Can I use ground switch for switchable negative feedback without adding noise?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 17, 2019.

  1. tubelectron

    tubelectron Tele-Meister

    Posts:
    382
    Joined:
    Nov 25, 2010
    Location:
    France
    You do what you want and follow the advice you want. But If a SBY was really useless, no tube amp maker would use it... :rolleyes:

    I don't know who is lord valves, but I rely on my vintage radio and tube manuals, service guides, etc...

    But it's me, OK ? :D

    -tbln
     
  2. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    35
    Posts:
    2,435
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    I tried the ground location and did not use shielded wire. The noise level was unacceptable. I took out the extension Jack and used it for the switch with now noise problems.

    Sent from my XT1585 using Tapatalk
     
  3. itsGiusto

    itsGiusto Tele-Meister

    Posts:
    208
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    That's not quite true. Mythos has a large effect on people (too large) especially in music gear. Amp makers might simply be adding these switches because people have come to expect them, even if they're basically useless. Amp makers might just be trying to avoid the stigma of people thinking it's bad to not have a standby switch.
     
    moosie and corliss1 like this.
  4. Wally

    Wally Telefied Ad Free Member

    Posts:
    32,578
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Tubelectron, as we all are, I agree.....be free, mon ami.
    Just curious...I grew up with a use radios....even in the automobiles. There were no standby switch anywhere. Nor were there any standby switches on any of the tube TVs I grew up with. My first guitar amp, a single ended GA5, had no standby. My second amp, a 1964 Rickenbacker B16 Supersonic, did have a standby. The myth came with that new amp...warm it up. I did it for a long time until I read something about the reality of applying voltage to cold tube elements.
    I have no idea who was the first to put a standby switch on a guitar amp, but I would lay odds that they were thinking about having the ability to unplug at the guitar...or switch to standby during a break...whatever. Imho, once you turn a tube device on it should be left in the fully operational state until one is through using it....through for a long term as opposed to a break. I do believe as I stated that there can be an advantage to having a standby switch. Warming tubes is not one of them from an engineering point of view as I read in more than one paper on the subject. Nor..imho..is it advisable to use in between sets. Let that amp operate right where it is...at full operating temperature.
    As for prolonged standby, it has been proven to cause a detrimental effect...over a long time. The eff3ct is not catastrophic or immediate. Hey, one can run a redplating tube a certain amount of time and it still will work in the circuit. It just lives a shorter life with degraded performance.

    I like the quality of your builds, Brother, and the standby switch is useful for certain things, I agree.
     
    JohnnyCrash likes this.
  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    3,244
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    I've always thought a "mute" switch that grounds the signal just before the phase inverter would be a good implementation, as long as you make it known through your manual that it is NOT the same function as standby, which may open up even more confusion.
     
    JohnnyCrash and King Fan like this.
  6. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    35
    Posts:
    2,435
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    Y'all jogged me memory on this one. From the Valve Wizard:

    "There are two plausible explainations for the added standby switch. One is that is helped to protect the power supply capacitors from excessive voltage before the valves start to conduct and pull the voltages down to normal levels. Fender often used a reservoir capacitor rated for higher voltage than the smoothing caps after the standby switch, which must have saved money and space. Take a look at the 5E6 Bassman for example. The 135 Bassman schematic even shows labelled voltages, exceeding the cap ratings during standby.
    The other possibility is that the 5E6 was rhe first amp where Fender used a DC-coupled cathode follower. This stage will sometimes arc between grid and cathode at switch-on if the cathode has not yet warmed up. (These days you should put a diode or neon-lamp between grid and cathode to prevent this, not rely on the user).

    Everyone knows Marshall copied the Bassman -complete with standby switch- so we then had the two biggest names using standby switches, only one of which knew why. The other big players, Vox and Gibson, traditionally never used standby switches. Only very recently have they given in and started adding them, presumably out of customer expectation, even though no modern designer (who values his reputiation) uses under-rated power supply capacitors. In fact, when Vox added a standby switch to its re-issue AC30CC they made the big mistake of putting the standby switch between the valve rectifier and the reservoir capacitor (something Fender was careful not to do). This hot-switching lead to several catastrophic rectifier failures on the first production run. Vox had to hastily add a resistor in parallel with the switch to allow the reservoir to charge up during standby (neatly demonstrating the pointlessness of adding the standby switch in the first place). This is just another way in which a standby switch can reduce the life of a valve.

