Results 1 to 24 of 24
Like Tree17Likes
  • 1 Post By pdf64
  • 1 Post By Jazz P Bass
  • 1 Post By pdf64
  • 3 Post By eschertron
  • 1 Post By stokes
  • 4 Post By Enzo
  • 1 Post By pdf64
  • 1 Post By stokes
  • 2 Post By pdf64
  • 2 Post By g1

Thread: microphonic resistor, or end of shielded cable?

  1. #1
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539

    microphonic resistor, or end of shielded cable?

    From suggestions by Merlin, I put grid stops on the second half of V1 and V2 preamp tubes on a Ab763 type deluxe build (the one Ive been posting too many threads about ). For the grid stops on pin7 that go do the volume control, I soldered one lead to the pin, and the other lead I soldered the shielded wire to and put some heat shrink over.

    While debugging the amp, I had the chassis upside down, running, and poked around gently with a chopstick. I was very surprised to find that tapping on that resistor gave an audible sound through the speaker. I didn't get that anywhere else in the amp, even the tubes themselves. Only that component.

    Is this common, a microphonic resistor? I don't recall what kind it is only that it is 68k. Should I replace the resistor with a different mfr/type?

  2. #2
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,164
    68k is a way higher value than is beneficial for a 12AX7 grid stopper; with early stages it will lead to hiss.
    There's a baseline level of noise that's directly related to the equivalent series resistance of the source, and any grid stoppers will obviously add to that.
    Ian suggests '8/gm' as a guideline value, so at least 6k8 for a 12AX7, but above 22k will just add noise and treble roll off without benefiting stability (unless the lead dress / layout / design is REALLY bad).

    With enough gain, almost anything can be microphonic; maybe there happens to be good mechanical coupling between the grid socket terminal and the actual grid?
    Any kind of film resistor should be fine.
    Certainly don't use CC for grid stoppers, early stages at least.
    They are little tubes of carbon granules.
    I remember as a teenager (we didn't get a phone until my brother went to uni in the late '70s!), telephone mics were also made from carbon granules, every so often they would go noisy and you would need to give the mic a bit of a knock to re-settle the granules into a happy place.
    Last edited by pdf64; 12-04-2017 at 02:55 PM.
    ric likes this.

  3. #3
    Old Timer
    Join Date
    May 2006
    Location
    NYC
    Posts
    1,630
    Why did you add these resistors between the grid and vol controls?

  4. #4
    ric
    ric is offline
    Senior Member
    Join Date
    Dec 2013
    Posts
    145
    Mike, in your amp pics it looks like you are using some Vishay/Dale rn series metal film resistors. Those are an excellent choice for anything in the signal path.

    Similpost with stokes. Yes, if you chose to use grid stops in the pre amp, those little brown ones are low noise.
    Last edited by ric; 12-04-2017 at 02:36 PM. Reason: similpost

  5. #5
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,164
    Quote Originally Posted by stokes View Post
    Why did you add these resistors between the grid and vol controls?
    Small grid stoppers mounted at the tube socket terminals of the input, 2nd and 3rd stages can help to maintain stability if a few tweaks for higher gain are done.
    I prefer to avoid screened cable inside an amp, especially the long run from a BF fx channel volume control to tube socket, as I suspect that cable capacitance acts to roll off noticeable treble when the volume control is set around the electrical halfway region and the source impedance goes up around 250k.
    Miller capacitance will act on that anyway, but no pint exacerbating the effect unnecessarily.
    I think that the Custom 68 DR etc have additional grid stoppers too, but use rather higher values which lead to hiss.
    Last edited by pdf64; 12-04-2017 at 03:05 PM.

  6. #6
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Quote Originally Posted by pdf64 View Post
    68k is a way higher value than is beneficial for a 12AX7 grid stopper; with early stages it will lead to hiss.
    There's a baseline level of noise that's directly related to the equivalent series resistance of the source, and any grid stoppers will obviously add to that.
    Ian suggests '8/gm' as a guideline value, so at least 6k8 for a 12AX7, but above 22k will just add noise and treble roll off without benefiting stability (unless the lead dress / layout / design is REALLY bad).

