Members rock_mumbles Posted January 12, 2011 Members Share Posted January 12, 2011 Originally Posted by turnituploud So do you suggest decreasing the values of both resistors in order to have more gain? and bout the dry signal blend, does that mean its not recommended to install an effects loop?im still new to this, so i dont understand much of this stuff yet... I wouldn't mess with R23/R24 they are a voltage divider (attenuator) that matches the dry (no reverb) signal to the level of the reverb signal. The place to play with the circuit is R32/C23, the "treble peaking" circuit ... If you want more gain and wouldn't mind cutting some of the highs remove C23 and replace R32 with a lower value ... between the recommended 680k to probably a 470k etc. People have posted that they are removing the "treble peaking circuit" from JCM 800 like circuits, they say they end up with smoother distortion, better tone and less fizz. As far as an effects loop, the stock amp splits the signal part goes straight through (dry signal) and part gets sent to the reverb circuit. Then it mixes the dry, non-reverb signal with the reverb signal before the driving the tone stack ... not what you want for an effects loop ... the circuit could be rewired to use the reverb driver/recovery stages as an effect loop ... you'd also need to remove the dry signal mixer from the circuit. Link to comment Share on other sites More sharing options...
Members GRB Posted January 12, 2011 Members Share Posted January 12, 2011 Originally Posted by eyebanez333 what other chokes will work for the JCA20H? I can't find the Hammond 156M anywhere...or one that has the same specs lol Check out Mouser I got a Hammond 156m choke there two weeks ago and they had three left. good luck! just checked mouser they have two left Link to comment Share on other sites More sharing options...
Members SciHi Posted January 12, 2011 Members Share Posted January 12, 2011 Originally Posted by rock_mumbles ... you might want to try increasing the value of that resistor (R24) incrementally 22k, 27k 33k etc. and see if that helps ... this would increase the reverb tank signal into the recovery stage ... I brought my combo home after practice tonight so tomorrow I will try this, and also switch out R32 and remove the treble cap (C23). I will let you know how it turns out... Link to comment Share on other sites More sharing options...
Members turnituploud Posted January 12, 2011 Members Share Posted January 12, 2011 just installed a 100k resistor in series with c23- ice picks gone! i boosted the treble from 6 to 9 and presence from 2-3 to 6 and now the amp is really alive! lol gain, reverb, bass,middle,treble are now cranked: reminds me of a marshall it also decreased the hum a lot, in fact i dont hear any now Link to comment Share on other sites More sharing options...
Members axe_2_grind Posted January 12, 2011 Members Share Posted January 12, 2011 I finally got a reply from Jet City regarding which tubes are which on the combo.Here's what they gave me:JCA2112 TUBE LAYOUTTube Locations (V1 is closest to input jack and V2-V7 move away from the input jack sequentially).V1 12AX7 - Input and 1st gain stageV2 12AX7 - 2nd gain stage/ 3rd amp stage (also provides drive to reverb tube)V3 12AX7 - Reverb wet and dry mix + drives tone controlV4 12AX7 - Reverb drive and returnV5 12AX7 - Phase InverterV6 EL84 - Power TubeV7 EL84 - Power TubeThough this doesn't seem to follow what is actually printed on the circuit board. The board shows V5&6 as the power tubes (which does physically correspond) and V7 is smack in the middle of the reverb send & return. As soon as my tubes get here I'll try it both ways, and see what happens. Link to comment Share on other sites More sharing options...
Members rock_mumbles Posted January 12, 2011 Members Share Posted January 12, 2011 Originally Posted by axe_2_grind I finally got a reply from Jet City regarding which tubes are which on the combo.Here's what they gave me:JCA2112 TUBE LAYOUTTube Locations (V1 is closest to input jack and V2-V7 move away from the input jack sequentially).V1 12AX7 - Input and 1st gain stageV2 12AX7 - 2nd gain stage/ 3rd amp stage (also provides drive to reverb tube)V3 12AX7 - Reverb wet and dry mix + drives tone controlV4 12AX7 - Reverb drive and returnV5 12AX7 - Phase InverterV6 EL84 - Power TubeV7 EL84 - Power TubeThough this doesn't seem to follow what is actually printed on the circuit board. The board shows V5&6 as the power tubes (which does physically correspond) and V7 is smack in the middle of the reverb send & return. As soon as my tubes get here I'll try it both ways, and see what happens. They gave you the tube designations for the Astroverb not the 2112 ...From looking at the 2112 and Astroverb circuit I don't completely agree with their circuit description either ... in the 2112 V7 is the reverb driver/recovery tube (V4 is the reverb tube in the Astroverb) and V4 is the phase inverter (V5 in the Astroverb)My guess is this ... the reason they are different from the Atomic 16/Astroverb amps is because most of the component values and designations are the same in the JCA20H and the JCA2112RC the treble peaking circuit R32/C23 is the same in both amps, so when they added the reverb to the 2112 the reverb tube became R7.here's what I found ... I don't have the amp to compare to but I think this is correct ... http://rh-tech.org/public/JCA2112RC_tube_layout.pdfWhat I see looking at the circuit is this; the signal is split right after V2a part goes to V7 the reverb tube and the other part goes to V2b which is the mixer stage for the dry (no-reverb) signal. The reverb signal comes back into V3a which is the mixer stage for the reverb both the dry and wet signals go into V3b the cathode follower which drives the tone stack.Here is a picture that follows the signal through the preamp BUT it is from an Astroverb schematic (which is a lot easier to follow) so the reverb tube is V4 and the phase inverter is V5 in this schematic ...http://rh-tech.org/public/soldano_as...6-edited-2.png Link to comment Share on other sites More sharing options...
