Members SciHi Posted February 26, 2011 Members Share Posted February 26, 2011 My heads mods are heading a little different direction from the combo, in addition to the preamp mods, I added a 150R sag resistor (no choke), an additional power node for the PI (2.7k dropping resistor), upgraded the screen node cap to 33uF, and decrease the preamp dropping resistors to 4.7k. I biased the PI with conventional EL84 values, R18 = 820R and R16 = 47k, and reduced the amount of nfb by increasing R12 to 68k (on 8 ohm tap). These mods have really improved the power amp for my use. I still have a few more changes that I plan on doing, increasing the screen resistors (R5 and R7), 1-2k range, increasing the grid resistors (R6 and R48) for EL84 (minimum of 10k), and trying out 470k's on the PI for R17 and R19. With the PI gain reduced, I increased R13 to 390k for more preamp signal. The combination of reducing the PI gain and nfb provides much more life to the power amp (IMO). Link to comment Share on other sites More sharing options...
Members turnituploud Posted February 26, 2011 Members Share Posted February 26, 2011 Originally Posted by SciHi My heads mods are heading a little different direction from the combo, in addition to the preamp mods, I added a 150R sag resistor (no choke), an additional power node for the PI (2.7k dropping resistor), upgraded the screen node cap to 33uF, and decrease the preamp dropping resistors to 4.7k. I biased the PI with conventional EL84 values, R18 = 820R and R16 = 47k, and reduced the amount of nfb by increasing R12 to 68k (on 8 ohm tap but used for 16 ohm speaker). These mods have really improved the power amp for my use. I still have a few more changes that I plan on doing, increasing the screen resistors (R5 and R7), 1-2k range, increasing the grid resistors (R6 and R48) for EL84 (minimum of 10k), and trying out 470k's on the PI for R17 and R19. With the PI gain reduced, I increased R13 to 390k for more preamp signal. The combination of reducing the PI gain and nfb provides much more life to the power amp (IMO). I have been modding the crap out of my combo, and in the beginning i was increasing the gain(in preamp) to areyouserious? amounts. Then i completely removed the reverb circuit, since i knew i was going to install an effects loop(tried a bunch, they mostly sound good, but had other problems in the preamp so i removed it temporarily) and have delay in there, so there was no need for reverb, especially before delay. That got the amp sounding gainier and dirtier, and kind of more stable and dynamic. But then i fuked up somewhere, so because of that and curiosity i just simply copied a jcm800 preamp with a few mods into my amp. Holy crap, what tone! I have really lost a lot of gain, and the amount of it is really low now so im going to mod it again for gain, just moderate amounts while preserving tone now. I've also increased the nfb resistor to 68k, then to 100k. This is where the thinness and harsh top end is at imo.One thing i havent found anywhere is what is this r13/r14 voltage divider combo for? Is it just to attenuate the signal a bit before the master volume? Why do so? The jcm didnt have that and most of the amps i saw didnt also, so i didnt implement that combo. Oh, and whats that sag resistor for? Link to comment Share on other sites More sharing options...
Members SciHi Posted February 27, 2011 Members Share Posted February 27, 2011 Originally Posted by turnituploud ...One thing i havent found anywhere is what is this r13/r14 voltage divider combo for? Is it just to attenuate the signal a bit before the master volume? Why do so? The jcm didnt have that and most of the amps i saw didnt also, so i didnt implement that combo. Oh, and whats that sag resistor for? Most amp designs just use a 500k pot for the master volume but my guess is with the moderate gain amp and the wrong PI values, the signal needed attenuate a significant amount before it hit the PI. If you fix the values on the PI, and/or reduce the preamp gain, I found you need more gain from the preamp. I just adjusted the value of R13. But theoretically, with a JCM800 style preamp and correct PI values, you should be able to remove R14 and just put a 1M in R13. I might do it one step at a time, first change R13 to 1M and see what that gives. If you don't reduce the PI gain, then you might to to have R13 470k (I am using a 390k). Also, depends on how much nfb you keep, I reduced the nfb and that gave more signal to the power tubes (and life to the amp). I found I had more usable clean-ish volume when I reduce the voltage divider, but I can't dime both gain and master volume now without insane feedback... Increasing the el84 grid stoppers to at least 10k should help, what does Vox use? (AC15/AC30). The sag resistor is to give the amp "tube rectifier" feel. If you want the power amp totally stiff, you wouldn't do that mod. I pulled the output sides of the two rectifier diodes and inserted a 150R 5watt resistor... Link to comment Share on other sites More sharing options...
