Mark IV clas A idea

[lerxst88]

I recently bought a myriad of different tubes to experiment with in the power section and was thinking of a "what if?".

What if I just leave the amp in class A all the time so I could use any set of tubes in the outer sockets and then put in EL-34s or something in the middle sockets to keep the current draw the same but leave them in standby.

Does this work?

What about taking the middle tubes out to reduce strain on the PT?


This is all assuming I leave it in Class A mode!!!

 

[topcat0399]

I recently bought a myriad of different tubes to experiment with in the power section and was thinking of a "what if?".

What if I just leave the amp in class A all the time so I could use any set of tubes in the outer sockets and then put in EL-34s or something in the middle sockets to keep the current draw the same but leave them in standby.

Does this work?

What about taking the middle tubes out to reduce strain on the PT?


This is all assuming I leave it in Class A mode!!!

I don't know the details of current draw on the center sockets when in class A but, I do know the amp works in class A with only an outer set installed; I've done it.

Some gorilla busted one of my tubes one night so I pulled the center 2 and ran her in class A with tubes just in the outer's (my mark IV is almost always in class A anyway). Made no difference to the way I usually operate. When I noticed 2 months later I still didn't have tubes in the center I said to myself - hmmmm - no adverse effect it seems, but I threw another pair back in the center sockets just in case I suddenly hit a BIG BIG room and I need to kick in the giant killer and show those guys with cab stacks who really is BOSS!

 

[>Photi G<]

I recently bought a myriad of different tubes to experiment with in the power section and was thinking of a "what if?".

What if I just leave the amp in class A all the time so I could use any set of tubes in the outer sockets and then put in EL-34s or something in the middle sockets to keep the current draw the same but leave them in standby.

Does this work?

What about taking the middle tubes out to reduce strain on the PT?


This is all assuming I leave it in Class A mode!!!

If you leave the amp in Class A mode, there really is no need to have any tubes in the inner sockets, since Class A mode removes them from the circuit altogether. One thing to keep in mind though is that the output impedance doubles. If you have an 8? cab, you need to plug it into the 4? jack now. If you have a 4? cab, you can't use it anymore (at least alone). One advantage to pulling the center tubes is that it reduces strain on the heater supply. So if you put tubes that have a high heater current in the Class A sockets, like, say KT88's, you won't blow the PT. Just make sure that the bias is within range of the tubes being used. You might need to install the adjustable bias mod for this.

 

[lerxst88]

...I threw another pair back in the center sockets just in case I suddenly hit a BIG BIG room and I need to kick in the giant killer and show those guys with cab stacks who really is BOSS!

I think its awesome how little the Mark IV combo is and how HUGE it sounds! I cant wait to play it live and see peoples mouths drop! lol

Thats really cool to hear! I recently got KT77s and its probably a good move to do what Photi G said to ease the strain on my amp. Good note to plug into the 4 ohm output too!

Thanks guys!

 

[swbo101]

I recently bought a myriad of different tubes to experiment with in the power section and was thinking of a "what if?".

What if I just leave the amp in class A all the time so I could use any set of tubes in the outer sockets and then put in EL-34s or something in the middle sockets to keep the current draw the same but leave them in standby.

Does this work?

What about taking the middle tubes out to reduce strain on the PT?


This is all assuming I leave it in Class A mode!!!

If you leave the amp in Class A mode, there really is no need to have any tubes in the inner sockets, since Class A mode removes them from the circuit altogether. One thing to keep in mind though is that the output impedance doubles. If you have an 8? cab, you need to plug it into the 4? jack now. If you have a 4? cab, you can't use it anymore (at least alone). One advantage to pulling the center tubes is that it reduces strain on the heater supply. So if you put tubes that have a high heater current in the Class A sockets, like, say KT88's, you won't blow the PT. Just make sure that the bias is within range of the tubes being used. You might need to install the adjustable bias mod for this.

Do you mean that you HAVE to use the 4 Ohm output everytime your in class A? or just if you run Class A WITHOUT the center tubes?

Thanks

 

[>Photi G<]

Do you mean that you HAVE to use the 4 Ohm output everytime your in class A? or just if you run Class A WITHOUT the center tubes?

Thanks

Really, yeah, You should use the 4? out whenever you're in Class A, or in older amps, 60W mode. Mesa used to put it in all the manuals, but over time, they developed their output transformers to handle the mismatch better, or they just saw that there weren't any problems with people letting it go, so they just stopped giving the warning.

 

[swbo101]

Do you mean that you HAVE to use the 4 Ohm output everytime your in class A? or just if you run Class A WITHOUT the center tubes?

Thanks

Really, yeah, You should use the 4? out whenever you're in Class A, or in older amps, 60W mode. Mesa used to put it in all the manuals, but over time, they developed their output transformers to handle the mismatch better, or they just saw that there weren't any problems with people letting it go, so they just stopped giving the warning.

Thanks...never knew that. I've had my Mark IV for 20 years now. Never switched to 4 ohms when running in class A, never saw that in the manual, also never had a problem. Appreciate the knowledge!!

 

[mr_fender]

When in Class A, the cathodes of the two inner power tubes are disconnected from ground, so no current flows through the tubes at all. They are quite literaly turned off. They only remaining draw on the power supply they have are on the heater filaments, which are always on. That's why the center tubes still glow when in Class A. It's perfectly OK to pull the center two tubes when runing in Class A. It makes no difference.

In a pinch, like if you blow a tube at a gig and don't have any spares, you can swap the good remaining pair into the outer sockets and just run in class A to finish the gig.