Thursday, 19 January 2012

Jet City 333 JCA20H HSG Mod (high saturated gain) with schematic circuit diagram.

There have been a few threads on how to get  a full high gain no-holds-barred design from the Jet City JCA20H amplifier.

I find this topology does a good job of going clean-ish when the volume from the guitar is backed off nearly all the way but has more than enough overdrive to use with my strat. I have a bleed cap fitted on the guitars volume BTW.

Compared with the original first three gain stages setup of x69  x23.6 x59 the individual gains are now x55.7 x47.6 and x54.6 so no valve is pushed too hard to spec. What has happened is a attenuation between stages has been removed and the treble voicing moved to after the tone stack. If we had infinite supply voltage and valves that didn't arc then the original cascaded gain to the start of the tone stack was around a factor of 12.5 thousand, with the modifications it would be around 118 thousand.

The mods are presented as achievable without cutting tracks or removing the PCB from the chassis. Simple to try and simple to put back. From the official schematic here are the changes made.

C27, 1uF. Fit an additional 1uF in parallel to make it 2uF.
R28, 220k. Fit an additional 220K in parallel to make 110K.
C26, 20nF. Fit a 22nF 400V cap in parallel to make 42nF in total.
R25, 2K2 fit a 1uF 25V cap  in parallel.
R53 1Meg. Sort circuit this part
Gain Pot 1Meg Log. Fit a 62K or 68K resistor across the outer track connections.
R53 to Wiper of Gain pot. 1.8Meg and 120pF in series (was100k and 1nF)
C23, 470pF Short circuit this part.
R14 470K Wire 220pF and 220K in series to restore some of the treble lost shorting out C23.
R12 47k (NEG Feedback Resistor, Power Amp section) Fit 330pF in parallel to quash harshness.

And two mods from The Valve Wizard Pages
R20 100k. Fit an additional 100K in parallel to make 50K
I also fitted a neon between pins 2 and 3 as protection.

The amp also has a depth or resonance modification fitted which warms the amp in the correct place.It also has a triode switch which mellows the tone and an unbalanced phase splitter to allow 2nd Harmonic distortion in the output stage.I'll detail the output stage modifications in another post.


I found that the treble bleed at the gain pot helped establish a usable clean sound lifting the mid to treble but not the bass which often pushes the stage into grit. For now I have fitted R-3 as 100k and C-3 as 1nF. However this may change as it is still work in progress. (UPDATE: Fitted R-3 = 1.8Meg and C-3 = 120pF as final value)

Below is the response at the cathode follower with the gain pot set to about 2. The lowest plot is without a bleed modification. The green upper plot is 100k and 1nF which gives a 12dB lift beginning at the lower mid frequency. The intermediate red trace is 100k and 220pf.



33 comments:

  1. What is that "opt 220uf" paralled at c28?
    Would I be correct to assume that this would also require an electrolytic same as c28? Hmmm??? I just so happen to have an expensive and nice 650v snap-in stand up cap @ 220uf that looks real cool and needs to find an application!!!???
    Actually have two of them just dying for a cool place to reside!
    Hmmmm???? Is this worthy????
    What does this extra umph of capacitance add to the tone????
    Also,,, "neon" ??? I am not familiar with that component,,, is this a neon lamp? How does this provide protection? What is it protecting?
    Please be gentle,,, My electronics knowledge is very modest, but I am very inquizzative and accomplished at monkey see monkey do style of homebrewing....!!!!
    I am very thorough at researching the directions and wieghing my options!!!! I dont just do something for the sake of doing it or jumping on everyones latest bandwagon because everyone else thinks it's the greatest...
    Seems as if alot of these things take on a life of their own and alot guys are hacking up gear and believe that they really have done something great but fail to come to terms with the reality of their folly. Just listen to some of the crap on you tube!
    Anyways,,,, it seems like you got something going on here and ya seem to back it up with some trick engineering simulations on the graphs and plots.
    It sure makes it nice to have a target to aim at and some good basis for a roadmap to get there. No voodoo here,,, maybe just some crosstalk/intermod or some other kinda treble,,, uh I meant to say trouble!

