Cathode Bypass Capacitor Voltage

JohnnyCrash

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JC wrote: "How many old JCM800s have you cracked open to see the cathode resistor for V1 was a huge-ass factory installed 2-3 watt 10k?"

Off topic...but what JCM 800 amp uses 10K cathode resistors on V1?



It's the unbypassed "cold" (via the 10k resistor) bias resistor of the cascaded preamp in the 2203/2204 amps - the second gainstage (V1's 2nd triode).
 

ThermionicScott

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There's nothing wrong with using a 50V cap in a 2-volt circuit. Just don't suddenly show it 50 volts after using it for years as a preamp bypass cap. ;)

- Scott
 

piece of ash

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A 50 Volt alum. 'lytic cap in a 2 Volt application WILL lose capacitance... that is a fact. Whether or not anything is with that "wrong" is an opinion. Perhaps it is best to oversize the cap to allow for the degradation?

What I would like to know from the tube gurus is: under what reakistic scenarios could the voltage run high? I mean way high... ... not silly stuff like plate-to-cathode shorts etc...
 

Wally

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Okay, I see it now, Johnny. That circuit actually began in the JMP SEries 2203/2204's. Thanks for the tip...
 

laird

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What I would like to know from the tube gurus is: under what reakistic scenarios could the voltage run high? I mean way high... ... not silly stuff like plate-to-cathode shorts etc...

The most obvious real-world situation I could think of is substituting in a current-driving tube like a 12AU7 or 12AT7 in place of a voltage-gain tube like a 12AX7... I could see the cathode voltage rising up from 1-3v to maybe the 15v range, but not much past that unless you're talking about some very uncommon tube swaps. 25v seems like a reasonable conservative rating to handle that scenario, especially when considering current spikes that often occur as tubes are first warming up.

BUT... If the previous stage were to start passing DC the cathode voltage will rise to maintain bias... If suddenly 100-200v started making its way through from the previous stage that bypass cap could turn into a firework real quick!

-Laird
 

printer2

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Lets look at your words "And yet Fender uses 50V caps for these positions." and "And yet the reissue uses 50V. Must sound like trash. ". Could you please show me where I missed a question, or anything remotely helpful. Both statements look like the conversational equivalent of a grenade, something destructive lobbed in the hopes of getting a reaction. I was not sure about the first, so I asked, but the "must sound like trash" in the second cleared the ambiguity right up. The effects of which we speak are well documented, even in the guitar amp world. It is discussed in the first of Merlin Blencowe preamp book, which is on his Valve Wizard site for a free down load.

That is a grenade, making a statement to punctuate a point? Of course we all would not consider that amp to sound like a piece of trash, well then, if it does not maybe using up to 50V capacitors in that position is not really a detriment. If you did not even remotely think of the possibility I do not know what to say. If it is trolling to you best to ignore my posts. I used the same method when I was teaching, it may seem like trolling but it caused the students to analyze the concept and think things out for themselves rather than just repeat what they have been taught.

As far as being helpful, check my other posts. Other than making the mistake of not checking out JC's building ability, which I apologized for immediate, what posts do you have a problem with? Ask the poster (I think electrician) that was troubleshooting his amp in real time and I tried to direct him through it. Spent a couple of hours swapping posts back and forth when I really wanted to work on my own amp.

Seeing that any whiff of personality is frowned on here, I will try to keep my posts simple and to the point. Never knew musicians to be so dry before.
 

piece of ash

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The most obvious real-world situation I could think of is substituting in a current-driving tube like a 12AU7 or 12AT7 in place of a voltage-gain tube like a 12AX7... I could see the cathode voltage rising up from 1-3v to maybe the 15v range, but not much past that unless you're talking about some very uncommon tube swaps. 25v seems like a reasonable conservative rating to handle that scenario, especially when considering current spikes that often occur as tubes are first warming up.

BUT... If the previous stage were to start passing DC the cathode voltage will rise to maintain bias... If suddenly 100-200v started making its way through from the previous stage that bypass cap could turn into a firework real quick!

-Laird

Thanks...
 

