Sunday, January 8, 2012

AC15HW1 Modifications


I recently made a couple of simple mods to my AC15HW1 amp. The amp sounds fantastic out of the box but with a couple of simple changes it can be amazing and sound closer to the old 1960's AC15. There are four basic changes :-

Reducing the DC filtering capacitor values and using better quality caps.
Changing to better quality signal capacitors.
Adding grid stoppers to the Top Boost and Phase Inverter to avoid blocking distortion.
Reducing R17 to give more gain in the Normal channel.

These simple changes really liven up the amp and remove some the modern "stiffness" associated with too much DC filtering. All components should be replaced with voltage ratings no less than the originals.

WARNING - These amps contain lethal high voltages which can remain present even when the amp is unplugged from the mains supply, do not attempt any modifications unless you know what you are doing.

Signal Cap Changes:-
C13 - Silver Mica 220pF (Bright Switch Cap)
C22 - Silver Mica 100pF (TB Bright Cap original value 120pF )
C21 - Silver Mica 500pF (TB coupling cap)
C23 - Silver Mica 47pF (Treble Cap)
C24 - Mallory 22nF (Middle Cap)
C25 - Mallory 22nF (Bass Cap)
C7, C10, C12, C14, C27 - Mallory 47nF (Stage Coupling Caps original values 100nF)

Filter Cap Changes:-
C1 - F&T 500v 22uF
C2 - F&T 500v 22uF (original value 47uF)
C3, C4 - 630v 100nF (original values 10nF)
C5, C6 - F&T 500v 10uF (original values 47uF)
C19 - F&T 500v 30uF (original value 47uF)

Resistor Changes:-
R17 reduced from 330K to 160K
Add 7.5K 27K resistors to the input pins 2 & 7 of v2 Phase Inverter
Add 7.5K 27K resistor to the input pin 2 of v9 TB channel

UPDATE: I have tested 27K grid stoppers with no detrimental effects to high frequency roll off and these have helped smooth the break-up when the amp starts to overdrive so I would recommend them over the 7.5K.

I used 500v Silver Mica for the small signal caps, 630v Mallory 150M caps for the larger signal caps and 500v F&T caps for the filtering. Resistors are all 1/2 watt. The grid stoppers are soldered directly between the tube base pin and its associated connecting wire. Click on the images below for larger versions, I have highlighted the changes with green and magenta circles.

Also see AC15HW1 Modifications (Part2)

UPDATE: Feb 2014

This is what I'm running now for those interested ;-)

Signal Caps:-
C13 - Silver Mica 220pF (Bright Switch Cap)
C22 - Silver Mica 100pF (TB Bright Cap original 120pF)
C21 - Silver Mica 500pF (TB coupling cap)
C23 - Silver Mica 47pF (Treble Cap)
C24 - Mallory 22nF (Middle Cap)
C25 - Mallory 22nF (Bass Cap)
C7, C10, C12, C14 - Mallory 100nF (Coupling Caps)
C27 - Mallory 47nF (TB coupling Cap)

Filter Caps:-
C1 - F&T 500v 22uF
C2 - F&T 500v 22uF (original value 47uF)
C5, C6 - F&T 500v 10uF (original values 47uF)
C19 - F&T 500v 47uF
C3, C4 - Mallory 630v 47nF (original values 10nF)

Cathode Bypass Caps:-
C16, C28 - Sprague 25uF 25v (Low ESR)

Resistors:-
R17 120K (Original value 330K)
Add 20K resistors to input pins 2 & 7 of v2 Phase Inverter
Add 20K resistor to input pin 2 of v9 TB channel








90 comments:

  1. Hi!

    I saw your post on the vox forum. I was wonder where you got the schematics? I am looking for the schematics for my AC30HW head. Any idea where i can get that? Thanks in advance!

    Jeffrey (stratman1976)

    ReplyDelete
    Replies
    1. Hello, were you ever able to obtain the schematics the the ac 30 hw? I really need them for changing out the power transformer.

      Delete
  2. Hi, I emailed Vox and asked them nicely, I didn't expect them to send me anything to be honest but they sent the Service Manual as a secure PDF.

    ReplyDelete
    Replies
    1. Could you be so kind as to send me the service manual. My amp tech is doing a complete transformer change and is having a hell of a time not knowing which pt wires are going to the various fuses underneath the chassis. I am putting in a mercury universal pt. if you send me that service manual I would be forever indebted please help

      Delete
  3. BTW... the AC30HW is identical to the AC15 except for two extra tubes in the power section (and a couple of resistors).

    ReplyDelete
  4. Cool, thanks for your reply. I think i'll try to get the official schemtic from Vox....enjoy your amp!

    Jeffrey

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  5. My AC15HW just arrived.

    It sounds fantastic but the distortion seems as a little "Buzzy" and I am experiencing ghost tones. Do these mods address anything in these concerns?

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  6. The buzziness may be the fact its new and the speaker will take some time to break in. This mod is more likely to produce ghost tones as it reduces the DC filtering. Are you really getting ghost tones though or are you just not used to an amp that gives so much harmonic content ?

    ReplyDelete
  7. Thanks, Nik. Sorry, for the delayed response.

    I replaced the original G12M-25 - 55hz with a partially broken-in G12-65-80hz that I had laying around. I have not noticed the Ghost tones nearly as much and cone cry is all but gone. The tone is also "clearer." Possibly, the lower end response of the Geenback made the Ghost tones more noticeable.

    Thanks, for the amazing work you have done to present the MOD's! At some point I will experiment with these MOD's but, for now I am enjoying the AC15 as it is +,- a speaker or tube...

    ReplyDelete
  8. thanks for all those info

    have you thought to push a little bit more the mod, by adding new transformers ? (or maybe those one are already good quality ?)

    do you know a place on the web to buy all those components ? why don't you fit a set of sozo vintage caps ?

    Thanks,
    mickael

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  9. New transformers are expensive and I'm pretty happy with the way the amp sounds with its original transformers. Sozo caps are an option too but again more expensive and I find the the Mallory caps to be great value for money. I bought my caps locally here in New Zealand. There are plenty of places online that sell them, just google ;-) Another cheap mod is to replace all the carbon comp resistors with carbon film, this will cut down on the hiss the amp makes. Carbon comp resistors don't add much to the tone except perhaps in positions were they are subject to high voltage signal changes like the anode loads.

    ReplyDelete
  10. I've bought that set from Sozo :

    (1) .0047 400v Mustard Caps
    (2) .022 400v Mustard Caps
    (3) .047 400v Caps
    (3) .1 400v Cap

    would it be possible for you to give me the location of them (ie. C12, C17, etc)

    I can solder, but my knowledge is very poor in amp technology

    THANKS

    ReplyDelete
  11. Click on the first and last photos in my post and you will see where they go.

    ReplyDelete
  12. ok thanks :-)

    what are C15, C17 and R29 on the high input of the normal channel ?

    ReplyDelete
  13. They are to reduce RF interference, you don't need to touch them, they are not really part of the signal path.

    ReplyDelete
  14. my amp is a AC30HW2 :

    May I remove C3 and C4, because they are not on the old AC30 circuit ?

    I plan to change R61 and R101 with 1K5 like AC30 schematic and also R79 and R104 with 100 ohms ; what will b ethe effect ?

    ReplyDelete
  15. You can remove C3 and C4 but they are really not changing the tone of the amp, they are reducing noise in the power supply. I would probably just leave them in there. R61 and R101 grid stoppers are unlikely to make much difference either. R79 and R104 will make a difference, these are the screen grid resistors and raising the voltage will raise the gm (transconductance) and lower the Ra (anode resistance) of the pentode. This will make them run hotter and under more strain. I haven't changed these in my amp so can't comment on the tonal changes. These new values may have been chosen to help with extending the tube life so you may want to keep these values if the tone isn't improved to your liking.

