Mark V distortion loss

[justplay]

I have a Mark V which I bought new less than a year ago. Just recently, at a show, I've noticed that I have some distortion loss issues. When using channel 3 in the Mark IV mode, my distortion doesn't have that nice saturated tone as before. I've tried 3 different guitars with both active and passive pickups in them and also plugging directly into the head with no out board effects. Still a noticeable loss of distortion. Does this sound like a preamp tube issue? Also the same amount of loss is apparent in channel 2 as well.

Any ideas what to try?

 

[screamingdaisy]

Sounds like worn out power tubes. Usually it results in a loss of top end and a dull sounding tone, which I'm thinking may be your loss of saturation. If you find yourself turning up the treble and volume and still not getting the sound you expect I'd use that as confirmation.

 

[justplay]

Definitely do not have any treble or volume issues. Just that the distortion just isn't what it used to be ... :|

 

[justplay]

I guess the only thing I can really do is to start swapping out each 12xa7 pre amp tube with a known good tube and trying the amp after each swap to see if the sound goes back to the way it used to be.

Any other suggestions?

 

[Karl Houseknecht]

I'd start with V3 since that is the 3rd gain stage for CH3 and 4th for CH2. That's the one they both have in common.

 

[justplay]

Thanks Karl, will try that, prolly some time this weekend, when I have the house to myself.... Crank it! :lol:
But if the wife were there, she's..... :roll: :mrgreen:

Anyways, seriously tho, I really do hope I can get this sorted out. I HAD a really sweet saturated tone with really nice sustain and now it's dry and drab... Will let you all know how it goes.

 

[justplay]

After doing a preliminary look at the tubes, starting with the power tubes, I found that one of the 6L6 tubes ( far right, looking at the back ) had a slight burnt mark on the glass as well as the metal inside the tube. Replaced this tube and all is well. Played a little with bed room volumes in the 45 watt setting and it seems like the tone and sustain are back to it's full glory. I will try again tomorrow afternoon at gig volumes and test again!

 

[Karl Houseknecht]

Very interesting! I wouldn't have thought a power tube would do that. You're not even using the outer pair of power tubes in 45 watt mode, so are you sure that it really fixed it? Only way to tell for sure is 90 watt mode. And I have to be honest, unless you're cranking it, the power tubes shouldn't contribute much to any kind of distortion.

Also, you should replace the power tubes in matched pairs. The outer set is a matched pair and the inner set is another.

 

[justplay]

Very interesting! I wouldn't have thought a power tube would do that. You're not even using the outer pair of power tubes in 45 watt mode, so are you sure that it really fixed it? Only way to tell for sure is 90 watt mode. And I have to be honest, unless you're cranking it, the power tubes shouldn't contribute much to any kind of distortion.

Also, you should replace the power tubes in matched pairs. The outer set is a matched pair and the inner set is another.

Karl, I think it also stemmed from the fact that I run channel 1 on 90 watts for maximum clean head room and had that switch selected for 90 watts the whole time. Since that was set that way, I had the issues as described above. It seems that a blown 6L6 caused me distortion loss and lack of sustain, along with the lack of that boogie growl, if you can call it that. One of the reasons that I like the boogie and the 6L6 over the EL34's, tho there are times when I do like that Marshall EL 34 sound, I still prefer the Boogie sound!