5F1 Mods

Discussion in 'Glowing Bottle Tube Amp Forum' started by Thomthom098, Apr 7, 2020.

  1. Thomthom098

    Thomthom098 TDPRI Member

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    Hi All, I am preparing an order for tubes (preamp tubes for a Marshall 6101 LM - 5881 based, needs 7 !) and would like to add bits and pieces to tweak / test some voicing changes on a 5F1 kit by Tube Amp Doctor. The kit is a pure 5F1, same components as on Fender scheme with negative feedback loop and no V1a bypass cap (as per scheme, even if I read a lot of 5F1s did have a bypass cap in reality).

    I made the kit with a Jensen C8R - 4 Ohms (which I put in for clean sounds I was after) and so am considering a couple tweaks to see if I get other sounds I like - let me know your thoughts:

    #1 Jensen C8P - 4 Ohms - my understanding is it would break earlier than the Ceramic, could be good as I'd love the amp to break a bit but don't play it beyond 4-5 volume as even 5W is loud in the apartment...

    #1a Was planning to build separate little cabinet and plug-in either the Ceramic or the Alnico one. Worth looking at power resistances and jack socket(s) to arrange both to be plugged in your opinions ?

    #1b Or would you here recommend a power attenuation to try to get the Ceramic to break instead

    #2 Negative feedback loop switch - an obvious / easy one (RobRob's favorite ! love the site !). While at it, do I buy 13, 27 and 39kOhm resistances to try vs the 22k original ? How many would you leave in situ (so I know what switch to buy: SPST, SPDT, SP3T ?...)

    #3 V1a bypass capacitor: a 25uF / 25V as shown on the Fender 1E1 scheme ? With an SPST switch to be able to go back to "original" or you'd also recommend trying other capacity values while at it ?

    #4 V1b bypass capacitor: I've read posts on this forum of people having this but can't see it on any champ schems, what would it do, what values do people test here ?

    #5 Add a tone like on Princeton 5E2 ?

    Thanks !
     
    Last edited: Apr 7, 2020
  2. King Fan

    King Fan Poster Extraordinaire

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    i can't tackle all the questions, but a couple thoughts:

    V1A bypass: Try the 25uF / *50V*. If you don't have a cap there now, try this first. This is gonna boost your gain and may markedly alter your search for breakup.

    NFB: it's easier and more useful to start with a pot that spans your range of test values, then see a) what values you like and b) if you really need more than one alternate resistor to swap in when you're sure.
     
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  3. Thomthom098

    Thomthom098 TDPRI Member

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    Thanks,
    - Any technical reason for 50V cap vs 25V on Fender schematics ?
    - Ah yes good idea the pot instead of resistance pick and trials. Is there a minimum resistance to keep in the line to avoid the pot at zero Ohm, not sure what it would do... ?
     
  4. King Fan

    King Fan Poster Extraordinaire

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    Yeah, the reason is 50V is more thoroughly 'up to the job' -- Leo just used 25V because it cost less back then to make 'em low voltage-tolerant. Doubling this safety margin costs us nothing and has zero effect on sound.

    Good question on the 'minimum' NFB resistor. Most people are just looking to decrease NFB and increase drive/dirt/signal, so just put a pot *in addition* to the existing 22K. If you wanted to 'increase' NFB for a quieter cleaner amp, I guess you could use a smaller resistor, but I'm not hearing that's what you want...
     
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  5. dougstrum

    dougstrum Tele-Afflicted

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    I like 10uf/25v. You still get the gain of bypass cap but maintain tighter bass.
    Easy to try out see what works best for you.
     
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  6. Thomthom098

    Thomthom098 TDPRI Member

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    Thanks, is this how I should do V1a bypass caps and Negative Feedback Loop Mods ?:
    - buying 2x 25V or 50V capacitors
    - a 1/2W resistance 12k or 39k (depending on more or less feedback I want) and
    - 2x SPDT on-off-on switches ?

    Do the two mods interact w/ each other ? I.e. I took NFB loop resistance values from RobRob's site for a 4 Ohm speaker but would those values be affected by the change on V1a bypass cap ?

    Any thoughts on V1b bypass caps I read some people have it as well ?


    2020-04-09_111420.jpg

    Thanks
     
  7. Thomthom098

    Thomthom098 TDPRI Member

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    I guess SPDT On-Off-On best represented like this as center is the Off connection 2020-04-09_111420.jpg
     
  8. Guitarteach

    Guitarteach Poster Extraordinaire

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    I have modded mine with v1a and v1b bypass caps on switches, a tone control and variable NFB.

    i would say they will just make it louder earlier, though not break up more.

    Biggest and best mod though by far was a VVR on the power tube. This lets you dial in serious dirt at lower volume.

    i did a DIY vvr using some tagstrip. Some scale the whole amp, but I just scaled with powertube voltages not the preamp. That keeps the edge.

    C66BB289-62C0-4E55-ACBA-A7C56CC2A9D3.jpeg
     
    Last edited: Apr 9, 2020
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  9. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    .