JohnnyCrash

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Okay, I see it now, Johnny. That circuit actually began in the JMP SEries 2203/2204's. Thanks for the tip...



Yeah, the 2203/2204 designation trips me out too. The JMP series do not have that weak Low input, but they also don't have the cascaded gainstage. I wonder who figures out (and how they decide on) circuit designations...

If I build another "Marshall" I'm considering one of the MV JMPs (probably a 2204) - I don't have one of the early MV Marshall circuits, but I'm a huge fan of the bands that used 'em (the Ramones, etc).



That is a grenade, making a statement to punctuate a point?



If you are confused why everybody else is the problem, perhaps you should re-examine your approach. I used to bug people too, now I realize how I came off sometimes... I was not a pleasant person early on in the forum (and I ocassionally still have bad, grumpy days).



Of course we all would not consider that amp to sound like a piece of trash, well then, if it does not maybe using up to 50V capacitors in that position is not really a detriment. If you did not even remotely think of the possibility I do not know what to say.



"Of course," "we all," "well then," "if you did not even remotely think of the possibility" that these kinds of statements have a whiff of superiority, sarcasm, and may come off as arrogant to others (as it has)? If it's not meant that way; cool.

The folks here aren't the ultra-sensitive type though, maybe it's just cold text on a monitor that makes things seem a way they are not originally meant to be.

celeste has been one of the kindest, most helpful members here for years - so, I do not understand your seeming reaction. She simply explained the facts... I could be wrong, and if so, I apologize, but it seems to some, that you're concerned about showing how right you are and how wrong others are instead of answering the OP's post.

She never said it sounds like "crap," she merely mentioned how, over time, it can be less than ideal, and why.



If it is trolling to you best to ignore my posts.



Will do. Most of your posts are indeed helpful, even if others have answered them and you're adding new perspectives, but sometimes they appear to have a healthy dose of smugness - which for people new to building amps, is yet another discouragement.

If this is not the case, perhaps you can at least understand how others may read into them as such? I've learned the hard way (I've been banned HAHA!) that in public forums, I have to reread my posts every now and then, so as to avoid insulting others. I also don't make fart jokes around the elderly, or curse around children, but we all have to adapt to various social situations :)



I used the same method when I was teaching, it may seem like trolling but it caused the students to analyze the concept and think things out for themselves rather than just repeat what they have been taught.



I'm not sure if your open to opinions other than your own, but sarcasm and insulting others' perceived stupidity seems like a horrible teaching method to me. At the least though, it is bad internet forum etiquette.



Spent a couple of hours swapping posts back and forth when I really wanted to work on my own amp.



Out of the kindness of your heart? I apologize that hackworth1's simple question somehow took you from your work (for "a couple of hours?")... you need not undercut fellow members though. I'm sure your hours of input are appreciated by the OP and others.



Seeing that any whiff of personality is frowned on here, I will try to keep my posts simple and to the point. Never knew musicians to be so dry before.



Personality is all over this forum (greatest members of all musician forums I've found), it is what some may mistake for superiority complexes and/or sarcasm that makes others bristle.

If this is not where you are coming from, I've found a simple apology and explanation (rather than further defenses for hostile teaching methods) always goes along way here... this is the best forum I've ever been on and the folks here are rarely looking to be offended, and often quick to respond to honest apologies.

They put up with me when I was going through a tough few years in my life where I was accidentally being a bitter PITA to others (I was acting like a know-it-all, smart-aleck, opinionated jerk). It wasn't a divorce or midlife crisis, but I was still acting like I was in andropause HAHA!

Anyway, celeste and piece of ash's advice is still good info. To hackworth1, it won't make your amp garbage (and nobody said it would), but it's ideal to use elecytrolytics that are rated a bit closer to actual voltages, with a safe little bit of extra. 20uF is fine. If you use bigger it won't ruin anything either.
 

celeste

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Yeah, the 2203/2204 designation trips me out too. The JMP series do not have that weak Low input, but they also don't have the cascaded gainstage. I wonder who figures out (and how they decide on) circuit designations...