    More here. http://www.valvewizard.co.uk/standby.html

    Sent from my XT1585 using Tapatalk
     
    JohnnyCrash, moosie, King Fan and 3 others like this.
  7. Paul G.

    Paul G. Friend of Leo's

    Posts:
    2,669
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    What Wally said.
     
    JohnnyCrash likes this.
  8. Wally

    Wally Telefied Ad Free Member

    Posts:
    32,578
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Jason, I need to keep that aspect in mind. Although, in those big Fenders for instance I would have 700-900vdc in that first stage.
     
  9. BobSmith

    BobSmith Tele-Meister

    Posts:
    127
    Joined:
    Feb 12, 2018
    Location:
    San Jose,CA
    I’m very interested to hear how the NFB switch in this location turns out for you. Pictures would be great!
     
    itsGiusto likes this.
  10. tubelectron

    tubelectron Tele-Meister

    Posts:
    382
    Joined:
    Nov 25, 2010
    Location:
    France
    C'est n'importe quoi ! :lol:

    These improvised tribunes should read (serious) litterature about tubes before publishing their free assumptions - they would among other discover the importance of the warm-up time, notably about glass micro cracks around pins. o_O

    Sorry, my old books (Tungsram memento, RCA and Western Electric tube manuals, among others) are not scanned... :(

    Again, you do what you want : it's your amp. You asked for a suggestion : I gave mine. If you believe that's a myth, it's your choice.

    But it's me, OK ? :D

    -tbln
     
    Wally likes this.
  11. King Fan

    King Fan Friend of Leo's Ad Free Member

    Posts:
    3,850
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    LOL. Now that this has become a standby thread, let me quote the rest of what Merlin says about standby, including the myth of cathode stripping, how the old tube manuals have been misunderstood, and the actual ways standby is unneeded and may be harmful.

    “Many guitar amps (too many) include a standby switch. This is meant to let the heaters warm up before the high voltage is switched on. Old books called it 'preheating'.
    But let's get one thing straight: a standby switch does not extend the life of the valves, in fact it is more likely to reduce their useful life. The valves do not care if you switch on the heaters and HT at the same time (with a couple of exceptions explained below). Now, I know what you're thinking, "but every guitar magazine in the land says the exact opposite?" Yes, they do, but guitar magazines know next to nothing about electronics, they just repeat the same old wives' tale each year.

    If standby switches really did have magical life-extending properties then we would expect to see it mentioned from time to time in proper textbooks and valve manuals, yet such discussion is conspicuous by its absence. Indeed, some texts explicitly exclude audio valves from discussion of similar pre-heating switches (mainly with regard to radio transmitters). Despite this, promotional materials glorifying the supposed effects of standby switches are often published (always on the internet and in non-technical magazines, never in real academic work) even by well-known guitar amp manufacturers, presumably with good-but-misguided intentions. So such myths become self propagating.

    You may have heard of 'cathode stripping', which is a specious argument wheeled out by standby-switch obsessives. In its purest form, cathode stripping occurs when particles of the oxide coating are physically torn from the surface of the cathode when it is exposed to a powerful electrostatic field from the anode. This would happen if the valve is operated at saturation, without a usual space-charge of electrons to protect it. Fortunately, this effect does not exist in receiving valves, even when operated at saturation, because it requires an electric field strength of at least 4MV/m (yes, 4 million volts per metre!). No guitar amp ever comes close to this.
    Another type of cathode stripping occurs when stray gas molecules in the valve become ionised by the electron stream. The positive ions will then be accelerated towards the more negative grid and cathode. If these manage to miss the grid then they may crash into the cathode, physically damaging its surface. The proper name for this process is cathode sputtering. Sputtering is a known problem in gas tubes and transmitting valves operating at kilovolt levels, near saturation. It doesn't occur to any significant degree in ordinary audio circuits. Note that even the RCA Transmitting Tubes Technical Manual No. 4, p65, states: “Voltage should not be applied to the plates or anodes of vacuum, mercury-vapor, or inert-gas rectifier tubes (except receiving types) until the filaments or cathodes have reached normal operating temperature” [My emphasis].
    Receiving valves are the small kind used in radio receivers, i.e audio valves like those in guitar amps, in case you were wondering.

    Cathode stripping should not be confused with cathode poisoning. Cathode poisoning refers to chemical –rather than mechanical– processes occurring at the cathode. There are several forms of cathode poisoning, including absorption of gas into the oxide coating, but the most pernicious type is the growth of interface resistance. When a valve cathode is fully heated but no anode current is allowed to flow for long periods of time (several hours), a high-resistance chemical layer can grow between the cathode tube and the oxide coating. This has an effect like an unbypassed cathode resistor; it increases noise and reduces the useful gain of the valve even though the oxide coating may have plenty of life left in it. This really does happen in receiving valves, and once formed it cannot be removed again.

    The two main causes of valve ageing are natural barium evaporation from the cathode, and interface resistance growth. Barium evaporation continues as long as the cathode is heated, so an ordinary standby switch has no effect on this. But a standby switch does encourage interface resistance growth. In other words, the standby switch is more likely to shorten the life of the valves!

    And there's more. Valve (vacuum) rectifiers should always be allowed to charge the reservoir naturally from cold. If the valve is preheated before the reservoir is allowed to charge, the valve will have to supply the full inrush current when the switch is finally thrown. This is called hot switching and causes sudden cathode saturation that can lead to catastrphic arcing inside the tube. Hot switching of rectifier valves was usually forbidden by valve manufacturers. Opening a standby switch can also induce a ghastly flyback voltage across the transformer winding, large enough to cause arcing in a valve rectifier (a precaution against this is to add ordinary silicon diodes in series with each anode of the valve to reduce the reverse voltage across it). Yet more reasons why standby switches are bad news.”

    http://www.valvewizard.co.uk/standby.html


    Sent from my iPad using Tapatalk
     
    moosie, BobSmith and Snfoilhat like this.
  12. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,088
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    In an amp with built-in reverb, I'll take the noise reduction from global NFB over the change in voicing and breakup from removing NFB every time. I've installed NFB in every wannabe Vox NFB-less amp with reverb that I've built so far, despite setting out each time not to include it.

    I'll be standing by for others' perspectives on the matter...


    ...

    :p
     
    D'tar likes this.
  13. dan40

    dan40 Tele-Afflicted Silver Supporter

    Posts:
    1,274
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    I was just looking through an old RCA tube manual earlier today, and noticed that none of the amplifier schematics printed in the back had standby switches drawn in. I know that Leo Fender used some of these amplifier schematics as the basis for his first designs, and I often wonder if he would later come to realize the controversy that he sparked by his decision to include a SB switch in his larger amps...lol.

    Edit: I typically don't include them in my builds anymore, but if I do use one, I like to place a 100k resistor across it to allow the first cap to charge slowly.
     
    King Fan and corliss1 like this.
  14. wabashslim

    wabashslim Tele-Holic

    Age:
    68
    Posts:
    666
    Joined:
    Nov 26, 2005
    Location:
    Sonorous Desert
    Pictures? How about a sound clip? I'd like to hear the big difference some here are claiming their NFB switch makes. Mine makes a small difference, I wouldn't call it negligible, but if someone flipped the switch while my back was turned I might not notice. That's with cutting the NFB, never tried increasing it.

    That which we feared has come upon us...
     
    King Fan and corliss1 like this.
  15. King Fan

    King Fan Friend of Leo's Ad Free Member

    Posts:
    3,850
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Heh, now that we’ve put standby to bed, back at the NFB Ranch...

    Our wise friend @Wally touches on the key thing about comparing NFB mods. Different amps differ a *lot* in how much negative they feed back, the size (and balance) of the top (and tail) resistors and other details of the NFB circuit as well as the amp's sound target.

    A total NFB cut on my 6g2 is pretty subtle — turns it from a rock singer into a blues singer. Just a bit more growl and gravel and depth in the voice.

