Members axe_2_grind Posted February 7, 2011 Members Share Posted February 7, 2011 Originally Posted by Nobody Told Me Question about bias; I'm reading a plate voltage of 360, and getting 23.6mA from one output tube and 22.9mA from the other. According to the Weber bias calculator, that puts my right in the 70% neighborhood. Am I figuring this correctly? Sounds about right to me. When I first got my combo my plate voltage was about 360 or so, and I biased it about the same. In fact my blue wire was the one I went by. The brown wire always seems to be slightly lower. Could just be the slight differences in the tubes I guess. Link to comment Share on other sites More sharing options...
Members monster265 Posted February 8, 2011 Members Share Posted February 8, 2011 Anyone apply the choke mod to a JCA2112RC Combo? Installed the Hammond 156M Choke to replace R3. Attached the choke to the power transformer and routed the wires from top through the eyelet where the power transformer wires enter into the chassis, removed R3 and soldered in place. Performance of the amp degraded as a result, 60HZ hum, and when powered on, splash of white noise for 1-2 seconds, until circuits stabilized. (my best account in writing). The plate voltage is 380 for my combo. Re-checked Bias. No change in plate voltage. Link to comment Share on other sites More sharing options...
Members SciHi Posted February 8, 2011 Members Share Posted February 8, 2011 Originally Posted by SciHi ... I added a 200R 5 watt sag resistor, then 1K 5 watt before the 10H choke, and changed out the screen resistors to 1.2k 5 watters. I think about half the problems was getting power tubes that sounded good with such high plate and screen voltages, mine was running 365 v on the plates and 350+ on the screens. Dropped those all down now to reasonable levels, 335v on plates and under 320v on screens and it sounds great with any power tubes... I do need to go back in later and change out the dropping resistor for the PI, 10k is really too big now that all the voltages are dropped up front, probably something in the range of 2.7k-4.7k... Regarding choke on combo, I will save you the time going back to post #401... Bottom line, you need to keep R3, and maybe even add a sag resistor, well it you want more Marshall EL84 character out of the power amp... Run the choke after R3, drop the voltage feeding the choke. I run the sag right off the diodes, pull the legs and add the 5 watt resistor... Link to comment Share on other sites More sharing options...
Members monster265 Posted February 9, 2011 Members Share Posted February 9, 2011 SciHi, thanks. I did not know you were mod-ing a combo for the choke mod, requiring a Sag resistor to reduce the plate voltage. Since this forum is geared towards the JCA20H. Will the Depth mod work for the JCA2112RC combo as described by mblooper45.http://acapella.harmony-central.com/...uctions-please Link to comment Share on other sites More sharing options...
Members SciHi Posted February 9, 2011 Members Share Posted February 9, 2011 Originally Posted by monster265 SciHi, thanks. I did not know you were mod-ing a combo for the choke mod, requiring a Sag resistor to reduce the plate voltage.Since this forum is geared towards the JCA20H. Will the Depth mod work for the JCA2112RC combo as described by mblooper45.http://acapella.harmony-central.com/...uctions-please I would do the choke first with just R3 replaced, then check voltages on the plate and screens, see how it sounds, it should be better, maybe good enough. I added the sag resistor to drop the plate even further, not really required just something I did to alter the nature of the beast, and I added larger screen resistors to drop screens voltage with the higher B+. You really don't want to drop all your rail voltage very much, note that the vh4 mod has all the preamp dropping resistors reduced to ~1k (I don't think I would do this without the choke). That will keep the voltage to the preamp nice and high. But the combo probably should of had another power node so the PI could run really high and have its own node, now it is sharing with 2 other preamp tubes. Bigger power rail caps might help. Lots of little things, not sure which are really important... My choke is also a 10H with about 250R resistance, yours much smaller and less resistance, so maybe not enough to do the job on the combo without the drop resistor also to help filter the power rail. You are replacing a 1K resistor, R3, with only 80R choke...I did the depth mod on my combo also, I just pulled the nfb wire from the 16 ohm jack, removed one 8 ohm jack, mounted the caps right to the back of the pot and mounted the pot in the empty speaker jack hole. Works great. The amp stock has too much nfb (IMO) so even moving the nfb wire to the 8 ohm jack will help, but really you should be running your 16 ohm speaker on the 8 ohm jack, but that is another story. I left the presence but it is pretty limited in usefulness, nothing until around 7 then gets ugly over 8... Link to comment Share on other sites More sharing options...
Members monster265 Posted February 9, 2011 Members Share Posted February 9, 2011 SciHi, very insightful feedback, being a newbie on mods. I did have the concern that the 80R choke was to small with the higher voltage rail when compared to the JCA20H; as after the choke mod the amp had to stabilize when switched on from standby. Great idea on the Depth mod pot placement, if I add it my plan was to perform it as you describe. Link to comment Share on other sites More sharing options...
Members axe_2_grind Posted February 11, 2011 Members Share Posted February 11, 2011 Just thought I'd share this video with y'all.Although we knew this already but here it is from the man himself. Link to comment Share on other sites More sharing options...