    Unlike the AC15 the AC30 will have 4 grid stoppers and 4 screen grid resistors, you can see the space on the main board in my photo where they reside. You can move the 3k3 grid stoppers from the board and solder them directly to pin 2 of the tube sockets, this is a common modification that will stop any problems with interference when using different brands of EL84 tubes. Just solder a link across where the resistor used to sit on the main board, once you have moved them.

    Note that the old Vox AC30 circuit was designed around the components of the day, the new Vox's have different transformers and modern tubes so just matching some of these components to the old values will not always result in what you expect.

    ReplyDelete
    Replies
    1. Dear Nik !

      Great job ! Bravo !! I am intersted in the filer caps mod looking for a looser feel, but i record home at low volume and wondered if lowering the filter caps would bring noise/hum ? (as my amp is super quiet for the moment)
      Also will i loose some of the bass? I just tried today a 1978 AC30 that had 32uf everywhere (which is not that low) and yet it had looser feel and more bass than mine (AC30HW2) tested with same speakers.

      Thanks for your precious help !

      Delete
  16. Is it 1/2 watt resistors fitted everywhere in the amp or 1 watt ?

    ReplyDelete
  17. Replies
    1. Thanks a lot Nik,

      I've ordered all components, but I must say I'm not sure to fit those 27K grid stopper resistors, which were not in the original circuit ...

      why have we never seen those kind of resistors in the previous versions of the AC Vox ???

      Delete
  18. I used 68k grid stoppers eventually. Back in the day they used to scrimp on anything they could to save a few pennies. You can take them out easily enough but the amp is better for them being in there. They prevent blocking distortion at high gain settings and also help with reducing interference. You'll notice they put them on V1 as that is more prone to issues.

    ReplyDelete
  19. Hi Nik,

    I've mod my amp, in 2 steps (1st signal caps,then 2nd filter caps)and I must say I've been more impressed by the effect of the filter caps on my tone !!!

    Do you think we can remove R10 and R16, because I don't find them in the original circuit of the AC30 ?

    Thanks,
    Mickael

    ReplyDelete
  20. I found the filter caps made more impact too. R10 & R16 are there because of the master volume. Don't remove them, they are not changing the tone but they are stopping the output signal of the PI becoming a dead short when the MV is at zero.

    ReplyDelete
  21. if I keep R10 and R16, that means I'll add a resistor of 10K with the 3k3 grid stoppers

    In that situation, it will not make a big difference if I fit a 1K5 grid stoppers instead of 3K3 ... ???

    ReplyDelete
  22. The MV is between these resistors so the 10K resistors will act a bit like grid stoppers at certain volume levels. The grid stoppers should really be moved to the pins on the tube socket for best results. Changing them to 1k5 won't make any audible difference so I would leave them at 3k3.

    ReplyDelete
  23. my last question ... and it will be finished :-)

    may I ask you why you moved from 27K to 68K grid stoppers ?

    THANKS FOR ALL YOUR ANSWERS NIK :-)

    ReplyDelete
  24. The higher you can make the grid stoppers they more effective they are. If you make them too high they can start to roll off the higher frequencies as they form an RC network with the input capacitance of the tube. 68K is a very common value. Some high gain designs use much higher values to tame the high frequencies on purpose to smooth out higher order harmonics which sound harsh.

    ReplyDelete
  25. Hi
    It's great that you share your experiences with moding ac15hw1. Is it possible to resend to me this secure pdf containig service manual you've got from vox ??
    I woul be very greatefull. I'm just trying to built this amp on my own. I need detailed schmatic. My email is akarmedicus@gmail.com

    ReplyDelete
  26. Hi Nik,

    on the diagramm "schematic_mods" there's a grid stopper resistor on the pin 2 of V9, but on the "Layout_mods" the resistor seems to be on pin 7 ; which one is correct ?

    why there's 2 grid stopper resistors on V2 and only one on V9 ?

    Thanks for your help :-)

    mickael

    ReplyDelete
  27. It should be on pin 2, I have corrected the layout diagram - thanks. Pin 7 of V9 is DC coupled to the first half of the tube so doesn't have a grid stopper.