    By contrast, a total NFB cut on my PR is a giant leap — turns it from Paul McCartney into, I dunno, Ted Nugent with a virus? Louder, noisier, less refined? Not very BF or PR. I don’t give it a lot of use, but there's no mistaking the diff when you flip the switch. This is why smart builders often use a pot or a multi-level NFB switch.

    And Rob's 'medium' amount of NFB in his well-loved 5e3 mod? Medium difference, but your amp is still very much a 5e3.

    Our empirical friend @Snfoilhat makes a good point. NFB may be needed in many circuits to allow reverb and other effects to do their thing. So we can’t generalize thatNFB cuts are huge, or trivial, or nice, or needless....




    Sent from my iPad using Tapatalk
     
    corliss1 likes this.
  16. Wally

    Wally Telefied Ad Free Member

    Posts:
    32,578
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX

    Okay, so I was still asleep this morning, and I made the same mistake I made years ago when muchxs had to correct me. My numbers are correct, but my conclusions are in error.....I wasn’t paying attention. Apologies.
    The BF Princeton’s NFB loop has a WEAKER cancellation effect due to the selection of resistances in the circuit when compared to the 6G2’s loop....the factors are .017 versus .046 respectively. The 6G2 is a gainier amp in the preamp due to that simpler tone control.
    I can still remember Much s straightening me out on this circuit years ago. I mistakenly was looking at the top resistance only thought that the 5F6A cancellation was greater than in an AB763 circuit of some sort. the Bassman has more cancellation. A 100K pot in series with that 56K top resistor would be interesting. I miss muchxs’ input here.
     
    Last edited: Aug 19, 2019
    King Fan and dan40 like this.
  17. wabashslim

    wabashslim Tele-Holic

    Age:
    68
    Posts:
    666
    Joined:
    Nov 26, 2005
    Location:
    Sonorous Desert
    Seems like my old PR isn't behaving like the others in this thread so I just hauled it out for some critical listening. Bear in mind this has a 12" Weber ceramic, a Deluxe Reverb output transformer and a mids pot if any of that matters. I plugged in my Weber Mini-Mass for the neighbor's sake. The NFB-cut seems more obvious at volumes above 6 or whenever distortion is evident, because of the uppers freqs boosting, but still nowhere near 5E3 territory. Wally's note about the PR's weaker NFB cancellation effect makes a lot of sense here from what I see...or hear.
     
  18. BobSmith

    BobSmith Tele-Meister

    Posts:
    127
    Joined:
    Feb 12, 2018
    Location:
    San Jose,CA
    Folks in semiconductor processing are fairly familiar with these concepts, particularly sputtering and plasma etching. There is a definitely plasma going on in our audio vacuum tubes, although the glow is faint. On experiment, I took a 30 second time lapse photo of the power tubes on my Fender Deville 212. (See below) You can clearly see the bulk plasma glow. I can promise you some of those positively charged ions (not sure what gas, but that doesn’t matter) are traveling past the electron sheath and banging on the cathode and making it nice and clean (etching). I’m not sure how significant the etch is, or if the oxide layer doesn’t grow back right after turning it off (I can’t image the gas contents inside the tube are that pure).

    But I suspect much of this is theoretical hair-splitting. Come on, they’re just cheap vacuum tubes after all, they won’t and don’t need to last forever. I say if you like a standby switch for what it functionally does (mutes the amp), use it. And if you don’t, that’s fine too.

    (The photo was taken at night with the lights off. The only light is collected from ambient nighttime and the tubes themselves. I like the photo, but it officially got zero likes on my Facebook page)

    E4F4B70F-4C6B-48DF-BED3-02A4CE7D6D48.jpeg

    Ok...now back to NFB!
     
    Last edited: Aug 19, 2019
  19. moosie

    moosie Doctor of Teleocity Silver Supporter

    Posts:
    15,213
    Joined:
    Jul 18, 2010
    Location:
    on the roof
    ^ Well, I like the photo. Might even say it's the best part of the thread... :)
     
    King Fan likes this.
  20. itsGiusto

    itsGiusto Tele-Meister

    Posts:
    208
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    (Side note, reference to your signature)
    Oh man, I thought I was the only one who realized that new Gibson guitars smell exactly like cake batter!
     
    moosie likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.