Carvin Bass Amp tube question

[Simon Barden]

With no tube, you should get no sound from the amp at all in the tube position, only in bypass. If you got sound in both modes, then the switch circuit isn’t working. It’s because you said that it sounds the same that there’s the possibility of the switch not working as it should.

I don’t know what voltage the tube is working at, but it won’t be high tube amp voltages, certainly under 100v, probably nearer half that. Preamp tubes are pretty long-lived (or they were in the good old days) and run at lower voltages, last even longer. Even so, the cost of a valve base wouldn’t have added much to the amp’s build price rather than solder the tube in.

[PJSprog]

I found a picture on the inturwebs of the pcb, and there definitely should be a tube socket there.


The leads on the tube will sometimes corrode over time, making them not slide in and out of the socket so easily (which also kind of helps to keep them in place, a tight friction fit). Give it a firm tug - but don't crush it in the process - and it should come right out.

[DrNomis_44]

The best way to remove a 12AX7 tube from it's socket is to gently move it from side to side while pulling so you basically are walking it out gradually, try to be gentle while you're doing that so you don't put a crack in the glass envelope near the base of the 12AX7.

[fender3x]

May have to hold my breath when I do it... Thanks guys!

[Simon Barden]

Looks like you may have to undo a couple of screws to get that board off its mountings, but you'd have to do that anyway if you wanted to swap out the 'tube' (even if it is really a 'thermionic valve' ).

[DrNomis_44]

They are usually called Tubes in places like the United States, they are more commonly known as Thermionic Valves in places like the United Kingdom and Europe, but it basically means the same thing whether you call it a Tube or a Thermionic Valve.

[Simon Barden]

Well, we normally shorten the term to just 'valve' in normal usage, though sometimes you need to be more specific to differentiate them from valves for hydronic/hydraulic use.

[fender3x]

I think Doc Nomis is right...I have even seen them referred to as "thermionic tubes." Among the many things I don't understand about them is why we call them tubes. Both Fleming and De Forest refer to their inventions as a "valve." This makes sense not just from a functional standpoint, but because they were made in bulbs rather than tubes. I have not been able to find out why we say tube rather than valve.

[fender3x]

Ok, popped off the top and wiggled the tube out of the slot. As Simon predicted, no sound with the valve out on the tube setting (splitting the difference with the terms here ;-) ). Put the valve back in. Sounds the same on both settings.

In the process I learned a couple of things. The published schematics are versions F and K. My board is a G made in 2010. The only difference seems to be an added cap in the DC protection circuit, so that should not effect anything we've been talking about.

I also learned that the tube as a Sovitek 12AX7WA. Not sure if any other tube would make a difference in this slot. Right now it's the only 12AX7 I have ;-)

[dave.king1]

Ok, popped off the top and wiggled the tube out of the slot. As Simon predicted, no sound with the valve out on the tube setting (splitting the difference with the terms here ;-) ). Put the valve back in. Sounds the same on both settings.

So, from this rather remote distance I would suggest that the switch is probably stuffed and stuck in one position