    ReplyDelete
  28. Nik
    I've noticed a substantial increase in Bass response in the Normal channel of my amp in doing the mods you've mentioned.
    Any thoughts of what might have caused that.
    It's a little too much with my Humbucker equipped guitars.
    Thanks
    Richard

    ReplyDelete
    Replies
    1. The changes will loosen the amps response a bit but shouldn't increase bass response. If anything it should reduce it slightly as we are using smaller coupling caps. I would just double check the values of the caps you used and that you changed the correct ones. C16 for example could increase bass response if it was made larger.

      Removing R17 in the normal channel will reduce the treble boost at higher preamp volumes. This could make the amp feel less bright, effectively sounding bassier but it shouldn't actually add any bass.

      I haven't noticed any extra bass in the couple of conversions I have done. It is possible that one of the components you replaced was way out of spec and now the amp is functioning correctly.

      If it still seems too bass heavy after checking everything then you can reduce the bass by reducing the values of C16 and C12. Try 10uF for C16 and see if that helps. Then try 22nF for C12 to cut it back some more.

      Delete
  29. Hi Nik,
    The stuff you are posting is great. I bought an AC15HW1 and will pick it up next Friday. I will check some things out you mentioned. But i have a differnt Question. Did you checknit out if it makes sense to bring in a FX Loop? I wnat, better need, to use a Reverb and a Delay and i heard that its not the best option before teh Amp. Other told me to patch TB and normal qand use the pedals between both (TB distorted and Normal clean for effects).
    Do you have any experiences?
    Kind regards
    Dieter

    ReplyDelete
  30. Hi, to be honest an FX loop is probably not much use on these amps. Most of the distortion / overdrive is produced in the phase inverter and power tubes which would be after the FX loop anyway. Time based effects like delay & reverb usually sound better if they after the distortion stages, if you play clean (slightly dirty) then they are fine in front of the amp.

    You can use an A/B/Y switch so that you can switch between the Normal & TB channels and also run them together for even more boost if you like, this is what I do on my AC30.

    ReplyDelete
    Replies
    1. Hi Nik,
      I picked up the AC15HW1 I bought last Friday. After playing first time I was very disappointed. One issue is that the normal channel crunches at 10 o'clock and overall the Amp is not as loud as I expected (really quiet - compared to a 15 Watt Vox VT15 Hybrid practicing amp). So something not seems ok. I opened the Amp and found out, that the pre owner put in other Tubes (JJ's and harmonics) and he also replaced R61 and R101 with wire Jumpers. Can you give me an impression if that can cause the mentioned effekt? Or can the replacement destroy something. If I use Normal or TB channel (with MV bypassed) from 11 o'clock upwards the Amp dont get louder (my impression is that it gets more quiet) but compressed. I owned a AC30 a few years ago and that was a beast - or is the AC 15 so less louder.
      Thanks
      Dieter

      Delete
  31. Hi Dieter, R61 & R101 are the grid stoppers for the output tubes, they may not have been removed but just moved so that they are soldered directly to the tube sockets. This is a common mod as it is better to have the grid stoppers as close to the tube as possible to avoid external interference being picked up.

    If they have been removed rather than just moved then this will not cause any damage and shouldn't cause any volume drop, it just makes the amp more susceptible to interference. I would put them back in though.

    These amps break up pretty quickly with humbuckers maybe around 10 o'clock and usually start to break up at around 12 o'clock with single coils. I haven't heard the VT15 so can't comment on how loud it is. The AC15 is generally a pretty loud amp, I have an AC30 too which is a fair bit louder and can do cleans quite a bit louder as it has far more headroom.

    I would try a new set of tubes first just to make sure. Then maybe try and find another AC15HW that you try out, maybe in a shop and see what thats like. Check very carefully through the circuit and see if anything else has been changed.

