Jet City 20a head repair and question

wagdog

wagdog

Ack
My buddies jet city 20 blew a tube the other night. It took the screen resistor and first B+ dropping resistor with it. I cleaned it up tonight and replaced the resistors. I'm going to install a new tube socket, but just scraped the cruft off and cleaned it with cleaner for now. I put some known good EL84's in it and biased it, as well as some known good 12ax7's (still had the original chinese 12ax7's in it).

The amp, honestly, sounds like fizzy dog poo. I don't like it at all. I played it through my mesa 12" cab w/a mesa black widow / celestion speaker. I couldn't dial in a decent tone at all. Sounded thin, and when gain was added it just got fizzy. The tone controls, especially the bass were pretty unresponsive. Do these amps just sound crappy?

Any, here's the repair. If you've heard the terms "burnt screen resistor" or "burned dropping resistor" and wondered what they look like, here it is... Will update when I get the new socket.

Also, this is a case where you MUST drain the filter caps. The dropping resistor being burned open left the screens and preamp B+ supply charged due to the draining resistor being disconnected. I ALWAYS drain the filter caps and check them with a DVM anyway, even if the circuit is "self-draining." Better safe than sorry.



The culprit:


Socket (replacement coming - cleaned up for now)


B+ Dropping resistor on the left, screen resistor on the right


Got very hot. I removed all the debris from the board and cleaned it


Replaced dropping resistor and both screen resistors
 
I've generally had pretty good luck with jet city amps. I had the 20w and the 50w. If you can't find a decent sound in there, there's definitely something seriously wrong.
 
That's a JCA20H, right?

I've played a few of those. Even with a smaller cabinet they have a pretty solid bass response. Sounds like something more may be wrong with it.
 
I'll have to go through it more thoroughly this weekend. Everything looks good, but perhaps it's not. I hate the stand on end filter caps - wondering what the underside looks like. It's possible the first one or two got ganked. Will also check the diodes in the rectifier, however I think they're ok as the voltages checked out.

And yes, jca20h
 
I am surprised that the JCA20H sounds bad after replacing with better tubes. I use JJ's in mine and 5751M in the V2 - the tone stack. Run the tone and prescence controls at 12 o'clock, master at 6-7 and adjust the preamp for the appropriate volume - usually from 4-6 for gigs. Works like a dream through my Mesa Boogie Thiele cab.
 
GilmourD said:
Does this help at all?

http://starin.info/Product Info/Jet City/Manuals/JCA20H - Schematic.pdf
Click to expand...

Had it - thanks for the thought though. Very simple, no frills design.

Modern Saint said:
I am surprised that the JCA20H sounds bad after replacing with better tubes. I use JJ's in mine and 5751M in the V2 - the tone stack. Run the tone and prescence controls at 12 o'clock, master at 6-7 and adjust the preamp for the appropriate volume - usually from 4-6 for gigs. Works like a dream through my Mesa Boogie Thiele cab.
Click to expand...

I'll have to spend some more time going over it and trying to dial in a good tone this weekend. I was kinda rushed yesterday.
 
I had a JC20 a while ago and it wasn't really for me. It was a little fizzy for my tastes, part of it could have been the guitar I had at the time. I think it would have been great for hard rock or punk, but it wasn't good for what I normally play.
 
Chad said:
I had a JC20 a while ago and it wasn't really for me. It was a little fizzy for my tastes, part of it could have been the guitar I had at the time. I think it would have been great for hard rock or punk, but it wasn't good for what I normally play.
Click to expand...

The JCA20H is based upon Mike Soldano's crunch circuitry so if you are looking for a crunchy drive, this is your amp. It can be tamed with tube mods as well as amp setting. If you are looking for a really clean amp, the user needs to visit another brand amp.
 
So in the OP I mentioned using a known set of good 12AX7's in the preamp. :facepalm: - nope, I used one good 12AX7, and two 10 year old GT pulls that I had in my tube box. They had rolled into the bag of new, but tried out in various amps, tubes I had in the same box and I didn't look at 'em closely when I put them in. I just popped in two fairly new JJ's and it's like a whole new amp. Didn't have a lot of time to play it, but I will tomorrow.
 
Modern Saint said:
I am surprised that the JCA20H sounds bad after replacing with better tubes. I use JJ's in mine and 5751M in the V2 - the tone stack. Run the tone and prescence controls at 12 o'clock, master at 6-7 and adjust the preamp for the appropriate volume - usually from 4-6 for gigs. Works like a dream through my Mesa Boogie Thiele cab.
Click to expand...
So I'm finding that master must be above 6 to get any kind of decent tone from this amp. It's got a sweet spot between like 6 and just shy of 9 with the preamp adjusted to taste. Turning down the master and cranking the preamp for dirt just sounds kinda fizzy, not a pleasant preamp distortion tone.

I may be a little spoiled by my mesa though.
 
wagdog said:
So I'm finding that master must be above 6 to get any kind of decent tone from this amp. It's got a sweet spot between like 6 and just shy of 9 with the preamp adjusted to taste. Turning down the master and cranking the preamp for dirt just sounds kinda fizzy, not a pleasant preamp distortion tone.

I may be a little spoiled by my mesa though.
Click to expand...
You got it - That is the sweet spot! I spoke to Mike Soldano a few years ago either at Namm or the Amp Show and he too said the samething. I have dimed the amp into my isocab before and it sounded good too.
 
Modern Saint said:
You got it - That is the sweet spot! I spoke to Mike Soldano a few years ago either at Namm or the Amp Show and he too said the samething. I have dimed the amp into my isocab before and it sounded good too.
Click to expand...
Yeah, I'm finding there is a definite spot about 7 on the master vol where it goes from thin'ish to full pretty quickly - after that it's just a bit more full. Definite jump there.

I like the amp w/my strat, but it really comes alive with my les paul. The guy who owns the amp plays a les paul, so it's a good amp for him.
 
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