Gemini 1 ...my tubes are very hot!

[Fred Rocker]

my gemini 1 had some Realistic "lifetime" tubes in there, 7591's.
they work fine i guess but they are burning blue, and i figured i'd see what a new set of tubes would sound like so i bought a matched set of jj's 7591's and threw them in...
the amp lost almost all of the bass, and half of the volume.

i shut it down fast. after a few minutes of cooling i decided i could pull the tubes back out. they were so hot. i couldnt touch them with a rag in my hand.
after about 20 mins i swapped my old realistic pieces back in there.
the temp is alot cooler and sound is better.
so i suppose this is bias?

so...back in'66...every time you changed a tube in a gemini 1...you had to bring it in for biasing?

i had thought that these were "fixed bias" but i'm reading that these are "cathode bias"?
and cathode bias, in laymens terms, means what?

[Hangman]

no need to bias, they are cathode bias meaning "Self biasing". Rather than having the cathode at ground potential and a negative bias voltage on the control grid, the Gemini puts a resistor between the cathode and ground, raising the cathode potential, in this way the grid is still negative with respect to the cathode.
These are called "self biasing" because if the tubes you use end up drawing more current than the average set of tubes the voltage across the resistor goes up, and the potential between grid and cathode becomes more negative (same effect as reducing the bias in a fixed bias amp) the reverse is also true.

Thats my simple explanation. Basically a well designed cathode bias amp like the Gemini should work with any set of 7591s. same with an AC30. The down side is that the voltage across the cathode resistor ends up limiting the maximum output swing, so typically cathode biased amps have a lower output than a fixed bias amp.
example: a Gemini I you get only 22watts, but with a Gemini II which is a similar amp with a fixed bias configuration you get 35 watts.
pretty much any 50-100 watt amp you see is going to be a fixed bias amp. there are a few exceptions of course.

My guess is that you may have a bad set of JJs or something else wrong with your amp. you might have it checked out.