KT77's and EL34's at the same time?
- Thread starter lostcause
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Old BF Shred
Well-known member
As the tube base (socket) wiring diagram and most running specifications is the same for both these tubes, the real concern is the idling current (controlled by bias voltage).
The Mesa's don't have adjustable biasing and even if they did, it would be one adjustment for ALL output tubes not just per socket(s).
That said, the idle current rating of the 2 tube types needs to be the same or very close; within a few milliamps (ma.) anyway, and of course within reason for the amp.
I'm not sure what Mesa specs for the LSC, but it can't be far off from the Stiletto which is 38-40ma. You can likely get away with a lot lower or slightly higher than this range.
Just be sure to install the different types of tubes "symmetrically"; that is the outer most pair must be one type (77's OR 34's), and the inner pair, the other type. Note that this convention holds true with every amp I’ve run across and is “pretty much” standard (extremely unlikely that the Lone Star is any different).
I have modified my Mesa 50/50 power amp to run el34's and have run tubes with slightly different ratings (say, one pair at "35" and one at "39") with no problem at all. Try not to go with more of a difference with this, and preferably get the same rating.
Also, if you are at all unsure, it would be prudent to have a tech measure the idle current on each tube just to be safe.
Mixing the tube types may actually result in an interesting and unique sound quality (perhaps not unlike mixing speaker types in one cabinet….which is generally totally safe as long as the impedance is the same).
Whether your ears will like the results, or not, is subjective, but it won’t hurt the amp if the tube’s ratings are within the amp’s capabilities.
Let us know how this works out.
Good luck!
The Mesa's don't have adjustable biasing and even if they did, it would be one adjustment for ALL output tubes not just per socket(s).
That said, the idle current rating of the 2 tube types needs to be the same or very close; within a few milliamps (ma.) anyway, and of course within reason for the amp.
I'm not sure what Mesa specs for the LSC, but it can't be far off from the Stiletto which is 38-40ma. You can likely get away with a lot lower or slightly higher than this range.
Just be sure to install the different types of tubes "symmetrically"; that is the outer most pair must be one type (77's OR 34's), and the inner pair, the other type. Note that this convention holds true with every amp I’ve run across and is “pretty much” standard (extremely unlikely that the Lone Star is any different).
I have modified my Mesa 50/50 power amp to run el34's and have run tubes with slightly different ratings (say, one pair at "35" and one at "39") with no problem at all. Try not to go with more of a difference with this, and preferably get the same rating.
Also, if you are at all unsure, it would be prudent to have a tech measure the idle current on each tube just to be safe.
Mixing the tube types may actually result in an interesting and unique sound quality (perhaps not unlike mixing speaker types in one cabinet….which is generally totally safe as long as the impedance is the same).
Whether your ears will like the results, or not, is subjective, but it won’t hurt the amp if the tube’s ratings are within the amp’s capabilities.
Let us know how this works out.
Good luck!
Assuming the Kt77 are a matched pair and the EL 34 are a matched pair,and the two pair dont match each other installing them as Old BF says is okay in full power mode,but if you use the half power switch (assuming this amp has one,I dont have a schem for the LSC,only the LS) depending on how far apart the two pairs are,you could have a degredation in tone and hum level.When using the half power switch,you lift the cathode on the inner pair,leaving the outer pair working solo,in this arrangement you will have a KT77 running one side of the OT and an EL34 on the other.If the current draw for the EL is more than 5-10ma's different than the KT,you will have an increase in hum and a loss in watts and not likely get the best tone.Using the mismatch in full power will be fine since you have two different tubes on one side and the OT will see "an average"current draw of the two.
