Mark IV new tubes and less gain and volume !

[VINZ87]

Hello,
My old MK IV (1993) have been retubed last month with :
four 12AX7 Boogie, one 12AT7 Groove Tube (the original tube was a 12 AT7 Boogie like in all MK IV before 1994 in the left socket preamp tube). Four 6L6 Boogie power tubes (grey code like the originals). Since this operation, I have less volume and gain in tweed power (class A, pentode, mid gain). The sound is not "bad" but there is no punch !! I almost have the same gain in R2 and lead !! In full power, I don't have this problem, it seems...
Is the 12AT7 Groove Tube responsible of this fact ?
Other topics of this great forum concerning preamp tubes seem to turn me on this way.. When I change power tube (I have tried new 6L6 of my Road King 2), there's the same problem...
Thank You
VINZ - France

 

[kiff]

Yea, you said the left-most socket? If so, that would be V1 and really shouldn't have a 12at7...

I've had the problem where my lead channel had almost no crunch and a big volume loss. It turned out to be a tube issue. V1 and V2 positions were the cause...

 

[JOEY B.]

Yea, you said the left-most socket? If so, that would be V1 and really shouldn't have a 12at7...

Left-most looking at the front = V1

Left-most looking at the rear = V4


V4 in a Mark IVA should be a 12AT7, all other preamp positions shouls be 12AX7's. The Mark IVB uses all 12AX7 preamp tubes.

 

[angelo]

Yes, mine has an AT7 in it as well. I can't recall, but it is in the original manual. Would it make sense in the reverb socket? I think that is where it was.

The V1 was the 12axsp or something. It was the 12 ax7 with the rubber around it....

 

[VINZ87]

Hello,
My 12AT 7 groove tube is in the V4 position, like the original 12AT7 : my MK IV is a MKIVA. Do you think this tube is responsible of this situation ? The gain is weak, especially on lead channel. Volume is divided in two with my new tubes (simul class, pentode, mid gain). I need to play in full power during rehearsals !!
Thanks a lot !
I think I have to order boogie 12 AT7.....

 

[6L6C]

Suggestion:

If you have your old preamp tubes put them all back in.

Is the amp somewhat acceptable?

If so go to V1 and replace with one of your new tubes. Is the sound still acceptable?
Then move on to V2 repeat and keep going.
If one sounds a little off (as far as power) try one of your other new tubes, might be just a bad preamp tube, so you will have to find it through process of elimination. If V4 sounds off with a new tube just leave in the old one.

 

[mesa1360]

I have the same problem with my mk IVa. Lead channel gain is weaker than r2. I notice however that if I push upward towards the socket either the phase inverter (PI) tube or the V2 tube, I get the strong gain in the lead channel. The magnet of the EV speaker is directly under the PI tube so as a crude solution, I inserted something between the top of the EV speaker magnet and the the PI tube to push the PI tube upwards towards the socket.

I wonder if the permanent solution is something that does not require a tech. What do you suggest I should do?

 

[Restless Rocks]

The new Mesa tubes are relabled JJ's. They are suppose to be of Slovakian manufacture. The first thing I noticed about the 12AX7's was 1) less gain 2) slightly darker tone. A reminder of why I never used JJ tubes in preamp positions. The 6L6's sound pretty good but in my opinion not as good as the previous STR 440'. The Russian and Chinese tubes had both become unreliable in the cathode follower positions so a change was in order. I've found if I raise my gain and treble settings by one on the MkIII it puts me back into the territory I'm used to.

 

[stokes]

I have the same problem with my mk IVa. Lead channel gain is weaker than r2. I notice however that if I push upward towards the socket either the phase inverter (PI) tube or the V2 tube, I get the strong gain in the lead channel. The magnet of the EV speaker is directly under the PI tube so as a crude solution, I inserted something between the top of the EV speaker magnet and the the PI tube to push the PI tube upwards towards the socket.

I wonder if the permanent solution is something that does not require a tech. What do you suggest I should do?

Try cleaning and re-tensioning your sockets.

 

[kmanick]

I have the same problem with my mk IVa. Lead channel gain is weaker than r2. I notice however that if I push upward towards the socket either the phase inverter (PI) tube or the V2 tube, I get the strong gain in the lead channel. The magnet of the EV speaker is directly under the PI tube so as a crude solution, I inserted something between the top of the EV speaker magnet and the the PI tube to push the PI tube upwards towards the socket.

I wonder if the permanent solution is something that does not require a tech. What do you suggest I should do?

Try cleaning and re-tensioning your sockets.

how do you re tension a tube socket?

 

[stokes]

To re-tension the sockets you have to remove the chassis and drain the filter caps first.If you cant or dont know how to do that safely,dont go any further.Once that is done get a dental pick or something similar,remove the tubes.Look into the socket and you will see the receptacle in the socket for each tube pin.You have to use the point of the pick to kind of bend/reshape the individual receptacles to make a tighter fit for the tube pins.To clean the sockets you simply spray a tubes pins with a non-lubricating contact cleaner and while the pins are still wet,put the tube in and out several times to "scrub" the sockets.Again,if you cant safely drain the caps dont attempt this,there are lethal voltages present at some pins in the sockets and you could get yourself killed.