    ReplyDelete
  32. Dieter, also check R17 hasn't been removed / bypassed, this is another common mod that I describe above. Removing / bypassing R17 gives the Normal channel more gain which most people prefer but you may not ;-)

    ReplyDelete
    Replies
    1. Nik, thanks for your inputs. But I think the Amp I owne has a major problem. I compared the Amp with a friends AC15HW1 (same as mine) and it was astonishing - my Amp is est. a quarter as loud as the other. If my Amp is fully blown on the normal channel with MV bypassed you can nearly talk with someone. So something here is very strange. But The Amp at itself works fine (both channels and all features ...). Have you ever heared about this??? If feels using an attenunator with full damping. Sounds crazy, but its reality.
      Can You help? I know remote diagnostic ist nearly impossible but maybe you have heard about this before.
      I think I will return it to the pre owner.
      Cheers
      Dieter

      Delete
  33. Its hard to say without getting it on the workbench. I would take it back and find another one. You could take it to an amp tech and ask for quick a check up, it may be something quite simple / cheap to fix.

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  34. Hi Nik, I have a question about the volume differences between Cool and Hot. Is ther a posibility to get nearly the same loudness? If I switch from cool to hot the it is much louder. Is there a way to make the Hot more silent without loosing distorsion?
    Cheer
    Dieter

    ReplyDelete
  35. Not very easily no. The hot switch lifts most of the tone stack out of the circuit so that the signal is not loaded down and thus overdrives the phase inverter. The only way to do it would be to add another master volume and change the hot/cold switch so that it also swaps between the two master volumes. Then you could set them independently.

    ReplyDelete
    Replies
    1. Hi Nik,

      I picked up an AC15HW1X in January. I needed a smaller to go amp so I thought the amp would be a great choice. Well, after owning it for a while I am starting to realize it's not what I thought it would be. It's great clean, but when you play it with pedals or drive the amp on its own with or without pedals, it starts to sound kind of "meh". I describe it as sort of nasally, stiff, and harsh. I also realized the amp needs to open up so it will sound decent. WIth the master volume engaged, the amp loses a lot of mojo.

      I came across your blog after doing a search on the internet for mods. How much better does the amp sound with these changes? Will it smoothen it out a lot and allow the amp to get great tones at any volume?

      Thanks,
      Patrick

      Delete
  36. I think the mod softens the amp a bit but its hard for me to say whether it will help you as tone is so subjective. AC15's are bright and brash and maybe that just doesn't suit your requirements. Maybe try some different tubes too that can change the amps character a little.

    ReplyDelete
    Replies
    1. I'm actually a Vox guy at heart. I love the clarity, breakup, and chime. I own an AC30TBX and have owned the AC15TBX (wish I still had that amp) and they are very smooth amps at breakup. I think the tone is good on the AC15HW, but I feel like it could be better. It also doesn't seem to take pedals as well as other amps. I did change tubes to all JJ's in the preamp (I put in Tung Sols in there initially) and it smoothened things up a bit, but the amp still has a stiff feel to it. I plan on performing these mods. I'm just trying to figure out whether I should do it myself or get my friend who has decent experience with this stuff to do it. Is it pretty easy to do? I have some experience with soldering, but I'm totally new at amp work. I would've gotten my amp tech to do it, but he moved away recently...Do you have any clips by the way?

      Delete
  37. Nik:

    I just got one of these amps and I'm loving the tone but fighting an annoying buzz from it that makes it useless for recording. It seems to come from the area of the rectifier tube. I've tried several rectifiers to no avail. I notice that the rectifier socket has a lot of play in it. Not like the pin holders are loose, but like the entire socket rocks in the chassis. I think the speaker waves are moving this socket and causing the ruckus. Additionally I notice that some of the socket screws have nuts on them and many others do not (?). Have you considered replacing the sockets in your amp, and if so, with what?

    ReplyDelete
  38. I don't have that issue as I don't use it for recording. I can only suggest tightening the sockets or using silicone sealant to stop the buzzing. Its quite common for these loud amps to rattle and buzz a bit.

    ReplyDelete
  39. I don't have any clips I don't have anything to record with. The mods definitely make it smoother in my opinion.