If I build another "Marshall" I'm considering one of the MV JMPs (probably a 2204) - I don't have one of the early MV Marshall circuits, but I'm a huge fan of the bands that used 'em (the Ramones, etc).







If you are confused why everybody else is the problem, perhaps you should re-examine your approach. I used to bug people too, now I realize how I came off sometimes... I was not a pleasant person early on in the forum (and I ocassionally still have bad, grumpy days).







"Of course," "we all," "well then," "if you did not even remotely think of the possibility" that these kinds of statements have a whiff of superiority, sarcasm, and may come off as arrogant to others (as it has)? If it's not meant that way; cool.

The folks here aren't the ultra-sensitive type though, maybe it's just cold text on a monitor that makes things seem a way they are not originally meant to be.

celeste has been one of the kindest, most helpful members here for years - so, I do not understand your seeming reaction. She simply explained the facts... I could be wrong, and if so, I apologize, but it seems to some, that you're concerned about showing how right you are and how wrong others are instead of answering the OP's post.

She never said it sounds like "crap," she merely mentioned how, over time, it can be less than ideal, and why.







Will do. Most of your posts are indeed helpful, even if others have answered them and you're adding new perspectives, but sometimes they appear to have a healthy dose of smugness - which for people new to building amps, is yet another discouragement.

If this is not the case, perhaps you can at least understand how others may read into them as such? I've learned the hard way (I've been banned HAHA!) that in public forums, I have to reread my posts every now and then, so as to avoid insulting others. I also don't make fart jokes around the elderly, or curse around children, but we all have to adapt to various social situations :)







I'm not sure if your open to opinions other than your own, but sarcasm and insulting others' perceived stupidity seems like a horrible teaching method to me. At the least though, it is bad internet forum etiquette.







Out of the kindness of your heart? I apologize that hackworth1's simple question somehow took you from your work (for "a couple of hours?")... you need not undercut fellow members though. I'm sure your hours of input are appreciated by the OP and others.







Personality is all over this forum (greatest members of all musician forums I've found), it is what some may mistake for superiority complexes and/or sarcasm that makes others bristle.

If this is not where you are coming from, I've found a simple apology and explanation (rather than further defenses for hostile teaching methods) always goes along way here... this is the best forum I've ever been on and the folks here are rarely looking to be offended, and often quick to respond to honest apologies.

They put up with me when I was going through a tough few years in my life where I was accidentally being a bitter PITA to others (I was acting like a know-it-all, smart-aleck, opinionated jerk). It wasn't a divorce or midlife crisis, but I was still acting like I was in andropause HAHA!

Anyway, celeste and piece of ash's advice is still good info. To hackworth1, it won't make your amp garbage (and nobody said it would), but it's ideal to use elecytrolytics that are rated a bit closer to actual voltages, with a safe little bit of extra. 20uF is fine. If you use bigger it won't ruin anything either.

Johnny, thank you very much for that. It gives me reason to examine my reactions and ask if I could have been a bit more compassionate. printer2's post did touch a hot button issue for me, so perhaps I was to quick on the trigger. Mr. printer2, if you are still reading, Johnny has given us a bridge to exchange olive branches if you care to.
 

printer2

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Johnny, thank you very much for that. It gives me reason to examine my reactions and ask if I could have been a bit more compassionate. printer2's post did touch a hot button issue for me, so perhaps I was to quick on the trigger. Mr. printer2, if you are still reading, Johnny has given us a bridge to exchange olive branches if you care to.

No problem. I have read many of your posts and I find you knowledgeable and very helpful to people. I never saw you tie into anyone (not that you really beat me up too much :oops:). I guess I was a little too sensitive of the remark, I help moderate a forum on a trade website and, well I have been called many things, but for some reason the T word gets to me. Mind you the ones we have, well they are bad. But as long as they do not break any rules...