Members turnituploud Posted February 27, 2011 Members Share Posted February 27, 2011 Originally Posted by SciHi Most amp designs just use a 500k pot for the master volume but my guess is with the moderate gain amp and the wrong PI values, the signal needed attenuate a significant amount before it hit the PI. If you fix the values on the PI, and/or reduce the preamp gain, I found you need more gain from the preamp. I just adjusted the value of R13. But theoretically, with a JCM800 style preamp and correct PI values, you should be able to remove R14 and just put a 1M in R13. I might do it one step at a time, first change R13 to 1M and see what that gives. If you don't reduce the PI gain, then you might to to have R13 470k (I am using a 390k). Also, depends on how much nfb you keep, I reduced the nfb and that gave more signal to the power tubes (and life to the amp). I found I had more usable clean-ish volume when I reduce the voltage divider, but I can't dime both gain and master volume now without insane feedback... Increasing the el84 grid stoppers to at least 10k should help, what does Vox use? (AC15/AC30). The sag resistor is to give the amp "tube rectifier" feel. If you want the power amp totally stiff, you wouldn't do that mod. I pulled the output sides of the two rectifier diodes and inserted a 150R 5watt resistor... Well i was going to do some power amp mods today, try some of yours and others, but got into modding the preamp further. Actually, lol, i have been modding like 7 hours straight now. Guess i'll do them tomorrow.So i tried fiddling around with r13, r14 and pi gain, and went from 180k/220k plate load on the phase inverter(yeah, that much) do the stock 82k/100k. As for r14 i tried 100k, 220k, but didnt like it much, it was kind of holding back the attack of the amp(you dont want to hold a jcm on a leash, they are made for being punchy and raw). I'll try 1m r13 tomorrow.I increased the nfb resistor to 100k, and left it there. Like i said, it kind of cured the thinness of the amp on the top strings, and made is sound hairier. Didnt try 150k, and dont think i want to now. Oh, and thats all obviously for overdrive, dont play clean much(like at all, just for fun on occasions), but while i was playing around with bright caps and treble peaking circuits, i tried the clean(gainBut man, this amp kills now. My to do list now consists of finishing the preamp(well its pretty much finished), playing around with the power amp(based around your mods), installing a PPIMV, installing an effects loop(in the template left from the reverb circuit), and in the end resoldering it all so it lasts(if i were to shake the amp, there would be like 5-10 components that would connect to each other). Man this all is so fun, after i do a clean boost pedal, i would probably make my own amp.And btw, did you ever have a problem when even if the master is on 0 you still have some amplification going? It was really bad, like it was ~0,5-1 on master volume before, but really scooped and fizzy, just like only the preamp distortion was amplified, and after i did some mods and most notably used better shielded wire(still some random wire with thick isolation, but better than the previous, cause the previous would shock me if i touched the isolation) it got better. I pretty much cant hear it now because of the guitar itself ringing, but its there. So is it wire or something else? And what type of wire do you/ should i use for internal connections? Link to comment Share on other sites More sharing options...
Members 60bass Posted February 27, 2011 Members Share Posted February 27, 2011 Hi I'm a noob on this forum, but been playing for years. Hoping somebody here can help answer a few questions regarding my new Jet City jc2112rc combo? First off let me say that I love the tone and build quality on the amp, but there some noise issues I need advice about. First I'm getting a noticeable hum on the amp. Not unbearably loud but it's always present I'm guessing that some hum might be normal with the AC filiments in this design, but mine seems louder than I've heard from other amps. Any thoughts on this? Can I convert it over to DC filiments, or possibly beef the filter capacitors to reduce or eliminate the hum? Second More importantly, I'm also having issues with buzzing similar to ground loop buzz. This is very noticeable and only gets worse with any gain or volume set past 5. Adding any highs, mids, or presence along with boosting the gain and the amp becomes IMO unplayable. I'm wondering if it's a grounding issue with the amp, or possibly crosstalk between the heater traces and some other trace on the PCB, or the placement of the reverb transformer on the PCB? Again any suggestions? At first I thought the buzzing might be from a bad tube, but I changed the pre amp and power tubes out with "known good" JJ's I had here. No change still had buzzing. I also ran it with and without the reverb plugged in and visually checked the reverb tank to make sure there wasn't an issue with that and it was fine. Also made sure the ground lug on the chassis was nice and tight. I know this is a high gain amp and some noise is to be expected. I can live with a little hiss but not that hum and buzzing. I can't believe the amount of noise I'm hearing would be considered normal. Any ideas or suggestions? Anyone worked with the combo version, or own one that's noisy? You're help is greatly appreciated. I can ship it back to MF but I'm on the fence as far as getting another combo as I'd hate to get another one and have the same issues. I'm open to keep this one just to not have the hassle of shipping it back so if anyone can help me get it quieted down with mods, I'd be happy. Thanks Link to comment Share on other sites More sharing options...
Members SciHi Posted February 28, 2011 Members Share Posted February 28, 2011 The combo has some hiss, relative to the amount of gain, but I won't say it had loud hum. I guess I would notify MF, report it as defective, and they will pay for shipping back. I have both a head and combo, and the head is much more mod friendly, I really didn't like either in stock form. The price was right, but I think you need to plan on doing significant mods to the circuit, check out "turnituploud," sounds like he pretty much did a total overhaul. I just did mods to the stock setup. I think you can get it sounding good leaving the primary circuit intact, just changing out resistors and capacitor values, well probably most of the resistors, and a few caps... You can totally pull the reverb tube and the amp still works, see if that does anything. Link to comment Share on other sites More sharing options...
Members SciHi Posted February 28, 2011 Members Share Posted February 28, 2011 Originally Posted by turnituploud ... And what type of wire do you/ should i use for internal connections? The type of wire I use depends on the use. Jumpers on the pcb, I usually just run regular wire. Input, tone control, or long or complicated grid routing, then I usually use shield cable. But it does depend on how strong the signal is, input or weaker signal early in the preamp more prone to noisy. High voltage (plate), flying sockets, etc, just normal wire, twisted pairs if applicable. Link to comment Share on other sites More sharing options...
Members Edge11 Posted February 28, 2011 Author Members Share Posted February 28, 2011 use 18-20 gauge hookup wire. Link to comment Share on other sites More sharing options...
Members rock_mumbles Posted February 28, 2011 Members Share Posted February 28, 2011 @60bass Since you swapped tubes and pulled the reverb tube ... it must be something in the circuit probably a bad solder on a ground ... I have a stock 2112 combo and I didn't notice any excessive noise in mine Link to comment Share on other sites More sharing options...
Members turnituploud Posted March 1, 2011 Members Share Posted March 1, 2011 Originally Posted by SciHi My heads mods are heading a little different direction from the combo, in addition to the preamp mods, I added a 150R sag resistor (no choke), an additional power node for the PI (2.7k dropping resistor), upgraded the screen node cap to 33uF, and decrease the preamp dropping resistors to 4.7k. I biased the PI with conventional EL84 values, R18 = 820R and R16 = 47k, and reduced the amount of nfb by increasing R12 to 68k (on 8 ohm tap but used for 16 ohm speaker). These mods have really improved the power amp for my use. I still have a few more changes that I plan on doing, increasing the screen resistors (R5 and R7), 1-2k range, increasing the grid resistors (R6 and R48) for EL84 (minimum of 10k), and trying out 470k's on the PI for R17 and R19. With the PI gain reduced, I increased R13 to 390k for more preamp signal. The combination of reducing the PI gain and nfb provides much more life to the power amp (IMO). So what do all your power amp mods do in general? These power amp mods seem like they will only work as a combination, so you cant use the default try this and decide if you like it(that or i have become lazy over the days). Link to comment Share on other sites More sharing options...
Members SciHi Posted March 1, 2011 Members Share Posted March 1, 2011 Originally Posted by turnituploud So what do all your power amp mods do in general? These power amp mods seem like they will only work as a combination, so you cant use the default try this and decide if you like it(that or i have become lazy over the days). Yea, a 7 hour mod session, that serious!My goal was to bring the power section in line with EL84's power tubes requirements. I would say all but the PI cathode value change work independently (need to change both R18 and R16 at the same time). All other changes are independent, do them or not, some are related to the reduced gain of the PI, do one, then not enough gain, so mod the MV... For clean I actually like to run around one like you say and dime the MV, and by changing out R13 for more MV gain, it is clean and loud. The stock grid stoppers are too small for EL84's, and screen resistors need to be larger (see article). Link to an article to read, good general information on EL84's, and they do like the JJ's for a couple of reasons: http://www.aikenamps.com/JJ_EH_tube_tests.html So I actually probably started the process backwards, started doing all these mods to the preamp, changed just about every resistor value, better, yes but still not happy. Then modded the power section, now it sounds really good! So if I would have just modded the power section first, probably wouldn't of done 90% of what I did to the preamp (my head mods). So where is the fizz, fazz, uglies coming from???? Link to comment Share on other sites More sharing options...
Members robertkoa Posted March 1, 2011 Members Share Posted March 1, 2011 I am thinking of gertting the JCA 22 - the 2 Channel version and having a professional Mod BOTH channels - Turning one channel into a Fender type clean but of course it would have more gain when needed. Leaving the gain channel intact but maybe smoothing it a bit into more warm tones . What do you guys think - how well will it work ? Have you heard the Verellyn clean mods or Friedman gain mods on these ? Thanks. Link to comment Share on other sites More sharing options...
Members SciHi Posted March 2, 2011 Members Share Posted March 2, 2011 Originally Posted by turnituploud ... One thing i havent found anywhere is what is this r13/r14 voltage divider combo for? Is it just to attenuate the signal a bit before the master volume? Why do so? The jcm didnt have that and most of the amps i saw didnt also, so i didnt implement that combo... Be careful following the schematic on the combo, was doing the recent mods from the head, the R13/R14 voltage divider, and the combo schematic has the resistors switched. R13 is the resistor to ground and a 330k on the combo. R14 in the grid resistor and a 220k. I wanted more gain from the MV on the combo so changed R13 to 470k. The voltage divider values are significantly different between the head and combo. The head (470k/220k) is dumping over twice the gain to ground as the combo (220k/330k).I also see the original Atomic 16 schematic shows the separate PI power node that I added to my head, so you save about $1 to leave out a resistor and cap??? Link to comment Share on other sites More sharing options...
Members hotrat Posted March 2, 2011 Members Share Posted March 2, 2011 Hi, I am new here. I have the combo version (which did come with the speaker plugged into an 8ohm jack) and was wondering if there is a resistor to swap to add gain to the preamp? I have been running either a dod 250 or a nady td1 in front of the combo for boost, and while they both sound good; I would like to get more gain from the amp. More like a mini hotrodded jcm800. Currently I have two eh 12ax7's in v 1 and 2, and some vintage russian military el84's in the power. I have left the cheapo chinese tubes in the reverb and the phase inverter, as the phase inverter will eat them quicker anyway. I am considering the bitmo mod but all I really want is gain, I don't want this to sound like a fender, so I dont want to complicate the circuit with a tone stack bypass. Link to comment Share on other sites More sharing options...
