Amp sounds terrible after changing tubes

[Tyeetime]

I just had new power tubes installed into my Blues Jr and it sounds awful.
Even set clean when I pluck a note it has a real "frappy" distortion for about 3 seconds and then disappears a the note rings out. The only difference is these are Groove Tube #4 (originals #7).
The odd thing is it sounded fine when I brought it home. Next day it sounds terrible.

Any ideas? What might cause a very bad distortion that only rings for a few seconds then disappears?

[czgunner]

Not an expert, but did the bias get adjusted correctly?

[Steve DeRosa]

Originally Posted by Tyeetime

I just had new power tubes installed into my Blues Jr and it sounds awful...The only difference is these are Groove Tube #4 (originals #7)...

Swap the old tubes back in, play for a while over a couple days, and see if it happens again...

If it does - and it probably will, based on the fact that you moved to a tube with a quicker onset of distortion - ditto on the bias job...

[Parlorman]

Bias issue was my first thought as well.

[stokes1971]

Assuming the new tubes are actually good,its not unusual to get a bad set of new production tubes,it is most likely a bias issue.Even with the number system that Groove tubes uses and the color system Mesa uses the bias should still be adjusted to get the ideal tone.The premise of these systems is that if you use the same number or color there is no need to re-bias.While this is true from a safety standpoint,they will not neccessarily sound the best.I have seen new tubes that are in the same "bias range" differ by as much as 15-20 ma's.Bias should always be checked and adjusted as needed.Whenever I get an amp for repair I always suggest installing an adjustable bias mod.While we're talking Blues Jr I can also tell you that I've had way too many amps from this era,Blues Deluxe,Devilles etc that needed to have the filter caps changed.They use cheaper asian caps that just suck out loud.Even had a few that exhibited no issues and talked the owner into replacing them with good quality caps.The results were eye opening.

[Stevie C]

Like the others have said, it may be the bias, but you could have also gotten a bad tube(it happens). Try switching the position of the tubes to see if you can tell a difference. A good amp repair tech can set your bias and check your tubes.

[roylor4]

Originally Posted by Stevie C

Like the others have said, it may be the bias, but you could have also gotten a bad tube(it happens). Try switching the position of the tubes to see if you can tell a difference. A good amp repair tech can set your bias and check your tubes.

Yep, a microphonic tube can wreak havoc on an amp.

[Paleolith54]

Originally Posted by stokes1971

Even with the number system that Groove tubes uses and the color system Mesa uses the bias should still be adjusted to get the ideal tone..

Has Mesa added bias pots to their amps? Unless they have, I think you'd have to physically mod the amp in order to adjust bias on a Mesa amp, although you can still check the bias if you have the equipment.

[Jilusaco]

I too own a Blues Jr. There was a common problem with them call BJD Blues Jr. Disease. They suffered from bad soldier joints. Over time with heat & vibration the joints where the tube sockets were soldered would fail. It's quite possible that when the new tubes were put in the pressure of installing them caused the joint to go ahead and fail. Now when you hammer a note the vibration is causing a disconnect in the circuit. The Jr is fixed biased unless it has been modded, so no adjustment other than different value tubes. I buy all my tubes from Eurotubes they know there stuff. They have different tube sets for each amp. I bought the tweed set for my Jr and am tickled to death. Some much so I order a set for my Deluxe. It made totally different amps out of both. They can't help you with bad solder joints but they can tube you up. Hope you find the problem.

[Tyeetime]

Quite a few suggestions to bias the amp, but I understand this model doesn't require biasing when changing tubes. Admittedly I don't know much about this. I have read about the circuit board issues with this amp, but it looks brand new inside. The solder points opposite the pins looks good, but perhaps it isn't something you can see obviously. I've taken it into a different shop for them to have a look at.

I appreciate all the responses. I've posted on a couple of different sites and this one has generated the most replies.

[Jeff Scott]

Is the amp cathode biased? if not, get the amp biased by a QUALIFIED tech who knows how to do so properly.

[rmp]

Originally Posted by Jeff Scott

Is the amp cathode biased? if not, get the amp biased by a QUALIFIED tech who knows how to do so properly.

THIS!!!!!!

[DanR]

Originally Posted by Tyeetime

Quite a few suggestions to bias the amp, but I understand this model doesn't require biasing when changing tubes.

The Blues Jr. IS fixed bias so checking the bias is always recommended when changing the power tubes.

[Stevie C]

Also, check the pins on the tubes, in case one is bent.

[Tyeetime]

Turns out it was an open screen grid resistor.