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I really don't like JJ preamp valves - they have a dark, middy/muddy tone. But if your amp is too bright-sounding now, they might be what you need. Even then I would be inclined not to change all of them at once until you hear what effect each one has.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I got this amp off a lad on here a few years back. Was always fine but started to sound a bit lifeless a few weeks back so I changed the power tubes which helped greatly. While I was in there I noticed that the 5 preamp tubes are a mix of the cheap Chinese ones I assume came in it, a JJ and an EHX one. Was thinking new preamp tubes would help too? happy to be wrong though. That’s why I asked about tube positions. Dunno what difference the positions make.
Noise, randomness, ballistic uncertainty.
I know that doesn't help much, except maybe to say throwing lots of money at it might even make it worse.
Noise, randomness, ballistic uncertainty.
V1 - first gain stage for both channels
V2 - second and third gain stages for lead channel
V3 - cathode follower driving FX loop
V4 - cathode follower driving tone stack
V5 - phase inverter
I have heard that newer JCA50s may have the tone stack and FX loop the other way round (like the 20W models), but I don't know if that's true.
It's probably also worth mentioning that any Russian-made and possibly JJ valves are a bad idea in V3 and V4, whichever way round they are - they have a high failure rate in cathode followers.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Noise, randomness, ballistic uncertainty.
Usually the valve closest to the input jack is V1.
The Chinese valves don't actually sound too bad for a modern valve. A lot better than Sovteks and better than JJs I would say.
If you really wanted to swap a preamp valve I suggest swapping just the V1 to a TungSol 12ax7, but only if you don't think your amp is too bright already, as it will boost your top end chime, which can translate to more fizz. If you don't want to make your amp any more brighter, then I'd just stick to the Chinese 12ax7 in V1.
https://www.ebay.co.uk/itm/Un-Branded-12AX7-CV4004-Military-Grade-Vintage-Valve-NOS-Teonex/254104965991?hash=item3b29d61367:g:aJQAAOSwBahVF~-W
I've had the same set in my JCA50H for about five years now, and they're still going nicely
EDIT: In terms of an effect on the sound...relative to JJs, I've found them less fizzy. It's a minor thing, though - the fact that they seem to last forever is far more important to me.
Surprisingly, EH valves are made in the same factory as Sovteks, but sound much better. Although they still have the failure problem in cathode followers, and a shorter lifespan in general.
Overall I prefer old-production valves - they’re far more consistent than new ones, and there’s much less variation between different brands, so you don’t need to pay through the nose for the most hyped ones. Although that said, the ones that are probably most different from the norm are RFTs, which are my favourites in Marshall-type amps.
I also like the cheap bog-standard Chinese Shuguang 12AX7s, for modern valves. They have a nicely balanced sound and are fine in cathode followers.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Back in the day when I did tone comparisons using my Satch JVM, and a whole range of tubes, I was actually very surprised at how much of a difference they made to the final tone.
I've noticed more tonal variance when switching out preamp tubes than poweramp tubes if I'm honest.
What power valves affect is the dynamic response and presence at higher volume, especially if the power stage is overdriven - which I know is something both of us rarely do since we generally use powerful master volume amps.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
One thing I'd read about Jet City amps was that in comparison to the Soldano's they were fizzy sounding, with less body. I run the presence low, 2/3, but it's defo not fizzy to my ears. The shop where I got the tubes has a early Hot Rod 50 in stock, so I got to A/B them through a nice Mesa 2x12. Now, it had just had two new power tubs installed, but the Jet City sounded better to me. I set them both up identically, and the Soldano was a bit lifeless on comparison.
Checked the preamp tubes in mine. it goes:
V1 JJ ECC83
V2 no name 12ax7
V3 EHX 12ax7
V4 JJ ECC83
V5 no name 12ax7
Given whats been said here, the only thing I'd be tempted to do is swap the JJ out of V4 as @ICBM mentioned them not doing well as a cathode followers, so just swap 4 and 5 maybe?
Noise, randomness, ballistic uncertainty.
It's more important to get the EH (Russian made) out of V3, that's a definite failure risk.
I would put that in V1 as it's probably the nicest-sounding. JJ in V2 for the most midrange/chunky overdrive tone, and V5 (phase inverter), and the no-name ones (which are probably Chinese, so fine as cathode followers) in V3 and V4. Chinese 12AX7s are easily identifiable by the double chrome strips on each side of the internal structure.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein