Mark IV: Low gain in tweed mode on lead channel

[kiff]

Hi guys, I'm new here and have a question hopefully someone can help me with. I've had my MarkIV for a couple of years now, but I've only recently been playing with it since I got married and we've had 2 kids right after I acquired it. The head is a short version and I'm pretty sure it's one of the early (ver. A ?) heads since the power cord isn't removable. This is by far the best sounding amp I've ever had or played through. My JCM800 will be collecting some dust in the garage now

Problem: About a month ago I changed out most of the preamp tubes. Some were noisy and I was also testing tonage. I changed v1 + v2 with MB spax7's and v3 + v4 with mullard reissue 12ax7's - high gain (from tubedepot.com). After I did that the lead channel had very little gain when running in Tweed mode. With both drive and gain pulled out and set around 8, it had a little more gain than rythm 1 and less than r2. However, the gain was fine in full power mode. After some messing w/ it I found that the problem was from the new spax7's in v1/v2. I put the original spax7 back into V1 and most of the gain was restored, but the overall volume was really low. Then I took out the other new spax7 from V2 and replaced it with the original groove tube 12ax7 and also with a new mullard reissue (standard gain not high). Both of those fixed the volume issue. I left the mullard in since the groove tube was way too bright.

I have a hard time believing that both of the brand new spax7's are faulty, but I guess it's possible. Even if they are, would the lowered voltage really make that much of a difference from tweed to full power? Anyone heard of this happening? Ideas?

Thanks in advance!

 

[MesaDCLP]

I've got 4 SPAX7's in 1 though 4 and a matched GT-12AX7-R2 in 5, and I don't lose any preamp gain between tweed and full.

The tubes replaced all JJ's, so there was a change in tone ... slightly quieter with a bit less gain.

Since you replaced all the preamp tubes, maybe you should swap your power tubes to see if that helps. I think the Tweed/Full switch has the most effect on the power section.

 

[kiff]

Since you replaced all the preamp tubes, maybe you should swap your power tubes to see if that helps. I think the Tweed/Full switch has the most effect on the power section.

I was thinking the same thing. I did in fact swap out the all 4 6L6's with the spares that the guy I bought the head from thru in. It didn't make any difference. He said the spares were new, but I really don't know for sure.

 

[MesaDCLP]

Buy new power tubes rated for Mesa, and see.

If that doesn't fix the problem, you may need a cap job. That's all I can think of.

Worst case, send it to Mesa and get them to look at it.

 

[kiff]

Would you recommend getting 4 or just 2 (for class A)? I'll have to wait 'till next month probably since I just bought a les paul this month.

 

[pstuker]

If that fixes the problem then one of your new preamp tubes is bad.

 

[kiff]

yea, i put the old ones back in and it works fine. it's just a little hard to believe two brand new spax7's from mesa are both bad. it's also a little strange that this happens at all since (as MesaDCLP noted) tweed power should mainly affect the power section...

 

[JOEY B.]

tweed power should mainly affect the power section...

Tweed power lowers the internal voltages within the amp. Listen to see if your cooling fan slows down a bit when you switch from Full to Tweed.