#1
So, recently my amp, a jet city JCA50h seemed to be dying, and i took it to a tech, and he changed the tubes and set the bias. He put in JJ 6L6's and electro hamonix 12ax7's.

And it sucks. The crunch channel is now pretty much unusable, since i used it for clean settings. But now, even if I set the gain to zero, it still breaks up, so i cant for the life of me get an even somewhat decent clean tone out of it. I could easily do it before, with the stock tubes.

On the other hand, the overdrive channel now has much LESS gain. I used to run the gain at about 5, maybe even less, and used the overdrive to tighten up the sound a bit. Now i run gain at about 3/4 open, and i have the overdrive level at about 3/4 as well, just to get the same amount of gain. The tone is kinda the same, honestly i cant really tell, but i think its actually a bit worse now than with the stock tubes.

I also noticed, that the eq that i keep in the fx loop seems to be way more responsive now, so i had to lower the level on that as well (i use it as a boost for leads), and for some reason it also adds a bit of gain, not just loudness.

Now can anyone please tell me what the hell is going on here? How can i have more gain on the crunch channel, and at the same time way less on the overdrive? Can i fix this by changing the preamp tubes? I really dont wanna take it in for another power tube change, but i cant do it alone, since i cant set the bias (no equpment, and also dont wanna die). The tech did give me a matched quad set of power tubes, so i do have 2 spares, that i can use without setting the bias again. Anyway, help will be much apreciated, since now im not sure how i can gig with this thing, if it wont do cleans.
Joža je kul. On ma sirove z dodatki pa hambije.
#2
Buy a single JJ ECC83S and try it in every position and see what happens. It aint gonna be a power tube. Swapping preamp tubes is easy, just like changing a light globe.
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#3
sounds like he screwed up your preamp tubes (maybe put a 12at7 in the wrong spot) I had some issues when I used to run a Jet City amp with getting a clear cut answer of what tube goes where.
#4
As far as i know all the preamp tubes are 12ax7's. Ill get a few jj ones and ill try them in different places tho.
Joža je kul. On ma sirove z dodatki pa hambije.
#5
ECC83 is the international code for 12AX7
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#6
Oh, i know that, i was replying to Joseph regarding the 12at7.
Joža je kul. On ma sirove z dodatki pa hambije.
#7
Take it back to the tech, something is obviously wrong
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2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
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#8
If you take it to a tech, try another one, that guy screwed it up already...

I would go with the suggestion to get a preamp tube and try it in each position, only change one tube at a time so you know exactly which one made a difference. As already stated, swapping preamp tubes is as easy as changing a light bulb. Just don't grab it while it's hot...use a pot holder. And shut the amp down, don't change them while it's turned on. Yes I realize you may already know that but I have to err on the side of caution.

Edit - I don't know the amp, but if you have reverb and/or vibrato, they use tubes and shouldn't affect the gain, so find out which ones are the preamp tubes and which ones apply to the reverb section, no need (usually) to worry about reverb or vibrato tubes, just the preamp ones.
Hmmm...I wonder what this button does...
Last edited by Paleo Pete at Mar 10, 2015,
#9
^ you can remove and replace preamp tubes while the amp is on with no issues (besides the tubes getting hot). Power tubes you must shut the amp down

EDIT: this is not the typical way to do things, I was just speaking in a technical sense. I would also advise you probably don't want to do it either.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
Last edited by Robbgnarly at Mar 10, 2015,
#10
Quote by Robbgnarly
^ you can remove and replace preamp tubes while the amp is on with no issues (besides the tubes getting hot). Power tubes you must shut the amp down


Umm....yeah...not recommended practice though...
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
#11
Quote by Arby911
Umm....yeah...not recommended practice though...

Yeah, I should have put that in there. It will work though and that is all I was trying to convey. I'll edit it

I actually do this pretty regular when I roll tubes to find a bad one or trying different brands
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate