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Thread: Looking for Supro Thunderbolt 6420 Build info

  1. #36
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    The screen supply is already 1K below the B+.
    Quote Originally Posted by g1 View Post
    Yes, individual screen resistors is always good practice, especially with modern tubes.
    Quote Originally Posted by mikepukmel View Post
    Thanks everyone, for tech info, links, photos, this is awesome. So, for this amp, Id like to be able to run it dimed (as close to 11 as possible ), since that's supposed to be where its character is revealed. To that end, as seems to be with new tubes, new tubes aren't as durable running higher screen (voltage or current or both?). Im most interested in reproducing as close to the original tone as possible. That said, if all other components are close to original values (except a few that we can't get any longer), if I add 470R screen resistors, but also leave the power supply chain as is (1k followed by 27k followed by 100k) will this change the amp character much? Helmholtz mentioned that it will have less power, and the breakup will be different. Of course, this isn't 1965 and I can't get 1965 made RCA tubes, but all things considered...
    Well Hell son....save your self some money and ditch that Volume knob.

    In all seriousness you have proven yourself a tinkerer...try it both ways because as we all know the only thing that really matters is what YOUR brain processes and sounds good to YOU......

    Have fun and keep it up.

    nosaj

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    Thanks Nosaj, will do! This is way fun stuff to tinker with!

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    Quote Originally Posted by mikepukmel View Post
    Thanks everyone, for tech info, links, photos, this is awesome. So, for this amp, Id like to be able to run it dimed (as close to 11 as possible ), since that's supposed to be where its character is revealed. To that end, as seems to be with new tubes, new tubes aren't as durable running higher screen (voltage or current or both?). Im most interested in reproducing as close to the original tone as possible. That said, if all other components are close to original values (except a few that we can't get any longer), if I add 470R screen resistors, but also leave the power supply chain as is (1k followed by 27k followed by 100k) will this change the amp character much? Helmholtz mentioned that it will have less power, and the breakup will be different. Of course, this isn't 1965 and I can't get 1965 made RCA tubes, but all things considered...
    The Sovtek 5881 that everyone loves to diss on because of its lacking sound in comparison to many other 6L6 types actually sounds great in these Tbolt amps and they last a long time too. They can take the punishment, and you can run them without screen grid resistors too like the originals if you want. The guy I fixed those three Tbolts for used those Sovteks in one, and the other two had NOS Sylvanias, but when the Sylvanias died he put the Sovteks in both the other amps and they sound just as good in that circuit and last a long time. Whenever I get around to building my clone (I have the transformers and chassis already) then I plan to use those Sovteks myself.

    Greg

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    Quote Originally Posted by soundmasterg View Post
    The Sovtek 5881 that everyone loves to diss on because of its lacking sound in comparison to many other 6L6 types actually sounds great in these Tbolt amps and they last a long time too. They can take the punishment, and you can run them without screen grid resistors too like the originals if you want. The guy I fixed those three Tbolts for used those Sovteks in one, and the other two had NOS Sylvanias, but when the Sylvanias died he put the Sovteks in both the other amps and they sound just as good in that circuit and last a long time. Whenever I get around to building my clone (I have the transformers and chassis already) then I plan to use those Sovteks myself.

    Greg
    Tubes can't have a sound on their own. Tube parameters are independent of frequency up to the 10MHz range. Sound is the result of the interaction of the tube and its circuit. In other words, different circuit - different "tube sound".

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    The screen supply is already 1K below the B+.
    Quote Originally Posted by g1 View Post
    Yes, individual screen resistors is always good practice, especially with modern tubes.
    Quote Originally Posted by Helmholtz View Post
    Tubes can't have a sound on their own. Tube parameters are independent of frequency up to the 10MHz range. Sound is the result of the interaction of the tube and its circuit. In other words, different circuit - different "tube sound".
    I beg to differ have you ever dropped one?

    nosaj

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    I beg to differ have you ever dropped one?
    Such kind of tube sound testing I prefer to leave to those, who consider it significant.

