Retubing a 20 watt Jet City combo

[Birdman642]

I have a Jet City 20 watt combo amp, I bought it used from my old Bassist's dad about six years ago. He replaced the tubes, but swapped them back to the stock ones when he sold the amp to me. It was my first, and so far, only tube amp. I'm wondering if it's time for me to retube it.
I used to gig somewhat frequently (once every couple weeks) and practice pretty frequently (at least once a week).
It's been my go to amp all this time and I've been playing it at bedroom levels for the last couple years now. I am however getting back into jamming and have to run it really loud to keep up with my current drummer.
According to Jet City I need: Three 12AX7s (preamp), two 12AX7s (reverb) and two EL84s (power amp). What do I need to know about retubing/biasing the amp? What do I need to know about tubes? Do I need to replace the tubes at all? I was also thinking about swapping the speaker for a Celestion V30
Thanks

[guitfiddle]

Re: Retubing a 20 watt Jet City combo

If it's working and you like the tone, let it be. When it seems like it's losing power and struggling, start by replacing the power tubes.

[BenPerkins]

Re: Retubing a 20 watt Jet City combo

What’s the amp model? If it’s cathode biased, you can just swap out the tubes with no rebias needed. If the amp is fixed bias, you’ll need someone (ex: reputable guitar shop or amp tech) to bias the amp for you.

If it has EL84’s, it may be cathode biased. What brand of tubes are in there now?


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[Ascension]

Re: Retubing a 20 watt Jet City combo

Owned a couple Jet City 20 watters. Had the 1st gen JCA22H head first and now own one of the new revoiced JCA22H Customs so know these amps pretty well.
Start with a power tube change and when I change power tubes I also spring for a new balanced Phase inverter tube just to be safe.
In these amps I would very strongly recommend the Mullard RI EL84's by New Sensor. They are a different design than most EL84's with a slightly larger bottle and breakup a little earlier + feel differently than other 84's. To my ear the Mullards have a softer more 6V6 like feel when they breakup that really works well in these particular amps. I own 4 EL 84 amps currently have a box full of different EL84's to swap in and out and to my ear in these amps right now in current production tubes those Mullards are by far the most musical of all of the EL84's. You wont notice that huge of a difference between these at super low volume but it's pretty dramatic when you really crank one of these with these particular power tubes how they come to life a higher volumes and begin to open the amp up tonally.
Start with that as pre amp tubes normally last a LONG time but power tube need to be pretty regularly changed if you play out a lot.
Good luck and keep us posted.
PS while these do have a bias pot they are pretty much just plug and play as yes they are cathode biased.

[BenPerkins]

Re: Retubing a 20 watt Jet City combo

Owned a couple Jet City 20 watters. Had the 1st gen JCA22H head first and now own one of the new revoiced JCA22H Customs so know these amps pretty well.
Start with a power tube change and when I change power tubes I also spring for a new balanced Phase inverter tube just to be safe.
In these amps I would very strongly recommend the Mullard RI EL84's by New Sensor. They are a different design than most EL84's with a slightly larger bottle and breakup a little earlier + feel differently than other 84's. To my ear the Mullards have a softer more 6V6 like feel when they breakup that really works well in these particular amps. I own 4 EL 84 amps currently have a box full of different EL84's to swap in and out and to my ear in these amps right now in current production tubes those Mullards are by far the most musical of all of the EL84's. You wont notice that huge of a difference between these at super low volume but it's pretty dramatic when you crank one of these with thee tubes how they come to life a higher volumes and begin to open the amp up tonally.
Start with that as pre amp tubes normally last a LONG time but power tube need to be pretty regularly changed if you play out a lot.
Good luck and keep us posted.
PS while these do have a bias pot they are pretty much just plug and play as yes they are cathode biased.

+1 for Mullards. I’m wondering how Genalex Gold Lion EL84s sound in this amp

[Birdman642]

Re: Retubing a 20 watt Jet City combo

What’s the amp model? If it’s cathode biased, you can just swap out the tubes with no rebias needed. If the amp is fixed bias, you’ll need someone (ex: reputable guitar shop or amp tech) to bias the amp for you.

If it has EL84’s, it may be cathode biased. What brand of tubes are in there now?


Sent from my iPhone using Tapatalk Pro

I'm not sure what brand is in there right now. Just the stock, I'm assuming Chinese made ones, right now. I haven't pulled it apart to look at it

[Ascension]

Re: Retubing a 20 watt Jet City combo

+1 for Mullards. I’m wondering how Genalex Gold Lion EL84s sound in this amp

The Gold Lions would be great in a hi fi receiver but IMO are a little to clean for what these amps are all about. My second choice for one of these is the TAD STE big bottle 84's and is quite similar in design to the Mullard. I have JJ'S Sovetecs, TAD's Shunghangs and the Mullards all on hand so have run them head to head in my amp. At higher volumes where you started getting into break up was where the Mullards really shined as they were notably more musical and harmonically rich.

