sahlomonic wrote:Welcome to the club! Depth mod is an absolute MUST, and is actually relatively easy to do (not to mention cheap!). You'll see a broader improvement of tone with a tube swap than with the depth mod, but both are in my opinion essential to getting the most out of the Jet City amps.
Yep it's on the list... I get the feeling if I open it up I'll at minimum do the depth mod and the high gain channel part of the SLO mod. The clips of that sound pretty killer.
I believe the SLO mod is only on the Overdrive channel. I don't think it changes anything on the Normal channel, unless you change the gain pots. I could be incorrect on that, though
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Sterling AX40 x3 PRS SE245, SE Paul's Jet City amps - all of them modded 2204s Bogner XTC 3534 Kemper rack
sahlomonic wrote:I believe the SLO mod is only on the Overdrive channel. I don't think it changes anything on the Normal channel, unless you change the gain pots. I could be incorrect on that, though
Ok gotcha, I was looking at this thread and thought there were a couple component changes on the crunch channel
i will be documenting my jca2212c modifications. i notice there aren't a lot of current mod posts on this topic. hopefully this will save some tinkering for people.
problems i noticed with the combo amp. flabby bass, lack of clarity with high gain. the worst is the high end and high end fizz. there is no clean channel. over all the amp sounds pretty sweet and has lots of potential. by no means are any of these flaws major.
choke mod: hammond 156m replace r52 according to jca22 schematic. this mod tightens up the tone. helps reduce hum and bring clarity to the low end.
speaker change: replaced the jet city eminence speaker with a celestion vintage 30. tightens up the bass and provides more clarity with the overdrive. IMO, it is a must.
added channel toggle switch to front end. just connects foot switch lead to ground. a jack that grounds when not plugged in will allow the foot switch to bypass the toggle switch, if wired as such.
V1 biasing mod: beginning of preamp stage. replaced plate resistor r6 from 220k to 100k. this allows for slightly clearer and more usable lead channel. it adds some really nice harmonic content as well. IMO, clearing the lead as well as adding more tone is by far one of the best things i've done to the lead. it squeals with ease even on the neck pickup. it really just helps the tone over all. the downside to this mod is that its not as quiet as with the 220k. it also cuts a little too much gain for the crunch channel. while on the crunch/clean channel i hear a little bit of scratching from time to time. it does not get louder as the volume is turned up.
clean mod: the clean mods that are posted on the internet do not actually make the amp clean. it cleans it up a bit but there is a hint of high end clipping. you can literally achieve the same tone if you just roll the gain way back. while on crunch, the mod i've done is to bypass v3 (cathode follower) before r10 and introduce the signal back at the tone stack at r26. ground the circuit right before r12. this gives an actual clean tone that is usable. this mod is with r6 at 100k. similar to a vox kind of tone. you can get clean at 12'o'clock and get a little dirt after that. the downside is that it is quieter than the other channels but is still pretty decent. i will be experimenting with the biasing of the second half of v1 to obtain more volume gain. switching from this to crunch will require a double pole switch. any comments or help with this would be greatly appreciated.
one thing i will eventually try is playing with cathode bypass values. there is not a lot going on in this design as far as thats concerned and doing so may yield really fine results.
update: changed first stage bypass cap to 10uF tantalum cap. raised mid range bass frequencies. makes it easier to tame high end fizz.
thinking of placing a low pass filter in the circuit somewhere to tame highs more. any suggestions?
Last edited by shep on Sun Sep 07, 2014 7:18 am, edited 1 time in total.
Thought I'd see if anyone here was interested....selling my JCA100. Most of the good mods listed on this thread have been done to this head. It screams. Heyboer SLO spec output tranny. All quality components used for the mods. $300;)
I was thinking about changing and biasing the tubes and wanted to hear some opinions on my tube selection. I mainly play modern rock, grunge & alternative.
What do you think? I am unsure about V2 & V3 especially.
Thanks!
Welcome to the group!
I'm sure there's more qualified opinions out there (or is that opinionated qualifications? ), but the JJs I bought for my JCA50H have been serving me well. Jay sent me a few tubes a while back to try out, and I think they've made a pretty solid difference. Will have to check to remember exactly what they are, but I'm pretty sure I'm running a JJ special version of the AX7 in V1, and then a tung-sol in v2, along with another special JJ in PI. From what I gather, your best bet for tonal differences will come from V1 and PI (V5), along with the power tubes. That, and setting your bias correctly.
And Curt said he uses hammond trannys in his soldano~ish amp.
Still waiting to hear from Heyboer. I asked them as well if they had a transformer that would work well with a soldano amp.
FRPC back in the hood!