    With enough gain, almost anything can be microphonic; maybe there happens to be good mechanical coupling between the grid socket terminal and the actual grid?
    Any kind of film resistor should be fine.
    Certainly don't use CC for grid stoppers, early stages at least.
    They are little tubes of carbon granules.
    I remember as a teenager (we didn't get a phone until my brother went to uni in the late '70s!), telephone mics were also made from carbon granules, every so often they would go noisy and you would need to give the mic a bit of a knock to re-settle the granules into a happy place.
    Thanks PDF. yes, this amp does have a lot more Johnson noise than I expected it would, must be due to this case. The original suggestion might have been a type'o, sounds like he might have meant 10k? My lead dress isn't spectacular working on that.

    Im using metal films everywhere. The reason for the odd mix of resistor types was just availability for each particular value.

  7. #7
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Quote Originally Posted by stokes View Post
    Why did you add these resistors between the grid and vol controls?
    Hi STokes, well i read as much as I could about what musicians thought about this particular amp. I never actually played through one (did have a BF Princeton back in teh day, and a SUnn). The one complaint that was consistent was blocking distortion (farting out) at higher bass levels and volumes. I read all I could about blocking distortion and the one suggestion for reducing it was to add grid stops after the tone stack, and up the value of the grid stop on the output tubes. I thought to try the additional grid stops on pin7 of the preamp tubes first, see how that worked, then up the stops on the output tubes if necessary.
    Last edited by mikepukmel; 12-04-2017 at 05:55 PM.

  8. #8
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Quote Originally Posted by ric View Post
    Mike, in your amp pics it looks like you are using some Vishay/Dale rn series metal film resistors. Those are an excellent choice for anything in the signal path.

    Similpost with stokes. Yes, if you chose to use grid stops in the pre amp, those little brown ones are low noise.
    Thanks Ric. I will fish around and see what I have in the little brown vishay dale's. I have a really odd mix of brands and types just due to what Digikey and Mouser has available in small quantities.

  9. #9
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Quote Originally Posted by pdf64 View Post
    Small grid stoppers mounted at the tube socket terminals of the input, 2nd and 3rd stages can help to maintain stability if a few tweaks for higher gain are done.
    I prefer to avoid screened cable inside an amp, especially the long run from a BF fx channel volume control to tube socket, as I suspect that cable capacitance acts to roll off noticeable treble when the volume control is set around the electrical halfway region and the source impedance goes up around 250k.
    Miller capacitance will act on that anyway, but no pint exacerbating the effect unnecessarily.
    I think that the Custom 68 DR etc have additional grid stoppers too, but use rather higher values which lead to hiss.
    Thanks PDF. As mentioned the amp does have more hiss but the hiss only increases to an untolerable level when I put the volume way up (near max), and Ive never played at that level. I didn't have the knobs on when I was fiddling so I don't know the levels. If the size of the stop in ohms should be more like 10k, and that will do the job, reduce blocking distortion and help with stability, I will switch out the stops I have in there now.

    So far, however it turned out, this amp is *bright*, maybe a little too bright.

    I did notice, maybe this is to be expected, that the sound quality is good at very low volume. But it really picks up (sounds better) a little higher. I suspect it was less than 1/2 vol. I didn't have the vol up enough to get blocking distortion or clipping yet, though.

  10. #10
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    11,860
    Quote Originally Posted by mikepukmel View Post
    Hi STokes, well i read as much as I could about what musicians thought about this particular amp. I never actually played through one (did have a BF Princeton back in teh day, and a SUnn). The one complaint that was consistent was blocking distortion (farting out) at higher bass levels and volumes. I read all I could about blocking distortion and the one suggestion for reducing it was to add grid stops after the tone stack, and up the value of the grid stop on the output tubes. I thought to try the additional grid stops on pin7 of the preamp tubes first, see how that worked, then up the stops on the output tubes if necessary.
    From what I have seen on Fender amps of this era, the "farting out" is usually caused by the first tube cathode bypass capacitor value being too high.
    25uf will pass way too much low frequencies.
    Try a 15, a 10 or even a 4uf.

    Have you ever encountered that symptom on a Marshall amp?
    No?
    Look at there circuits. They use a 0.1uf cap.
    ric likes this.

