another except for bypassing the 470k/500p circuit. Doing this reduces the voltage division of low mid and bass frequencies with the volume controls load, BUT, since the coupling cap value is reduced the low bass frequencies get cut, having the effect of increasing mids.
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    Just thinking about amps that use switched coupling caps and another classic is the Orange OR120/OR80 that works nicely.
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    The NFB cap would go right before the OT tap as a resonance switch. I've also found I like upping the plate resistor on V1a in SPL's. This tends to at least sound to me like it adds gain and needs to be controlled downstream to prevent oscillation. Again, I'm still learning, and most of these tweaks are by ear with safety checks as I learn. Here's a schematic of my last build which sound great but is a one trick pony. (I'm a studio guy, so I like one trick ponies).spl-mod-g.png

    Pardon the crude nature of my drawings, they're just for my reference so I can remember what I like and don't like.
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    Supporting Member Chuck H's Avatar
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    The way your NFB circuit is set up now is like having a resonance control on ten all the time. You might benefit by adding a variable control to that effect to get the tighter bass you mention. Different caps would change the knee frequency, but everything below that doesn't get feed back at all so the actual bass or low bass will never tighten.
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    Quote Originally Posted by Chuck H View Post
    The way your NFB circuit is set up now is like having a resonance control on ten all the time. You might benefit by adding a variable control to that effect to get the tighter bass you mention. Different caps would change the knee frequency, but everything below that doesn't get feed back at all so the actual bass or low bass will never tighten.
    Thanks Chuck. That's kinda my plan at this point. 2 different values to change the knee strapped a across a push/pull 1M pot with a switch before it to take it out completely if I want.
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    Just thinking about amps that use switched coupling caps and another classic is the Orange OR120/OR80 that works nicely.
    Agreed Mick, I was just about to mention Orange's "FACS" control.
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    You guys have been extremely helpful as always. I feel like I owe you money at this point. If I can ask one more question for now, does anyone see any problem with switching the cathode like this? I may even make it more complicated by adding a second option, but here's my starting point. Thanks.

    cathode-switch.png
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    Supporting Member Chuck H's Avatar
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    Ordinarily you would, again, need to include an equalizing resistor for this circuit. This would be my more economical approach.
    Attached Thumbnails Attached Thumbnails swpos1a.png  
    Last edited by Chuck H; 01-06-2018 at 09:18 PM.
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    That's brilliant. I'm starting to see how you think and I like it! Again thanks for all your help.

    Is the basic idea behind an equalizing resistor that as R2 approaches infinity, that V2 approaches V1? So that for significant values of R2, it has no effect on the circuit? Also, in your economical version, does the .68uf not affect the tone at all with the switch open?
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    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Mr. Bill View Post
    That's brilliant. I'm starting to see how you think and I like it! Again thanks for all your help.

    Is the basic idea behind an equalizing resistor that as R2 approaches infinity, that V2 approaches V1? So that for significant values of R2, it has no effect on the circuit? Also, in your economical version, does the .68uf not affect the tone at all with the switch open?
    I never look at in that way. I just look at voltage potentials. You don't want a cap holding a charge or needing a charge the moment it's switched into the circuit. It needs to be compatible with the environment or the instant differential will impress itself on the audio (POP!).

    And yes, the .68u cap WILL affect the tone. But it is only bypassing less than 10% of the resistance in that circuit with the switch open. The other 90% of the resistance is still elevating the circuit and imposing local negative feedback. I doubt you'd even hear the difference (without wondering if you were ACTUALLY hearing it) if you put the .68u on a switch and A/B tested it.
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    Thanks again Chuck. Would you mind reposting your not as economical version you had put up first so I can compare?

    And as far as A/B testing. I do a before and after shot of every mod and change I make to amps with the same stereo guitar part reamped pre and post incremental change. I try not to fool myself, but if I'm doubting I heard a significant difference in something, I just assume I'm not. Too bad you can't do a full null test as a tube circuit will never give you the exact same thing twice.
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    Supporting Member Chuck H's Avatar
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    Trying to dig it out of my already uploaded images. It may give some trouble because it has the same file name... Nope. It replaced it. I'll redraw.
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    Supporting Member Chuck H's Avatar
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    Ok... This is the typical way of doing this. And it occurs to me that it's six in one and a half dozen in the other, beacause...

    Bypassing 1/10th of the load should be roughly the same effect as paralleling it with a bypass at 10X. The math would prove out the details, but it's mice nuts. This just occurred to me now looking at the circuits. And, incidentally, this is half the reason I post here. It keeps my observations in the game. Using a larger value resistance for the switch bridge would probably skew the differential enough to cause some noise? I'm not sure on this, but I know that 100k is a typical value for this sort of circuit. So in that light, I'd be VERY surprised if anyone could hear any difference between my economy circuit and this one.
    Attached Thumbnails Attached Thumbnails swpos1b.png  
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    And if I wanted to add a switchable cap only on the third stage. I should do it like this? Am I getting it?stage-3-cap-switch.png
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    Supporting Member Chuck H's Avatar
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    Nope. The point (to some circumstances, like this one) is to keep capacitors referenced to the circuit voltage potentials. See below.