[BenPerkins]

Re: Retubing a 20 watt Jet City combo

The Gold Lions would be great in a hi fi receiver but IMO are a little to clean for what these amps are all about. My second choice for one of these is the TAD STE big bottle 84's and is quite similar in design to the Mullard. I have JJ'S Sovetecs, TAD's Shunghangs and the Mullards all on hand so have run them head to head in my amp. At higher volumes where you started getting into break up was where the Mullards really shined as they were notably more musical and harmonically rich.

Mullards are good all around - I put some in my OR15 and it eliminated hiss while tightening up the gain a little bit. What tubes typically come stock in these amps?

[misterwhizzy]

Re: Retubing a 20 watt Jet City combo

+1 for Mullards. I’m wondering how Genalex Gold Lion EL84s sound in this amp

Those probably cost as much as the whole amp.

[Birdman642]

Re: Retubing a 20 watt Jet City combo

Those probably cost as much as the whole amp.

They aren't quite as expensive, but about a quarter for a matched set

[Birdman642]

Re: Retubing a 20 watt Jet City combo

I'm wondering what it means when they say "high gain" tubes. Does this mean they break up quicker or they actually give more gain to the preamp section? I'm playing in a metal band and I just started experimenting with tones. I'm setting the gain on the amp to get a crunch sound (Think AC/DC level) and then boosting that with an EHX Metal muff with the gain set around 8 o'clock

[Ascension]

Re: Retubing a 20 watt Jet City combo

Mullards are good all around - I put some in my OR15 and it eliminated hiss while tightening up the gain a little bit. What tubes typically come stock in these amps?

Chinese Shunghangs. Not bad tubes over all and in particular I like the Preamp 12AX7 Shunghangs a lot. Have a complete set of those in both of my Boogie Subway Rocket Preamps and those amps love them.
The Mullards I was referring to are the New Sensor current production EL-84 Power Tubes. Pretty inexpensive tubes but really sound good in these amps as they have a nice soft break up when you push them. Works very well in these amps in particular.

[Ascension]

Re: Retubing a 20 watt Jet City combo

I'm wondering what it means when they say "high gain" tubes. Does this mean they break up quicker or they actually give more gain to the preamp section? I'm playing in a metal band and I just started experimenting with tones. I'm setting the gain on the amp to get a crunch sound (Think AC/DC level) and then boosting that with an EHX Metal muff with the gain set around 8 o'clock

High gain is just that additional gain in the tube. It's a measured out put of the tube itself but higher gain is not always musical. Can be harsh on teh high end muddy and flat sounding overly compressed ect. Pre amp tubes are where you will see the most noticeable tone difference and in particular the earliest pre amp stages make the most difference as V-1 normally sets the stage for everything that follows and V-2 on most amps is the driver for the high gain channel on a switching amp.

[misterwhizzy]

Re: Retubing a 20 watt Jet City combo

My experience with my JCA20H had been that setting the bias on the power tubes makes a much bigger difference than the brand that's in there. The last set of cheap JJs in there sounded just as good as the Mullard, Tung-Sols, and EHX power tubes I tried in it. I found changing the preamp tubes to be a much more effective tone-shaping tool. The best advice I got was to try a 12AU7 as a phase inverter to give it just a bit more clean headroom.

I know old man Ascension wants to be the authority on these amps, but there is more than one point of view. Really my only gripe with that amp was lack of an effects loop and its slight thinness on the unwound strings. Both of those were solved with amp mods, but it turns out I just don't really like the way EL84s break up, so I upgraded to a JCA50H and modded that to SLO preamp specs with EL34s, and that's the ticket for me.

But Jet City amps are an excellent value, and I hope you play the hell out of yours.

[Birdman642]

Re: Retubing a 20 watt Jet City combo

But Jet City amps are an excellent value, and I hope you play the hell out of yours.

Oh trust me, I have haha. It's been with me almost every step of the way. I really need a crash course on amps as I was always far more interested in learning about guitars. But I'm learning (slowsly but surely) that amps play a much bigger part in your soud than a guitar. It seems like swapping tubes and such (as with almost anything musically) is a matter of taste/price bracket. Obviously some will be better built than others, but again, taste/price. I plan on getting a 6505+ but that comes after I get my big boy job haha