Anyway, no big deal. And thanks for offering the branch I appreciate it.
 

printer2

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So after a quick read of page 27 (thanks piece of ash, and I still get a kick out of the name) I wondered how much I could see if I scope two different capacitors with the same microfarad ratting but different voltage ratings. I wanted to use standard Fender values but the lowest value I had was 25V in 22uF, I also have 22uF at 350V. Now being that 25V is much larger than the voltage generated across a 1.5k resistor with a 12AX7 and a 100k plate resistor I went searching through some of my unused audio equipment and sacrificed one and came up with a 22uF at 6.3V.

Now this is not really a scientific test, I had a circuit that I was breadboarding and I just swapped out the capacitors I had and used the 6.3V and the 350V. I switched between the two using a SPDT switch and took some picture of the scope. The pictures are of an 80 Hz signal, the first is the generator at the input jack, the next is the 6.3V and the third is the 350V. I am really surprised at the results as I would have thought the 350V cap would look terrible.

Test setup/capacitors

BypassCapacitorTest63Vto350V.jpg


Signal Generator (channel 1)

CapacitorBypassTestSignalGenerator.jpg


6.3V capacitor

BypassCapacitorTest63V.jpg


350V capacitor

BypassCapacitorTest350V.jpg



I know this is just a simple test, obviously not the same capacitor manufacture, the 6.3V has not been used in years, the 350V has been within a year and it has seen severe service. I would have thought the gain would have been somewhat different, not sure what to think yet.
 

printer2

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Yeah, the 2203/2204 designation trips me out too. The JMP series do not have that weak Low input, but they also don't have the cascaded gainstage. I wonder who figures out (and how they decide on) circuit designations...

If I build another "Marshall" I'm considering one of the MV JMPs (probably a 2204) - I don't have one of the early MV Marshall circuits, but I'm a huge fan of the bands that used 'em (the Ramones, etc).







If you are confused why everybody else is the problem, perhaps you should re-examine your approach. I used to bug people too, now I realize how I came off sometimes... I was not a pleasant person early on in the forum (and I ocassionally still have bad, grumpy days).







"Of course," "we all," "well then," "if you did not even remotely think of the possibility" that these kinds of statements have a whiff of superiority, sarcasm, and may come off as arrogant to others (as it has)? If it's not meant that way; cool.

The folks here aren't the ultra-sensitive type though, maybe it's just cold text on a monitor that makes things seem a way they are not originally meant to be.

celeste has been one of the kindest, most helpful members here for years - so, I do not understand your seeming reaction. She simply explained the facts... I could be wrong, and if so, I apologize, but it seems to some, that you're concerned about showing how right you are and how wrong others are instead of answering the OP's post.

She never said it sounds like "crap," she merely mentioned how, over time, it can be less than ideal, and why.







Will do. Most of your posts are indeed helpful, even if others have answered them and you're adding new perspectives, but sometimes they appear to have a healthy dose of smugness - which for people new to building amps, is yet another discouragement.

If this is not the case, perhaps you can at least understand how others may read into them as such? I've learned the hard way (I've been banned HAHA!) that in public forums, I have to reread my posts every now and then, so as to avoid insulting others. I also don't make fart jokes around the elderly, or curse around children, but we all have to adapt to various social situations :)







I'm not sure if your open to opinions other than your own, but sarcasm and insulting others' perceived stupidity seems like a horrible teaching method to me. At the least though, it is bad internet forum etiquette.







Out of the kindness of your heart? I apologize that hackworth1's simple question somehow took you from your work (for "a couple of hours?")... you need not undercut fellow members though. I'm sure your hours of input are appreciated by the OP and others.







Personality is all over this forum (greatest members of all musician forums I've found), it is what some may mistake for superiority complexes and/or sarcasm that makes others bristle.

If this is not where you are coming from, I've found a simple apology and explanation (rather than further defenses for hostile teaching methods) always goes along way here... this is the best forum I've ever been on and the folks here are rarely looking to be offended, and often quick to respond to honest apologies.

They put up with me when I was going through a tough few years in my life where I was accidentally being a bitter PITA to others (I was acting like a know-it-all, smart-aleck, opinionated jerk). It wasn't a divorce or midlife crisis, but I was still acting like I was in andropause HAHA!

