Unexpected tone impact of plate resistors. Please tell me where I’m wrong!

Discussion in 'Glowing Bottle Tube Amp Forum' started by trouserpress, Jul 27, 2021.

  1. trouserpress

    trouserpress Tele-Holic

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    I wanted to cure intermittent (seemingly unprovoked) rumbling noise in the reverb signal of my pal’s 77 Princeton Reverb. So I swapped the two old CC plate resistors in the reverb circuit for new metal film ones.

    The noise was gone (perhaps due to my tightening the nuts of the reverb jacks as well) and I returned the amp. Soon my buddy rang me up and told me that all vividness and deepness of his beloved amp has gone away. I said it cannot be so he played it for a couple of days but the negative impression stayed/hardened.

    Today I put back in the old resistors and even my old ears could immediatly (without letting the PR warming up) hear that the amp ‚has come back to life’.

    How can that be???????? The difference in tone was profound!

    Many reasonable / credible articles deny the impact on tone by the types of resistors in that particular position. Or am I wrong??

    On my Fluke I could read that all 4 resistors where very close to 100K (+- 2%).

    Any thoughts?
     
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  2. Wally

    Wally Telefied Ad Free Member

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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    There are other reasonable / credible opinions as well. In addition to what aikenamps says about CC resistors. The CC will change resistance when confronted with more current. Other resistors will hold steady at 100K resistance. This means, as notes are being played, the CC resistance is responding in kind. It is one place where *mojo* can be measured.

    Johnny Cocheroo wants' to know if you are a convert now?

    :D:D:D
     
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  4. trouserpress

    trouserpress Tele-Holic

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  5. trouserpress

    trouserpress Tele-Holic

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    If that‘s what JC says then: Yes, he can count me in.
     
  6. Wally

    Wally Telefied Ad Free Member

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    Aiken describes the difference in the tone that some might prefer…..read ‘conclusions’.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    From aikenamps:
    Bear in mind, however, that many people prefer the "sound" of carbon comps, claiming they sound warmer than film or wirewound types. This is possibly due to distortions generated by the modulation of the contact noise current by the AC signal. Since this noise has a 1/f frequency characteristic (similar to pink noise), it is more pleasing to the ear than white noise.

    If you have a look at Rob Robinette's 5F6A mods site, there is a thermal image of an amp. The plate resistors are registering hot in the image. When they are hot they are more apt to display the characteristics that set them apart from other resistors. Their datasheets warn of this. In some camps that is not good because they do distort and vary in value. Of course we want them to be stable... or do we?

    Higher wattage CC will not get as hot so the variation is less.
     
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  8. Asmith

    Asmith Friend of Leo's

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    As someone else said the resistors may have a different resistance when a voltage is across them. If you get the chance you could measure the current and voltage drop across the CCs when the amp is running to see if they are still measuring at 100k.
     
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  9. trouserpress

    trouserpress Tele-Holic

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    OK …. the heat …. a) lets the resistance shift (towards more thus providing V3 with less voltage?) and …b) adds some kind of distortion to the signal (although these resistors are not in its path).
    In conjunction with the b+ (and maybe bias) situation this might or might not lead to considerable tonal changes. Thanks.

    But still .. that heating up got to be happening pdq after switching the amp on.
     
  10. Paul G.

    Paul G. Friend of Leo's

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    You're forgetting that plate resistors have 2 functions. Yes, they provide voltage to the plates, but even more important is their role converting the output of the tube which is AC Current, into the AC Voltage to drive the next stage. So, the plate resistor is not seeing a constant DC current, they are seeing a fluctuating current, so the variance of resistance with current is constantly moving. And that's the sort of thing that "character" is made from.
     
  11. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    There's a lot of yada on the web about resistor types. Like so many things, this means most folks simply choose a side with no science or evidence. The Aiken piece is good. Even better, though, especially if we're talking about plate resistors, is R.G. Keen, who shows why that's one place CC can make the most difference:

    http://www.geofex.com/Article_Folders/carbon_comp/carboncomp.htm

    Now, having said that, I'm impressed the difference in this case is *that* massive. Gotta ask: did you measure the actual resistance of the old carbon comp resistors? In addition to CCs' possible mojo potential, and well-known noise potential, they have *definite* drift potential.
     
  12. Jowes_84

    Jowes_84 Tele-Meister

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    Limited experience but kind of similar…
    My first Fender Super Champ (sold) sounded „best“ with a worn out, slightly microphonic 6U10 pre amp tube … even though it called for a 6C10. I think sometimes components on the edge introduce noise/artefacts that are favourable to our ears and we think mojo. Very likely those old cc do something similar. Would be interesting to compare those with new production cc s and see if they did the same. Thanks for education, I am basically a cap is a cap and resistor is a resistor believer, but I am here to learn.
     
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  13. Dacious

    Dacious Doctor of Teleocity

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    I'd bet some of the CC resistors are reading way funky in value. Measuring the voltage drop across them might be revealing.
     
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  14. fenderchamp

    fenderchamp Friend of Leo's Ad Free Member

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    but did the noise come back too?
     
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  15. trouserpress

    trouserpress Tele-Holic

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    The old CC resistors measured 98.3 and 102.4 kOhm and the new mf resistors both measured 99.smthg. I can tell you, King Fan that I really was surprised and nearly speechless by the difference.
    Now I really do regret not having documented the difference in voltage drop. I'll revive this thread if I'll catch up on this in the future.
    No the noise has not yet come back. Maybe tightening nuts or touching up some solder joints was responsible for the cure. Also the noise might come back - I don't know yet (high expectations in association with repairing/servicing tube amps can be real downers).
     
  16. trouserpress

    trouserpress Tele-Holic

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  17. mountainhick

    mountainhick Tele-Meister

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    Don't know that 4% could make a difference but, that there could be the reason. The variation/difference between tubes' output due to slightly off values. Maybe just enough unmatched harmonics between them.

    Anyone experimented with this?
     
    Last edited: Jul 28, 2021
  18. Wally

    Wally Telefied Ad Free Member

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    not to stray for very long from the subject of this thread to far, but that 6U10 does not have the same triodes as does the 6C10. Two of those triodes are close to a 12AX 7 triode while the other one is a low gain, high current 12AU7 triode. It would have been interesting to have identified where that 12AU7 triode was operating in order to fully understand what function in the amp it was affecting. I am going to think that that difference was more important than any microphonic artifacts.
     
    Last edited: Jul 28, 2021
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  19. sllsll

    sllsll TDPRI Member

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    I thought that the Princeton Reverb only has 1 100K plate resistor in the reverb circuit, on the recovery tube. The reverb circuit has very little "tone" by itself. Fender runs the reverb at the drive level set by the volume control and then mixes a selected amount of the low fidelity reverb signal with the dry signal. Since the dry signal contributes the majority of the tone of the amp's sound, why would the plate resistor of the reverb recovery circuit kill the amp's overall sound? That's what I find interesting.
     
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  20. Wally

    Wally Telefied Ad Free Member

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    Good catch….we were ignoring that reality!!! There are four 100k plate load resistors there…one each on the three preamp gain stages and that single load resistor in the reverb circuit on the 12AX7reverb recovery triode.
     
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