tag:blogger.com,1999:blog-8254144712200769850.post5527217217453110656..comments2023-02-05T23:02:49.201-08:00Comments on Valve Analogue, Tube Analog, Guitars and Stuff: Jet City 333 JCA20H HSG Mod (high saturated gain) with schematic circuit diagram.Roosvillehttp://www.blogger.com/profile/18346708485236938737noreply@blogger.comBlogger36125tag:blogger.com,1999:blog-8254144712200769850.post-87214534837184686922020-03-30T07:46:06.657-07:002020-03-30T07:46:06.657-07:00I just completed this mod, including the NFB bypas...I just completed this mod, including the NFB bypass cap, though not including the cathode bypass cap C-1, as I plan to be adding a depth control later, so want to keep things tight.<br /><br />I must say, it sounds fantastic! With the gain stages running more evenly, it seems to compress less on the highs at higher settings on the preamp volume. Overall far more balanced, and with more usable gain to play with. The presence control is also now usable, and the amp at the same settings is significantly less harsh, without losing any real top end definition.<br /><br />Thanks a lot for this! When I get a chance, and I've fitted the depth control and an FX loop, I'll probably get some tidy sound samples done. post some samples hereColmhttps://www.blogger.com/profile/03146344495951679740noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-46970949507846037242019-09-25T12:08:25.194-07:002019-09-25T12:08:25.194-07:00An intrinsic aspect with owning a demand appliance...An intrinsic aspect with owning a demand appliance is usually right repair in addition to health care. Furthermore, it suggests inspecting ones demand appliance normally to guarantee it truly is doing the job adequately. Areas suggestions about what exactly to take into consideration as soon as reviewing ones demand appliance intended for complications. <a href="https://pressurewasherguides.com/how-to-adjust-unloader-valve-on-pressure-washer/" rel="nofollow">Power Washer</a><br />John Doehttps://www.blogger.com/profile/08723000039778468723noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-800210147832316502017-11-04T01:28:23.275-07:002017-11-04T01:28:23.275-07:00will this mod get the amp into a more metal territ...will this mod get the amp into a more metal territory?Anonymoushttps://www.blogger.com/profile/11382938291768870132noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-74269454972643419582014-08-25T07:40:38.932-07:002014-08-25T07:40:38.932-07:00P.S.: I meant the VALVE WIZARD site, when I said ...P.S.: I meant the VALVE WIZARD site, when I said "-Master", earlier!<br /><br />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.Anonymoushttps://www.blogger.com/profile/11168093757771523412noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-31976202844712129452014-08-25T07:25:38.079-07:002014-08-25T07:25:38.079-07:00Thank you for the response.
Actually I had alread...Thank you for the response.<br /><br />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.<br /><br />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.<br /><br />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).<br /><br />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).<br /><br />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.?<br /><br />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?<br /><br />Thank you.Anonymoushttps://www.blogger.com/profile/11168093757771523412noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-60523366697819738012014-07-17T01:04:52.154-07:002014-07-17T01:04:52.154-07:00Felix. I like the wooliness to a valve amp which i...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.Roosvillehttps://www.blogger.com/profile/18346708485236938737noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-57102694793987516802014-07-16T07:52:43.441-07:002014-07-16T07:52:43.441-07:00I see I wasn't clear earlier. I used a 220pF c...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).Anonymoushttps://www.blogger.com/profile/11168093757771523412noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-91898157089428189952014-07-16T07:47:43.098-07:002014-07-16T07:47:43.098-07:00Hi Roosville.