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    Last edited by Helmholtz; 01-06-2019 at 01:15 AM.
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  7. #42
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    Quote Originally Posted by Helmholtz View Post
    Tubes can't have a sound on their own. Tube parameters are independent of frequency up to the 10MHz range. Sound is the result of the interaction of the tube and its circuit. In other words, different circuit - different "tube sound".
    I know some believe that, including Stan who often posts here. If you are swapping tubes in and out and using the same circuit, the amp sounds different when you swap tubes in and out. It would make sense that different tubes would have different internal resistances and capacitances, since they are a product that is produced from many individual parts that all can't be exactly the same. So if you want to call it interaction between the tube and circuit, thats fine, but since the sound changes when you swap the tubes, then it is likely a result of those resistances and capacitances interacting with the circuit. Everyone tends to simplify and say that it is the tubes that sound the way they do, and perhaps that is wrong, but if you have ever swapped between different tube types in an amp, you would know that the sound does change when you do so. Since the tubes are one of the variables and the circuit is not changing, then what is it that is changing the sound? The end result is that those Sovtek 5881's work well, last a long time, and sound good in this circuit, and that can't be said about some other circuits with those tubes.

    Greg

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  8. #43
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    I believe what Stan was usually saying is that a certain "brand" of tube cannot have a certain "tone" assigned to it across all zillion examples of that tube. He would say that Pair A would sound different from Pair Bin the same circuit IF all else was left the same. But, if Pair C is the exact same as Pair B, the two pairs may or may not sound the same. He also said that any differences that might be heard can be compensated for by small circuit tweaking.

    Or as Juan used to put, these particular tubes on this particular day dounded this particular way at these particular operating parameters, while my ears were in this particular state...

    Sure you could notice differences, but these vendor descriptions of "Tube A is creamy in the bass under distortion" or whatever just cannot be relied upon to sound the same in YPUR amp.

    All that said, yeah, those Sovtek 5881WXTs are tough as nails and take a TON of punishment. Uf it's any help I ran a pair in a VT-40 for 2 hours straight, dimed (yes, Master Volume too), at 600 plate volts. Not a single falter.

    Justin

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  9. #44
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    If you are swapping tubes in and out and using the same circuit, the amp sounds different when you swap tubes in and out.
    No doubt.

    Since the tubes are one of the variables and the circuit is not changing, then what is it that is changing the sound?
    The tube is an essential part of the circuit. "Sound" is the result of the (linear and non-linear) transfer characteristics of the circuit with a given tube. A tube having different parameter values and characteristics changes the circuit behaviour in many respects: frequency response, distortion, dynamics. In a different surrounding/circuit configuration the same tube may cause different sound changes. So the tube can't have an own sound character, but one tube may sound better in a given circuit than an other one.

    If you change the value of a single resistor in a circuit, the sound may change noticeably. This doesn't justify to attribute an own sound quality to the different resistors, even though one of them may make the special circuit sound "better".

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  10. #45
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    Quote Originally Posted by Helmholtz View Post
    No doubt.



    The tube is an essential part of the circuit. "Sound" is the result of the (linear and non-linear) transfer characteristics of the circuit with a given tube. A tube having different parameter values and characteristics changes the circuit behaviour in many respects: frequency response, distortion, dynamics. In a different surrounding/circuit configuration the same tube may cause different sound changes. So the tube can't have an own sound character, but one tube may sound better in a given circuit than an other one.

    If you change the value of a single resistor in a circuit, the sound may change noticeably. This doesn't justify to attribute an own sound quality to the different resistors, even though one of them may make the special circuit sound "better".
    So what I am saying is that since each tube is going to have different values (i.e. different internal resistances and capacitances, different gain, input impedance, output impedance, etc.) then swapping one 6L6 tube for example into a circuit vs a different 6L6 tube will change the sound, and the circuit hasn't changed at all, except in how it interacts with the new tube and its different internal parameters. So the tube and all that entails as far as different internal values between tube types and the fact the tube is an active device has changed the sound as far as how it is reacting to the circuit. Simply saying that the tube changed the sound is probably not accurate since there are all these other variables involved, but saying that the sound did change when you swapped the tubes is accurate, and the tube itself had an impact on that sound change is also accurate.