Members Edge11 Posted February 11, 2011 Author Members Share Posted February 11, 2011 Originally Posted by axe_2_grind Just thought I'd share this video with y'all. Although we knew this already but here it is from the man himself. Gonna have to agree with him on the flexibility part. I've been noticing more and more that the higher i put the presence and depth the more aggressive the gain appears to become.However I dont think it sounds voxish. On the clean channel i ussually crank the master and control the volume with the volume pots on my guitar, it gives me a really cool nmv refined plexi sound. Link to comment Share on other sites More sharing options...
Members axe_2_grind Posted February 12, 2011 Members Share Posted February 12, 2011 Originally Posted by mblooper45 Gonna have to agree with him on the flexibility part. I've been noticing more and more that the higher i put the presence and depth the more aggressive the gain appears to become.However I dont think it sounds voxish. On the clean channel i ussually crank the master and control the volume with the volume pots on my guitar, it gives me a really cool nmv refined plexi sound. Yeah. Actually I took my combo down to my studio today and cranked it a bit and threw on my ipod through the PA and just jammed. I've been using it clean and getting my distortion from my pedalboard. With the master cranked and gain on 1 it has a really sweet sound to it. I was able to get my metal zone dialed then use the amp's eq to tailor the sound. I jammed on everything from Iron Maiden to some 90's R&B stuff. This amp has a very sweet tone for hard rock, blues, R&B and funk. still needs a 4x12 for metal, but that's just part of the metal sound though too. No gettin' around it, haha!I'm actually pretty satisfied with this purchase now. I might not do the FX loop mod now. Just use it clean. Link to comment Share on other sites More sharing options...
Members vic Posted February 12, 2011 Members Share Posted February 12, 2011 will these mods still work on the jca22h? Link to comment Share on other sites More sharing options...
Members Edge11 Posted February 12, 2011 Author Members Share Posted February 12, 2011 Originally Posted by vic will these mods still work on the jca22h? Who knows, that amp isn't out yet, there's no schemes or info on it yet. Link to comment Share on other sites More sharing options...
Members SciHi Posted February 12, 2011 Members Share Posted February 12, 2011 Originally Posted by mblooper45 ... the higher i put the presence and depth the more aggressive the gain appears to become ... With depth and presence dimed you have basically negated all the nfb, so you get the full tube character. I still wonder why these EL84 amps have the higher gain PI values used for lower mutual transconductance octal tubes like 6v6 or 6L6 then kill it all with such a strong nfb circuit??? Link to comment Share on other sites More sharing options...
Members Edge11 Posted February 14, 2011 Author Members Share Posted February 14, 2011 Originally Posted by SciHi With depth and presence dimed you have basically negated all the nfb, so you get the full tube character. I still wonder why these EL84 amps have the higher gain PI values used for lower mutual transconductance octal tubes like 6v6 or 6L6 then kill it all with such a strong nfb circuit??? Well one of the first amps i cut my teeth on was a plate driven s.l.o style amp with no nfb and an ac-30 p.i. and power amp. It's one of the most open sounding hi gain amps i've heard. kinda like what my 6505 sounds cranked. Link to comment Share on other sites More sharing options...
Members SciHi Posted February 16, 2011 Members Share Posted February 16, 2011 Now I have seen everything: http://www.walmart.com/ip/Jet-City-A...-Head/15048468 Link to comment Share on other sites More sharing options...
Members Edge11 Posted February 16, 2011 Author Members Share Posted February 16, 2011 Originally Posted by SciHi Now I have seen everything: http://www.walmart.com/ip/Jet-City-A...-Head/15048468 Bold news.There was a pretty big discussion about it a while back and apparently dough was scratching his head because he didn't authorize it (at least as far as i remember) Link to comment Share on other sites More sharing options...
Members SciHi Posted February 16, 2011 Members Share Posted February 16, 2011 Yea, late to the party... Co-worker owns a head with the rocker switches, he came over to check out my modded combo and also listened to my stock head, both mine have metal toggles, got them around Thanksgiving on sale from MF. He said he can't run his gain/MV settings as high without feedback and said his presence comes on strong right at the start, mine nothing until about 6-7. So did they made any changes to the circuit when they redid the switch and pilot light ??? Link to comment Share on other sites More sharing options...
Members BSS Posted February 16, 2011 Members Share Posted February 16, 2011 Originally Posted by SciHi Yea, late to the party... Co-worker owns a head with the rocker switches, he came over to check out my modded combo and also listened to my stock head, both mine have metal toggles, got them around Thanksgiving on sale from MF. He said he can't run his gain/MV settings as high without feedback and said his presence comes on strong right at the start, mine nothing until about 6-7. So did they made any changes to the circuit when they redid the switch and pilot light ??? I don't remember them ever having rocker switches on the 20 watt head? Link to comment Share on other sites More sharing options...
Members SciHi Posted February 16, 2011 Members Share Posted February 16, 2011 Originally Posted by BSS I don't remember them ever having rocker switches on the 20 watt head? Yea, it was news to me, but again, I came late to the party. Could even be an early model I guess, I just assumed it was more recent, mine has the intermittant LED pilot. I have seen pictures with rocker switches, think maybe the 50-100 watt versions. Just seemed odd??? Link to comment Share on other sites More sharing options...