I love your blog. To me it is right...Hi Roosville.<br /><br />I love your blog. To me it is right up there with Aiken Amps' and the Valve Masters' sites.<br /><br />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?<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />Any help you could give me would be appreciated.Anonymoushttps://www.blogger.com/profile/11168093757771523412noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-80686562691359301152014-04-17T08:10:27.322-07:002014-04-17T08:10:27.322-07:00Hi, if I want to mod the slope resistor, which res...Hi, if I want to mod the slope resistor, which resistor should I replaced, R19, R20 or R31?<br />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?<br />Thanks a lot.Sidneyhttps://www.blogger.com/profile/13327846071816657875noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-19705061662074606852013-07-11T03:21:48.393-07:002013-07-11T03:21:48.393-07:00Hello Roosville,
My mistake!! I was looking at th...Hello Roosville,<br /><br />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 ;-).<br />Thanks for the fast reply, am about to order the parts needed. Anonymoushttps://www.blogger.com/profile/09104303260223055697noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-749215954945940132013-07-11T01:06:26.214-07:002013-07-11T01:06:26.214-07:00Hi,
Thanks for taking the time to read and comme...Hi, <br /><br />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.<br />Roosvillehttps://www.blogger.com/profile/18346708485236938737noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-67977115115529352502013-07-10T23:07:25.582-07:002013-07-10T23:07:25.582-07:00Hello Roosville,
First i want to thank you for yo...Hello Roosville,<br /><br />First i want to thank you for your expertice and the gesture of sharing this with all of us.<br /><br />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!<br /><br />Just one small question, you mention a change in value for C3 and R3...<br />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)"<br />Shouldn't that be C5 en R5 in your schematic?? Anonymoushttps://www.blogger.com/profile/09104303260223055697noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-1692806411609733892013-06-13T01:08:15.514-07:002013-06-13T01:08:15.514-07:00In the Jan 2012 entry “EL84 Triode / Pentode Switc...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. Roosvillehttps://www.blogger.com/profile/18346708485236938737noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-13380255294255279012013-06-12T08:40:38.824-07:002013-06-12T08:40:38.824-07:00i cannot find details on the depth mod , do you ha...i cannot find details on the depth mod , do you have any links to instructions on it?<br /><br /><br />thanksAnonymoushttps://www.blogger.com/profile/06619701303140483592noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-44035379725941495432013-06-07T01:19:01.060-07:002013-06-07T01:19:01.060-07:00Hi, Thanks for commenting. It's good to have h...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.Roosvillehttps://www.blogger.com/profile/18346708485236938737noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-24591615683677701082013-05-31T23:07:37.921-07:002013-05-31T23:07:37.921-07:00WOW I JUST DID THIS MOD AND MAN WHAT A DIFFERENCE ...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 !<br /><br />thanks<br /><br />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 , <br /><br /><br />thanks<br /><br />Anonymoushttps://www.blogger.com/profile/06619701303140483592noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-51569306409435674192012-04-23T13:32:06.122-07:002012-04-23T13:32:06.122-07:00Have a friend of mine which is sound engineer that...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 ;)Torses poilushttps://www.blogger.com/profile/17827152579933743093noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-9534391358279073382012-04-23T13:15:56.992-07:002012-04-23T13:15:56.992-07:00Torses. I am really grateful that you have taken t...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.Roosvillehttps://www.blogger.com/profile/18346708485236938737noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-19796612653665362112012-04-20T14:44:20.180-07:002012-04-20T14:44:20.180-07:00here's a sample at full gain, hsg mod kill. I&...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...<br /><br />Sound sample @ full gain : https://docs.google.com/file/d/0B4OAYlhxVIIyX2c4Y2pRYlVkUjg/editTorses poilushttps://www.blogger.com/profile/17827152579933743093noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-15096219138638187982012-04-17T14:52:24.433-07:002012-04-17T14:52:24.433-07:00A new post of interogation, i've hum since out...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<br />Thx by advanceTorses poilushttps://www.blogger.com/profile/17827152579933743093noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-62919971627660010782012-04-17T07:15:30.466-07:002012-04-17T07:15:30.466-07:00i've forgot but i've made it, the 330pf fo...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 GigTorses poilushttps://www.blogger.com/profile/17827152579933743093noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-63457479513502737792012-04-17T07:09:44.297-07:002012-04-17T07:09:44.297-07:00That was a potentiometer issue, no problem now. i ...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.<br />I'll give other feedback after trying on my real cab, for now i've try on my combo speakersTorses poilushttps://www.blogger.com/profile/17827152579933743093noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-71650068828617140072012-04-17T04:41:29.295-07:002012-04-17T04:41:29.295-07:00I think you have an issue with the variator (I ass...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. <br />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.Roosvillehttps://www.blogger.com/profile/18346708485236938737noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-63016830893255257042012-04-17T03:15:47.826-07:002012-04-17T03:15:47.826-07:00i finish the mod without the c1 because can't ...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.Torses poilushttps://www.blogger.com/profile/17827152579933743093noreply@blogger.comtag:blogger.com,1999:blog-8254144712200769850.post-72527092512673334052012-04-17T02:10:37.107-07:002012-04-17T02:10:37.107-07:00Torses, You don't need to worry about the neon...Torses, You don't need to worry about the neon. Please look on :-<br /><br />http://www.freewebs.com/valvewizard1/dccf.htm<br /> <br />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.<br /><br />All the very best.Roosvillehttps://www.blogger.com/profile/18346708485236938737noreply@blogger.com