Members SciHi Posted January 12, 2011 Members Share Posted January 12, 2011 Originally Posted by rock_mumbles ...Here is a picture that follows the signal through the preamp BUT it is from an Astroverb schematic (which is a lot easier to follow) so the reverb tube is V4 and the phase inverter is V5 in this schematic ...http://rh-tech.org/public/soldano_as...6-edited-2.png From that layout (Astroverb) and signal flow, you would say that tube 2 was also added for reverb, not present in the JCA20H, along with the actual reverb tube... Link to comment Share on other sites More sharing options...
Members UGQuest Posted January 12, 2011 Members Share Posted January 12, 2011 I just got the 20watt head today & out of the box it was very fizzy. I replaced the preamp tubes with tung sol 12ax7 v1&v2 & the PI with an eh12at7. This took out the fizz & lowered the overall gain a tiny bit. I was playing it through my JTM30 speakers 2x10. I think I will buy a used 12"Celestion g12m70 & build my own speaker cab for this head. Has anyone else built their own cab? So far I really like this head, I am going to try some jj/tesla 12ax7 just because of the comments I have read on here. I did the Bitmo Surfra mod and it is pretty cool. Link to comment Share on other sites More sharing options...
Members axe_2_grind Posted January 13, 2011 Members Share Posted January 13, 2011 Originally Posted by rock_mumbles They gave you the tube designations for the Astroverb not the 2112 ...From looking at the 2112 and Astroverb circuit I don't completely agree with their circuit description either ... in the 2112 V7 is the reverb driver/recovery tube (V4 is the reverb tube in the Astroverb) and V4 is the phase inverter (V5 in the Astroverb)My guess is this ... the reason they are different from the Atomic 16/Astroverb amps is because most of the component values and designations are the same in the JCA20H and the JCA2112RC the treble peaking circuit R32/C23 is the same in both amps, so when they added the reverb to the 2112 the reverb tube became R7.here's what I found ... I don't have the amp to compare to but I think this is correct ... http://rh-tech.org/public/JCA2112RC_tube_layout.pdfWhat I see looking at the circuit is this; the signal is split right after V2a part goes to V7 the reverb tube and the other part goes to V2b which is the mixer stage for the dry (no-reverb) signal. The reverb signal comes back into V3a which is the mixer stage for the reverb both the dry and wet signals go into V3b the cathode follower which drives the tone stack.Here is a picture that follows the signal through the preamp BUT it is from an Astroverb schematic (which is a lot easier to follow) so the reverb tube is V4 and the phase inverter is V5 in this schematic ...http://rh-tech.org/public/soldano_as...6-edited-2.png I completely agree with that assessment. The board itself has these labeled in the same order as that photo and that's also how was seeing them too.My tubes came in today and I tried the 12at7 in a few different spots (V2 lowers the gain slightly and gives it a vintage marshall tone) however I put it in the PI (V4) and replaced v1 and v2. I can't say that I noticed any significant change by placing the 12at7/ECC81 in v4 (and I did order it with balanced triodes) Think I'll try replacing V3 and V7 as well with JJ ECC83S's (12ax7) as well. Overall It's a lot smoother, and warmer and it tames the eq alot. I can adjust my mids back up and even turn up the presence a bit too. Link to comment Share on other sites More sharing options...