Members dowman Posted February 17, 2011 Members Share Posted February 17, 2011 Originally Posted by SciHi Yea, it was news to me, but again, I came late to the party. Could even be an early model I guess, I just assumed it was more recent, mine has the intermittant LED pilot. I have seen pictures with rocker switches, think maybe the 50-100 watt versions. Just seemed odd??? I just bought my JCA20H/JCA12S half stack on Monday to take advantage of Guitar Center's $75 off coupon. The head on mine also came with the plastic rocker switches. The power switch had the embedded blue lamp so the entire switch lights up when the amp's on.I wonder if this is a new design for them. I'd imagine the first run of the JCA20's probably sold out by now. They may be on a new "printing" at this stage of the game.So far, I love this amp, although it does break up rather early. By the time the gain knob is at 2, there's some pretty nice overdrive action going on, and I can't take the presence knob past 3 if the amp isn't in full clean mode (i.e. gain down around 0.5 - 1.0).This thread is a lot to take in. I'm pretty sure I'm going to swap out the stock Chinese tubes for something a little better. At least a pair of JJ's EL84's. Was there a general consensus on how to tame the gain beast just a smidge? I don't want full on clean for the whole range, but I'd like a little more headroom in the gain knob before the preamp starts breaking up (right now that happens around 1.5 - 2.0 on the gain knob. 3 or 4 would be better.). I think I saw some tips on taming the icepicks hiding behind the Presence knob a few pages back on the thread, so I'm going to look in to that, too, I think. Link to comment Share on other sites More sharing options...
Members SciHi Posted February 17, 2011 Members Share Posted February 17, 2011 Originally Posted by dowman ... The head on mine also came with the plastic rocker switches. ... Maybe someone who owns a new model can take a few gut shots of their amp. I think moving the nfb wire from the 16 ohm to 8 ohm speaker jack would help some, but I think they may have done some other changes to the presences, maybe removed the 4.7K tail resistor and just used a 5k pot like many designs???? Link to comment Share on other sites More sharing options...
Members dowman Posted February 17, 2011 Members Share Posted February 17, 2011 Originally Posted by SciHi Maybe someone who owns a new model can take a few gut shots of their amp. I think moving the nfb wire from the 16 ohm to 8 ohm speaker jack would help some, but I think they may have done some other changes to the presences, maybe removed the 4.7K tail resistor and just used a 5k pot like may designs???? I'll see if I can open mine up and snap a few pics tonight. Link to comment Share on other sites More sharing options...
Members detailman Posted February 17, 2011 Members Share Posted February 17, 2011 Originally Posted by dowman I just bought my JCA20H/JCA12S half stack on Monday to take advantage of Guitar Center's $75 off coupon. The head on mine also came with the plastic rocker switches. The power switch had the embedded blue lamp so the entire switch lights up when the amp's on.I wonder if this is a new design for them. I'd imagine the first run of the JCA20's probably sold out by now. They may be on a new "printing" at this stage of the game. Congrats on your new half stack! I got the same setup from Guitar Center online back in December and mine came with the metal toggles, not plastic. Yours must be a newer "printing", as you said. This is the thread to come to if you want to mod your amp. Personally I like mine as is and find it to be an awesome plug and play amp. Enjoy and welcome to the club! Link to comment Share on other sites More sharing options...
Members dowman Posted February 21, 2011 Members Share Posted February 21, 2011 Originally Posted by dowman I'll see if I can open mine up and snap a few pics tonight. Here are the pics I took the other night. Link to comment Share on other sites More sharing options...
Members dowman Posted February 21, 2011 Members Share Posted February 21, 2011 Originally Posted by dowman Here are the pics I took the other night. Here are some shots of the iron. Link to comment Share on other sites More sharing options...
Members SciHi Posted February 21, 2011 Members Share Posted February 21, 2011 Originally Posted by dowman Here are the pics I took the other night. Thanks for the gut shots. I can't detect any changes. From the color bands I can make out, looks like values the same, nfb still on 16 ohm tap, still looks like 47k nfb resistor. Guess they could have changed from a linear B25k pot on presence to a log type? Easy way to tell is check the resistance from center to either end post set at 12:00, if it is ~12.5k then it is a linear taper pot. I have been doing more homework on the presence-nfb setup, Merlin Blencowe has a nice discussion in his book, starting on page 187, and show the JCA type setup on page 193 as the preferred design, to keep the voltage off the pot, no scatchy pot noise. But he also goes over all the math, and presents an example using an early fender circuit, the tail resistor / nfb resistor form a voltage divider, fender was 5k/56k and the JCA is 4.7k/47k, so both basically 10% feedback. The huge difference is the fender was attached to a 2 ohm speaker tap, whereas the JCA is attached to 16 ohm speaker tap. So the JCA has just about three times the nfb. Marshall more generally used 5k/100k to either the 8 (same as Fender example) or 16 ohm tap (1.4 times more)... Link to comment Share on other sites More sharing options...
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