    Greg

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  11. #46
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    I think what most of us were arguing before (in the past) is that any changes in sound due to a tube change are real, but also adjustable by changing/tweaking the circuit to adapt to the different characteristics of the new tube. As you say, tge circuit didn't change, but how the tube and circuit interact with each other does. Stan & others would argue that any perceived sound changes could be reversed by slight tweaks to the circuit.

    And the #1 sin of all was how vendors & internet hype say, "put these tubes in your amp & it will sound This Way & put those tubes in your amp & it will sound That Way. A more honest assessment would be "these tubes made our amp sound this way at this set if operating parameters (list them all) in this circuit. Your results may (and most likely will) be very different..."

    Not arguing with anyone; I thunk we're all mostly saying the same things but from different angles... Now as far as relibility issues go, there might be more consensus. That Sovtek 5881WXTs are super-stout I think is pretty accepted in the guitar amp community.

    Justin

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    Quote Originally Posted by soundmasterg View Post
    So what I am saying is that since each tube is going to have different values (i.e. different internal resistances and capacitances, different gain, input impedance, output impedance, etc.) then swapping one 6L6 tube for example into a circuit vs a different 6L6 tube will change the sound, and the circuit hasn't changed at all, except in how it interacts with the new tube and its different internal parameters. So the tube and all that entails as far as different internal values between tube types and the fact the tube is an active device has changed the sound as far as how it is reacting to the circuit. Simply saying that the tube changed the sound is probably not accurate since there are all these other variables involved, but saying that the sound did change when you swapped the tubes is accurate, and the tube itself had an impact on that sound change is also accurate.

    Greg
    Yes.

    Where do we disagree regarding my original statement that tubes don't have a specific sound of their own?
    And the sound of the same tube in a different circuit is not predictable.

    BTW, the capacitances of power pentodes/tetrodes are too small to noticeably influence sound in the audio range.

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    Quote Originally Posted by Helmholtz View Post
    Yes.

    Where do we disagree regarding my original statement that tubes don't have a specific sound of their own?
    And the sound of the same tube in a different circuit is not predictable.

    BTW, the capacitances of power pentodes/tetrodes are too small to noticeably influence sound in the audio range.
    I guess in my original post where I said the Sovtek 5881 sounds great in the Tbolts, since I didn't say it was its interacting with the circuit and that was the reason for the sound difference between that tube and other tubes in this circuit, that you maybe assumed I was saying the tube's sound itself was the reason to sound good?

    Greg

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    Quote Originally Posted by soundmasterg View Post
    I guess in my original post where I said the Sovtek 5881 sounds great in the Tbolts, since I didn't say it was its interacting with the circuit and that was the reason for the sound difference between that tube and other tubes in this circuit, that you maybe assumed I was saying the tube's sound itself was the reason to sound good?

    Greg
    I have absolutely no reason to doubt your positive experience with the Sovtek 5881s in the Tbolts. Quite the contrary - my comments were meant supportive.

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    On a similar vein, Ive pretty much convinced myself (thanks to lots of your comments, suggestions) that the layout, e.g. point to point over terminal strips, vs a turret or eyelet board won't affect the tone, as long as one or the other isn't producing parasitic oscillation, or other odd effects due to wiring too long. So, Id like to take a shot at making a turret board for this project. Due to some great links you all posted, found a few starter layouts (for a board). Reading (a little knowledge is a dangerous thing), Merlin's book and MEF threads again on power supplies, and grounding, Id like to put the filter caps near where they are needed rather than way on one side of the amp, and running long wires.

    The fender like designs have a bundle of wiring come through the chassis, from the power supply caps, on the opposite side of the board from the tube sockets. Is there a problem with running the 'power rail' for lack of a better term, on the tube side? I.e. supply filter cap for the preamp tube would have its + side pointing toward the tubes. A dropping resistor would go from there, on turrets, to the next cap up the chain, for the phase inverter, and so on. Would tube wiring running under, and perpendicular to the HV wire be a problem (noise, oscillation, etc)?

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    e.g. something like this:

    The crummy hand modified (using paint) layout is missing some jumpers, maybe to go under the board to connect the power supply.

    Another trial at hand/MSPaint drawing. I think this is a little closer. The Fender amps twist the pair going from the PI to the output tubes. Can't draw that.

