Page 1 of 2 12 LastLast
Results 1 to 35 of 57
Like Tree73Likes

Thread: cathode bypass and grid stoppers/ when where and why

  1. #1
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146

    cathode bypass and grid stoppers/ when where and why

    Looks like there's a consensus here that the first stage cathode should be fully bypassed to minimize heater noise. Are we talking 22uf on a 100k plate 1k5 cathode? Are we saying smaller cap values are still fully bypassing as long as no resistor is in series with the cap?

    Also the grid stop question of how much and why, if at all.

    As a jumping off point I'll mention from my own experiments with 3 gain stage to concertina and 4 gain stage with reverb to LTP I liked 22k single input stopper, 47k second stage and 22k the rest of the way. With the cathode bypasses varying from adjustable up to 220uf to none.

    I'd like to know what the prevailing wisdom around here is, and why.
    mikepukmel likes this.

  2. #2
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,539
    Be sure to understand that these are two separate engineering issues, so they can be discussed apart from each other without confusion.

    I'm in the 'bypass the cr@p out of the first stage' camp. 22u takes the 1.5k cathode resistor's knee frequency to just about 5Hz. I have used some 10u/50v caps because they were available, the knee freq is about 10Hz. Probably still OK.

    What about forward-biased LEDs? Anyone have a sense of how this topology resists heater hum?
    ric and mikepukmel like this.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  3. #3
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    11,899
    Fully bypass the first gain stage cathode. If you need to mitigate LF from the first stage you should use the coupling cap. If you need to mitigate LF in the overall preamp you can usually partially bypass subsequent gain stages without notable consequence. The caveat here is that there is a big difference between vintage preamps, amps that use overall overdrive to achieve distortion and modern preamp based distortion generators that typically run the power amp clean. For the later it's best to keep the first two stages fully bypassed or just use DC for the filaments and partial bypass as you like on any stage.
    Last edited by Chuck H; 12-05-2017 at 04:07 AM.
    ric and mikepukmel like this.
    "The man is an incompetent waste of human flesh. He should donate his organs now to someone who might actually make good use of them." The Dude re: maybe I shouldn't say, but his name rhymes with Trump

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  4. #4
    Supporting Member loudthud's Avatar
    Join Date
    May 2006
    Location
    Near Dallas Texas
    Posts
    3,309
    A Cold Clipper stage will act badly if the cathode is bypassed when that stage is hit by a big signal. The bias will want to move when a signal is present and may be slow to recover between notes. An unbypassed cathode will recover instantly.
    Chuck H, J M Fahey, ric and 3 others like this.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  5. #5
    Senior Member
    Join Date
    Jan 2014
    Location
    Chiraq
    Posts
    704
    What would you consider a cold clipper stage to be, maybe anything higher than 3.3K cathode resistor, or maybe 10K?

    I have been messing around a little with what seems like ampeg style component values where it's something a little weird, something like 6.8K cathode resistor and 150-470K plate resistor.

    What is the advantage or difference with this compared to just doing 1K cathode 100K plate resistor which is way more typical?
    mikepukmel likes this.

  6. #6
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,539
    As the anode R goes up, the cathode R can go up too. Looking at a load line chart will help identify what's too far from center. Marshall's 100k anode, 10k cathode, no bypass cap (2203/2204) is a good example of cold clipping in action.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  7. #7
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    I think there are two different types of heater noise (maybe more). One type occurs outside the tube, and is caused by unwanted coupling between the heater circuit and the signal circuit. This will be affected by cathode bypass capacitors in the usual way (low-cut shelving filter).
    The other type occurs inside the tube and is caused by unwanted leakage current from the heater to the cathode. This will be affected by a cathode bypass cap in a different way. A big capacitor in this case will short-circuit the ac hum voltage to ground and prevent it being amplified by the tube. I think in this case there is no shelving effect, and it’s actually a high-cut filter (with regard to the heater noise getting into the signal), albeit with big attenuation at 50/60Hz. A very high valued bypass cap, such as 200uF, can then have a beneficial effect on this type of hum/buzz.
    pdf64, ric, SoulFetish and 1 others like this.

  8. #8
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    Quote Originally Posted by ric View Post
    Also the grid stop question of how much and why, if at all.
    The grid stopper on V1 is usually the main source of background hiss in an amp. Reducing the resistor value reduces the hiss level, but the lower you go the more chance there is of picking up radio stations. (Could be either humorous or embarrassing in a gig situation!)