    Another consideration is the cap value. The .68uf value will NOT pass the same frequencies across the the 820R resistor as it would across a 1k or a 1.5k. And certainly not the same as a 2.7k which is the "classic" sound. So if that is your goal you need to adjust the cap value to knee at the same frequency for the 820R as the .68u value does for a 2.7k if you want it to do anything similar. The knee frequency of the capacitor is relative to circuit impedance. Which in this case is the same as the circuit resistance (though it isn't always). I like to use on line calculators and spice to support my lack of electronics training and math skills. But a good rule in cases where the resistance is the relative impedance is that the cap value doubles when the resistance value halves.

    Another thing to be aware of is that the cathode resistor value dictates the level of affect by the bypass cap. With the 2.7k value you get a -1db (more on why I chose this measure later) at about 550Hz with a total boost over unbypassed of about 6.5dB. But with an 820R resistor and (as I guestimate) a 1.5uf bypass cap you get a -1db at about 500Hz and a total boost of a tad less than 3dB. I chose the -1dB level rather than the typical -3db level because this cathode bypass circuit is only capable of about +6dB. So what get's percieved as the boost effect (IMHE) is anything above the the -1dB knee. Other's can disagree as they wish. Take your time getting your head wrapped around this and DO download Duncan TSC and ltspice as soon as you're up to it. Until then... Here is the schematic representing the values specified. Know that there is NO crime or wrong is simply experimenting with almost any values in the preamp and going with whatever sounds good to you

    EDIT: P.S. This is your welcome entrance into the "rabbit hole". Enter at your own risk. Sometimes I think I might have been better off to just plug into stuff and use what I thought worked and omit what didn't without trying to "know" more about the actual mechanisms. But I'm not wired like that and my life/playing and other things are permanently affected by the path I've taken. Choose and prosper or lament
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    Last edited by Chuck H; 01-07-2018 at 06:00 AM.
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    Thanks again for the correction and useful information. Some of it I already understand a little, more is always helpful. Regarding caps, I guess they always need to be in circuit, but the switch determines whether or not they're actually affecting it. That makes sense. As for the rabbit hole.....the rabbit hole is the only part I care about. My best kept secret is that I don't even play guitar! I could fake it in a punk band, but my instrument is the drums. From a recording standpoint though, I'm obsessed with guitar tones. I've never been truly happy with any amp so I started building my own. I'm working on a bit of a arsenal here of great sounds for the studio where I work. The journey is the best part!
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    Wait. Serious question. How is my drawing in post 26 different from Dave's drawing in post 5?
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    Quote Originally Posted by Mr. Bill View Post
    Wait. Serious question. How is my drawing in post 26 different from Dave's drawing in post 5?
    I had an extra cap and 10M resistor to ground before the switched capacitors. You need to put the 100k across the switch.
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    Quote Originally Posted by Chuck H View Post
    Another consideration is the cap value. The .68uf value will NOT pass the same frequencies across the the 820R resistor as it would across a 1k or a 1.5k. And certainly not the same as a 2.7k which is the "classic" sound. So if that is your goal you need to adjust the cap value to knee at the same frequency for the 820R as the .68u value does for a 2.7k
    The knee frequency doesn't change as much as you might think with change in resistor value because the resistor is effectively in parallel with the "resistance looking into the cathode" which is a lower value (1/Gm?) but the low frequency (un-bypassed) gain changes a lot.

    rkck.png
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    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Mr. Bill View Post
    Wait. Serious question. How is my drawing in post 26 different from Dave's drawing in post 5?
    In post #5 the caps are referenced to ground with the 10M AFTER being isolated from DC. In the cathode circuit you drew the DC is across the 100k resistor which is simply parallel to the 820R resistor, which is just the same as if the cathode resistor were 813k with no other resistor in the circuit. Ideally Dave's circuit (and mine) would have the 10M at the switch end of the caps. But being as there are multiple caps and poles this is cumbersome. For the cathode bypass cap you need the resistor across the switch to keep the cap in the circuit and equalize the charge for when the switch is flipped. For Dave's circuit in post #5 the caps are referenced to 0V at either end in a strictly AC environment. They are different circuits.
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    Oh okay. I overlooked that they were isolated by the cap before the switching network. I'm learning more this week than I have in months. You guys will never get rid of me now, ha.
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    Just triple checking that I have my head wrapped around this. So if I go with Dave's switching idea since I have all those parts in shop, then follow that with a single mini toggle to take the 470k/500p in and out of circuit before the gain knob, I'm fine with the second switch because the .022 decouples the signal before any other switching? Here's my crude drawing. I really need to get a better grip on LTspice so my simulations actually work.double-switched-couplers.png


    Further still, my switch to take the the .0047 in and out of the NFB loop needs nothing more than a switch since the cap on the presence knob is already isolating it from DC taking the scratchies out of the pot?
    Last edited by Mr. Bill; 01-07-2018 at 06:29 PM.
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    Supporting Member Chuck H's Avatar
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    Good to go on the three way switch. Connect the two way so that it shorts either end of the 470k/500p circuit together. Effectively "shorting" it rather than trying to bypass it.
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