    Click image for larger version. 

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    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by mikepukmel; 01-11-2019 at 02:21 AM.
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    Just one thought I had (late in the week, running low on thought power) is to make sure the lead to V2's first grid is shielded - pretty low level on that signal wire.

    edit: OK, a couple more thoughts. Don't be afraid to put a 10k grid stopper on the first couple stages. And related to that, what's the function of the 47k resistor from the input? If it's a grid stop, then put it right on the socket. The cap after it confuses me as to its intended purpose.

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    Quote Originally Posted by eschertron View Post
    Just one thought I had (late in the week, running low on thought power) is to make sure the lead to V2's first grid is shielded - pretty low level on that signal wire.

    edit: OK, a couple more thoughts. Don't be afraid to put a 10k grid stopper on the first couple stages. And related to that, what's the function of the 47k resistor from the input? If it's a grid stop, then put it right on the socket. The cap after it confuses me as to its intended purpose.
    Hi Eschertron, Thanks for the tips! Yeah, will definitely put grid stops on the input tube pin, and also use shielded wire there. And, as to the 47k resistor and cap, oh man I tore my hair out (OK not quite tore my hair out), but thought deeply with a stern look on my face, no idea what that cap on the input is for. The schematics Ive been able to find, and also images of the chassis, they did put 47k near the input jacks (one per input jack). The wierd thing is that they did not put them right on the jack, like Fender did, they're on a terminal strip, after a wire a few inches long. And they both feed into a 0.005uf ceramic cap.

    Click image for larger version. 

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  19. #54
    Supporting Member eschertron's Avatar
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    I can see putting the cap in that order IF the grid stops are on the jack, a la Fender. Otherwise, I'm confused too. I can't actually tear my hair out, not enough left to grasp!

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    I just hope it doesn't blow up when I run it on 10.

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  21. #56
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by mikepukmel View Post
    I just hope it doesn't blow up when I run it on 10.
    Put an 11 on there then you'll be ok

    nosaj

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    Please let us know how it sounds! I'm curious about the paraphase phase splitter.

    Record some clips before it blows up

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    Quote Originally Posted by mikepukmel View Post
    Hi Eschertron, Thanks for the tips! Yeah, will definitely put grid stops on the input tube pin, and also use shielded wire there. And, as to the 47k resistor and cap, oh man I tore my hair out (OK not quite tore my hair out), but thought deeply with a stern look on my face, no idea what that cap on the input is for. The schematics Ive been able to find, and also images of the chassis, they did put 47k near the input jacks (one per input jack). The wierd thing is that they did not put them right on the jack, like Fender did, they're on a terminal strip, after a wire a few inches long. And they both feed into a 0.005uf ceramic cap.

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    The two 47k input resistors shown in the schematic are for mixing and decoupling 2 instruments when used simultaneously. They also act as grid stoppers, producing a low pass effect together with the input capacitance (mostly Miller) of the input triode. Mounting grid stoppers directly to the tube socket only makes a difference in the MHz range.

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    Quote Originally Posted by Helmholtz View Post
    The two 47k input resistors shown in the schematic are for mixing and decoupling 2 instruments when used simultaneously. They also act as grid stoppers, producing a low pass effect together with the input capacitance (mostly Miller) of the input triode. Mounting grid stoppers directly to the tube socket only makes a difference in the MHz range.
    Cool, thanks Helmholtz.

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    Quote Originally Posted by eschertron View Post
    Please let us know how it sounds! I'm curious about the paraphase phase splitter.

    Record some clips before it blows up
    Me too, will do. Ive heard raves about the overdrive tone, but haven't found anyone who has one to try out. Will get the old Nikon warmed up and recording before I dime it! This is the absolute last amp build for me. No more. Probably. Most likely one of the last ones I'll build. Getting near the end of the builds. Unless something spectacular comes along.

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    Pulling my hair out to try to figure out how this amp would work without shorting the PT secondary. Going back and forth between the various versions of this amp schematic, I think the CT should not be grounded. Wouldn't it provide a path to a dead short across one diode, depending on which half cycle?

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    "It must be right, is on the internet"

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  27. #62
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    Don't use that schematic. The ones from Sean Weatherford are known to be the most correct. That schematic you posted, and a couple unreadable others were why Sean created his in the first place.

    Greg

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    Quote Originally Posted by mikepukmel View Post
    Pulling my hair out to try to figure out how this amp would work without shorting the PT secondary. Going back and forth between the various versions of this amp schematic, I think the CT should not be grounded. Wouldn't it provide a path to a dead short across one diode, depending on which half cycle?

    Click image for larger version. 

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    "It must be right, is on the internet"
    You are correct. It's wrong.

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  29. #64
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    Quote Originally Posted by soundmasterg View Post
    Don't use that schematic. The ones from Sean Weatherford are known to be the most correct.
    And here's the link:
    http://www.angelfire.com/mech/beansamps/supro/supro.htm

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  30. #65
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    Thanks again for the angelfire link. Great info in the Thunderbolt there.
    Maybe CJ Poulos put that in there for anyone who breaks his copyright.

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  31. #66
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    I don't want my epitaph to read "Build an amp. Lasted exactly one complete AC cycle before the fire."

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  32. #67
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    Quote Originally Posted by Helmholtz View Post
    The 1k voltage dropper resistor in the PS chain is no (common) screen resistor. It does not allow the screen voltage to change with signal/screen current. The primary purpose of power tube screen resistors is to limit screen current at low instantaneous plate voltages/high plate currents. This way average screen dissipation is kept low.
    They might not have been necessary in this design at least as long as the power stage is not driven into clipping, which was probably considered abuse in the old days.
    Apart from the protection effect screen resistors reduce tube distortion as well as gain by local NFB. Also available clean power will be a little lower.
    Reading and re-reading all the supro posts, thanks again Helmholtz. Its the funny thing about these amps: original design was not intended for overdrive, but just about all usages Ive seen use these amps for their unique overdrive character. So (with my limited electronics knowledge) have gone in circles a few times. First, original amps had no screen resistors (reasons listed above by several posters), but those days, the tubes were built like cars were: heavy duty, could take much abuse. Todays tubes, meh at best. Goal is to abuse this amp well into overdrive, to see what the supro overdrive sounds like. So, protect the amp circuit so it won't blow up i my face would be nice, but not so that it changes sound character to a different amp. If it *is* driven well into clipping, then, it should have some resistor on the screens?

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  33. #68
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    it *is* driven well into clipping, then, it should have some resistor on the screens?
    Definitey. Screen resistors preserve the screens from excessive dissipation and will extend tube life. Most great clipping amps have them: '59 Bassman, JTM45.......
    I don't understand why you didn't try a couple 470R in the meantime and see what sound difference they make. You can always short them for comparison. JUst watch for "red screening".

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    Quote Originally Posted by Helmholtz View Post
    Definitey. Screen resistors preserve the screens from excessive dissipation and will extend tube life. Most great clipping amps have them: '59 Bassman, JTM45.......
    I don't understand why you didn't try a couple 470R in the meantime and see what sound difference they make. You can always short them for comparison. JUst watch for "red screening".
    Hi Helmholtz, thanks!! Well, its not built yet. I started collecting info last year, than ran into a whole cascade of household repairs to do (still not finished). But its next on my list. Will definitely try with and without 470R.

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  35. #70
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    Quote Originally Posted by eschertron View Post
    Just one thought I had (late in the week, running low on thought power) is to make sure the lead to V2's first grid is shielded - pretty low level on that signal wire.

    edit: OK, a couple more thoughts. Don't be afraid to put a 10k grid stopper on the first couple stages. And related to that, what's the function of the 47k resistor from the input? If it's a grid stop, then put it right on the socket. The cap after it confuses me as to its intended purpose.
    Re the 0.05 cap after the two 47k resistors on the inputs, I don't even know enough to ask why its there. I did check a few build photos that i could get and there are one or two other schematics floating around, might be from the same source, but they all have it. So, there are things that affect tone, and then stuff that doesn't affect tone, but noise, rf interference etc. And, maybe something to do with input matching to certain pickups, etc. Am I wrong in guessing that the components on the inpt side of the input tube are mostly not related to tone? Grid stops (even if put in the wrong place), grid leak, and that odd cap?

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