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    1. Hi,

      The 220uF is 220nF (nano-farad), sorry but your 220uF/630v caps will need to wait for a different day. I had a spare 450V/220nF cap and as electrolytic capacitors have a quite low frequency resonance then I placed it in to act as a short to any RF pickup.
      The neon is exactly as you describe an indicator lamp. It has a breakdown voltage of ~60V DC (measured). The issue is that for a cathode follower if you turn on the HT before the valve heater is fully up then the valve Grid is pulled up to HT via R44 but the cathode is pulled to ground via R20 until current flows through R20 and the voltage rises on the cathode. This can cause breakdown within the valve or shorten its life. The neon limits the difference to 60V. Google valve wizard DC Cathode follower, make sure you have the DC page and not the AC follower page. He also shows the diode/resistor alternative that I have fitted to another amp.
      One objective here was to balance the gain out a bit better. The original design has the first gain stage set to about 70. This is great if your valve can support this gain, but some won’t, so now you have a valve dependence where some valves will sound better than others. The design here has the gain set to 56 which is much more achievable so your valves will all sound good. In standard form the JCA20 valve stages for V1A, V1B and V2A have the gains set to X70, X23.6 and X59. In this schematic they are X56, X48 and X54. By removing the voicing attenuation and placing it (C-5 and R-5) after the tone stack the combined gain of the HSG mod is 118 thousand compared with the stock JCA20 of 13 thousand. The trick is to retain an uncluttered gain which retains tone and doesn’t sound cr*p. Hope this helps and you have fun trying it out.

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    2. Hey Roosville,
      Thanks for the reply! Great blog! Great Kharma! I've gained much from what you have so freely given!
      After,typing my previous reply, I already took notice and navigated to the Tone Wizard site. Thanks for another great tip! Found the neon trick! It's explaind with some LED trickery too! Seems like neon lamps arent so readily avail locally. Will have to order online. I am not as concerned with the JCA20h , I understand how to properly implement the stndby switch as provided. However, I have a SE project (ala s/s recto champ) that should have this protection!
      Regarding the HSG mods ,,,,, Uh---duhhhhhh 22,0000pf!!!! OOps!!! (BLOODY HELL of what to do with those big pretty standup caps! Wishful that they were multicaps tho!)
      Is RF big prob? Understood that the world is greatly more wireless than ever. However, not only has this made for a smaller world it also demanded technology to create a faster more frequent universe. Lower frequency electromagnetic pollution must be the lowest levels in decades. Assuming that more rf is generated and radiated at much higher frequencies, with smaller harmonics. Technological changes such as digital broadcasting, spreadspectrum, faster clocking CPUs, more efficient and faster switching power circuits, solid state illumination technolgy, underground electrical distribution.... and a dead russian woodpecker(LOL?) Of course I am imagining only man made sources and excluding other sources of noise. I was unaware of the potential for RF detection. It seems justified to exercise as much prudence as can be afforded to keep the foundation for the intial signal as quiet, clean and pure as possible.
      The rationale for spreading out the gain sound logical. It seems that in the stock form, you not only are pumping up the goodness but the bad too. Beside the whole beauty of lower wattage amp is for true output distortion not preamp clipping. Why does the great Soldano engineer V1a at X70 anyways? Is it because of some sort of mojo voodoo tone or expectation of such? Or,,, more likely good ol yankee "too much is never enough???" Or is that mentality carrying over into the evolution of certain popular tone circuits? Because, "this how we've always done it" as each patented tone guru, showman and shaman passes along his magic potion recipe? Interesting is your term "valve dependence". Certainly you have also noticed on every forum, discussion of this amp begins with how greatly the tone is altered by tubes replacement within the preamp. Definetly the most consistient inconsistency! I suppose it can be good or bad. If you took the time to blueprint the inconsistiency and used it to an advantage. If nothing else, this amp and my other experiments with a SE amp have recently honed my ability to percieve sonic difference between tubes. (gotta love those telefunken! "picked em" for free = better sonics!) Hopefully I can get around to trying out your HSG mod later on in the evening.
      Please keep your blog updated on your PI experimentation regarding unbalanced output and harmonic generation.
      I dunno? just seems to go against the conventional wisdom. Been influenced to avoid that and use matched tubes too! Obviously, we are conditioned to expect 60cycle. Also reminds me about old skool "hum-control" type bias balance circuits. What if instead we call them Harmonic adjustment or better yet patent a clever name like harmonic distillation (split the royalties) everyone patenting tube circuits these days!