  11. #11
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    11,892
    Quote Originally Posted by Jazz P Bass View Post
    From what I have seen on Fender amps of this era, the "farting out" is usually caused by the first tube cathode bypass capacitor value being too high.
    25uf will pass way too much low frequencies.
    Try a 15, a 10 or even a 4uf.

    Have you ever encountered that symptom on a Marshall amp?
    No?
    Look at there circuits. They use a 0.1uf cap.
    Since hum abatement was already an issue for this build...

    I did the partial on the first stage cathode with BF topology preamps for a long time and it's a good way to mitigate farting bass. There's just so much LF through the BF topology preamp. But lately I've been moving the partial bypass to the second (tone stack/volume recovery stage) on the principal that a fully bypassed first stage is more ideal for minimizing filament circuit hum injection on the signal path. I've never done any actual testing to see what/if the real world advantage to this is. Since there's no clipping of the second preamp stage at any volume setting with that preamp design partial bypass on the second stage works just fine. You do need to go to a lower value cap to get similar results. More like 1uf or 2.2uf.

    JM2C
    "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

  12. #12
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Thanks everyone!

    Boils down to 3 issues then?

    Grid stops help fix:

    1) keeping unwanted RF noise out of amp (e.g. radio stations etc)
    2) stability problems, e.g. HF oscillation
    3) changing time constants to reduce blocking distortion.

    Fender put a 68k near the input jacks but would this only help 1 since its not on the tube socket?

    After some more reading, still not sure where the blocking distortion occurs, which tube stage(s).

    Not sure about my wiring, don't know enough to judge which parts of the lead dress result in bad oscillations.

  13. #13
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,164
    Quote Originally Posted by Jazz P Bass View Post
    From what I have seen on Fender amps of this era, the "farting out" is usually caused by the first tube cathode bypass capacitor value being too high.
    25uf will pass way too much low frequencies.
    Try a 15, a 10 or even a 4uf.

    Have you ever encountered that symptom on a Marshall amp?
    No?
    Look at there circuits. They use a 0.1uf cap.
    Dunno about 0.1uF cathode bypass? Rather 0.68uF comes to mind.
    But for best heater hum rejection, early, especially input stage cathodes should be fully bypassed.
    For BF Fenders, the cathodes of 2nd / 3rd stages seem better places go 'go partial', both in terms of hum rejection and blocking distortion mitigation.

  14. #14
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,530
    JM2C
    Blocking shouldn't be an issue for the input stage, unless you're slamming the amp with a herzog-like pedal with 10vpp output or so. It's the stages that do get signals that exceed the input HR that need blocking mitigated.
    Likewise, with the typical small-signal at the input there will be no distortion of the LF content, so partial bypass can be reserved for a later stage. I modified a champion 600 reissue at the power tube (functionally the third gain stage) with a small coupling cap and a partial cathode bypass, and got good results. The signal simply wasn't large enough to be a problem in the earlier stages.
    pdf64, J M Fahey and mikepukmel like this.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  15. #15
    Old Timer
    Join Date
    May 2006
    Location
    NYC
    Posts
    1,630
    The original design had no blocking distortion problems,so I fail to see the need for them in the places you added them.You say they are microphonic,so I would suggest removing them.If they are microphonic you are adding noise that shouldnt be there.These amps are not hi-gain circuits so there shouldnt be any blocking distortion problem.
    mikepukmel likes this.

  16. #16
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Thanks everyone!! Oh boy, well, I've done a bunch of reading, some from musicians, and some articles like Aiken's and Merlin's sites and the one consistent complaint about blackface circuits was blocking distortion when you turn up the bass, and then turn up the volume. Most of them said "just turn the bass down". One even said "WHY would you turn the bass up that high, you know it will sound lousy" referring to blocking. I wrote Aiken and Merlin emails asked questions a bout blocking. Aiken says that the combination of smaller ohm or no grid stop resistors and larger coupling caps in the blackface Fenders cause the problem to a much greater extent than say the marshalls, that used much smaller coupling caps, had earlier preamp stages that rolled off the bass a lot more, and had bigger grid stops (which some of you wrote above about coupling caps). So, what could I do without changing the tone TOO much, the suggestions were to first add grid stops on the back half of the preamp tubes (after the tone stack), and up the value of the stop on the output tubes. If this wasn't enough, then add grid stops on the phase inverter and reduce the coupling cap on the down side of the phase inverter. Some of this will change the tone of the amp, ok, agreed. But funny that if there is too much blocking distortion, you can't turn up the bass anyway, so ... I guess id rather have the amp as close to designed as possible, but reduce the blocking without going too far. I like to play with a wide variety of settings on the amp, if turning up the bass kills he thing with blocking Id like to reduce that. Merlin and Aiken both suggested 100k grid stops in earlier stages to reduce blocking, but this really does increase audible Johnson noise, but only seems to be a big problem at volume 8 or 9 and I won't go that high.

