Plans for modding AC15HW1X to more vintage specs - Need advice

Discussion in 'Shock Brother's DIY Amps' started by HaroldBKNY, Jul 19, 2019.

  1. HaroldBKNY

    HaroldBKNY TDPRI Member

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    I'm hoping to mod my AC15HW1X to be closer to 1960 JMI specs. I could use some advice from people who know these amps better than I do. This amp really seems to be closer to being like a lower-powered AC30 with a top-boost section, not an AC15, so that's what I'll be targeting.

    Throughout this post, I'll be referencing components by their numbers in the AC15HW1X schematic, not the vintage schematic. See all referenced schematics below or attached to post.

    I'd like advice on all of this, but the parts that I especially need help deciding what to do will be in blue.

    For some of these changes, I'm uncertain if I should actually do them, if they'll just make it closer to vintage spec, but decrease the sound quality (like removing the RF-reduction portions from the input). Then again, it's possible that the components don't just reduce RF but also degrade the tone in some way, and should be removed.

    Proposed Changes:
    Input Section:
    Remove C8, C15, C20, C26.
    Move R40 to connect to input jack, before R37, and similarly move R23 to connect before R18.
    This will effectively make R18 and R19 act as a single 66k resistor (and same for R37 and R38)
    I imagine that these differences to the input were mostly added in by Vox for the purposes of reducing RF from getting into the input. Should I actually remove them? They're probably filtering and attenuating the tone in small ways, and maybe I'd be better off without them if I don't really care about small amounts of RF getting in?

    Grounding Section:
    Remove C17, C30, R46, and R29. I'm not certain why these components are here. They seem to be isolating the ground from the chassis. My only guess is once again that they are involved in reducing RF, but I don't really know. I don't really know what "Ferrite" resistors are. Is there any reason I shouldn't remove these?

    Preamp Tube Section (Tube V3):
    Reduce C5 from 47uf to 8uf. This filter cap value is way too high for vintage specs (like many other filter caps in this amp)
    Reduce C12 from 100nf to either 47nf or 500pf. I'm uncertain which would be best. In the AC30/6 schematic, 47nf is on the normal channel, and 500pf is on the bright channel (unless I'm reading it incorrectly). I'm leaning more towards 500pf, but I don't really know.
    Remove R27. This seems to be a pulldown resistor. I don't really know why Vox stuck this in.
    Reduce R17 to 220k, and move it to after VR3, in between lug 2 of VR3 and the junction of C13 and C14. Once again, I don't know why Vox changed this part's location from the vintage specs.
    Reduce C14 from 100nf to 47uf

    Top Boost Section:
    Reduce C19 from 47uf to 32uf.
    Reduce C27 from 100nf to 47nf.


    Phase Inverter Section:
    Reduce C6 from 47uf to 8uf.
    Change C7 and C10 from from 100nf to either 150nf or 10nf. The value of these phase inverter coupling caps seem to differ amongst the schematics I was looking at. In the AC30/6, it's 150nf (unless I'm misreading it, it's difficult to tell), and in the AC15 and AC30/4, it's 10nf. I'm leaning towards doing 10nf, not 150nf. Would that be a more vintage Vox sound?
    Remove R10 and R16? I'm uncertain why Vox added these. My guess is to isolate the tone cut circuit from the master volume circuit, which they added. Though I'm not certain why these resistors are strictly necessary for that. Would it be bad to remove them? They could possibly be attenuating the signal.
    Change VR2 from B250k to A250k. All of the vintage schematics specify log taper, not linear taper. But is the vintage spec better or not? Maybe making this change would cause there to be more usable positions on the tone cut knob? Or would it make it worse?


    Power Amp Section:
    Change R61 and R101 from 3k3 to 1k5. All of the vintage schematics specify 1k5, so I'm not sure why Vox changed this value. Is this okay to change back to 1k5?
    Change R79 and R104 from 470R to 100R. I'm really uncertain about this, but once again, all of the vintage schematics specify 100R for this resistor, not 470R.
    Change C74 from 220uf to 50uf? I'm uncertain about this one. It's difficult to read the value in the AC30/6 schematic, but in the AC30/4 and the AC15, this value is 50uf, not 220uf.


    Filter Section:
    Change C2 from 47uf to 16uf.
    Change C1 from 22uf to 16uf.

    Remove C3 and C4. These just seem to be overkill for filtering, adding onto the values of C1 and C2 respectively. Is it safe to remove these for a more vintage sound?
    Change the choke from 7h to 15h or 20h? I'm hoping not to do this one because I don't want to have to spend so much money on a vintage style choke, and also how to fit it onto the amp chassis. However, if it would make a significant impact on tone, I will try to do it. Would making this change be important? All of the vintage schematics specify that the choke should be between 10h and 20h, but aren't specific.


    The AC15HW1X schematic is below (don't pay attention to the pink and green circles, they're someone else's mods):
    Part 1:
    [​IMG]
    Part 2:
    [​IMG]

    The vintage schematics have been attached to this post for reference.
     

