sll Posted March 18, 2023 Share Posted March 18, 2023 When wiring up a Les Paul per the 1950s configuration, which connection should be the outside foil end of the tone capacitor? Thanks Quote Link to comment Share on other sites More sharing options...
stokes Posted April 7, 2023 Share Posted April 7, 2023 In tube amps we orient the outside foil to ground. i.e. coming off a plate a coupling cap gets the outside foil connected to the grid of the next stage, which has a ground reference via the grid resistor or vol pot. I dont think it makes much difference in a guitar tone cap, but in keeping with this scheme, I'd orient it the same, outside foil closest to ground. Quote Link to comment Share on other sites More sharing options...
sll Posted April 7, 2023 Author Share Posted April 7, 2023 I agree with connection to the lowest impedance connection to ground but wouldn't the plate side, through the power supply, be the lowest AC signal connection to ground in an amp? Thanks. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 9, 2023 Share Posted April 9, 2023 On 4/7/2023 at 9:46 AM, sll said: I agree with connection to the lowest impedance connection to ground but wouldn't the plate side, through the power supply, be the lowest AC signal connection to ground in an amp? Thanks. No, the plate is connected to the grid of the next stage via the coupling cap. The grid has around 1meg or less connection to ground, plate is much higher Quote Link to comment Share on other sites More sharing options...
sll Posted April 9, 2023 Author Share Posted April 9, 2023 My understanding is that the plate is connected to V+ through a 100K ohm which is in parallel to the effective AC impedance of the tube from plate to cathode. At V+ the filter caps are a virtual AC short to ground which makes the V+ node an equivalent AC ground. So the AC impedance to ground at the plate is less than 100K ohms which is lower than the impedance to ground of the following grid circuit. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 18, 2023 Share Posted April 18, 2023 The 100k plate resistor would be in series with the cathode-plate circuit, not parallel. If you put a meter between the plate and cathode of say, a 12AX7 there will be no resistance reading. The other end of the 100k is connected to ground via the filter cap much, much higher resistance. Now if you put the meter from plate to ground you will see about 6 to 10 meg. Grid to ground will be from 0 to 1 meg depending if you have a pot or fixed resistor on the grid. Quote Link to comment Share on other sites More sharing options...
sll Posted April 18, 2023 Author Share Posted April 18, 2023 Do not get confused by the DC circuitry. The AC signal output point is between the plate and the plate load resistor. At that point, from an ac perspective, the resistor and the tube are in parallel with the non-plate end of the load resistor tied effectively to the cathode, or the cathode resistor if there is one. Draw it out from an AC viewpoint. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 19, 2023 Share Posted April 19, 2023 If you say so. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 21, 2023 Share Posted April 21, 2023 On 4/18/2023 at 7:26 PM, sll said: Do not get confused by the DC circuitry. But that is exactly what the "outside foil" scheme is all about. you are overthinking this. The aim of orienting the cap with the outside foil closest to ground is so the outside foil becomes a kind of sheilding to reduce noise and microphonics. Yes, a cap is microphonic. Now you have a plate with somewhere around 250 vdc above ground on it and the grid of the next stage with maybe 1vdc above ground. Which is closer to ground? Quote Link to comment Share on other sites More sharing options...
sll Posted April 21, 2023 Author Share Posted April 21, 2023 If the 250VDChas a filter cap to ground and the 1 VDC doesn't then the 250VDC is closer to AC ground. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 22, 2023 Share Posted April 22, 2023 If by "AC" ground you are talking about signal ground, the signal ground is the same as the DC ground. To measure the ac signal voltage one meter lead connects to a chassis ground, with the meter connected to the same ground, you switch to dcv on the meter and measure any dc voltage in the amp. There is no seperate "AC ground" Quote Link to comment Share on other sites More sharing options...
sll Posted April 22, 2023 Author Share Posted April 22, 2023 The paths to ground are entirely different. One is through the filter caps and the other is through the power supply and associated resistors. Filter caps have a much lower impedance. Would you really use a high impedance voltage source to feed a low impedance load? A preamp has a low output impedance and a poweramp has a high input impedance. Same reasoning. This is basic. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 22, 2023 Share Posted April 22, 2023 You are over complicating this. The whole idea of grounding the outside foil is to create a sheild. The shield does not get grounded thru a filter cap, has nothing to do with ac voltage its a matter of dc resistance, the plate has no dc resistance to ground, the grid does. This is basic. Quote Link to comment Share on other sites More sharing options...