    A grid stopper of around 10k can be a good compromise. An additional small cap from grid to ground can also be introduced to allow a small grid-stopper while retaining the required high cut filtering to block radio interference.
    ric and mikepukmel like this.

  9. #9
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Malcolm, what range of sizes do you use for input cap-to-ground?
    mikepukmel likes this.

  10. #10
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    I would say about 300pF. The theory and technical details are covered in Merlin's book (which is highly recommended):
    'Designing Tube Preamps for Guitar and Bass', Second Edition, Merlin Blencowe
    ric and mikepukmel like this.

  11. #11
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,167
    It goes horribly against the grain with me to add capacitance to the input; fine with the guitar at full volume, but it would surely muddy the tone at mid settings, ie when the equivalent series impedance of the source is >100k? Assuming a 'plug straight in' no pedals scenario.
    Also not an issue for the players who just use the instrument volume as a mute, sometimes even replace it with a switch.

    But for the players such as myself, who spend time and money sourcing low capacitance guitar cables in order to minimise the 'muddy at mid volume' thing, it seems counter-productive to then add capacitance at the amp input.
    Last edited by pdf64; 12-07-2017 at 04:13 PM.

  12. #12
    Supporting Member
    Join Date
    Sep 2007
    Posts
    2,860
    Quote Originally Posted by Malcolm Irving View Post
    I would say about 300pF. The theory and technical details are covered in Merlin's book (which is highly recommended):
    'Designing Tube Preamps for Guitar and Bass', Second Edition, Merlin Blencowe
    A filter such as you describe is most effective if the interfering signal is coming in on the center conductor since the R and C act together in this case. But it is the shield of the cable that picks up most of the interference. The C still has some effect, of course. Guitar amps usually have jacks that are isolated from the chassis. This is what you need to prevent ground loops resulting in power line buzzing, but it leaves you open to rf on the shield. One solution is to put a .01 microf cap from jack ground to the chassis, making the jack and chases at nearly the same rf potential, while leaving them isolated at the frequencies of significant power line harmonics. Post #6 here (https://www.thegearpage.net/board/in...ignals.281178/) describes this. Ferrite might be useful if used properly, like those lumps you see on some USB cables.
    ric, SoulFetish and mikepukmel like this.

  13. #13
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Quote Originally Posted by pdf64 View Post
    It goes horribly against the grain with me to add capacitance to the input; fine with the guitar at full volume, but it would surely muddy the tone at mid settings, ie when the equivalent series impedance of the source is >100k? Assuming a 'plug straight in' no pedals scenario.
    Also not an issue for the players who just use the instrument volume as a mute, sometimes even replace it with a switch.
    I'm picking up radio on one amp and plan to include this feature on future builds. I would, however, make it switchable.

    A friend and trusted source of info who retired from the local university after a career of building and working on tube amps there suggested no more than .001. 300p seems quite reasonable and conservative.

    Guitar amps are a special case, and it's cool to be able to get all these different perspectives from people all over the planet that build and play them. For my specific guitar related questions I used to go to the local tech, some guy who called himself "Enzo". He really seemed to know quite a bit and was willing to answer questions.

  14. #14
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Quote Originally Posted by Mike Sulzer View Post
    A filter such as you describe is most effective if the interfering signal is coming in on the center conductor since the R and C act together in this case. But it is the shield of the cable that picks up most of the interference. The C still has some effect, of course. Guitar amps usually have jacks that are isolated from the chassis. This is what you need to prevent ground loops resulting in power line buzzing, but it leaves you open to rf on the shield. One solution is to put a .01 microf cap from jack ground to the chassis, making the jack and chases at nearly the same rf potential, while leaving them isolated at the frequencies of significant power line harmonics. Post #6 here (https://www.thegearpage.net/board/in...ignals.281178/) describes this. Ferrite might be useful if used properly, like those lumps you see on some USB cables.
    Mike, this brings up something else I wonder about. An isolated input jack leaves the guitar ground floating from earth ground. We want the amp chassis unearthed for safety, wouldn't we want the guitar grounded for the same reason?

  15. #15
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,491
    For my specific guitar related questions I used to go to the local tech, some guy who called himself "Enzo". He really seemed to know quite a bit and was willing to answer questions
    .