  17. #17
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,458
    Note too there is a difference between potential problems and actually HAVING the problem.
    J M Fahey, g1, eschertron and 1 others like this.
    Education is what you're left with after you have forgotten what you have learned.

  18. #18
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,164
    Quote Originally Posted by mikepukmel View Post
    ...one consistent complaint about blackface circuits was blocking distortion when you turn up the bass, and then turn up the volume. Most of them said "just turn the bass down". One even said "WHY would you turn the bass up that high, you know it will sound lousy" referring to blocking...
    Actually those responses seem reasonable to me. The flipside of having controls with a lot of range is that some settings will likely sound bad. Perhaps there's blocking in there, but get rid of the blocking distortion and those settings will still likely sound bad.

    Some folks can tend to fixate on making their amps sound bad, whereas good musicians concentrate on making the best of what they've got, maybe there's a degree of bias shift in there, on the verge of blocking distortion, they work with it rather than against it.

    I think that experience of these things will be your best education, eg build the thing stock and analyse it, then tweak to make it block, oscillate etc, so you get a better idea of what the cause and symptom of these effects are.

    Quote Originally Posted by mikepukmel View Post
    ...Merlin and Aiken both suggested 100k grid stops in earlier stages to reduce blocking...
    I suspect you may have misunderstood / misinterpreted that.
    Whatever, it's misapplied, a BF Fender preamp is pretty resilient to blocking distortion.

    Can you cite what you read that led you to that understanding?

  19. #19
    Old Timer
    Join Date
    May 2006
    Location
    NYC
    Posts
    1,630
    Quote Originally Posted by Enzo View Post
    Note too there is a difference between potential problems and actually HAVING the problem.
    In other words,if it aint broke,dont fix it.
    mikepukmel likes this.

  20. #20
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,164
    Quote Originally Posted by mikepukmel View Post
    ...Grid stops help fix:

    1) keeping unwanted RF noise out of amp (e.g. radio stations etc)
    2) stability problems, e.g. HF oscillation
    3) changing time constants to reduce blocking distortion.
    ...
    As the context to the thread is a classic / vintage design, bear in mind that mitigating blocking distortion was unlikely to have been a consideration, as the amps weren't intended to be overdriven heavily enough for it to manifest.
    And the primary reason for the 68k input resistors was likely as input mixers, as they're not mounted at the socket terminal, and with the values apparently most suitable to that purpose.
    And that many (earlier) designs didn't have the power tube grid stoppers, likely because parts were expensive and all had to 'earn their keep'.

    Grid stoppers don't really change the time constants that relate to bias shift / blocking distortion, rather that by limiting grid current they reduce its effect, see https://www.ampbooks.com/mobile/ampl...ias-excursion/
    Not sure how accurate the actual calculator is, but you may perceive that it's more effective to tweak the Cg and Rg values.

    Quote Originally Posted by mikepukmel View Post
    ...not sure where the blocking distortion occurs, which tube stage(s)...
    Blocking distortion is a bias shift that occurs when a capacitively coupled grid is overdriven, ie the preceding stage tries to pull the following grid positive WRT its cathode, thereby inducing grid conduction (ask if you don't understand this).
    In a non master volume amp, as with all regular classic guitar amps, as signal level rises from 0, at any reasonable control setting the output tube control grids will be the first place at which overdrive occurs; with most designs, signal levels have to go a lot higher for any stages before that to overdrive (with a BF reverb channel, the reverb driver stage is usually next).
    Bias shift / blocking distortion is therefore primarily an output tube grid thing.

