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Got a friend that called me Saturday with an interesting dilemma. He's designed a solid state preamp that he says sounds absolutely great running into solid state power amps. However when he tries running it through tube power amps it sounds thin and weak. Does anyone know of any general concerns or obstacles that would present themselves when trying to use a solid state preamp into a tube power section?

You might be asking yourself WHY would anyone want to do this, but suspend that notion for the time being....

Is it an impedance thing and or drive thing
where the phase invertor tube is not being loaded
or driven well enough for it's liking ?

Isn't the voltage output of most SS preamps much lower than a typical 12ax7 or similar preamp?

The input sensitivity of any external power amp unit (whether it was tube or solid-state design) should be pretty much a standard, that is: designed for line level signals. It would be a tremendous inconvinience if the sensitivities were all over the place.

Impedance "mismatch" isn't too likely either, there really isn't need to match impedance but to "bridge" it insted, meaning the power amp should have high enough input impedance no to load down the source. Naturally they are again all designed this way for conveninece so tube power amps should have plenty, solid-state power amps too but usually a bit less, so with that in mind the effect of loading down the signal and causing unneccessary attenuation should really be opposite.

Without knowing much of the details its pretty much impossible to tell what the issue is. Solid-state amps usually have nore headroom and do not start introducing distortion as early as generic tube power amps, they also may have a way flatter response (though both characteristics really depend on the overall design). Maybe he is hearing these differences...

Ben N wrote:Isn't the voltage output of most SS preamps much lower than a typical 12ax7 or similar preamp?

Ben, you hit the nail on the head! He said adding an additional gain stage to increase the voltage out of the preamp was a step in the right direction. thanks!

Dr Tony Balls wrote:You might be asking yourself WHY would anyone want to do this, but suspend that notion for the time being....

Well he's not the first. Leo Fender designed a whole line of hybrid amps for Music Man in the mid-late 70's with solid state (op-amps actually) preamps and tube (valve) power amplifiers. The only thing unusual about the design is that the tube power amp is a "cascode" design and not the typical grid controlled power amp. I have no idea why Mr. Fender designed it this way, perhaps this is how they get 150 watts out of four 6L6GC tubes, although running a B+ voltage of 750 has to help. In any case, these amps are big on clean headroom, might as well be solid state for as clean and loud as they are. Not much tubey warmth to my ears.

My guess as to why they designed a hybrid amp like this is because even in the mid-70's high-power solid-state amps were not yet as reliable as good ol' tubes. Hard to imagine now.