    My bad. Move along. Nothing to see here.
     
  10. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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  11. tubegeek

    tubegeek Tele-Afflicted

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    Nice, thank you for that.

    Is that from Kevin O'Connor or did you design it yourself or...? Haven't really seen what K O'C's circuit looks like though you do hear a lot sbout it on the various forums
     
  12. sudogeek

    sudogeek Tele-Meister

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    You can also add a (switchable) cathode bypass cap to V1B, a la the blackface/silverface Champ and Vibro-Champ. This was done to boost the signal after the losses in the tone stack. If you add a tone control/stack (imo, not necessary), this is useful. But you can add it to the 5F1 circuit to push the power tube harder.

    My Champ has a 0.68 uF bypass cap on V1A, a switchable blackface-like tone stack, a switchable 10uF bypass cap on V1B, and NFB on a switch but, over time after trying all the variations, I’ve found I mostly just leave it stock 5F1 except for the V1A cap. Oh, I do use a 10” speaker which also helps.
     
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  13. Thomthom098

    Thomthom098 TDPRI Member

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    Thanks GuitarTeach, this is way over my level, I'll start with just Resistances, Capacitors and Switches for now :)
     
  14. Thomthom098

    Thomthom098 TDPRI Member

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    Thanks, 0.68uF on V1A would then boost more highs and less lows vs the previous values mentioned like 25uF or 10uF ? Does this apply to the V1b bypass capacitor as well, to boost all levels 25uF and 0.65 for highs only ?
     
  15. Guitarteach

    Guitarteach Poster Extraordinaire

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    i don’t know where the original circuit came from.

    i made two ampmaker kits with VVRs as stock components on the turret board and liked the effect, so was looking for a diy retrofit version for my other builds. You could buy a pcb from Hall or somebody but this was same idea using discrete components i had spare.
     
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  16. tubegeek

    tubegeek Tele-Afflicted

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    True. Yes to both q's.
     
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  17. King Fan

    King Fan Poster Extraordinaire

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    If you want, I'd do your mods one at a time -- and I'd keep each one simple. Why?

    Well, this is a Champ, after all. A dozen chrome accessories hanging off a Vespa is cute -- but that's all it is.

    But more than that, it's really nice to be able to defeat each mod and return to stock (the exception being a 10" speaker, a great 'mod' I'd have on my list). BTW staying at 8" is simpler, and I do like your alnico idea. I see you're in Monaco? What can you get for 8" vintage-style alnico besides reissue Jensen?

    It's nice to do 'em one at a time so you can get to know how each works and 'dial it in' (NFB cut, bypass cap size, etc.) without other interactions. Also, you may definitely reach a point where you know two chrome mirrors is plenty!

    For these reasons I'd focus on SPST switches if at all possible.

    If it were me I'd shorten the list and make sure I had the most logical order. Adding an on-off stock-value V1a bypass cap (idea #3 in your first post) is almost a no-brainer, since as you pointed out, many, maybe most, Champs were actually built that way, and it's a great sound. Then sure, add an alternate value with a switch later.

    A single-value on-off NFB cut would be simple and likely sufficient, and wouldn't negate your V1a bypass. For my purposes I just ciip in a pot for a few days or a week to select the right value, then on-off that value. If you fall in love with two values, you can complicate the switching a little.

    Finally, if you do get all the way to VVR or MVs or bucking transformers or zeners or tone stacks or attenuators or power scaling or extension cabs… you'll get there on a fun, useful, stepwise journey!
     
    Last edited: Apr 9, 2020
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  18. Thomthom098

    Thomthom098 TDPRI Member

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    Thanks King Fan, fully agree, I guess I'd only like to try 3 things, V1a bypass cap, NFB Loop and an Alnico speaker. And all being activated one by one with switch to go back to like I am now as I like it a lot already.

    SPST or SPDT don't know, I guess handy to install two values of cap and another value of NFBL resistance while going into the amp to do things, does not cost much, same space the switch takes on front panel etc...

    And speaker will take a bit of time as I'd rebuilt a small separate little cabinet. I only looked at Tube Amp Doctor site to see Alnico reissue, they normally recommend P8R for the champ. Haven't checked other sites too much, a lot are in US (Weber !) and customs an issue when ordering to Europe, 20% tax etc...

    2020-04-09_165920.jpg
     
    Last edited: Apr 9, 2020
  19. Guitarteach

    Guitarteach Poster Extraordinaire

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    Mind sounds spectacular through a 12” Celestion alnico blue. Zero boxinesss.
     
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  20. Thomthom098

    Thomthom098 TDPRI Member

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    Ah did not yet think of that, I have a Marshall 6101 home with a 12" Celestion Gold S303 speaker, can take 200W, should do ! but 8 Ohm, not sure I can try it from the champ 5F1 designed for 4 Ohm. And at the Garage, I have the 4x12 Marshall cabs, they have the switch 4 - 8 - 16 Ohm so could try that - I read the Champ is likely to sound more bassy through these larger speakers... :)
     
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