different pre tubes

[marshman]

I tried searching this topic, but have no idea where to start. I thought I'd seen this topic here before, but can't find it.

Aside from the basic set of 12A?7/ECC8? tubes, there are apparently other tubes in this family like 12BH7 and that sort of thing. I also recall a hazy mention of tubes with two different mu triodes inside. So, perhaps, a 12AX7 on one tri and a 12AT7 on the other. I would think that if that's the case, they could be pretty useful for fine-tuning gain to taste. Is there a reason this topic isn't brought up much? Are the tubes just too inconsistent to be useful or something?

[Trout]

A lot of audiophiles buy the 12BH7's. The exact reason I have no idea, But mine all sold fast on ebay.

I have however seen them in several old console stereos, Mostly Zeniths, They were used in a line stage on those amps. Perhaps they have a certain quality clean use.
Trout

[novosibir]

Aside from the basic set of 12A?7/ECC8? tubes, there are apparently other tubes in this family like 12BH7 and that sort of thing. I also recall a hazy mention of tubes with two different mu triodes inside. So, perhaps, a 12AX7 on one tri and a 12AT7 on the other.

No! That's the 12DW7, but not as you've described! The 12DW7 has one system of the 12AX7 (µ = 100) and the other one from a 12AU7 (µ = 17).

However the 12BH7 is nothin' other than a 'stronger' 12AU7 with a max. plate dissipation of 3.5W instead of 2.75W and the double heater current.

Larry

[zaphod_phil]

Aside from the basic set of 12A?7/ECC8? tubes, there are apparently other tubes in this family like 12BH7 and that sort of thing. ... I would think that if that's the case, they could be pretty useful for fine-tuning gain to taste. Is there a reason this topic isn't brought up much?

The regular/classic 18W and Lite basically have a single-stage preamp, followed by a PI. So they have to pretty much squeaze out every possible drop of gain from the 12AX7s they use, which means there isn't a lot of point to split-gain tubes. For fun you could try a 12BZ7 which has the same voltage gain as a 12AX7 but delivers twice the current, in the V1 position. Some folks like the tone they give.