Jet City users - to mod or not to mod?

McToot

Eh up.  I picked up a Custom 22 from @p90fool a few months ago and have been having a lot of fun re-discovering my rock and metal days. I even bought a pointy-Floydy thing and a pink mock-croc strap to make me sound better (erm...). 

It's a cracking bit of kit for the money, but it has one main weakness in my eyes. The crunch channel has very little clean headroom and goes to full-on distorted, while the overdrive channel starts at face-melt and adds hot sauce. I typically set the two channels up with the gain on 1 (crunch) and 2 (OD)... 

What would be really nice would be to tame the gain on both and ideally get a bit of a more organic richness to the structure of the drive. There's a guy in the US that does a whole variety of mod kits for the enthusiast and there could be a couple of interesting options for $50-75 (plus shipping). @p90fool also shared a simple set of mods to reduce the gain on the OD channel.

I don't have the skills but I have a tame amp guy nearby, so all sort of things are possible.  The amp cost me so little that I'm happy to invest a bit in it, even though it's not my main amp. 

Any experiences of mods? Any advice?

ICBM

If you want to reduce the gain on both channels you could try a lower-gain preamp valve (or valves). I haven't done this personally but it's an easy thing to try and from the size/quality of the components in these amps there shouldn't be any worries about overloading low-power resistors as there can be with some amps (eg Fender reissues).

In descending order of gain the types that can replace a 12AX7 are: 5751, 12AT7, 12AY7, 12AU7. A 12AU7 is probably too drastic, but if you want to try one they're fairly cheap.

All these types should be OK to use in either the V1 or V2, or both, positions - V1 affects both channels and V2 just the overdrive channel. (Don't try them in V3 or V4, these are cathode-follower positions which don't really affect the gain much and where a 12AT7 or 12AY7 can't be safely used anyway.) V5 is the phase inverter and also won't affect the gain, although it can affect the clean headroom at high volume - a 12AT7 is probably the best choice if you want to try changing it.

McToot

Many thanks for that @ICBM ; - a re-valve with a couple of different options is certainly the first stop... The guys on the JCA forum on Facebook recommend a 5751 or 12AT in V1 and V2.  

I haven't even opened the amp up to see what the stock tubes are, but will do.  

not_the_dj

Very interested to hear how you get on. I have the same amp so might try the same at some point. 

McToot

not_the_dj said:

Very interested to hear how you get on. I have the same amp so might try the same at some point. 

What I know so far (via the Facebook group) is that if you want to tame the ludicrous gain on the OD channel and you know how to use a soldering iron (I do not), then follow the following:

"Try this off of V2: R17, change to 680K from 470K; R18, change to 220K from 470K and remove C57 which is a 470pF across R58"

That might as well be in Swahili for me, but my amp guru would get it...

@p90fool tried that and it worked well, though still leaves more gain than most would need. He also let me know the following:

"You can also tame the brightness on that channel by lifting one leg on C6. "

This sounds like a game of Twister, but again I'm sure it'll make sense to some people

p90fool

I have some 12AT7s somewhere, I'll dig them out and try them early next week.

peteri

I had one for ages, changed the valves and it made all the difference in the world. 

AT in V1 made the cleaner channel really usable for me, putting a better quality valve in the phase inverter made the dirt channel much nicer. 

From memory I also put a better quality V2 in as well. 

Bang for with three valves changed it was the best amp I’ve owned, nice crunch on the ‘clean’ which cleaned up lovely with the guitar volume and great lead sound 

took pedals very well

only thing I didn’t like so much was the effects loop which is after the MV?

ICBM

peteri said:

only thing I didn’t like so much was the effects loop which is after the MV?

Yes, which is where it should be. (Or at least after the distortion - just before the MV also works as long as it's after the previous valve stage.)

The 50 and 100W models have it earlier in the circuit which is worse, because the loop is then before some of the distortion.

Kebabkid

Hi Jules,
If you want someone else to do the mods, Dan Gower is fantastic and has some history with these

http://www.metalireland.com/community/viewtopic.php?f=2&t=53075

p90fool

Well I tried a 12AT7 in V2 and it's excellent in conjunction with the resistor changes I'd already made. Being able to have the gain control higher also reduces the need to cut out C6, it's much warmer when you don't have to have the knob down at 3 or 4. 

Without my mods I'd be tempted to try a 12AY7 in V2 first, or maybe a 12AT7 in V1 and V2 if you want to affect the crunch channel too. 

It'll cost a few quid to grab a handful of preamp valves to try, but still way less than having a tech mod the amp. 

McToot

p90fool said:

Well I tried a 12AT7 in V2 and it's excellent in conjunction with the resistor changes I'd already made. Being able to have the gain control higher also reduces the need to cut out C6, it's much warmer when you don't have to have the knob down at 3 or 4. 

Without my mods I'd be tempted to try a 12AY7 in V2 first, or maybe a 12AT7 in V1 and V2 if you want to affect the crunch channel too. 

It'll cost a few quid to grab a handful of preamp valves to try, but still way less than having a tech mod the amp. 

Thanks Julian. I intend on doing that. I've also found out that the feller from Epic on the JCA forum doesn't have to send his kits from the US as he simply sources the components here and has them sent to you; presumably the instructions are just emailed owner. Sounds like a neat idea.  I may look at one of his mods too.  In time.  

ICBM

p90fool said:

It'll cost a few quid to grab a handful of preamp valves to try, but still way less than having a tech mod the amp. 

And has the advantage that you can try several different combinations of gain reduction at different places in the circuit, very quickly and with the option to put it all back to stock at any point.

Apart from the totally unnecessary bright cap on the overdrive channel of the 50 and 100W models - which I honestly think may be to deliberately differentiate it from the SLO which has to somehow justify being ten times the price! - even the one on the crunch channel corresponds to the SLO's bright mode - I think these are really well-voiced amps generally. There's a lot of gain, but it's much easier to reduce it than to add it. (Other than by sticking a TS in front .)

p90fool

ICBM said:

p90fool said:

It'll cost a few quid to grab a handful of preamp valves to try, but still way less than having a tech mod the amp. 

And has the advantage that you can try several different combinations of gain reduction at different places in the circuit, very quickly and with the option to put it all back to stock at any point.

Apart from the totally unnecessary bright cap on the overdrive channel of the 50 and 100W models - which I honestly think may be to deliberately differentiate it from the SLO which has to somehow justify being ten times the price! - even the one on the crunch channel corresponds to the SLO's bright mode - I think these are really well-voiced amps generally. There's a lot of gain, but it's much easier to reduce it than to add it. (Other than by sticking a TS in front .)

Yeah I put my bright caps on a 3-way switch - both on, both off, or crunch channel only on, so I could have jangly rhythm and fat lead. 

They're fun and easy to mod though.