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Tube problem?

outoftune

Member
Messages
65
I have been having problems with my Gibson GA-30rv head for a while now. I've had it fixed a couple times for a couple different problems. The last time it was fixed the note about the repair said something along with the lines of tube sockets were not properly alligned and that it was a manufacture flaw or something. It said that the same issue would probably present itself later.

I have just experienced a problem again where the amp powers up but there is no output at all. I opened up the amp and found that when powered up, all 4 power tubes fire up but 1 or 2 of the preamp tubes do not. Should I just try replacing them? Or would I still be getting some sort of output even if one was bad?
 

Blue Strat

Member
Messages
30,192
When you say that the preamp tubes aren't "firing up", what exactly do you mean? Are they not lighting up? If so, they're not getting heater voltage/current from the power transformer. This would mean that there's an open or intermittent connection in the heater wiring.
 

outoftune

Member
Messages
65
sorry I should've been better in my description. Yes I mean that it is not lighting up and is not warm upon touch.
 

outoftune

Member
Messages
65
i found that one of these is the phase splitter tube. i'm not sure what these do exactly. any suggestions would be appreciated. here is the link to the schematic.
[SIZE=-1]
schematic
[/SIZE]
 

outoftune

Member
Messages
65
I measured two of the pins of the phase splitter, pin 1 and pin 6 whose voltages should have read 280 and 270 read 350 and 250 respectively. Is this definitely something I can't do myself? I'm thinking of just taking it in but am really short on cash right now.
 

Blue Strat

Member
Messages
30,192
If the tubes really aren't lighting up, the voltages on pins 1 and 6 will be off because the tube isn't doing anything.

I highly recommend a trip to the tech.
 

John Phillips

Member
Messages
13,038
This amp uses a complex DC supply for the preamp tube filaments, and it's likely that the rectifier or voltage regulation circuit has failed. This is a job for a tech, you'll have to get the circuit board out, and it's not a simple circuit either.

While they're in there, have them replace all the power tube sockets with ceramic ones, not the same plastic crap they used at the factory, since then the problem (which is that the plastic sockets shrink and pull the tube pins out of alignment and can even crack them) will not present itself again. I find it hard to believe any competent tech wouldn't do this the first time. I would even consider doing the preamp tubes as well, although they don't get as hot.
 

outoftune

Member
Messages
65
Yeah I wish I had my final 3 semesters of ee and the proper equipment and time to try to fix it myself. Thank you for the advice about the sockets. I will tell them to replace them.
 




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