    ReplyDelete
  40. Ok. Thanks for putting this stuff online. I got a pretty noob question. Silver Mica and Mallory are just brands right? It doesn't matter what I replace those caps with as long as theyre high quality? I was thinking Sozos.

    ReplyDelete
  41. Mallory is a brand, Silver Mica is a type of construction. Sozos will be fine but will probably cost more ;-). The amp is fairly easy to work on but removing the original "lead-free" solder takes a bit of time as it doesn't flow well. If you have decent soldering iron and solder sucker you should be fine.
    The amp discharges to safe voltages in a few seconds when the power is switched off but you should check it with volt meter before touching anything. Check the voltages across the large filter caps before putting your fingers in there. The big filter caps are also glued to the board so you may find they get damaged when you try to pull them free, but I wouldn't worry they are pretty cheap and nasty.
    If you want to try the mods in stages do the filter caps first, they make the biggest difference to the tone and feel of the amp.

    ReplyDelete
    Replies
    1. Oh cool. Thanks a lot man. If I could I would just pay you to do mine lol...this kind of reminds me of why my friend told me to not get Mercury Magnetics. He says they're overrated. He said the caps they send you with the kit seem to do most of the difference in tone which brings me back to what you said about the filter caps making the biggest difference.

      Delete
  42. MM transformers are good quality but it depends on what your replacing with them as to whether they are good value. If you swap out the OT of an Orange Tiny Terror with a MM you will notice a difference and it could be said to be a worthwhile upgrade. If you swapped the OT of a Rockerverb your probably wasting your money as the RV's have really good transformers already. There is no one rule with stuff ;-)

    ReplyDelete
    Replies
    1. Very true. I should swap out the the transformers in my Matchless =P.

      Delete
    2. Well, I just purchased the filter caps. I went with Mallory's since those Sozo caps are very expensive. I noticed the Mallory's are rated at 630v while the Sozo's are rated at 400v. Does that matter? I'll let you know what I think once I stick them in.

      Delete
  43. As long as the voltage is equal or higher than the originals it will be fine.

    ReplyDelete
  44. Nik, I want to thank you for putting this info up for the rest of us to benefit from. I've done most of the mods described here and my amp is sounding fantastic. I'm running both channels together, both about halfway up, with the Cut on about 9 o'clock and the MV to taste. The only additional mod I performed was changing the MV pot to a slightly different value of audio taper pot as someone recommends on the Les Paul Forum. I was using a Blue in mine for a while, but the speaker was making some odd noises so I'm running a WGS Green Beret which sounds remarkable. No reason to seek out another Blue IMHO.

    Thanks again,
    Mike in Tennessee

    ReplyDelete
    Replies
    1. For the curious, here is the info on the MV pot from Lyle at the Les Paul Forum:

      "Here's an easy improvement for this amp - replace the stock master volume pot with a 500K audio taper pot (Alpha 16mm knurled shaft is what you want).

      Vox used a 200K audio pot for the master volume.

      This is a very small resistance between the opposing output signals, and it attenuates the signal a noticeable amount even when on 10.

      Vox put a switch in to defeat the MV to avoid this attenuation.

      They chose 200K as there is a very smooth taper with that low a value but again there is a good bit of attenuation.

      Changing this pot to a 500K Audio pot decreases the attenuation when on full quite a bit more but still has a good taper, even though it's not as gradual as the 200K.

      The "perfect" pot for the AC30 MV would be a 600K audio taper pot. At 600K you cannot hear when the pot is in or out of the circuit. But the only way to get 600K is to use a 1MA pot with a 1.5m resistor in parallel, and that sadly gives an almost useless taper. If you use a 1M pot by itself the circuit is transparent but there isn't any volume reduction from 5:00 to 2:30.

      So, 500KA is the best compromise IMO."

      Delete
  45. Hi there.

    Can anyone give a link to the ac30hwh schenmatic and layout?

    thanks
    Paul

    ReplyDelete
  46. I don't have the AC30 schematic. The AC15 is the same except for the two extra output tubes in the AC30.