    Attached Files:

    Last edited: Jul 19, 2019
  2. HaroldBKNY

    HaroldBKNY TDPRI Member

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    I've put together an edited version of the schematic to take out the pink and green circles connect the power and preamp sections. This is the AC15HW1X circuit completely stock:

    [​IMG]
     
    Last edited: Jul 22, 2019
  3. gusfinley

    gusfinley Tele-Holic

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    I finally gota change to take a good look at this....

    As for the question of why all of these extra components are installed a lot this has to do with meeting European Electrical Standards (CE Certification). The inputs stage and grounding for example, have a lot of EMI filtering, which could certainly be removed if that is what you want. If you do run into problems, you could always just reinstall them.

    C5 is wise to lower to either 8, 16, or 22uF.

    I don't have any input on C12, you could tune it to taste - there is a bright switch in the circuit so 500pF might be a little to low for that value.

    for C14, I only remember seeing .1uF in this postion.

    C19 to 32uF would match the vintage TB value.

    C7 and C10 were usually 150nF Or 100nF. I don't recall ever seeing them lower for an AC30.

    I'm not sure about R10, R16 and VR2. I've kept the 10K isolation resistors in my MV + Tone Cut builds. I believe I have used log Tone cut pots in my builds also.

    The input resistors to the power tubes would block out higher frequencies and could reduce blocking distortion, but you could go 1.5K if that is not a problem.

    The 470R screen resistors are to increase reliability and shouldn't change tone too uch. If you're using good tubes the 100R would be okay.

    220uF should be correct for a 1964 era AC30 cathode bypass cap.

    For C1, C2 I've seem recommendations of no more than 32uF for these. The 32uF still gives a more vintage sound than higehr values but helps combat ghost notes in the preamp.

    C3 and C4 can certainly be removed.

    I'd leave the choke as is.

    Best of luck to you! Let us know what you decide and how it changes the tone of the amp!
     
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  4. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Thanks so much for the great advice!


    Still curious about this more. I will do more research into how screen resistors work, but is there an advantage to using 100R? Does this impact tone at all? What does "good tube" mean?
     
  5. gusfinley

    gusfinley Tele-Holic

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    There was a discussion on the now defunct Plexi Palace Vox Forum about the values of input and screen resistors and their effects on tone and reliability. Some early vox amps didn't even get screen resistors! These resistors limit screen current during max output. The higher the value the more current is limited. I stick to the 100 ohm resistors myself and try to use a solid tube like JJ in the power section ( of course NOS would be better! ).
     
    Last edited: Jul 24, 2019
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  6. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Cool, from what I'm reading online, it sounds like I may like the lower screen resistors, it may be closer to what I'm going for.

    The stock resistors in the AC15HW1X were rated for 1/2 watt, but it looks like in the vintage AC30/4, each resistor is rated for 3w! I guess to play it safe I should use 3w resistors as well? Do they even make 3w carbon comp resistors?
     
  7. gusfinley

    gusfinley Tele-Holic

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    Anything above 1/2 W would be better. Carbon Comps above 1/2 W are expensive and I can't see vox wanted to pay three times as much for a resistor there. In my current AC15 project, I found a cool Higher Powered Metal Film Resistor that looks like a Carbon Comp.

    Remember that resistors don't just have wattage ratings - they have voltage ratings, too! 1/2 W resistors are often rated around 250-300 VDC. Since you'll have voltages around 300 VDC on your screens, a little extra voltage rating would be better.
     
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  8. HaroldBKNY

    HaroldBKNY TDPRI Member

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    If it only has to be more than 1/2 watt, I think I can go with these 1w 500V ones from where I'll probably be buying all the caps for these mods:
    https://www.tubesandmore.com/products/resistors-1w-carbon-composition
     
  9. cgharrison

    cgharrison TDPRI Member

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    I’ve been down this road - reducing the filter caps is by far the most impactful mod for the amp. It really takes the sound to a much more vintage place. The other mods are fine, but make relatively small differences in the sound. I removed that input ferrite bead network, and didn’t notice any change. Good luck!
     
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  10. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Just got my parts in the mail today, and I'm excited to do the mods, hopefully early next week. I've actually never worked with a turret board before, only eyelets, so if you have any advice, please let me know.

    For C12, I've actually decided to change up that whole part of the circuit. The bright switch on the stock amp is a Fender-style bright switch, a cap across the two legs of the volume pot. I've decided that I'd rather have the bright switch be able to swap between the two values of coupling cap for C12 that I was deciding between, the 500pf value from the vintage bright channel, and the 47nf from the vintage normal channel. I've done this sort of mod before on Fender amps and I find it can help voice a channel for really good distorted tone, to go down to small valued coupling caps.

    Also, for the log tone pot, I could only find a push pull pot in the correct size, so now I'll have an extra switch available to me on the amp. I don't know what I'll want the switch to do, and I'm open to suggestions.
     
    Last edited: Aug 2, 2019
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