Someone make sense of this quote for me please..

[griff]

I posted on the Jet City forum on FB that I was gonna use a 5751 in my preamp.
A guy named Mark who's a well known amp tech and customizes jet city amps in the US, responded with..
"Just remember & I can't say it enough, all preamp tubes have an optimal bias point, just like power tubes do.
There's always a compromise in tone and response (from the optimized biasing of each particular type of tube) from one type to another.
Generally, improperly biased preamp tubes will sound flatter and more lifeless."

So is he saying find tubes that are originally designed for said amps preamp, or am I missing something here?
As far as I'm aware, most types of preamp tubes can just be thrown in to an amp.

[kwhelan]

http://proguitarshop.com/andyscorner/amp-bias-explained

[sizzlingbadger]

The amp was designed (biased) for a 12AX7 not 5751 is what he is saying.

[Slowy]

Might explain why some pre amp tubes are, well, tubes and others are portals to Nirvana.

[griff]

Ok but most preamps are self adjusting bias so...?
I'm guessing he means if you replace a 12ax7 for a 12aT7 or something, the amp isn't gonna respond the same maybe?
I reckon preamp valves are a good way to get closer to the sound you're after.

[sizzlingbadger]

No they are not self adjusting to a "designed" bias point, they are usually cathode biased which means they settle to a "bias state" based on the tube characteristics and circuit design. Different tube types will settle to a different bias point.

In my experience a 5751 is pretty close to a 12AX7 and will usually work quite well to reduce a gain stage in a high gain cascaded design. They can sound a bit flat in V1 though.

[griff]

Ok. Yeah was planning on V2 for that puppy. So we'll see how it goes.