Help! - My Seminar amp has lost its Mojo, with all new tubes

[blumuz123]

My brand new Seminar head worked great for 6 months then I began I notice that I would have to begi to crank
the Gain knob and the Master Knob up 1/4 turn to get near the same results (but with more sag)

so......I slapped all new tubs in same brands except 2 tungsols swapped for Ruby pre amp tubes, biased it at 36 and it seemed that
the mojo was back fo 2 gigs. Now its lost its Mojo again and I have to turn the Master and Gain knob at least up 1/3 turn
to get the mojo back.

whats up with that?
any Ideas

 

[jmgman69]

What power tubes are you using?I'm running E34Ls and have then biased at 75....36 sounds low unless that is per tube.

 

[Buckeyedog]

As Jeff has pointed out, the latest EL34's have not been good. Do you have a spare set to try? Does yours have the solid state rectifier or the tube??

I also had 4 brand new Tung Sol's go bad within minutes of playing them. If you have a known good pre, swap them out one at a time and see if you may have a bad pre amp tube. Most likely it's a tube issue somewhere.

 

[jmgman69]

As Jeff has pointed out, the latest EL34's have not been good. Do you have a spare set to try? Does yours have the solid state rectifier or the tube??

I also had 4 brand new Tung Sol's go bad within minutes of playing them. If you have a known good pre, swap them out one at a time and see if you may have a bad pre amp tube. Most likely it's a tube issue somewhere.

The last of my tung sols just died,great sounding tube,but absolute garbage,preamp tubes that is!!

 

[Buckeyedog]

The last of my tung sols just died,great sounding tube,but absolute garbage,preamp tubes that is!!

Yep, sucks man. I love the tube as well......had great luck with the first few I bought like a year ago, then just recently, suck city!!

 

[blumuz123]

I put Ruby el34bht's in like the originals biased at 36 each

from left to right on the preamp tubes tung sol 12ax7,tung sol 12 ax7, JJecc83s, JJecc83s

all brand new

I try swapping tube out one at a time and see

I have Sovtek 12ax7's and 5881's,tung sol el34b's that I know work in my Mod 50
I dont know if I have the tube or solid state rectifier, I will find out.
how does the tube or solid state rectifier affect the amp?

Thanks

 

[guitarslinger]

theoretical question.

Depending on which seminar in which it was built, it might or might not have a cap after the fx loop cathode in parallel with a resistor.

if you have a cap go bad there, the volume will go down.

 

[Jeff Hilligan]

For starters, use one known good pre amp tube and swap it into all positions till you find the problem one. If you have no luck with that we then move to the next test.