Blew out V3 in my Solo 50 (JJ 12AX7 high-gain)

[cellardweller]

Blew out the tube at V3 in my Solo 50...no idea how it happened.
Powered 'er up, let it warm, plugged in...not a peep on channel 2.
No glowing at v3 so I swap it out with the tube I believe to be from that position.
Practice continued but I hated my sound and want to do some experimentation ...changing about the preamp tubes.

I have plenty of spare 12ax7's laying around, and just wanted to make sure (as I believe I remember it to be) that the preamp tubes CAN be mixed and matched without detriment to the amp.

Thanks in advance.

 

[crane]

For sure they can. Mixing brands and type can really help finding your sound.

 

[cellardweller]

I've been fortunate to inherit a box of old tubes....Some RCA 12AU7A's, Two 12AT7's (one is GE, the other Sylvania) some 12AY7's...and I don't know squat about any of these!

 

[94Tremoverb]

V3 is the cathode follower position in the Solo 50, which is probably why the tube blew, and why the replacement sounds bad. A lot of Russian-made 12AX7s have a problem with the cathode to filament voltage rating being below the correct spec, and a cathode follower runs the cathode at a couple of hundred volts above the filament. A poor rating will allow current to flow from the cathode to ground via the filament - if enough current flows, it will blow the filament (which would explain it not glowing) or if not enough it will still end up biasing the tube completely wrongly, which will sound terrible.

Avoid any Russian 12AX7 in cathode follower positions unless you know for sure that the particular tube is OK with it. This may apply to other tubes too, but so far I've only seen it with Russian ones. Use a Chinese, or better still a NOS tube (which were made properly to the right spec) and you should have no problem. You can't substitute any 12AT7 or 12AY7 in cathode follower positions either for the same reason, although this time by design - the spec was lower, 90V for these two, as opposed to 200V for the 12AX7 and 12AU7, so *don't* try them in V3 - but you can use them in the other positions. In particular you may find the 12AY7 sounds good in V1 or V2 - if you want a cleaner and clearer tone, although with less gain - and the 12AT7 in V4 or particularly V5 (the phase inverter) which will also increase clarity and headroom but not affect the distortion gain as much because that mostly comes from the first three tubes. The 12AU7 is probably too low-gain for your amp, unless you really want to bring down the gain and volume a fair amount, but they have a nice tone so it may be worth trying if you don't run the amp wide open usually - you'll just have to turn it up further.

 

[Neptical]

What do you think of that Solo 50? I had one for over 10 years and tried experimenting with all sorts of different tubes but could never get it out of that boxy fizzy tone no matter what I did.

All the Best,
~Nep~

 

[cellardweller]

V3 is the cathode follower position in the Solo 50, which is probably why the tube blew, and why the replacement sounds bad. A lot of Russian-made 12AX7s have a problem with the cathode to filament voltage rating being below the correct spec, and a cathode follower runs the cathode at a couple of hundred volts above the filament. A poor rating will allow current to flow from the cathode to ground via the filament - if enough current flows, it will blow the filament (which would explain it not glowing) or if not enough it will still end up biasing the tube completely wrongly, which will sound terrible.

Avoid any Russian 12AX7 in cathode follower positions unless you know for sure that the particular tube is OK with it. This may apply to other tubes too, but so far I've only seen it with Russian ones. Use a Chinese, or better still a NOS tube (which were made properly to the right spec) and you should have no problem. You can't substitute any 12AT7 or 12AY7 in cathode follower positions either for the same reason, although this time by design - the spec was lower, 90V for these two, as opposed to 200V for the 12AX7 and 12AU7, so *don't* try them in V3 - but you can use them in the other positions. In particular you may find the 12AY7 sounds good in V1 or V2 - if you want a cleaner and clearer tone, although with less gain - and the 12AT7 in V4 or particularly V5 (the phase inverter) which will also increase clarity and headroom but not affect the distortion gain as much because that mostly comes from the first three tubes. The 12AU7 is probably too low-gain for your amp, unless you really want to bring down the gain and volume a fair amount, but they have a nice tone so it may be worth trying if you don't run the amp wide open usually - you'll just have to turn it up further.

THANK YOU FOR THE EXTENSIVE INFO!!!

The tubes in question are JJ's.

Before having read this, I tried swapping several 12AX7's and noticed all glowed significantly less-bright than the 12AX7's in location V1 and V2...An actual "mesa" branded 12AX7 glowed by far the least-bright...what is the significance of this?

 

[cellardweller]

What do you think of that Solo 50? I had one for over 10 years and tried experimenting with all sorts of different tubes but could never get it out of that boxy fizzy tone no matter what I did.

All the Best,
~Nep~

I probably do not have a fair basis for comparison, and nothing else even approaching decent to compare it to (only other amp is really bad S.S.).

 

[cellardweller]

Use a Chinese, or better still a NOS tube (which were made properly to the right spec) and you should have no problem.

Any way of knowing what the "mesa" branded tubes are (chinese...russian...)???

 

[94Tremoverb]

Chinese 12AX7s are easily identifiable by the double bright chrome strips on the sides of the plate structures.

It's interesting you've had the voltage rating problem (which I think it is, the symptoms match perfectly) with JJs - I haven't come across this yet but I will bear it in mind. I don't like JJ 12AX7s anyway though, for tone reasons - I find them muddy and lifeless.

V1 has a DC filament supply (to reduce hum), which is why it will glow slightly less brightly. I only have the schematic for Series 1, but it's likely that Series 2 has this on V2 as well if you're seeing the same thing. If it's serious it may be that there is a problem in the filament rectifier circuit, but I wouldn't worry about it if the amp seems to run OK otherwise.

 

[cellardweller]

Chinese 12AX7s are easily identifiable by the double bright chrome strips on the sides of the plate structures.

It's interesting you've had the voltage rating problem (which I think it is, the symptoms match perfectly) with JJs - I haven't come across this yet but I will bear it in mind. I don't like JJ 12AX7s anyway though, for tone reasons - I find them muddy and lifeless.

V1 has a DC filament supply (to reduce hum), which is why it will glow slightly less brightly. I only have the schematic for Series 1, but it's likely that Series 2 has this on V2 as well if you're seeing the same thing. If it's serious it may be that there is a problem in the filament rectifier circuit, but I wouldn't worry about it if the amp seems to run OK otherwise.

V1 and V2 were glowing MORE brightly than the replacement tube I put in the V3 position.

 

[94Tremoverb]

Sorry, I misread that, but it's the same reason - the DC supply for V1 (and probably V2, if this is a Series 2) just produces a different visual glow. If it's brighter it's unlikely there's anything wrong with it either.