Members SciHi Posted March 3, 2011 Members Share Posted March 3, 2011 Originally Posted by hotrat ... was wondering if there is a resistor to swap to add gain to the preamp? Only one resistor swap? You could more than double the gain of v1b stage with a 1K cathode resistor (R25) bypassed with 1uF cap... (And to cheat, just piggy back another 2.2k and cap right to the existing cathode resistor, don't even need to pull the board...) I think that is pretty much what vh4 did on the head... But I also reduced R52 and R32 to 470k and removed the treble peaking cap C23), that is a little gain and more to my tastes... However, be careful because the combo schematic is all messed up... Also, I think the reverb pretty much messes up the preamp circuit in v2 and v3. If I was after a high gain amp, I would pull the reverb, tube 7 (who uses reverb with high gain preamps?), use v2a for the SLO100 clipping stage, and parallel recovery stage v2b-v3a (already shared cathodes), to feed the cathode follower. Then you have a real SLO... But its not a beginners project...But let me also point out that the great high gain amps generally have a rock solid, clean power amp. The JCA 20w power amp is really not setup for a high gain preamp... Link to comment Share on other sites More sharing options...
Members SciHi Posted March 3, 2011 Members Share Posted March 3, 2011 Originally Posted by hotrat ... so I dont want to complicate the circuit with a tone stack bypass. So lifting the tone stack is simple "overdrive" trick but doesn't really help if you want preamp overdrive, you are really adding gain to the PI/power amp and power section overdrive. My guess is with the values used on the JCA 20's, you will get heavy PI distortion. You need to work on the preamp gain up front, just overdriving the PI or power stage is not that high gain preamp character... I think the secret is lots of moderate gain stages versus one really excessive one. The secret to the SLO100 is that 39k cathode clipping stage (5 preamp stages, the stage missing on the Atomic and Astroverb's...), basically a near unity gain stage but clips agressively at any signal level. But it is also a love it or hate it type of sound. I actually did one of the SLO mods and converted back to a 10k clipping stage, a more universal value, adds more gain, but not the aggresive clipping tone... Google SLO100 mods and you can find some of the suggested mods to the original SLO100 preamp circuit... Link to comment Share on other sites More sharing options...
Members Edge11 Posted March 3, 2011 Author Members Share Posted March 3, 2011 If you want something closer to the original s.l.o with less hassle you can always plate feed the tone stack and moake the rest of the amp a slo clone minus the cathode follower. If would also change he p.i. to ac30 spec and disconnect the nfb if i did this. Link to comment Share on other sites More sharing options...
Members Edge11 Posted March 3, 2011 Author Members Share Posted March 3, 2011 Originally Posted by SciHi So lifting the tone stack is simple "overdrive" trick but doesn't really help if you want preamp overdrive, you are really adding gain to the PI/power amp and power section overdrive. My guess is with the values used on the JCA 20's, you will get heavy PI distortion. You need to work on the preamp gain up front, just overdriving the PI or power stage is not that high gain preamp character... I think the secret is lots of moderate gain stages versus one really excessive one. The secret to the SLO100 is that 39k cathode clipping stage (5 preamp stages, the stage missing on the Atomic and Astroverb's...), basically a near unity gain stage but clips agressively at any signal level. But it is also a love it or hate it type of sound. I actually did one of the SLO mods and converted back to a 10k clipping stage, a more universal value, adds more gain, but not the aggresive clipping tone... Google SLO100 mods and you can find some of the suggested mods to the original SLO100 preamp circuit... If you added a clipping stage where your supposed to on a stock jca you just basically made a hotter jcm 800.I found this interesting link tonight just browsing around the net.http://rh-tech.org/public/JCA20H_par...-stage_mod.png Link to comment Share on other sites More sharing options...