    Thank you for the kind thoughts. Now retired, I am trying to pass along the accumulated lore of the trade to local fellow RJ.
    ric and mikepukmel like this.
    Education is what you're left with after you have forgotten what you have learned.

  16. #16
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    Quote Originally Posted by ric View Post
    ... . An isolated input jack leaves the guitar ground floating from earth ground. We want the amp chassis unearthed for safety, wouldn't we want the guitar grounded for the same reason?
    The amp chassis should be solidly connected to mains earth for safety. The chassis should also be connected to signal ground to help with screening (at one point only - to avoid a ground loop). The best place to make the connection between signal ground and chassis is close to the input jacks, IMHO. This would be via a dedicated wire to a solder lug on the chassis (i.e. the jack socket itself is still isolated).

    A lot of people recommend grounding at the output stage, but I disagree with that for the following reason: The non-zero resistance of any signal grounding scheme means that a small ac voltage will exist between the chassis and the signal ground at the opposite end to where the connection is made. I would rather have that small ac voltage at the power output stage where it will have negligible effect, rather than near the input stage where it will be amplified if it is linked into the signal path via stray capacitance.
    Last edited by Malcolm Irving; 12-07-2017 at 04:05 PM.
    pdf64, ric and mikepukmel like this.

  17. #17
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    I suppose ‘signal ground to chassis close to the input jack’ only applies in conjunction with the ‘daisy chain of local star points’ method of grounding.
    If people prefer ‘a single star point’ for the whole amp, that may work better with the star point at the power output stage. Otherwise, there would be some long ground wires carrying buzz current all the way to near the input jack and potentially linking into the signal path by stray coupling.
    pdf64, ric and mikepukmel like this.

  18. #18
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Quote Originally Posted by Enzo View Post
    .

    Thank you for the kind thoughts. Now retired, I am trying to pass along the accumulated lore of the trade to local fellow RJ.
    You let me take up many hours of you're shop time over a year or two.

    I also remember my cousin (also a tech) calling me back about voltage rating on a switch as I'm checking out at the hardware and telling him:"Thanks, Enzo already called back and told me that". And you pointed me at Ampage where "I answer these kind of questions". Yes you do.
    mikepukmel likes this.

  19. #19
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Anyone care to weigh in on an isolated input jack leaving the guitar's ground floating, not at earth ground?
    mikepukmel likes this.

  20. #20
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,491
    Chassis should be at earth, What we usually call "ground" is really the circuit common. On many amps those are the same, on many others it is not. For example many Marshall amps have a ground common to chassis link that is a 10 ohm resistor, a cap and a pair of diodes all in parallel.

    But in all cases the earth and ground common are linked together, as Malcolm said. So the guitar is not floating, it is just not directly connected to chassis at the input jack. In some places the isolated jack ground is linked to earth (chassis) right at the jack, but through a link with a ferrite bead or some such. In other cases, the jack ground is connected to common which trails over to the power supply, where it is earthed to chassis.

    So, no float, Pepsi.
    ric, Malcolm Irving and mikepukmel like this.
    Education is what you're left with after you have forgotten what you have learned.

  21. #21
    Supporting Member
    Join Date
    Sep 2007
    Posts
    2,860
    Quote Originally Posted by Malcolm Irving View Post
    The amp chassis should be solidly connected to mains earth for safety. The chassis should also be connected to signal ground to help with screening (at one point only - to avoid a ground loop). The best place to make the connection between signal ground and chassis is close to the input jacks, IMHO. This would be via a dedicated wire to a solder lug on the chassis (i.e. the jack socket itself is still isolated).

    A lot of people recommend grounding at the output stage, but I disagree with that for the following reason: The non-zero resistance of any signal grounding scheme means that a small ac voltage will exist between the chassis and the signal ground at the opposite end to where the connection is made. I would rather have that small ac voltage at the power output stage where it will have negligible effect, rather than near the input stage where it will be amplified if it is linked into the signal path via stray capacitance.

    I do not think the issue is stray capacitance at 60 (50) Hz and significant harmonics. A connection between the low side of the supply (which has a transformer) and the chassis carries some current (blame it on Faraday). The voltage from the guitar should be the only voltage across the input of the first stage, which is the grid and the bottom of the cathode resistor/bypass capacitor. If the input jack is the connection of the power supply to the chassis then that current consisting of power harmonics flows in the wire connecting the low side of the jack to the low side of the first stage input, and thus makes a voltage in series with this input at the most sensitive point in the amp.

    It seems that some really smart people can make that current very small with proper layout, etc. I prefer to be safe and make that connection at a high level point, preferring the center tap of the transformer itself, which its essentially the same as the output stage.

    As discussed earlier, the jack isolated from the chassis is not good for rfi rejection. But there are ways around that.
    ric likes this.

  22. #22
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,167
    Quote Originally Posted by ric View Post
    Anyone care to weigh in on an isolated input jack leaving the guitar's ground floating, not at earth ground?
    Malcolm wasn't proposing that the guitar's ground be floating; in all cases it would be referenced to ground, either directly by means of a conductor (wire, chassis etc), or via a low value resistor//cap//diodes, the latter to mitigate for ground loop hum.
    But really it's safer if the guitar (and hence the guitarist) isn't grounded.
    If someone is playing out a lot, especially in venues where the electrical system may be less than ideal, then use of a wireless system is excellent electrocution mitigation.

  23. #23
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    Quote Originally Posted by Mike Sulzer View Post
    ... . A connection between the low side of the supply (which has a transformer) and the chassis carries some current (blame it on Faraday). ... .
    Can you elaborate a bit on that? Are you thinking of stray capacitance between the transformer winding and the core/frame and hence to the chassis? Or eddy currents induced somewhere?

  24. #24
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Quote Originally Posted by Malcolm Irving View Post
    Can you elaborate a bit on that? Are you thinking of stray capacitance between the transformer winding and the core/frame and hence to the chassis? Or eddy currents induced somewhere?
    It sounded to me like Mike is addressing what you mentioned in post#18, PA ground at the back end, only V1 grid and cathode/ bypass cap across input. Only at another angle/ perspective. I've been known to be wrong and I'm sure he can speak for himself, but that's what I understood.

  25. #25
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    I’m sure Mike is right that there is some current between the PT secondary 0V side and the chassis, but I am not clear about the closed path which that current takes and how significant that current is.
    If the connection from the PT secondary 0V and the chassis is the only such connection, we need to find some closed loop path for the current to return in. This path could be through inter-winding capacitance in the PT via the mains neutral and then the mains earth (which are connected somewhere by the power company). Or it could be via stray capacitance and/or insulation leakage to the transformer core/frame and hence to the chassis.
    I don’t think that current would be a result of magnetic induction (i.e. Faraday's Law) but maybe I’m wrong.
    ric likes this.

  26. #26
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    I’ve made a quick experiment. I have an amp which hums a bit more than I would like (100Hz hum). It’s a 2xEL84 class AB push-pull, with a 12AX7 LTP PI and an EF86 input stage. It has silicon rectification.
    I tried 4 options for connecting the chassis to signal ground:
    (1) near the input jack,
    (2) at the 0V of the reservoir cap,
    (3) both of the above (not recommended as it creates a ground loop),
    (4) both of the above, but with the ground daisy chain broken between the LTP and output stage (i.e. separate pre-amp and power-amp grounds to chassis).
    To cut a long story short, all 4 options gave me the same 23mV ac at the 8 ohm load, with nothing plugged in and the volume control at max.
    The hum level is acceptable and is subjectively about the same as the hiss level. The input grid stopper is 10k, but an EF86 is known to be noisier than a 12AX7.
    I suspect that the hum could be reduced by using better matched EL84s and/or replacing the 47uF reservoir with 100uF.
    Last edited by Malcolm Irving; 12-08-2017 at 03:37 PM.
    ric likes this.

  27. #27
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    OK. Tried increasing the reservoir by tacking in another 47uF in parallel with the existing one.
    Output level (hiss plus hum) at full volume now reduced from 23mV to 13mV.
    Still no measurable difference between options (1), (2) and (4). (Didn’t bother with option 3 this time.)
    ric and nsubulysses like this.

  28. #28
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Quote Originally Posted by Malcolm Irving View Post
    OK. Tried increasing the reservoir by tacking in another 47uF in parallel with the existing one.
    Output level (hiss plus hum) at full volume now reduced from 23mV to 13mV.
    Still no measurable difference between options (1), (2) and (4). (Didn’t bother with option 3 this time.)
    Aluminum or steel chassis?

  29. #29
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    Quote Originally Posted by ric View Post
    Aluminum or steel chassis?
    Aluminium.
    ric likes this.

  30. #30
    Supporting Member
    Join Date
    Sep 2007
    Posts
    2,860
    Quote Originally Posted by Malcolm Irving View Post
    OK. Tried increasing the reservoir by tacking in another 47uF in parallel with the existing one.
    Output level (hiss plus hum) at full volume now reduced from 23mV to 13mV.
    Still no measurable difference between options (1), (2) and (4). (Didn’t bother with option 3 this time.)
    I agree with your analysis that leakage such as capacitance at the transformer would be necessary for the current I am hypothesizing. But nothing does it like an experiment! Some difference might be noticeable with a quieter first stage, but you have shown that it cannot be a big difference, if any at all.

    If the hum is due to poor balance between the El34s, should it not be just as audible with the volume all the way down?

    When you make the cap bigger, you are reducing the hum at all the successive stage of the RC filtering (if the circuit is the usual one or a variant), and so the question of where the hum is introduced is still open, I think.
    ric and Malcolm Irving like this.

  31. #31
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    Quote Originally Posted by Mike Sulzer View Post
    ...
    If the hum is due to poor balance between the El34s, should it not be just as audible with the volume all the way down?

    When you make the cap bigger, you are reducing the hum at all the successive stage of the RC filtering (if the circuit is the usual one or a variant), and so the question of where the hum is introduced is still open, I think.
    Good points, I agree. With the vol. all the way down, the output drops to 0.5mV ac. The hum (as well as the hiss) must be coming from the first stage (which is the only one prior to the vol. control).
    ric likes this.

  32. #32
    Supporting Member
    Join Date
    Sep 2007
    Posts
    2,860
    Quote Originally Posted by Malcolm Irving View Post
    Good points, I agree. With the vol. all the way down, the output drops to 0.5mV ac. The hum (as well as the hiss) must be coming from the first stage (which is the only one prior to the vol. control).
    So perhaps the hum is from the filament (how they are heated, that is)?
    ric likes this.

  33. #33
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,167
    But Malcolm notes that it's 100Hz hum (post #26).

  34. #34
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    146
    Quote Originally Posted by pdf64 View Post
    But Malcolm notes that it's 100Hz hum (post #26).
    USA is 60 cycle 120v for residential, you are 100hz 220v? So you are talking possible mains hum?

    Malcolm is using an aluminum chassis, not as good as steel for magnetic shielding. Could this be how hum is getting through? Are preamp tube shields being used? If aluminum, would steel shields make a difference?

  35. #35
    Senior Member Malcolm Irving's Avatar
    Join Date
    Dec 2014
    Location
    Derbyshire
    Posts
    276
    The amp uses AC heaters. We’re on 50Hz here in the UK, which makes the hum 100Hz on B+, so I expect it is coming from the B+ rail. As Mike said, doubling the reservoir cap reduces the hum throughout the B+ supply, including that to V1. The EF86 has an internal shield, which in this case is grounded.
    I’ve tried 4 old EF86s and 2 new ones, producing a range of hiss and hum output levels, at full volume, from 13mV to 20mV. (All with 2 x 47uF reservoir.)
    It is interesting that the hiss and hum are subjectively at about the same level, while the earlier test suggests that hum is a bigger voltage component of the output. (Doubling the reservoir reduced the hum but does nothing to the hiss.) I expect this is due to human hearing being much less sensitive to 100 Hz than to the higher frequencies in the hiss, at these low sound pressure levels.
    The hum (and hiss) are already at an acceptable low level, but I will try tacking on an additional smoothing cap on the B+ for V1 and report back, when I have some time, just out of academic interest.
    ric likes this.

Page 1 of 2 12 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. grid stoppers and scren resisters?
    By catnine in forum Theory & Design
    Replies: 2
    Last Post: 12-25-2012, 05:45 AM
  2. Grid stoppers with AB2
    By Tage in forum Theory & Design
    Replies: 12
    Last Post: 11-07-2012, 10:48 PM
  3. Replies: 10
    Last Post: 01-21-2011, 01:39 PM
  4. Treble shunt - before or after grid-stoppers?
    By Gaz in forum Theory & Design
    Replies: 20
    Last Post: 08-29-2010, 04:32 PM
  5. Grid stoppers?
    By EETStudent in forum Theory & Design
    Replies: 26
    Last Post: 09-12-2008, 08:37 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •