ShortScale.org

Ok, so I have had my Blues Deville for 7 or 8 years and have only had to "maintain" or fix it 3 times thus far. That is with being regularly gigged for the last 5 or 6 years. I had the power tubes replaced, fixed a melty thing that happened (possibly due to me throwing a tantrum onstage and kicking the amp over) and I also had the tube socket tightened. I don't feel this is an excessive amount of service for a $450 amp that's nearing 20 yrs old and it has not been babied the least bit. However, I expect everything I own to operate as it should at all times regardless of reality. Which brings us to my current reality. I am getting reduced volume and increased distortion. All of the tubes light up and seem to be secure in their homes. It seems like a power tube issue to me, or maybe a preamp tube problem, or maybe I melted some shit again despite the fact that I have now made it a point not to kick my amp over. I am hesitant to do any of the work on the amp myself and my amp tech lives an hour away. I am a little confused as to what is all involved in changing out the tubes. Is it just pulling out the old ones and popping in the new ones, or is there biasing and all of the black magic voodoo bullshit involved? Ultimately the solution is for someone to give me a 50 watt bassman head, but until that happens I suppose I'll continue to tango with my tweedy little beast.

So I guess the question is about changing tubes. is it hard?

Changing tubes is easy and something anyone that owns tube gear needs to know.
You only need to bias power amp tubes if you switch them on a fixed bias amp. I'm guessing your amp is cathode biased which means you don't have to do anything after switching tubes.

You might want to do some troubleshooting with the preamp tubes first, swap them around and try to see if the sound changes. If your amp has reverb, there is a chance that it is driven by a tube - the amp will not need that tube to produce a sound. What this means is that there is one spot for a preamp tube that doesn't really affect the sound, so if the bad one happens to land there during a tube swap everything would sound better. When you find the bad tube, you can just replace it with your reverb tube and not have reverb for a while.

If you swap around every preamp tube in every position and do not get a change, then i would next suspect the power tubes. Just get a new matched set. Make sure you find the bad power tube, but keep the other good ones for backups, never throw away a tube that works; you might need it in an emergency!


Here is a blues deville schematic:
http://www.ampwares.com/schematics/blues_deville.pdf

Those almost half circles with dotted lines inside represent each preamp tube, BUT 12ax7's are 'dual triodes' - this means they are basically two tubes in one package, hence why you have V1a and V1b. Both are each side of the physical V1 tube. You can have a tube with one bad side in it that works in some spots and not others (though uncommon). Like if a gain stage only utilized half a tube.

Physically on 99% of amps, V1 is the tube closest to your input jack. Then the they would go V2, V3, V4 etc...
The 12ax7 next to your power amp tubes is almost always the 'phase inverter'. It typically doesn't have much tonal effect like your first few tubes in the preamp would.


It is totally safe to swap your your preamp tubes with the amp turned on, just make sure your master vol is pretty low because there will be loud pops that could possibly damage speakers if too loud.


add this page to your bookmarks, it's a great reference:
http://www.geofex.com/ampdbug/ampdebug.htm

Ok, looks like the reverb is not tube driven. From from the owner's manual, there should be 3 preamp tubes. V1 is most important and will have the biggest impact on the sound of the amp. V2 is next, and then V3 is your phase inverter. I typically give the least amount of priority to the phase inverter (PI) when it comes to troubleshooting preamp tubes as they seem to not have as many issues.

Awesome. Thank you Sloan.