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    3. OK Roosville,
      I have completed all of your HSG mods with exception of the 220nf input bypass cap/rf filter, the neon protection mod and I overlooked the valve king mod on R-20 (reduced from 100k to 50K by paralleled 100k)
      I will add that later along with the Neon.
      I included the much publicized "depth pot" mod. (intial impression is that the pot can be done away with and a fixed value of personal preference installed instead.) Also included the extra 47uf filter cap on the +B and the 330pf bandpass on the feedback.
      My intial impression so far is very good! It's very close to what I would prefer. In the stock form this amp was just not to my taste and basically unusable to my preference.(Never understood the hype and popularity of this amp in its stock form)
      I purchased it because of the fact that it was a decent deal and offered alot of value for the money. The circuit is very basic and I figured it would make a decent platform to build a TMB or something.
      Your changes as engineered work very well and make this amp very versatile and much better sonically overall. The preamp gain is very usable and is more than extra generous. The preamp overdrive really sounds decent throughout its range. The poweramp cleans up well and has some headroom but easily breaks up into a pleasant amount of good distortion and generates great overtones and harmonics for a PP output.
      Unfortunately, I should have followed a more step by step approach instead of a complete wholesale change at once. In other words I changed everything all at the same time without taking note of what sonic effect was induced or most pronounced by which individual change. I also changed tubes from new JJ's to used vintage telefunken longplate 12ax7 in V-1, a vintage oem RCA 12ax7 in V-2, a new JJ 12ax7 in V3. The output tubes are likely a mismatched pair consisting of a used vintage Telefunken and a OEM Sylvania EL-34s.
      However, I suppose the end result is what truly matters the most. As it stands now, it is off the bench and in rotation in my living room lineup of gear where it will now set some use instead of collecting dust. Very much more desireable sound, with maybe just some fine tuning with the value at your "c-3" and a bit less bias!
      THANKS!!!!
      Oh yeah my style of playing is heavy/hard blues (think zep) and classic rock.

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    4. Hi again Roosville,

      I dont know how to corrospond with you via email and I feel as if I am monopolizing your blog.
      I just wanted to update that I did the final changes to include the bypass/Rf filter cap 220nf @ C-20 ,,, I did seem to perceive a small reduction in the noise floor of the amp. Maybe my imagination but it didnt hurt anything. I also added the Valve Wizard mods reducing the cathode resistor R-20 to 50K, and including the neon lamp protection, as it seems more prudent now that the resistance is lowered to half of what the grid is seeing at r44????? (the intial current flowing the path of least resistance? Correct?) I dunno.... But I do know that this amp sounds great and I really put it to task today, with a real workout, pushing it really hard, fully testing its limits and endurance.
      I JUST WANT TO THANK YOU VERY MUCH FOR TAKING THE TIME TO SHARE YOUR EXPERIENCE, KNOWLEDGE AND EXPERTISE. You have made a great difference helping me get some great tone from this amp. This amp has become so much more desirable to play through. I just cant say enough about how incredibly impressed I am!
      For what its worth,,,, my settings are as follows...
      Gain = 3-1/2, Bass = 9, Mid = 5, Treble = 7, Volume = 9, Presence = 7, DepthPot 3/4 full

      Standard Les Paul Humbucker in bridge position volume and tone at 10 the amp is very brite and crispy clean and at the edge of power amp breakup. No icepicks or brassy yuckiness. Strum agressively and very nice crisp distortion.
      Gain??? Oh my goodness! More gain than I could ever imagine. And its all usable throughout the whole span of the dial. Incredible, and I'm the type that usually dimes all the controls. Five is the new ten for me!
      THANKS THANKS AND THANKS ROOSVILLE YOU ARE A MOD GENIUS, I am so grateful for finding your blog during a google search! With so many mods published, yours is where its at!
      Good Job!!!!
      Brian aka Southbound, email: dougroundup@gmail.com

