Mixing output tubes?

[moltenmetalburn]

Hey resident RT amplifier gurus, can someone explain pros and cons of mixing output tube types?

(My amplifier CAN do this safely. )

Specifically about mixing EL34 types with KT77, KT88 types.

There is not much information available, judt a bunch of naysaying from laymen.

Is this inducing assymetry? Phasing? Crossover distortion problems possible?

 

[CrazyNutz]

I've experimented with this a couple times on my rm100, it has a bias pot for each tube.

Combos I've tried:
KT88 KT77 KT77 KT88
KT88 EL34 EL34 KT88
KT77 EL34 EL34 KT77

I never noticed any weird problems.
Asymmetry not a problem as long as you have the configuration mirrored on both sides i.e. kt88 el34 | el34 kt88

As far as the sound, really was just a mix of the two tube types. Wasn't my thing, just ended up going straight el34 on that amp.
But then again I'm a el34 guy and really don't like any other tube type.

 

[ChurchHill]

About the only thing I can add is just to make sure that the tube can handle the plate voltage of your amp. Your amp may be able to handle mixing tubes, but blowing a tube because the tube couldn't handle your amp could blow the fuse, the socket, associated resistors, or output transformer. That would suck!

Just in general, I'd mix'n'match to refine an amp, but I wouldn't expect any major changes. I'd be very curious to find out what combinations you end up trying and what your thoughts about them are.

 

[Loudness250]

On my RM 100 I mixed 6L6's and El34, no problem.

 

[petejt]

I use two EL34s and two 6L6s in my Mesa/Boogie MarkIV, running on Simulclass.

 

[stephen sawall]

If the amp is designed for it should be no problems. Most amps are not designed this way. If the tube measurements are too far apart even if the same tube can cause issues in many amps.

 

[thegame]

Some amp designers are adamant about not mixing or changing tubes types in the same amplifier such as Bogner and Wizard. Their reasoning is that even if an amp's transformer can handle the varying current demands of various tube types, the transformers are usually designed with a specific tube type in mind to get the most performance out of it. That is why they offer the same amp with different transformers for specific tubes (Bogner Ecstasy 6L6 vs EL34, Wizard Metal EL34 series vs. 6L and KT series).

To put another way, if one of those EL34s models I mentioned could safely handle running 6550s, it wouldn't necessarily sound as good as one designed specifically with 6550s in mind.

Any thoughts to add to this ?

 

[CrazyNutz]

Some amp designers are adamant about not mixing or changing tubes types in the same amplifier such as Bogner and Wizard. Their reasoning is that even if an amp's transformer can handle the varying current demands of various tube types, the transformers are usually designed with a specific tube type in mind to get the most performance out of it. That is why they offer the same amp with different transformers for specific tubes (Bogner Ecstasy 6L6 vs EL34, Wizard Metal EL34 series vs. 6L and KT series).

To put another way, if one of those EL34s models I mentioned could safely handle running 6550s, it wouldn't necessarily sound as good as one designed specifically with 6550s in mind.

Any thoughts to add to this ?

There are a couple of things:

1). Primary Impedance of the output transformer.
For a quad of el34 type tubes this would be around 1.8k (some are less)
For a quad of 6l6 type tubes this would be around 2.2k

This small difference usually won't make or break anything, you'll just have slightly less efficiency.

2). Screen resistors
el34 amps traditionally have 1k screen resistors
6l6 types traditionally have 470r

470r screen resistors can shorten the life of el34's
Many modern amps just use 1k screen resistors now, because this is safe for both el34 and 6l6

3). Dont use big glass (kt88's, 6550s) in an amp not designed for it. You can usually have it modified to work properly.
These kind of tubes require more bias current. The fix is usually just to use 100k bias feed resistors inplace of the common 220k's

Thats my thoughts.