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Thread: New here, first silly question about variable resistors.

  1. #1
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    New here, first silly question about variable resistors.

    Hello all,

    I have lurked here for a while, and finally got past the intimidation to actually sign up. I am currently an engineering student with a strong interest in music electronics. I have built a compressor and a couple amps so far. It is quite addictive to say the least.

    Anyway, I have a question regarding prototyping a guitar amp. Would there be any downside to laying the entire preamp section out on breadboard using variable resistor trimmers for the plate resistors, cathode resistors, and voltage dividers while I tweak an actually sound that I like? Any foreseeable problems with noise, tonal issues being too far off for reliability, etc.?

    Also, what would be the best way to add socket type clips on a board to easily swap out caps while experimenting with different sounds during a prototyping stage? I really would like to have a way to teak things on the fly without having to solder and desolder over and over again ruining components and turrets.

    Thanks in advance, I hope my question here doesn't expose me as too much a novice at this.

    Billy

  2. #2
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    Well, one time I blew up an electrolytic cap in a bias circuit I was working on... that 100V cap didn't like it when I dialed the pot down to zero ohms, and the cap saw 400+V. I would say try to establish some safe minimum tor your proposed circuit and use a fixed resistor for that value, then tweak from there. Like, you probably don't want your preamp cathode bias resistor top be variable down to zero... or any resistor, for that matter. That's my only concern...

    Justin
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  3. #3
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    That's excellent advice. Thank you. Do you think trimmers affect the tone in any appreciable way from a regular carbon film or metal film?
    Last edited by Mr. Bill; 07-19-2017 at 11:18 PM. Reason: My spelling is horrible with kids in my lap.

  4. #4
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    Not enough for me to care... just don't leave em in?
    By the way, I'm more of a philosopher than a tech... so everything is allowed until it explodes.

    Justin
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    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  5. #5
    Supporting Member mozz's Avatar
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    Using a pot for a variable plate resistor could be trouble, you are talking up to 300v or so, most aren't rated for that. You can buy these prototype board and they work great.
    Attached Thumbnails Attached Thumbnails 12ax7proto.jpg  
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    Supporting Member loudthud's Avatar
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    A friend of mine was working on a Masters degree in oscillator design and showed up early to a lab class that was taught by graduate students. The "professors" were standing around, one was holding a soldering iron to two wires twisted together and another was saying, "I have seen this, the two wires just melt together". Don't be those guys, learn how top solder. And practice! You'll need a decent soldering iron, get one with variable temperature control.
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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  7. #7
    Supporting Member eschertron's Avatar
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    Decide which elements in the design are likely to "want" to be adjustable, in other words - prioritize your tweaking.

    Plate resistors (as mozz mentioned) may not be ideal candidates. For those, prototype with a high value (220k?) and then clip resistors in parallel to get lower resistances as you see fit. I've had good luck using alligator-clip "jumper" cables to parallel a resistor or cap into a preamp circuit. Some designers tack-solder the parallel components, their choice. Also, regarding plate resistors, changes of much less that a factor of 2 aren't as audible here as they might be in other circuit locations. Just my opinion, though.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Quote Originally Posted by mozz View Post
    Using a pot for a variable plate resistor could be trouble, you are talking up to 300v or so, most aren't rated for that. You can buy these prototype board and they work great.
    Those look fantastic. I'll be picking some of those up for sure. Ideally I would like a big slab of wood I can just tinker around on and come up with different variations and combinations for amps. I had the idea for using the variable resistors for prototyping (I'm sure I'm not the first), but having a minimum safe value would make it even easier. Of course I would make sure they are rated for higher voltages than they will see.

    Are those solderless breadboard suitable for these kind of projects? I've never actually used them, but built things either on turret boards or kits that come with pcb's.

  9. #9
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    Here are a couple threads with similar discussions. Also, near the bottom of the page in each thread is a section headed "similar threads", which can be very helpful.

    Prototyping board

    frustrated by eyelet/turret board prototyping
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

  10. #10
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    I've built and developed a few amp and reverb prototypes using trimmers and replaceable components, both solid-state as well as tube. You need to work safely and pay attention, as working on an experimental amp is more likely to be hazardous. The way I do it is I have a simple aluminium panel with an assortment of holes to take pots, sockets and switches This mounts to a really basic grounded chassis (just a folded sheet of 16g aluminium) that has an output transformer and various sized cutouts for tube sockets. Make it a generous size so you have plenty of room. My power supply is off-board. I determine my voltages beforehand, based on my intended transformers and tube compliment. I use tag-board - the type that Vox used to use. It has two rows of double contacts;

    | Barrier Strip, 36 Contact, 6.35mm Pitch, 2 Row |

    I do a rough sketch of my layout and mount the appropriate tube sockets/pots/input jacks etc and wire all of these up to the outside terminals and connect the heater string. This forms the rough skeleton of the particular amp I'm building. All the components solder onto the inner terminals. Regular low-voltage breadboards are just not good enough for tube amps. I have a little DIY plastic jig to bend the component leads to the correct spacing and leave the leads over-length. It's as quick to tack these in place as it is to fiddle with clips or something else, though in the old days of radio you used to get Fahnestock clips for breadboarding and if you can get these they're excellent; mount them onto a piece of FR4 for areas that you want to experiment with extensively (such as tone stacks). Plate resistors should be fixed and I establish what I need beforehand through gain calculations. Trimmers are fine for voltage dividers and especially establishing optimum cathode resistor values. Try to get the larger type as they're easier to locate on an experimental chassis.

    Make sure your power supply has bleed resistors and that it can drain pretty quickly. You don't want it to hold a charge. I also make sure I additionally bleed the circuit before making changes to be doubly-sure.

    I always begin with a baseline configuration and make sure the amp will pass an audio signal; maybe 100k plate resistors, 1.5k cathode with 22uf bypass and no tone stack. There's a good reason why pretty much every tube amp boils down to a narrow range of component variables. If you start of with wacky values you may end up troubleshooting before you even start. Also, make notes of what you change and what sounds good. There's nothing so frustrating as losing a brilliant idea because you cant remember what you did. Change control; the ability to recreate a product at any point in its existence.

  11. #11
    Senior Member Enzo's Avatar
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    Breadboarding has its place, but in gainy circuits like guitar amps, having the circuit spread all over a breadboard will never be as stable or as quiet as a built circuit. SO use it for trying ideas, but not for a build.

    Used to be the little GK amps were popular, the 250ML or similar. All Solid state op amps. The feedback resistors of each stage were trim pots, so your could tweak the gain of each stage. More than once I had one brought in with the complaint it squeals. And yes indeed, all the trimmers had been maxed.

    I did have one customer with one of those ubiquitous Silvertone amps, and he was chasing his tone. He wanted to change this resistor, then that one, and maybe the other stage, and he kept asking for my time. I eventually changed the resistors in that circuit to trimmers and pointed out which was which, and said, "fiddle away."
    Education is what you're left with after you have forgotten what you have learned.

  12. #12
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    a decade resistor box is nice, you have to watch your power lever so you don't have smoke coming out,

    you can also make a turret board with different value caps, not too many values used in a tube amp, .001 .0047 .01 .047 and .1, maybe a 250, 330, 500 pf thrown in for tone stacks,

    and lets not forget the 3.3 Meg/ 10 pf gain dropper, a lot of signal going thru that 10 pf if you use the Fender stuff for experiments,

    having a bunch of capacitor types also help, go to an electronic surplus store if you have one in your area,
    Fender used those molded blue caps but they are expensive, Pikatron caps were used in later Fender amps and sound good, of course orange drops are a must,
    try different 250 pf caps for the tone circuit, the old RMC caps sound very nice, not a big Mica fan but some people like them, look for distortion by putting the scope on the downstream side of the 250 pf, some caps cause distortion here in the form of a little dip at the top of the sine wave, this might be a wire harness issue, still trying to sort out why some amps have it and some don't, the ones that do not have it seem to sound better,

    get a variac for voltage tweaking, you can play with noise values by changing heater voltages,

    look at a PV 5150 circuit, there are voltages dividers all over the place, engineers like these because the can put in set values for the tubes and tweak the gain afterwards with the dividers,

    experiment with different output transformers, they can make a big difference in the sound,

    Ever hear a Fisher 100 stereo amp output transformer used in a 6BQ5 git amp? Lord have Mercy.

    this box is big bucks but they have cheaper, no biggy if you already owe citibank 30 grand for student loans,

    decade.png
    Last edited by cjenrick; 07-24-2017 at 07:30 AM.

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