Carvin Bass Amp tube question

[fender3x]

My main bass amp is a Carvin BX500. Carvin stopped producing amps in 2017, but up to then was pretty well known in the US. I gather that it's not that well known outside the US... Mine has class D power, with lots of tone shaping knobs (drive, parametric eq, graphic eq, treble, bass and contour and compression), and weighs in under 6lbs.

It's mostly solid state, but has one 12AX7 tube which can be engaged or disengaged. I thought this might make the sound a little warmer, or change the sound of the overdrive. What it seems to do, however is nothin at all. I am thinking of swapping out the 12AX7 for some other tube, but I don't know much about this and wondered if anyone here had advice about that...and what I might expect by changing the tube?

[DrNomis_44]

My main bass amp is a Carvin BX500. Carvin stopped producing amps in 2017, but up to then was pretty well known in the US. I gather that it's not that well known outside the US... Mine has class D power, with lots of tone shaping knobs (drive, parametric eq, graphic eq, treble, bass and contour and compression), and weighs in under 6lbs.

It's mostly solid state, but has one 12AX7 tube which can be engaged or disengaged. I thought this might make the sound a little warmer, or change the sound of the overdrive. What it seems to do, however is nothin at all. I am thinking of swapping out the 12AX7 for some other tube, but I don't know much about this and wondered if anyone here had advice about that...and what I might expect by changing the tube?

You can replace it with a 7025 (basically a low noise version of the 12AX7), an ECC83 (the European equivalent of the 12AX7), a 12AX7A, a 12AX7WA tube and they all should work fine.

You might notice a slight change in the tone, or amount of gain you get, the 12AX7 is basically a high gain (theoretical maximum of 100) small signal twin-triode tube with a nine-pin base, it is used a lot in guitar amplifiers and some high-end stereo audio equipment.

Best thing to do is get hold of a few 12AX7, 7025, and ECC83 tubes and try each of them out one by one till you find one that has a tone you like.

[Marcel]

With the tube switched in it is doing something, but how much of what is the question.

Totally agree with Doc on swapping the tube with any of the variants he suggested. Normal design has these types of tubes a cathode biased or self biased so there is nil need to set the bias on each change, they will correct themselves. Typically it is only output power tubes that need biasing adjustments done by a amp tech.

I doubt you will hear any tonal differences, but you will most likely hear gain changes with different tubes. Actually it makes me wonder about your gain structure and if you are driving the tube hard enough to generate some level of overdrive sound when the tube is switched in. While a clean bass is the most popular option an over driven tube bass is popular in some genres of music which I'd suggest is why the tube was included in the amp in the first place.

Just power down the amp when you do the swaps

And have fun...

[Simon Barden]

I doubt you'd get anything different by changing the tube. It may be that the switching circuit isn't working properly so it's not being brought in. If it is, and you are getting sound, then the tube must be working. You could pull the tube out and see if you get any sound with the tube selected. If you do, then the switching circuit is probably faulty. You won't do any damage.

[DrNomis_44]

I seem to remember doing some work on a Hartke bass amp a few years ago, if my memory serves me right it had a control on the front panel which allowed you to blend between a solid-state preamp and a tube preamp circuit, I do remember that the tube in it was a 12AX7, the rest of the amp was solid-state, to my ears the tube preamp sounded a bit warmer than the solid-state preamp.

[Simon Barden]

You can find a circuit diagram for it on line (though there have been quite a few revisions so yours might be slightly different in components). This tube section looks more like an extra preamp stage (i.e. not in parallel with anything or instead of another solid state stage) with both halves of the 12AX7 in use, but I can't tell how exactly much gain it's been configured for, if any. No extra controls for it so it's dependent on upstream signal levels for pushing any drive from it. It appears to be right before the power amp section, between the pre-amp and the power amp, and is after both the master volume and the limiter. So to get it to overdrive, you'll need the master volume up as well as a reasonable level of input drive. It only looks like this is going to make much difference at loud gig levels to me, much the same as a loud non-master volume valve bass amp would behave.

Don't forget that tubes when not overdriven, can be almost as non-distorting as solid state devices, just a bit noisier.

[fender3x]

I downloaded the schematic. Thanks, Simon, for also taking a look. I came to the same conclusion, but my read on such things is not to be trusted. so nice to have somebody who knows what they're doing take a look too.

It also looks to me like the tube circuit comes in after everything else, just before the power amp. No need to pull it out to find out what it sounds like without the tube. It has a switch that will do that. To my ear it sounds exactly the same with the tube in or the tube out. But it may be, as Simon suggests, because I am playing at garage volumes with the master barely cracked open.

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[Simon Barden]

It’s not a simple switch as it’s driving transistor switching circuits, which is why I suggested taking the tube out to check the switching circuits are working properly and are bringing the tube in and out.

[Simon Barden]

Been looking at the circuit diagram and as far as I can make out, neither the DI or headphone out are taken after the valve circuit, so you can't even disconnect the speaker, wind up the amp and listen via a mixer or audio interface.

[fender3x]

It’s not a simple switch as it’s driving transistor switching circuits, which is why I suggested taking the tube out to check the switching circuits are working properly and are bringing the tube in and out.

OK... I popped the top off, and wiggled the tube... it's really stuck in there tightly. Which made me wonder what I would learn if I took the tube out? Right now I can't hear any difference between tube in and tube out. That's what at least a dozen reports about Carvin BX amps on the internet report. So, if I take the tube out, and it sounds the same on both settings, what will I have learned? Also, it's on it's own PCB board, and I wonder if the tube might be soldered in? I can't see anything that looks like a socket from the top, and it would be a pain to get under it. Tube in or out the tone from this amp is pretty good, particularly once it goes through my Sansamp BDDI. I was really just wondering if it might be worth a try to coax a different tone out of the amp by trying a different tube, but at the moment I am thinking that maybe I should let this sleeping dog lie.