Jet City 22H Help Requested

[David Raye]

Hello everyone! I picked up a used Jet City 22H head today from a guy who says his son was trying to do modifications to it and it works but is unplayable. I gave him 75 bucks for it and brought it home and plugged it in to a 1x12 cab and everything turned on and seemed fine, including the footswitch working, until I strummed a chord.

The amp is so saturated and overly-distorted on both the crunch and lead channels that it sounds like a fuzz box an steroids is inside it! The volume knobs work as well as the tone controls but the gain knobs have absolutely no affect whatsoever on the insane amount of distortion. They don't make any change at all.

I looked for schematics on the 'net but the sites I found couldn't agree if they were accurate or not so I don't have one available... sorry.

The amp has 5 12ax7's and two el84's in it and I listened to a couple videos on you tube and they were nowhere near this full-blown. It isn't playable at all in this condition. I haven't had a chance to pull the chassis out and have a look but thought maybe someone could head me in the right direction before I do.

Thanks for everything in advance. You guys have been great!

[xtian]

I think this amp has an FX loop. This will help you see if the preamp is OK, by listening at the FX SEND. And you can test the power amp by plugging in a known good preamp signal into the FX RETURN.

[alexradium]

It's an SLO,just undo all the mods, check out the gain pots, sometimes they wear out and short.

[Jon Snell]

I would also check the anode load resistors on all ECC83s. (12AX7). If one has gone high value you will get a weak horrible noise when playing.

[Mick Bailey]

I would be looking for a wiring/modification problem first-off. You have to know the circuit is correct otherwise nothing will make sense. When I get this kind of job in I firstly appraise what's been done, making a note of any cut tracks, lifted pads, unusual or different component types and any damage. Basically - a visual inspection of changes or damage. If I'm not sure about something I try to find a picture of the same type of amp - sometimes it can give a clue about what's happened. It may be that the person who modified it attempted to follow instructions for a popular modification that went wrong. When I find an area that's been worked on I do a search for (say) 'R36/C12 mod' and sometimes this will turn up a reference to something that's been attempted.

You do need to pull the chassis and inspect the amp to decide on a course of action.

[copperheadroads]

There was quite a bit of info in this thread about the common mods like the depth mod
http://www.guitargearforum.com/forum...615&start=1925

[nickb]

Here is the schematic I have for the 22H. As far as I know is correct.

jet-city_jca22h_sch.pdf

It's hard to give good help when you don't know what you dealing with. Once you've gone through it and can provide a new schematic, we can only give general guidelines. The first thing I'd measure are the voltages on each cathode, grid and plate to see if they are in the expected ballpark. Are you able to estimate those or do you need help? Takes 5mins and gives a lot of info. Then address any discrepancies. Next, I'd mark up the stock schematic with the mods as seen and post it.