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    5. Brian,

      I am glad that I helped someone out, and pleased that you found that the amp is in fact well worth the money! Just a few notes for you though. The treble bleed on the GAIN control, if you find that you get a bit too much top with your fairly moderate setting of 3.5 then try R3 = 180k and C-3 = 220pF. It still gives a treble lift as the GAIN control is decreased but it isn’t as aggressive. For a more modern sound you can remove the C-1 _1uF cathode bypass capacitor. It will give a perceived tighter bass. However this depends on the cab and speaker. A closed cab with a speaker with a resonance of say 70Hz will sound a lot tighter than an open cab with a speaker with a resonance of 85Hz which is within the bottom E range. I prefer a bit of flab. The feedback bypass capacitor of 330pF can be reduced to 220pF if you wish. It still kills all the output harshness but allows a bit more air. This is what I currently have fitted.

      One thing I would encourage you to try is the Pentode – Triode switch. In the EL84 stage is does sound smoother. I had a Marshall JCM800 50W Jubilee years ago (EL34) which sounded the same in either mode. However the JCA20 sounds a lot nicer in Triode operation and is my preference if I don’t need the wattage. If you do try this don’t buy a DPDT switch that is too small both in terms of volts but also in dimension. A half inch spacing between adjacent switch mechanisms avoids the chance of cross pickup and unwanted instability appearing. Note that I used 1.5K resistors in lieu of the 470R fitted in the schematic. This was driven by the fact that the 1.5K were 5watts so will be safe to use (if valve fail short) but more importantly they had enough lead length to reach from the board to the switch that I mounted in the rear panel. The holes on the PCB are through plated so you can extract and fit components with a soldering iron if you don’t fancy pulling the PCB.

      Thank you for your kind comments, it’s appreciated.

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    6. PS Above I said the optional value of R-3 = 180K, I should have said 1.8Meg. It will sound a high value but it does still work, honest!

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  2. Hi Roosville, thanx for this perfect explanation mod. I plan to modify my Jet to have more Gain without using pedal, is this mod permit to have a kind of lead channel at full gain? Did you made some sound samples at full gain thanks
    Best regards

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  3. Hi Torses,

    The amp has more than enough gain which doesn't get at all messed up and muddy, it can also go clean from the volume of the guitar if you use a treble bypass on the volume pot (lots of info on google). There is a youtube clip I did, the sound is pretty dire compaired to the amp in the room but it will give you an idea. Look for JCA20H HSG mod on youtube and you shold find it. Many Thanks for taking the time to coment, I would do the Triode Switch mod too, it really makes the amp sound better if tone-tone-tone is what you are looking for.

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    1. Just have seen the youtube clip, and it sound incredibly, i'll begin the mod this week and post comment after that, Thanks to share with us, good job and continuation ;)

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  4. i've start the complete mode without neon (can't find in the near store), finish and test it tomorrow can't wait GRrrrr, i appreciate this non destructive way of modding ;), i'll let comment and maybe sound sample or video of the future beast

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    1. Torses, You don't need to worry about the neon. Please look on :-

      http://www.freewebs.com/valvewizard1/dccf.htm

      On the above link is a full explanation about cathode follower protection. The mod doesn't alter the sound it just helps to extend the life of the valve. The diode and resistor works better than a neon. It is just that I had a neon to hand and it looked a fun experiment. Just for information, if I power both power and standby together the neon will light for about 7 seconds which shows the issue and the neon giving protection. Get a diode and resistor after you have finished the mod. Depending on your cab you may wish to not include the C-1, 1uF capacitor if the bass is too loose. If it sounds poor then re-check the mod list as a mistake is very easy to make. I would really recommend a Triode switch afterwards, it gives a much nicer tone. The youtube clip was in Pentode as I hadn't finished the mod at this time.

      All the very best.

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  5. i finish the mod without the c1 because can't find 1uF @400 or more they only have 1.5 as you say it's optional i decide to not include this and it's sound great very punchy, the gain increase is terrific, exactly what i need, the tone adapt more to the way of playing, punchy to smooth, all is ok the only issue i'm experiencing is when i crank my bass variator more than 7.5 i'm loosing bass when i'm on 8.5 the bass come back and loosing it again at 9, don't know if it's the mod or a variator issue. Terrific mod anymay more of what i need without more hum, Thank you very much to share.