Anyway, celeste and piece of ash's advice is still good info. To hackworth1, it won't make your amp garbage (and nobody said it would), but it's ideal to use elecytrolytics that are rated a bit closer to actual voltages, with a safe little bit of extra. 20uF is fine. If you use bigger it won't ruin anything either.

Yeah I'll watch it. In the other forums where I am known, well they kind of got used to me and realize I am more the laugh at myself kind of guy and I see the humor in a situation even if it is dark humor. It did take them some time to pick up on it though. I was really surprised on one site that after a year that some of the people that did not care for me at first (mind you we were at opposite ends of the spectrum in our posts) nominated me to be one of the moderators.

Just too used to post like with the people who know me. Hope you guys have patience with me, I do mean well.
 

Telenut62

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We all have our off days ;) Anyway what your showing on the scope is that the voltage rating of the cap has no bearing on the signal?
 

Wally

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Yeah, the 2203/2204 designation trips me out too. The JMP series do not have that weak Low input, but they also don't have the cascaded gainstage. I wonder who figures out (and how they decide on) circuit designations...

If I build another "Marshall" I'm considering one of the MV JMPs (probably a 2204) - I don't have one of the early MV Marshall circuits, but I'm a huge fan of the bands that used 'em (the Ramones, etc).


I know this is off the topic a bit, but you might want to compare the JMP 2203/2204 to the JCM SEries 2203/2204 preamps.....they look quite simular to me, Johnny. The Low input feeds into v1B. When plugging into V1A for the hi input, there is a cascaded gain situation with those two triodes in both amps, isn't there? I am looking at a schematic from the '70's for the JMP 2203 and a schematic from 1988 for the JCM 800 Series 2203.
 

printer2

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We all have our off days ;) Anyway what your showing on the scope is that the voltage rating of the cap has no bearing on the signal?

Not sure yet. I was thinking about this one this morning. I do not know enough about the effect or what I should be looking for. I thought the muddiness of a too high a voltage cap might be manifest by a increase in gain. It appears not. I am wondering though if I should try it again at a lower frequency.

I did read an article once about distortion due to the use of ceramic capacitors with a bias voltage but I am not sure if it would be the same effect we are looking for here. I would like to spend more time looking into it but I have too many things on the go now. Maybe in the winter when it is -30 below out. I'll put these two capacitors aside to play with later.
 

ThermionicScott

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A 50 Volt alum. 'lytic cap in a 2 Volt application WILL lose capacitance... that is a fact. Whether or not anything is with that "wrong" is an opinion. Perhaps it is best to oversize the cap to allow for the degradation?
Interesting, what degree of capacitance loss do you generally see? Is it drastic within the 20-year replacement window for electrolytics? I was under the impression that this effect was mostly a bygone thing, passed on in lore along with things like the need to "form" caps.

- Scott
 

JohnnyCrash

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I know this is off the topic a bit, but you might want to compare the JMP 2203/2204 to the JCM SEries 2203/2204 preamps.....they look quite simular to me, Johnny. The Low input feeds into v1B. When plugging into V1A for the hi input, there is a cascaded gain situation with those two triodes in both amps, isn't there? I am looking at a schematic from the '70's for the JMP 2203 and a schematic from 1988 for the JCM 800 Series 2203.



The JMP schematics I see do not cascade gainstages... maybe I've got the wrong schematics?

I've got this for the JMPs:
http://www.drtube.com/schematics/marshall/2204u.gif


It looks like both Low and High have their own triodes and no cascaded gain stages... which I guess is why the JMPs have a fatter sound (with less gain).


As compared to the JCM800 2203/2204s as seen on the bottom half of this schematic:
http://www.drtube.com/schematics/marshall/jcm800pr.gif


The JCM800's Low channel skips the first stage and goes directly into the second (via the 470k+470pF, missing the 0.022uF output cap of stage 1), whereas the High channel throws the first stage into the 2nd's grid (where the Low taps directly into, bypassing the 1st stage entirely).

I've only built and owned JCM800s. The JMPs are all new to me :)
 




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