    ReplyDelete
  47. Hi,

    I own a AC15h1tv with a EF86 circuit, Is the filter cap mod possible
    in the same way as shown here?

    thanks,

    Gerd

    ReplyDelete
  48. Hi the AC15h1tv uses partly PCB construction so it will be harder to work on. Looking at the schematics I can see that it is a very close reproduction of the original AC15/30 EF86 normal channel + the AC30 top boost channel circuits. I would leave it as it is to be honest.

    ReplyDelete
    Replies
    1. Hi Nik,

      Thank`s for reply.
      The thing is, I have a tone problem with my amp, after playing for
      ap. 30 minutes the tone gets dull and edgeless, I´m afraid a filter cap
      could be faulty ? therefore I will replace them.
      (With the same values?)
      I changed already all tubes with no succsess, I`m rather frustrated.
      Or have you an other good idea for me?

      Thank`s a lot,,

      Gerd

      Delete
  49. Could be a cap or even a resistor warming up. Changing them all with the same values won't hurt. Getting it to an amp tech might be a better option though.

    ReplyDelete
  50. Hi Nik,

    Thank`s for kind support,

    Cheers,

    Gerd

    ReplyDelete
  51. Does less stiff means that vintage less clarity thing? And why do I expect as a change for swapping the coupling caps?

    ReplyDelete
  52. Hi guys. I have a AC15C1.
    What modification to make,
    to increase gain (make more distortion) of TB channel?

    ReplyDelete
    Replies
    1. To be honest.. The C1 might be quite hard to mod

      Delete
  53. I wouldn't mod a C1, just use a pedal for more gain.

    ReplyDelete
  54. This comment has been removed by a blog administrator.

    ReplyDelete
  55. hi, love your ac15HW mod page... lot of great mods here...
    i have a problem with mine: when i switch on the master volume, there's a nasty distorted, cutting out sound, i suspected the 200K pot, changed for a fresh one, but the distortion still occurs...
    could it be the switch? or something else?? all ideas are welcome...
    i love this amp, but it's a shame the master vol is useless on mine...
    Ben

    ReplyDelete
  56. Its hard to say without having a look at it but it could be the switch. You could solder the two wires together that are connected to the switch and if the MV works ok it would prove the switch is faulty.

    ReplyDelete
  57. i did try to bypass the switch, the problem still occurs...
    distortion, cutting out sound, come back, lower volume, cut out etc... and without the master volume, perfect sound...
    very strange... i don't understand what it could be...
    Ben

    ReplyDelete
  58. I'd like to add an ef86 preamp into the quation...is there a way to do it? If so, I really just want to keep the normal channel.

    Thanks

    Alex

    I'd also like to change the light to blue...ha

    ReplyDelete
  59. You could change the circuit to use an ef86. You would have t look at the older AC15 circuits and copy it.

    ReplyDelete
  60. hey Nik..
    still didn't solve my master volume problem but i gave up trying to use it, you really don't need it on this amp.. i even consider using the master vol pot and switch for something else...
    anyway, i played with the treble caps, my final choice is 47pF for the treble boost chanel, and 100pF for the normal channel... it remove some extra brightness.. love it... i must say i use a telecaster... ;)
    Ben

    ReplyDelete
    Replies
    1. Ben, did you try changing any of the tubes ? The MV will load the PI a bit maybe its that, could be worth swapping th etudes about if you don't have any spares.

      Delete
  61. Can anyone tell me what power taps there are on the mains transformer. I have a Japanese ac15hw1 (100v) but now live in Taiwan (110v). I guess the 120v tap would be better.

    ReplyDelete
  62. There are 4 different part numbers for the mains transformer so you may be out of luck. I can't check my AC15 now but will have access to it at the end of the week.

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  63. Thanks Nik, I did guess that something like that maybe the case. As I am originally from the UK I may just bite the bullet and grab a drop in MM replacement so I can be more flexible in the future. Cheers Lee.

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  64. Checked my AC15HW today, no extra primary taps on the PT.

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  65. How does the amp sound with what your currently running vs your original mods?