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    1. I think you have an issue with the variator (I assume potentiometer in English) The bass is linear with the control. It could be some cleaning fluid may restore correct operation. The C-1 1uF is a low voltage (25V or above) and is not a high voltage device. The optional C-OPT is 220nF / 400V and probably contributes very little. I hope this means that the amplifier is of more use to you now.
      If correct you should get a lot of sustain but without it becoming indistinct. Please try the harshness mod from my separate post and the Triode mod if you do not need the full power output. I did note that the diagram doesn’t highlight C-4 in red and I was worried that you wouldn’t fit it. It has a big impact. Thanks for taking the time to give feedback, it is appreciated.

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  6. That was a potentiometer issue, no problem now. i made an error in my last message, i've put the c-4 (1uF 50v) and don't put the c-opt and c-1 "for more modern sound" as you said in previous comment. I'll give a try to Harshness mod.
    I'll give other feedback after trying on my real cab, for now i've try on my combo speakers

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    1. i've forgot but i've made it, the 330pf for R12. Now i'll certainly fit a fx loop to insert my noise suppressor to prevent hum on Gig

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  7. A new post of interogation, i've hum since out of box, no more hum with your mod... when i put more than 2 on master and more than 5 on gain, i've constant hum appearing with or without guitar plugged in, is this issue can be modded? i've change the preamp tubes to JJ and put a noise gate between the guitar and the head but it's problematic on gig at full gain and master at 4. I have read that a choke can decrease a little this hum... Have you experience this, do you have an idea of what can be done? i was thinking to fit an fx loop but after few search i realize that it's expensive for a cheap amp like this one
    Thx by advance

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  8. here's a sample at full gain, hsg mod kill. I'm not a sound engineer or good guitar player so be clement. The sound is better but have to take some sound recording lessons...

    Sound sample @ full gain : https://docs.google.com/file/d/0B4OAYlhxVIIyX2c4Y2pRYlVkUjg/edit

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    1. Torses. I am really grateful that you have taken the time to post a sample. I agree that it's difficult to get a decent recording without proper tools. One thing I do find is that the tone controls are quite sensitive, I have since spent time adjusting the tone and resonance control balance and found it time well spent. I hope you continue with the JCA20 sounds like it's going well so far.

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  9. Have a friend of mine which is sound engineer that come tomorrow to hear the change and give me some tips for a better recording ;)

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  10. WOW I JUST DID THIS MOD AND MAN WHAT A DIFFERENCE I bought this amp based on reviews and etc, and although it sounded ok , it was kinda thin and lacking gain on the lead , but after your mod wow , the leads are sweet , gainy and thick and when i lower the gain it evens cleans up nicely !

    thanks

    now if i just figure out how to get a line level out of it to feed another power amp and effects, maybe ill tap off the master vol pot , either that or if i can find an effects loop mod and use the send ,


    thanks

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    1. Hi, Thanks for commenting. It's good to have helped someone else with this amp. I would suggest you also include a depth (resonance) modification in your amp. The details are freely available on the net but also included on the pentode / triode switch schematic here.

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  11. i cannot find details on the depth mod , do you have any links to instructions on it?


    thanks

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  12. In the Jan 2012 entry “EL84 Triode / Pentode Switch” there is a schematic at the bottom. It will show a pot and capacitor inserted in the feedback loop AKA depth mod. This will add bass and also change the interaction between the amplifier and speaker (hence resonance control). The pot is a 1Meg Log and the capacitor is 4.7nF. The feedback tap wire is moved (Yellow wire) from the 16R output jack to the 8R output jack. If you wish to simplify just use a 330k/360k resistor and not a pot. I removed one speaker jack and used the hole for the pot. Note; there is also a capacitor that I place across the feedback resistor, use a 220pF the original diagram was 330pF. It counters a treble peak due to the less than ideal K factor in the output transformer. Some people find the sound a little harsh and this allows you to better control the treble with the tone control. Google JCA20H depth mod and there are plenty of images and advice.

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  13. Hello Roosville,

    First i want to thank you for your expertice and the gesture of sharing this with all of us.

    Was looking to build an amp and after long hours of searching on the web i found your schematic. This is what i was looking for!

    Just one small question, you mention a change in value for C3 and R3...
    quote "I found that the treble bleed at the gain pot helped establish a usable clean sound lifting the mid to treble but not the bass which often pushes the stage into grit. For now I have fitted R-3 as 100k and C-3 as 1nF. However this may change as it is still work in progress. (UPDATE: Fitted R-3 = 1.8Meg and C-3 = 120pF as final value)"
    Shouldn't that be C5 en R5 in your schematic??

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    1. Hi,

      Thanks for taking the time to read and comment. It really is appreciated. The text is correct. R-3 and C-3 allow more treble through as the gain control is lowered. With the Gain at 10 they have no effect. The network C-5 and R-5 form a fixed treble lift usually termed “voicing” by a lot of people. The schematic was a way of changing a stock Jet City 20W without needing to remove the board or any components. The idea was you could very quickly do the mod, and return to stock if it wasn’t for you. To get the gain I required I needed to remove some of the attenuation fitted between the input and the V2 valve where all the clipping takes place. The “Short C23” takes out attenuation but also the treble lift that C23 provides. The way round this was to fit C-5 and R-5 which replicates this voicing after the clipping stage. There is no requirement to re-equalise as the master volume (1 Meg log) is changed. It is a lot easier to work with a three valve pre-amp than a two valve, but I wanted the three valve gain so this was the compromise that I found worked. As another note, depending on your guitar and pickup combination C-1 can be left out if the bottom end is found to be a bit flabby. Many Thanks.

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  14. Hello Roosville,

    My mistake!! I was looking at the output stage and was confused with the C3 and R3 over there. Couldn't understand that fiddling with those had effect on the clean sound... oeps...bad reading of your great comment ;-).
    Thanks for the fast reply, am about to order the parts needed.

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  15. Hi, if I want to mod the slope resistor, which resistor should I replaced, R19, R20 or R31?
    Also, for the choke mod, it is not easy to find the hammond 156m in my city, any ideas for the choke I should use to get tue right set?
    Thanks a lot.

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  16. Hi Roosville.

    I love your blog. To me it is right up there with Aiken Amps' and the Valve Masters' sites.

    I tried the NFB OP Harshness mod, but tried it with a .1uF cap (not across a resistor) inline with it to kill any stray DC (the switch for 16 ohm/no feedback/8 ohm pops and I wanted to keep the depth pot from crackling; it doesn't crackle now though; I suppose the switch popping is b/c of arcing, but IDK). Could this .1uF cap have killed my treble?

    I any case, the amp sounded dead in the treble range, so I cut both the 220pF cap and the .1uF cap out. I may reattach the 220pF cap at a later time. However, that won't help when I am running without NFB, so I am looking at the 120pF cap over/between the anodes of the P.I. tube; 120pF there has a definite effect upon the tonality of the amp, though the actual corner frequency and thus the frequency band which is totally eliminated is high enough to be inaudible to most humans, I gather. I understand that the slope starts low enough that there is a gradual increase of attenuation from little at low frequencies, to more as the frequency climbs (almost linearly AFAICT), until the corner frequency, at which point the oppositional nature of the P.I. tube eliminates those frequencies.

    Could you comment upon alternate methods of smoothing a high-frequency peak without affecting the lower frequencies? I have performed several mods which make my amp fairly bassy (a 33k resistor subbed for the stock 47K at R21 for a less lossy tone stack in the lower registers, primarily), so I need a solution for that high-up crispy/crunchy "screaming baby/breaking glass" sound ("fizz?") - distortion, but not _pleasing_ distortion.

    I am also looking for a mod which will allow hi-freq transients some room to express themselves, but will smooth them and warm them up, if that makes sense.... sort of like a tube does with hi-amp signals: to a large degree they are compressed; they are attenuated rather than cut off, and rendered less harsh by this process, If I am understanding what I am reading.

    Also I need a way to increase the treble component of my amp so that it is more balanced than bassy. I am looking at coupling caps, but could use some pointers.

    Any help you could give me would be appreciated.

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    1. I see I wasn't clear earlier. I used a 220pF cap across the 47K NFB resistor AND I put a .1uF cap inline with it. When I cut them out, I cut the 220pF cap first, then cut out the .1uF cap. Both helped the tonal balance (when I cut them out).

