Mark IV Lead Channel Problems

[geese_com]

So....I just got a Mark IV, but there is one problem. After playing on the lead channel for about 5 minutes, the sound just dies. No distortion or anything. It sounds more like R1. I have 2 6L6s inside and 2 EL34s on the outside. I have it currently on Harmonics, Simulclass, Triode, and Full Power.


Help please!!!!!!!!!!!

 

[trem]

Had similar problem, though mine wasn't transient, just lack of gain period. It was the pre-amp tubes.

 

[Shep]

yeah replace your V1 to v5 and you should notice a difference in tone and it should clear up your problem..

 

[geese_com]

Replaced what I had in the V1 with V5. Still died. Is the only solution to buy 5 more preamp tubes?

 

[geese_com]

Had similar problem, though mine wasn't transient, just lack of gain period. It was the pre-amp tubes.

Did you just purchase a set of new ones?

 

[geese_com]

Out of these choices: http://www.musiciansfriend.com/navigation/accessories?N=100001+304273&Ntk=All&Ntt=12ax7+tube&Nty=1

Which would be the best to buy?

Also, is there a way to spot just one bad preamp tube?


Thanks for all the help guys.

 

[trem]

I think what Shep meant was to replace the whole V1 to V5 with a new set of tubes. I missed something in your original post. You said that after 5 minutes of playing the gain went down. Does this occur again after you switch off and on the amp again?

Yeah, I bought a new set of pre-amp tubes. Out of those in the MF page, my personal choice would be the Tung-Sol, but any one of the 12AX7s will do. FWIW, nowadays Boogies are shipped with the Electro-Harmonix 12AX7EH (though relabelled as "Russian-2"). Replace all 5 pre-amp tubes. Do note that if you have the revision A of the Mark IV, you'll need 12AT7 in the V4. Whereas if yours is the B, use 12AX7 like the rest.

To roughly tell which tube is bad or not, you have to put a known good set of tubes at V2 to V5, then place the tube to be tested at V1. If it's bad, it's either going to be noisy, causing lack of volume, microphony, or a combination of those.

 

[MrMarkIII]

Get out your wallet, because if you replace all the pre-amp tubes at once, you'll never know which one is bad, and so will have to discard all of the old tubes. You also won't have any spares for next time, so you'd better buy two new sets.

If you had a chandelier with one bad bulb, would you replace all of the other bulbs as well?

Any Boogie manual describes how to trouble-shoot tube problems, and replacing pre-amp tubes one-at-a-time, starting with V1, is SOP. If the sound doesn't improve, put V1 back and move on to V2. Rinse and repeat.

But, it's your money.

 

[MrMarkIII]

Maybe it's the foot switch.

 

[geese_com]

After turning my amp off for about 3 minutes will return things back to normal.

I think I'll just buy 5 new tubes and then do the testing one at a time method. So that I'll have 4 more tubes for spares.

What do you think?

 

[geese_com]

Anyone have suggestions on the best preamp tubes to get (not just on the ones from musicians friend that I posted)

 

[Ednolb]

Do note that if you have the revision A of the Mark IV, you'll need 12AT7 in the V4.

True, but you can just replace the 12AT7 with a 12AX7, right?

 

[trem]

True, but you can just replace the 12AT7 with a 12AX7, right?

Yes, you can. But the circuit was designed around a 12AT7, so the gain and impedance of the 12AT7 would make everything work properly. IOW, it will sound its best using the 12AT7.

Anyone have suggestions on the best preamp tubes to get (not just on the ones from musicians friend that I posted)

Each tube has their own sound, you should let your ears decide. JJ, EH, Shuguang and NS Tung-Sol seems to be the most popular nowadays. There's also the popular tube concoction from Doug's Tubes that some love, and others hate.
http://www.dougstubes.com/tonekits.html

And to add to your confusion, there's always the NOS route. Good luck.

 

[geese_com]

I just bought the Tungsol tubes from Musiciansfriend since there were other things that I needed to buy anyways.