Members rock_mumbles Posted January 13, 2011 Members Share Posted January 13, 2011 Originally Posted by axe_2_grind ... I can't say that I noticed any significant change by placing the 12at7/ECC81 in v4 (and I did order it with balanced triodes) ... You'd probably have to have the master up quite a bit to notice a difference, since the master is before the phase inverter ... I think that it depends on how the PI and PA are set up whether an AT7 gives more power tube drive or less ... a friends Mesa had more power tube crunch with the 12AT7 than with a 12AX7 and he didn't like it because he wants a dirty preamp with clean power amp ... I'm old school (actually just old) and tend to favor the other way around Link to comment Share on other sites More sharing options...
Members eyebanez333 Posted January 13, 2011 Members Share Posted January 13, 2011 http://acapella.harmony-central.com/...t-New-JCA-Head JCA22H 2 channels, effects loop, $399 Link to comment Share on other sites More sharing options...
Members turnituploud Posted January 13, 2011 Members Share Posted January 13, 2011 Originally Posted by SciHi Reverb is a blended signal, whereas a FX loop is 100% of the signal. The combo actually has a significantly different setup because of the reverb so it has 2 - additional preamp tubes. It is hard to see how they have used the tubes, the original Soldano Astroverb schematic shows better how the extra tubes are used to send/split, and recombine the signals. I guess if you were really smart and cleaver, you could probably figure out a way to use v2 for a send-return fx using the extra gain available from that stage, but I am neither... well what i leave the reverb as it is and install the fx loop Right before the part that splits the signal to wet and dry like in this schematic?[ATTACH=CONFIG]330328[/ATTACH]yeah, i know this is not the jca2112, but i think it splits the signal into dry and wet just like the astroverb.oh, and wont putting in and out jacks after the master volume work as a loop? And is it better to put the loop like in this schematic or after the mv?and wow, that jca22h looks like a killer amp! Cant wait to hear how it sounds! Link to comment Share on other sites More sharing options...
Members rock_mumbles Posted January 14, 2011 Members Share Posted January 14, 2011 Originally Posted by turnituploud well what i leave the reverb as it is and install the fx loop Right before the part that splits the signal to wet and dry like in this schematic?[ATTACH=CONFIG]330328[/ATTACH]yeah, i know this is not the jca2112, but i think it splits the signal into dry and wet just like the astroverb.oh, and wont putting in and out jacks after the master volume work as a loop? And is it better to put the loop like in this schematic or after the mv?and wow, that jca22h looks like a killer amp! Cant wait to hear how it sounds! You'd have to be careful putting the unbuffered effects loop there (before the coupling cap) as you're hooking up to about 250V or so ... I think in the amp's I've looked at the effects loop is after the tone control and volume control ... right before the phase-inverter. Link to comment Share on other sites More sharing options...
Members Edge11 Posted January 14, 2011 Author Members Share Posted January 14, 2011 Originally Posted by eyebanez333 http://acapella.harmony-central.com/...t-New-JCA-HeadJCA22H2 channels, effects loop, $399 What power tubes does it run? Link to comment Share on other sites More sharing options...
Members turnituploud Posted January 14, 2011 Members Share Posted January 14, 2011 Originally Posted by rock_mumbles You'd have to be careful putting the unbuffered effects loop there (before the coupling cap) as you're hooking up to about 250V or so ... I think in the amp's I've looked at the effects loop is after the tone control and volume control ... right before the phase-inverter. Yeah, the loop in these kinds of amps is right before the phase inverter, but i cant see myself using a reverb and a delay from a loop like this, because delaying a signal with already a reverb on top of it is kinda wrong. Afaik, you run the signal through the loop with a delay, then you reverb it. Link to comment Share on other sites More sharing options...
Members eyebanez333 Posted January 14, 2011 Members Share Posted January 14, 2011 Originally Posted by mblooper45 What power tubes does it run? I'm assuming EL84's...it's still 20 watts and Doug made it sound like it was just a channel and FX loop addition. Although he did say one channel was the soldano crunch channel, and the other was the soldano super lead overdrive channel...that's not the case on the JCA20H is it? Either way...it may have the preamp of a SLO (if it's exact, it's going to need another couple tubes in the preamp, right?), but it's not going to 'be' a SLO without power tube and tranny swaps as well IMO. I'm sure it'll sound close, and should be amazing for $399. Link to comment Share on other sites More sharing options...
Members axe_2_grind Posted January 14, 2011 Members Share Posted January 14, 2011 Originally Posted by eyebanez333 http://acapella.harmony-central.com/...t-New-JCA-HeadJCA22H2 channels, effects loop, $399 F!@#1ng Bastards! Here we all are busting our humps to install and FX loop and an extra channel, then they come out with that? WTH!?! And it's gonna drive down the price of the old amps now too! Link to comment Share on other sites More sharing options...
Members axe_2_grind Posted January 14, 2011 Members Share Posted January 14, 2011 Originally Posted by rock_mumbles You'd have to be careful putting the unbuffered effects loop there (before the coupling cap) as you're hooking up to about 250V or so ... I think in the amp's I've looked at the effects loop is after the tone control and volume control ... right before the phase-inverter. Looking at my schematic it looks like you could tap in (in series) right before C7 which feeds off the master volume pot. But I want to get a second opinion on that since I'm hardly the expert here.Also, for an unbuffered loop, couldn't you tap in just after the tonestack before the master volume? Link to comment Share on other sites More sharing options...
Members ZZ007 Posted January 14, 2011 Members Share Posted January 14, 2011 Hello, I just want to add an extra channel with adjustable gain with pot , not only the master ... Somebody test it ? Is possible to use a 16 ohms Speaker with the Depth Mod ??? Best regards ... Sorry for my bad english cause i'm french .. Link to comment Share on other sites More sharing options...
Members eyebanez333 Posted January 14, 2011 Members Share Posted January 14, 2011 Originally Posted by rock_mumbles That's just pretty much a Fender 125C1A choke which were 3 to 4 H rated to about 90mA and about 90 ohm DC resistance...Here's a link to the Classic Tone: http://www.classictone.net/40-18003.htmlAllen Amps has the TRCH http://www.allenamps.com/parts.php#transformersthere are many others Would a Hammond 154M work? 2H, 100mA, 175 ohms, 300VDC Link to comment Share on other sites More sharing options...
Members rock_mumbles Posted January 14, 2011 Members Share Posted January 14, 2011 Originally Posted by ZZ007 ...Is possible to use a 16 ohms Speaker with the Depth Mod ???Best regards ...Sorry for my bad english cause i'm french .. Yes it only changes the Negative Feedback circuit, it doesn't affect the output transformer speaker taps. Originally Posted by eyebanez333 Would a Hammond 154M work? 2H, 100mA, 175 ohms, 300VDC I don't know what the 300V rating means on a choke ... but it'll be at about 400V in the amp ... also with 2H of inductance it won't stabilize the voltages as well and with the higher DC resistance I'd expect it to lower the voltages in the power supply some ... if that choke is known to work in a 20 watt or so Fender push-pull amp then it should be OK ... Link to comment Share on other sites More sharing options...
Members SciHi Posted January 15, 2011 Members Share Posted January 15, 2011 Originally Posted by eyebanez333 Would a Hammond 154M work? 2H, 100mA, 175 ohms, 300VDC I just put a Triad C-7X, 10H, 90mA, 270R on my combo, because I had it on hand for a Plexi-6v6 build. Works fine but screens on the EL84 are even worse, now 358 volts. I think they recommend more like 325 volts max. Anyone leave the dropping resistor, R3, on with the choke? I have seen builds where the choke is placed after the dropping resistor. The other thing I am wondering, if a sag resistor right off the diodes should be added, the plates are 365v, so more B+ then you need for EL84's???? Link to comment Share on other sites More sharing options...
Members SciHi Posted January 15, 2011 Members Share Posted January 15, 2011 Originally Posted by rock_mumbles ... in the combo circuit R50 is a 15k resistor to ground which really "loads down" the recovery stage, you might want to try increasing the value of that resistor incrementally 22k, 27k 33k etc. and see if that helps ... this would increase the reverb tank signal into the recovery stage ... I used my single component tester to evaluate different values for R50, and going up to just the 22k provided a little extra life to the amp without impact reverb or tone degradation. Any higher an it was too much gain boost and got muddy. If you think about it, even the 22k is a 50% boost over the original 15k. I am not sure I would recommend this to anyone unless you have done significant gain reduction mods to the first gain stages...This could stage could actually be used as a gain stage by playing a little more with the voltage divider values... Link to comment Share on other sites More sharing options...
Members diagrammatiks Posted January 16, 2011 Members Share Posted January 16, 2011 Hey guys excuse the playing I'm not nearly as good as most of ya'll. I just need a quick opinion...I just grabbed this jca20h from a buddy of mine who had been raving about and I need to know....is this jca20 possessed by the devil? I've tried 3 or 4 jca's already and none of them have sounded like this it's bone stock as far as i can tell. Link to comment Share on other sites More sharing options...
Members axe_2_grind Posted January 16, 2011 Members Share Posted January 16, 2011 Just replaced the remaining stock tubes (v3 & v7) with Mesa 12ax7's and replaced the power tubes with Mesa's as well. Rebiased, and now the thing is smokin!! Sounds like a marshall now. m/Next step, figure out how & where to put in the FX loop. Link to comment Share on other sites More sharing options...
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