    Are these mods based off a jmi era ac15 circuit?

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  66. The bright switch on the Normal channel is more effective with R17 reduced 120K instead of being shorted out. Making C27 smaller stopped the TB channel getting too flabby in the low end. It is based loosely on the JMI specs.

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  67. These changes are very subtle, I didn't actually set out to change the values in my amp. I sold my original amp and then ended up getting another one and just did the mods again but slightly differently. Probably not worth changing the original values except maybe R17 as thats cheap and easy and has the most effect.

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  68. Hi Nik, Apologies if this is a double post, but it didn't publish at first so I've had to retype from memory!

    Thanks for posting all of this - it's very useful info!

    I just bought a used AC15Hw1 from eBay. I don't think it's been played much at all. The speaker is still pretty stiff and even the paper labels on the valves look brand new and free from discolouration. I've just finished a weekend of gigs so it's been working hard for 12 hours and the speaker sounds better already.

    I love the clean tone and the glassy top end, but there's a spikiness in the upper mids that becomes crashy with any kind of natural overdrive from the preamp. It's fine with a decent drive sound in front of it, but even so on the TB channel I'm rolling the treble back to 3 o'clock and the bass is pushed to 9 o'clock to tame the top end a bit.

    I'm looking to retain the presence, which is such a key part of what I like about the Vox tone, but I'd like to smooth out the upper mids. I'm guessing that most of the problem comes from the amps compression characteristics. the crashy distortion has no sustain - it's a bit like an old silicon fuzz pedal, in that the note fizzles out. the opposite of the creamy Brian May distortion. Even on loud clean tones there is no 'bloom' to the notes. My old Ac15C1 amp with SS rect and PCBs all over it had more bloom.

    I think these problems are all inter-related - could it be as simple as over-filtering? I've changed the Ruby preamp vaves for some Russian Tung Sols I had in my spares kit and it did smooth htings out a lot and give me a little more sustain, but still quite a way off any previous AC amp I've had.

    Thanks in advance for any help you can give.

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  69. Hi, its hard to tell without getting the amp and testing a few things. I would be careful with the Tung Sol tubes in V2 as they may not last long in that position. I found JJ's work very well in these amps. You could try changing the EL84's to something else too. The overdrive in these amps is produced predominately in the PI and power tubes so they will make the most difference to the distortion. These amps are pretty stiff sounding and reducing the filtering does help. The speakers take a long time to break in too.

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  70. Thanks Nik. I had my Dad look at it who knows his valves and circuits (he trained as a TV engineer when they only had valves and has worked in music shops too). I killed the V1 Tung Sol within a few days! I was hitting the front end with a Wampler Pinnnacle set to clean boost - it didn't like it much at all. The crashy breakup was made worse, so I'm thinking there must be a component fault somewhere. We found one of the little brown ceramic disc caps was noisy if if you put your hand anywhere near is, especially when the V1 was on it's way out. He had a good poke around with a meter, but without a circuit diagram he was a bit lost. For example, the plates were reading 345v on 15w setting and 369 on 7.5w. He also thought the amp was overall way too bright - and he's seen and heard a lot of top boost Voxes through the years!

    Could you help with a circuit diagram? Also, I've seen you talk here and there about smoothing the breakup, which of the mods do you think will have the most effect on smoothing it out?

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  71. Stevie, the circuit diagrams are on this post.

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  72. Hello Nik,

    Thank you for the information and the mods.
    I am thinking to build an AC15HW based on your schematic that you will uploaded and I would like to ask you where can I find to buy the original power transformer and the output transformer.
    The transformers are the same with the AC15 or with the AC30TB?

    Thank you very much in advance.
    Nikos

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  73. Hi! I was wondering why you didn't get rid of the crappy output tranny and choke and replace those with the replica JMI types from Mercury Magnetics? Your output tranny alone is 40% of your amps' sound/tone.

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  74. Replacement transformers aren't so easy to come by outside the US. Shipping a MM tran to Europe or NZ/Aus is prohibitively expensive.

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