    As I mentioned, why not just build it stock and get to know it?
    If in use, you happen to notice nastiness that you think may be blocking distortion, then test to identify it, and then implement mitigation if it is indicated.
    Reducing Cg to 47nF / Rg to 100k is usually sufficient IME.
    Last edited by pdf64; 12-06-2017 at 02:33 PM.
    g1 and mikepukmel like this.

  21. #21
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Thanks, everyone. I will go back and dig up comments about the Fender designs and blocking. Great suggestion to get it built and learn to know it first, before doing stuff. The two technical resources were Aiken's articles and Merlin's book. Both mention as you have above, that Fender never designed or intended his amps to be driven to distortion. And also at least a few other technical articles talk about the preamp designs on Fenders compared to Marshalls, the Fenders didn't cut off the low end like Marshall did. So, this would compound blocking, if you crank the Fenders. Went back and re-read the section on blocking in Merlin's book, and now looking at the grid bias calculator (thanks for the link) After the re-reads, and read, I have a little better idea what causes blocking distortion.

    It looks like (please correct me if Im wrong), that the size of the grid stop that would reduce grid current enough to have an effect on blocking, would be much larger than the grid stop designed to reduce unwanted oscillation. Both Merlin and Aiken talk about roughly 100k for blocking, but much smaller < 20k for oscillation problems.

  22. #22
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    These took a long time to go through, probably 6 months before I started buying parts to build a Deluxe:

    https://www.thegearpage.net/board/in...reverb.389128/

    There is some chatter at first, but at least a few musicians commented like this:
    "That said, the best way around it is to keep the bass knob on 4 or even less, mic the amp and make up for the lack of bass with some EQ in the mains.

    Many players overlook this. I think we obsess sometimes over getting lots of bass in our guitar amps, forgetting that it's the bass player's job to play bass. Seriously, I keep my BFDR bass set between 2 to 4 and the amp sounds rich and full in a mix playing in a band. I use a Weber 12F150."

    Ive read this from many musicians: keep the bass below about 4. (or it will make the refried bean sound)

    I'll dig up some more links, but there are many like that.

    So, before I started the build, I emailed a few of the gurus (it wa before I found this awesome site!!!) and thought, well if its only adding a few grid stops and it doesn't kill the tone, and it will help (doesn't have to completely get rid of) blocking, then why not add the grid stops?

  23. #23
    g1
    g1 is offline
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    8,944
    I always considered the "flabby bass" and blocking distortion as separate issues.
    Most common responses I've seen on this forum to flabby bass are usually bass cuts via cathode bypass cap reduction or coupling cap reduction. (aside from "you can't set bass above 4 with a Fender" )
    For blocking distortion, larger grid resistors are often mentioned, but as others have mentioned, this is usually with high gain type amps.
    Chuck H and mikepukmel like this.
    Certified Dotard

  24. #24
    Senior Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    539
    Quote Originally Posted by g1 View Post
    I always considered the "flabby bass" and blocking distortion as separate issues.
    Most common responses I've seen on this forum to flabby bass are usually bass cuts via cathode bypass cap reduction or coupling cap reduction. (aside from "you can't set bass above 4 with a Fender" )
    For blocking distortion, larger grid resistors are often mentioned, but as others have mentioned, this is usually with high gain type amps.
    Thanks g1, yeah, and it gets worse when you talk to more musicians (flabby bass vs blocking).
    I have to get a setup where I can record some sounds and let you all hear them. I may very well be mistaking farts for flab.

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Shielded cable
    By Jarvini in forum Guitar Amps
    Replies: 13
    Last Post: 10-04-2017, 09:58 PM
  2. Shielded cable
    By Jarvini in forum Guitar Amps
    Replies: 0
    Last Post: 10-02-2017, 08:49 PM
  3. Shielded wire grounded at one end
    By Johnrcurry in forum Theory & Design
    Replies: 5
    Last Post: 07-10-2010, 12:24 AM
  4. Shielded Cable
    By Amp Kat in forum Theory & Design
    Replies: 3
    Last Post: 12-03-2008, 07:16 PM
  5. Shielded cable
    By mbach72 in forum Build Your Amp
    Replies: 1
    Last Post: 02-23-2007, 06:28 PM

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
  •