If you can get Jay's attention, he was working with a company called Edcor (I think) that was going to build a correct transformer to whatever spec needed - correct bolt holes, custom winds for whatever current needs / power tube desired, etc. Last I remember, he was pretty stoked about the quality and tonal improvement it would bring.
By the way, are you still running KT66s in your 100H? Did you ever do anything with the SGRs, and has it been reliable all this time?
FRPC - I'll pm him for you. He's one of our resident experts, and all-around good guy. Not the only one, mind you, but I mention him due to the work he'd started with that transformer manufacturer.
SGR = screen grid resistor. There's been talk throughout this thread and even just a few posts back that upgrading them was a good idea when running other power tubes, due to the increased current requirements.
yeah I'd definitely be interested in more info about that edcor OT
Didn't mess with the SGR. The JJ's kt66 draws the same current as a 6L6GC so they were a direct swap.
In the brief stint I had trying the amp out with EL34's and KT88's, they sounded great but red-plated when the amp was pushed to gig volume and honestly, the 66's sound so similar in feel and compression to the 88's, I didn't bother pursuing any sort of mod to allow for 88 use.
Tokai LS150, Charvel San Dimas, Strandberg Boden Standard, Martin GPC-Aura GT Splawn "MUFASA" Nitro, Friedman Runt 50, Brit-Tone SLO+, MadRyan 50w Silver Jubilee
What are the pots that are used in the Amp? I didn't open it yet and the schematic that I have has no values for them. Are they 1M pots? Which one fit as replacement?
Sovtek 12AX7 (WA, WB and WC - in order of gain rating) are really bad tubes, sound dark, flat, lifeless and just "meh". Sovtek 12AX7 LP/LPS are ok, but I only would use them in the PI.
22H gain pots are A1M, if you change them to A500k you would also have to replace the two 1M resistors with 470k and the two caps 1nF and 470p to 2,2nF and 1nF.
FastRedPonyCar wrote:yeah I'd definitely be interested in more info about that edcor OT
Didn't mess with the SGR. The JJ's kt66 draws the same current as a 6L6GC so they were a direct swap.
In the brief stint I had trying the amp out with EL34's and KT88's, they sounded great but red-plated when the amp was pushed to gig volume and honestly, the 66's sound so similar in feel and compression to the 88's, I didn't bother pursuing any sort of mod to allow for 88 use.
The main premise for changing the SGRs for the KT66s is because the screens on the 66 are more delicate than the 6L6s. The stock JCA value is 470R. I usually use at least a 1k5 3-5w for the screens on the bigger tubes to keep them happier at high volumes. Longevity will be more apparent if you play the thing at high volumes. If you don't crank it up a whole lot, there may be no discernible difference in lifespan.
I actually contacted Heyboer a year ago or so and they said they could make an SLO-spec tranny set for $250~ish or less for the 100H. From what they told me, they would be able to fit in the stock location without drilling holes. I gave them the dimensions of the Transformers and they said it would be possible to follow the same footprint.
Took the 50 apart today (after safely discharging the caps) to check the board for bad solder joints. I couldn't really find anything that looks bad, and certainly didn't find any burn marks or any evidence of a failed component. I really don't know what to look for so hopefully someone can lend me an opinion. Some solder joints weren't mirror-like shiny like they are supposed to be, but didn't look dull and brittle either.
Looked over it maybe half a dozen times front and back. The problem I'm having, is Bass and Middle controls are non-functional when I adjust them. All I hear from the amp that works is Gain, Treble, Channel volume, and Presence. Channel switching works, volumes work. Just all treble coming out. Every now and then for a split second the full signal kicks in like a short in a wire when you bump it just right, then back to all treble.
Close-up of the Middle, and Bass controls, respectively.
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I know nothing
Sterling AX40 x3 PRS SE245, SE Paul's Jet City amps - all of them modded 2204s Bogner XTC 3534 Kemper rack
Sahl - This is common to both Crunch & OD channels, right?
If it is something with the tone stack, my guess is that it'd be in the area highlighted below:
Does the treble control work?
Check slope resistor R24, the two caps (C18 & C17), and the mid & bass pots that make up the rest of the components. I'd check continuity of everything to ground as well (make sure the circuit paths and ground wire are all doing what they should).
), but the JJs I bought for my JCA50H have been serving me well. Jay sent me a few tubes a while back to try out, and I think they've made a pretty solid difference. Will have to check to remember exactly what they are, but I'm pretty sure I'm running a JJ special version of the AX7 in V1, and then a tung-sol in v2, along with another special JJ in PI. From what I gather, your best bet for tonal differences will come from V1 and PI (V5), along with the power tubes. That, and setting your bias correctly.