sll Posted April 22, 2023 Author Share Posted April 22, 2023 You're just wrong, I don't know how else to explain it to you. Anyway, the foil in the guitar cap goes to the end away from the amp. A typical amp has an input impedance of about 1M which is much higher than the pickup and tone pots. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 22, 2023 Share Posted April 22, 2023 Damn, I been doing it wrong for the last 58 yrs? How did I ever get this far without you. I'm sorry but you are wrong, I dont know how else to explain it to you. Impedance, ac grounds, whatever you think that is has nothing to do with orienting the outer foil of a cap. Quote Link to comment Share on other sites More sharing options...
sll Posted April 22, 2023 Author Share Posted April 22, 2023 Okay then, just two cranky old guys having a discussion. I studied tube circuits in school also. Do you agree that R load is << than R grid leak? Do you agree that the filter cap is almost a short for ac signals such as noise? Then how can the point I labeled "outside foil" not be closer to ground than the other end of the coupling cap? Quote Link to comment Share on other sites More sharing options...
stokes Posted April 22, 2023 Share Posted April 22, 2023 No school for me, I went to work at 14 yrs old in a tv,radio repair shop and have been inside amps ever since. I have run a repair shop since 1982 and was the go to for Mandolin Bros for the last couple years before they closed. This has nothing to do with the ac signal.It is all about dc resistance. If the filter cap, or in this case the decoupling cap has almost zero resistance to ground, how do you suppose the signal being amplified gets past the plate, by your thinking that stage wouldnt pass the signal, it would be grounded at the plate,no? Quote Link to comment Share on other sites More sharing options...
sll Posted April 22, 2023 Author Share Posted April 22, 2023 Nice. I took the school route, not a judgement. I graduated RCA Institute tech school in NYC in 1974 worked as a tech went to Stony Brook U in 1978 and after graduating as an ee worked as design, systems, and planning engineers until I retired at the end of 2019. The load resistor prevents the short circuit of the signal to ground. I had said it was the lower resistance point to ground not a short to ground. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 22, 2023 Share Posted April 22, 2023 But its not a lower dc resistance. Put your ohm meter on the plate and then the grid and see. Again for the outer foil to ground we are only concerned with the dc resistance to make the outer foil a shield. Quote Link to comment Share on other sites More sharing options...
sll Posted April 22, 2023 Author Share Posted April 22, 2023 I never said DC resistance. I've clearly said AC. The DC resistance is not important as the noise on the capacitor foil which you try to bleed off will be AC not DC. Let's put a fork in this one, it's done. Quote Link to comment Share on other sites More sharing options...
sll Posted April 22, 2023 Author Share Posted April 22, 2023 Have a good one @stokes. It's been fun. Take care. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 23, 2023 Share Posted April 23, 2023 OK, you win, I guess I should get all the amps I've built and repaired over the years and ground the cab shields via the power supply caps since they bleed off stray ac noise. Quote Link to comment Share on other sites More sharing options...
sll Posted April 23, 2023 Author Share Posted April 23, 2023 Hahaha. We better hide this thread so nobody sees it. 😂🤣😂 Quote Link to comment Share on other sites More sharing options...
badbluesplayer Posted April 29, 2023 Share Posted April 29, 2023 On 4/23/2023 at 8:27 AM, stokes said: OK, you win, I guess I should get all the amps I've built and repaired over the years and ground the cab shields via the power supply caps since they bleed off stray ac noise. Dude - you don't know what you're talking about. And awful ignorant and stubborn about it. Foil on the coupling caps goes to the previous plate. The plates of preamp tubes are at ground, AC. That's different than ground, DC. You don't understand that, do you? For a guitar, the outside foil goes to the lowest potential, the tone pot. You learned something today. Quote Link to comment Share on other sites More sharing options...
stokes Posted April 29, 2023 Share Posted April 29, 2023 No, you guys are awful ignorant. This from Gerald Webers book Tube Guitar Amplifier Essentials. Although I learned about this outside foil thing over 40 yrs ago I knew I saw it somewhere since. Quote Link to comment Share on other sites More sharing options...
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