Members hotrat Posted March 3, 2011 Members Share Posted March 3, 2011 Wow, thanks for all the info. I'll pop it open again and pull reverb tube 7 and see what that does first before I have to drain the caps and get out the iron. I wanted the reverb but I guess I should have scored the head, or even waited for the jca22, but I got a great deal on it and don't mind rebuilding it considering the quality of the platform. I am going to have to do some more learning about it but this combo's a work in progress from here on out. Link to comment Share on other sites More sharing options...
Members SciHi Posted March 3, 2011 Members Share Posted March 3, 2011 Originally Posted by mblooper45 If you want something closer to the original s.l.o with less hassle you can always plate feed the tone stack and make the rest of the amp a slo clone minus the cathode follower.... Now that I think about the mods I did, the amp is really more like a SLO100 preamp just without the clipping stage. The Crate v18/v33 slo mod was basically what you describe and it did sound pretty darn good!!! Link to comment Share on other sites More sharing options...
Members Dukeofibanez Posted March 4, 2011 Members Share Posted March 4, 2011 I just got my JCA20H today. With the new front panel switches. One of the transformers had a long drop of solder hardened to it. Whatever didn't hurt anything. Out of the box it sounded great. Perfect for Bush, Offspring, Foo Fighters. To get some heavier tone I would suggest a HBE Big D. With this the amp turns into a rock monster. Since then I have changed the EL84s to a matching pair of Mesa Boogie's and the V1 to a Ruby 12ax7a. Wow, I'm glad I got this for $250 new because now I will never sell it. lol I wouldn't be able to justify selling it for any less than $450. This amp is a keeper. Cheers! Link to comment Share on other sites More sharing options...
Members StudioPlsCherry Posted March 5, 2011 Members Share Posted March 5, 2011 Got an odd one... My jca20h developed a weird noise where whenever i play loud a rattling hissy noise comes out of my speaker. I can also recreate the noise if i tap on my amp with my finger with both power and standby on. I dont think it is a chassis problem, I checked all my screws. Any chance a loose weld could be causing this? The only thing i have done to date is swap tubes, and this noise only developed in the last couple days. It is almost as if a noise is being picked up inside the amp itself and being amplified. Upon further futsing around with the head....I think it is power tube rattle, weird this only started happening recently. Link to comment Share on other sites More sharing options...
Members rock_mumbles Posted March 5, 2011 Members Share Posted March 5, 2011 I've had several preamp tubes do that and have one jj EL84 that does that ... pull the back panel on your head or with a combo open up the top vent and with the amp on and volume up some use a piece of doweling and tap on each tube, that will tell you if a tube is microphonic. As built these things are going to eat up good EL84's every 6 months or so, the screen voltage limit on an EL84 is 300V and these are above 325V IIIRC ... the only currently available tube that can survive that kind of abuse is a Sovtek EL84M or bought direct from former Soviet states the 6P14P-EV tubes. These are 14W dissipation tubes instead of 12W dissipation tubes so if you've got enough power supply which these amps do, you can get a slightly cleaner louder output section. A loose/bad solder which is very common occurrence with lead free solder could cause a problem but you'd generally have a buzzing if a ground connections was loose or intermittent or a crackling sound if a signal path component was intermittent. Link to comment Share on other sites More sharing options...
Members StudioPlsCherry Posted March 6, 2011 Members Share Posted March 6, 2011 I did the old chopstick test and found that the right el84 gives off a little rattle but the real culprit was v1. Quick swapout with a new jj and all is well in the world. I did have another goofy one... The cable rattled out of my cab today... anyone else have a loose connection on their cab? Link to comment Share on other sites More sharing options...
Members SciHi Posted March 6, 2011 Members Share Posted March 6, 2011 Originally Posted by rock_mumbles ...As built these things are going to eat up good EL84's every 6 months or so, the screen voltage limit on an EL84 is 300V and these are above 325V IIIRC ... The Aiken's Amp testing with JJ's and EH's recommended 2-2.5k plate resistors for similar B+ voltage, ~365v... Link to comment Share on other sites More sharing options...
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