RobRob Bassman Micro question

Discussion in 'Shock Brother's DIY Amps' started by Hyakuya, May 13, 2021.

  1. Hyakuya

    Hyakuya TDPRI Member

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    Hi folks

    A general question about resistors really but with reference to the Bassman Micro....

    The design uses carbon comp resistors pretty much throughout and I'm thinking of using carbon film instead (Kiwame to be exact). Are carbon comp specified here for mojo purposes or is there a specific reason for them beyond their vintage appeal.

    Merlin Blencowe describes metal film resistors as "the resistor of choice for nearly all audio applications" and from what I can see carbon film are close. His description of carbon comp resistors in 'Designing High Fidelity Preamps ' is pretty off putting but then again, the characteristics he talks about (distortion, etc.) may be the exact reason why they are specified in the Bassman Micro BoM.

    What are the thoughts on resistor choice and more importantly, am I really going to notice.
     
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  2. sds1

    sds1 Tele-Afflicted

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    Without side by side comparison in a controlled environment, I'm not sure how you could rationally determine there to be a difference. And if you could perceive a difference, who's to say you'd personally prefer CC over MF? The conclusion will be subjective.

    Carbon comps may have properties that make a difference in certain spots of the amp, RG Keen talks about it too:
    http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

    I think you'll see RG's analysis lines up with the choice CC resistors @robrob suggests in his builds.

    As far as Johnson noise is concerned, carbon comps induce the most hiss of any other resistor type by far.
     
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  3. Hyakuya

    Hyakuya TDPRI Member

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    The RG Keen article makes for interesting reading....

    @robrob uses CC resistors almost exclusively (unless I'm reading the layout wrong)

    upload_2021-5-13_13-49-51.png

    but RG is saying you need enough voltage across the resistor to get the distortion they offer and so they aren't ideal for the input stage, grid resistor (except output stage) or cathode resistors.

    So, places where the signal is large are suited to CC resistors like the plate resistors or phase inverter except where there is a feedback loop which goes around the inverter. I plan on fitting a 3-way feedback switch to my Bassman so when set to none the CC resistor would add it's qualities.

    I think selective use of CC is the way ahead. I used Kiwame CF resistors throughout my Champ Micro and that has no hum at all.
     
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  4. 2L man

    2L man Tele-Meister

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    Carbon composition resistors are "noisier" and if used in pre amp total noise can come too high. However there is a theiry that some of the noise in later amp stages can be good and some builders use CC resistors in phase inverter anodes and power tube grid stoppers.

    I just bought 82k and 100k which are common anode resistors and few common power tube grid stopper sizes so next few will have them. When I have used my CF resistor storage I will use only Metal Film in signal path and Metal Oxide only if MF power is not enough.
     
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  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I use MF high wattage/voltage resistors exclusively. Besides the Blencowe and RG info, Aiken amps gives similar advice.
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I doubt Rob would *require* you to use CC. :) In fact given the two ways you can draw resistors in DIYLC, I think he may be drawing generic ones there, neither tubular nor tablet shaped, nor chocolate nor blue nor tan.

    The smart guys above nicely cover the science and opinions of experts. Among the experts, Keen is the only one to discuss the science of the mojo, which he kinda notes *may* make sense, but even he's very far from actually pushing you to use the very few CCs he discusses.

    It’s an ancient and well-trod argument in these forums. Luckily, there’s (kinda) no wrong answer. Science/noise purists use MF; budget/science hybrids and folks who like brown use CF. (No, AFAIK there’s zero logic to suggest it’s carbon that has the mojo; they're just quieter than CC and can be cheaper than MF). But despite their instability and noise, many builders use CC. This includes many pro shops. Except — if asked why, most of those pros don't cite science, they cite marketing: “Because my customers want 'em.” Amps are used mostly by humans. Beliefs beat evidence and logic every time. :D
     
    Last edited: May 13, 2021
  7. Hyakuya

    Hyakuya TDPRI Member

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    I was never suggesting Rob was saying CC were required for the build, just working through the logic of resistor selection.

    From the comments here and the reading I've done I've decided to go with Kiwame CF througout except for the Voltage drop resistor and 1M resistor on the input socket which I'm going MF.
     
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good, yeah, I was joking about Rob requiring them; if he specifies them somewhere you were smart to look into it.

    MF for the input resistor is good, and *might* also be notably quieter on that 33K grid stopper and the first plate load. No big tho. Many people like MO for the output cathode 3W, and out of superstition I often use MO over sockets (hot).
     
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  9. sds1

    sds1 Tele-Afflicted

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    Well I spoke out of school on this one, indeed this layout is all CC's.

    I was thinking of some layouts where CC's are strategically used in a few spots only. I thought these were Rob's?

    Maybe I'm thinking the opposite, Rob using non-CC's in choice places. :)
     
  10. brucerbc

    brucerbc TDPRI Member

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    I recall Dave Hunter suggesting it may be worth using CC resistors in strategic locations in his "The Guitar Amp Handbook: Understanding Tube Amplifiers and Getting Great Sounds". Haven't gone back to the source recently, but have followed his build advice on amps and been pleased with results. Though ... I didn't also NOT try his build advice and make comparisons, so can't say with confidence there's more mojo with CCs ...
     
  11. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Hey, I still think you were right. :):):)

    Here's Rob's optimized 5E3 -- note how he draws the lone (Keen-style) CC plate load, and the CF (or Dale MF?) resistors everywhere else.

    upload_2021-5-13_13-20-19.png

    Likewise note how that lonely CC is specifically drawn as the DIYLC "tubular" or cylindrical type; the brown-black resistors in the Bassman are like the tan resistors here, in the "standard" or oblong shape. (OK, sure, we could ask Rob what he intended, but where's the fun in that???)

    FWIW here are the default resistor types in DIYLC; note the shapes and colors. Of course you can do either shape, and any colors you want -- I drew in one like Rob does on the Bassman Micro, which I stoutly maintain (without any evidence) may be intended to be generic...

    upload_2021-5-13_13-39-16.png
     
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  12. sds1

    sds1 Tele-Afflicted

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    Hmm that's a good one. My theory now that I said out loud "what the heck are those?!" is they are maybe originally carbon comps that got round-y after a DIYLC upgrade. I'm remembering that the round-y feature was added later.
     
  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    LOL, maybe now instead of arguing about what type of resistors sound best, we can argue about which DIYLC shape and color sound best....
     
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  14. Hyakuya

    Hyakuya TDPRI Member

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    It was the fact that other layouts Rob has produced have different resistors placed around the design that got me thinking about this....

    I'm going with the Bassman layout is using round-y generic resistors and that the resistor value is what really counts.
     
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  15. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I usually use age appropriate resistors when I do a layout. I'm not trying to suggest we should use carbon comp resistors in a modern 5E3.

    I typically use metal film for the input resistors (first stage grid leaks and grid stops), carbon film everywhere else except the power supply and one carbon comp for mojo late in the circuit (usually a plate resistor) but before the NFB loop as suggested by R. G. Keen.
     
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