fender vibro champ XD volume loss

[redcoats1976]

i have a vibro champ 5 watt XD .been a great amp for around 10 years but lately it seems to be down on volume.have tried different guitar cords,and wondering if the tube (12ax7) is starting to go bad.is there anything else i should check,or a way to see if the tube is going bad before i spend the money to replace the tube?

 

[mdubya]

Changing the tube would be a very easy thing to try.

 

[strayedstrater]

It's more likely to be the 6V6 power tube than the 12AX7 preamp tube. Power tubes wear out faster than preamps, especially in single-ended Class A amps.

Although it could be the preamp tube, or even both tubes. Tubes can fail at any time, in a wide variety of ways. Sometimes they totally stop working, sometimes they get noisy, sometimes they get dull sounding, sometimes their output drops.

VCXD is cathode biased, so you don't need to check/adjust bias when replacing the power tube. Just stick a new one in and play. (All preamp tubes are cathode biased so also plug & play.)

 

[strayedstrater]

There are tube testing machines that can evaluate tubes. But they cost many times more than a 12AX7 plus a 6V6.

Some technicians have tube testers. Others will just pop in new tubes to check, and if that doesn't fix it they'll just put the old tubes back in and not charge you for a set of tubes. But most techs charge a minimum "bench fee" just to check your amp, and that's higher than the cost of a 12AX7 and a 6V6 (unless you go crazy buying super expensive NOS tubes).

All tubes eventually fail. Preamps typically last decades but many don't last anywhere near that long. It's good to have spares on hand because you'll need them eventually.

 

[cybermgk]

It's more likely to be the 6V6 power tube than the 12AX7 preamp tube. Power tubes wear out faster than preamps, especially in single-ended Class A amps.

Although it could be the preamp tube, or even both tubes. Tubes can fail at any time, in a wide variety of ways. Sometimes they totally stop working, sometimes they get noisy, sometimes they get dull sounding, sometimes their output drops.

VCXD is cathode biased, so you don't need to check/adjust bias when replacing the power tube. Just stick a new one in and play. (All preamp tubes are cathode biased so also plug & play.)

Yep, this would be my first place to try. 10 years for the one, single-ended output tube, is a LONG time, if the amp was used regularly. Tubes, do wear out.

 

[redcoats1976]

i thought this amp only had one 12ax7 tube in it. would tung sol or JJ tubes be a good choice to replace it with?might pull it apart tomorrow and see what i find.

 

[efstop]

I had to replace the EL84 in my Class 5 after 8-9 years and the amp was barely used. It was buzzing and spitting with nothing plugged in or the volume turned up at all.
A new tube fixed it.
So, yeah, yours might need a new 6V6. Doesn't hurt to have a spare, anyway.

 

[strayedstrater]

i thought this amp only had one 12ax7 tube in it. would tung sol or JJ tubes be a good choice to replace it with?might pull it apart tomorrow and see what i find.

 

[strayedstrater]

View attachment 693000

JJ and Tung Sol are both fine. I find the differences between brands to be pretty subtle (some people hear big differences). With "multi-voice" amps like the VCXD I find that different tubes can make some of the voices sound better while making other voices sound worse.

But assuming one or both of the tubes is the source of the problem, replacing the weak tube(s) will likely make all the voices sound dramatically better.

 

[redcoats1976]

@ strayedstrater...thanks,a great help.

 

[cybermgk]

i thought this amp only had one 12ax7 tube in it. would tung sol or JJ tubes be a good choice to replace it with?might pull it apart tomorrow and see what i find.

It has one of each. 1 12AX7 and 1 6V6.

The preamp is digital, and produces all the voicings. Then 1/2 of the 12AX7 is used as a buffer and driver into the output section, which is the 6V6. If you are getting new make tubes, I for one like JJ 6V6s. They sound a little 6L6ish, which I like.

 

[strayedstrater]

It has one of each. 1 12AX7 and 1 6V6.

The preamp is digital, and produces all the voicings. Then 1/2 of the 12AX7 is used as a buffer and driver into the output section, which is the 6V6. If you are getting new make tubes, I for one like JJ 6V6s. They sound a little 6L6ish, which I like.

Back when the VCXD and SCXD/SCX2 were new, a lot of people really wanted to believe the 12AX7's were part of the preamp. Some people still cling to the mistaken belief that the clean channel on the SCXD/SCX2 is "all tube". But as you said, all the preamp voicings are digital and all the tubes come after that as part of the power amp.

Personally the 6V6 is my favorite output tube and JJ would be my last choice because of its somewhat 6L6-ish character. (Not enough that I'd recommend against JJ's, just enough that they aren't my choice of 6V6's.)

 

[cybermgk]

Back when the VCXD and SCXD/SCX2 were new, a lot of people really wanted to believe the 12AX7's were part of the preamp. Some people still cling to the mistaken belief that the clean channel on the SCXD/SCX2 is "all tube". But as you said, all the preamp voicings are digital and all the tubes come after that as part of the power amp.

Personally the 6V6 is my favorite output tube and JJ would be my last choice because of its somewhat 6L6-ish character. (Not enough that I'd recommend against JJ's, just enough that they aren't my choice of 6V6's.)

And if you don't like the 6L6 tone JJ 6V6s bring to the table. I totally agree, don't go JJ. I like 6L6 tone in some amps. For the fendery voices/pream[s, for my ears I'd want the JJs. But, for some more Marshally tones/preamps, maybe not. NOS or Old Stock Pulls (not never used, but tested strong) RCA would be a good overall choice. BUT, they will cost some.

@redcoats1976 you might try these Soviet era NOS 6v6 GT at Tube Depot. Only $20 a tube. Also they are shorter, which for the VCXD is plus. They would get less back blast from the speaker *whuch can rattle and batter a tube over time).,

They APPEAR to be very similar to those older RCA 6V6s

 

[Seven]

Volume loss usually equals power tube.

 

[redcoats1976]

got a 6V6 coming from tube depot,should be here this week sometime...