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  17. Felix. I like the wooliness to a valve amp which is why I have placed some extra cathode capacitance at the first stage (total 2uF). If you wish to tighten up the bass then I would suggest that as a first step you look at R29 (1k8) and reduce the capacitance here. Try a total capacitance of 0.68uF first. This will help to rebalance the bass but also modify the compression character. The reduction of low frequency should allow the high frequency dynamics a tad more room. The SloClone forum is a very good place for this type of work. If you are not already on the forum you may find it a good place to sign up to. WRT the 220pF. Try 120pF by itself if you end up with not-enough cut. It is a ballancing act. One issue with the JCA20 is the output transformer. The coupling factor K which is basically measuring the losses varys. If the K value drops it can lead to a high frequency peaking, hence reports of shrillness by some people. I don’t sign up to the fact that it is good simply because it was specified by M.Soldano. It is made in China, unless you sit and watch it being made anything can happen. This alone makes it impossible for a one-size-fits for the treble.

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    1. Thank you for the response.

      Actually I had already put a .47 uF cap there (across the R29/ stage 1 Rk / as the Rkbp)! -So we were thinking along the same lines; that's gratifying. I wasn't aware of the bass being "tighter", but I see that the new frequency profile does modify the overall character of the sound in that direction, now that you have pointed it out.

      A .47 uF Rkbp is Very bright but I actually like it quite a lot. -I have reduced the cathode resistor there to 888 ohms, too, so it is nicely warm and bright at the same time.

      In general I have followed your strategy re the "HSG" mod: keep gain moderate at each stage; don't stress out any one tube for its "tone" (tortured tubes screaming, maybe? -Ha). -However, I have modified my own JCA20H with distortion in mind: the "interstage attenuators"/superfluous voltage dividers (such as R14 and R53) are now gone (jumpered - and I've dropped R36 to 68K, just as a grid stopper; the rest of its resistance I saw as limiting gain unnecessarily); ground refs are now uniformly 1Mohm.) What I have found, using the "all valves in moderation" strategy, is that the distortion seems to be more "pure", if that makes sense (especially as regards replacing the stage 1 Ra with a reasonable value; 220K seems to me to be a poor -and puzzling- choice): if it were an analog synth I would say that there was less noise modulating the signal, but I'm not sure how much this is just my own experience coloring my perception. It *seems* to make sense, however: if there is extra noise on the first tube it may well seem to be a sort of distortion, of a textural, fizzy sort - exactly how the JCA20H's tone is described most often (in the literature I have seen, at least).

      For now I have removed the "treble harshness" cap in the NFB loop (I run without NFB most often in any case). -If it isn't broken... right? -And I do like the amp's treble character (now, anyway - the harshness in the upper registers seemed to go away with the replacement of that 220K Ra on stage one!) - though I have the hearing of a Cocker Spaniel, & "silent" dog whistles disappointed me greatly by not being silent, lol- so I am going to leave the NFB loop mostly as it stands, perhaps adjusting the R12 value (though I do have it switchable between the 16- and the 8-ohm tranny taps for NFB level changes already, as I found the resonance pot I had installed (as well as the 16-ohm/lifted-NFB/8-ohm switch) to be rather redundant).

      Speaking of transformers, I understand that the JCA20's OPT runs about half the plate-to-plate impedance spec'd for a set of EL-84s... so I run the speaker out of the 8-ohm tap, for just about the spec'd impedance. -Correct and O.K.?

      And, if you would allow me to impose further upon your good nature, would you mind commenting upon the tonal effects of bypassing a large-value filter cap with a smaller-value cap - for example, bypassing a 100uF cap (with its firm bass but poor treble performance) with a 10uF cap (which is presumably better at treble response but with terrible, flubby bass performance) - can a balance be achieved? -That is, can there be a combination which yields firm bass and crisp treble with a clear and present midrange - a "best of all worlds" cap combo?

      Thank you.

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    2. P.S.: I meant the VALVE WIZARD site, when I said "-Master", earlier!

      I'm a tyro, obviously, though I have learned a lot from the 3 sites I mentioned (one of them being this one). Again, I appreciate being given the opportunity to educate myself.

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