Lower gain preamp valves

John_A

Just seen an ad for a jca22h that's had V1 swapped out for a lower gain valve. Was thinking this might be a good idea, but common sense tells me it's no different to just turning down the gain. Am I missing something or is that the case?

ecc83

No John it is not. Firstly, guitar amps do not have "gain" controls, they have VOLUME controls in "silly" places and call them "gain" controls. A true gain control to an engineer alters feedback around a stage.

Secondly, fitting the "wrong" valve for which the amplifier was designed *&^$ks with the bias and produces unpredictable results, some you might like, others not. In rare cases (don't think the jca22h is one?) ad hoc valve swapping can cause damage to anode load resistors.

Putting a Moggy Minor 998cc engine in an AC  Cobra is going to do a bit more than make it go slower! Other way around of course would smash the gearbox.

Dave.

Maynehead

It can help to expand the usable range of the gain control. If you never turn your gain past 5, then putting a lower gain valve in v1 will let you let you use more of your gain dial, and hence give you finer grained control over your tone.

Similarly, if you have an amp that goes from nothing to holy-shit-that's-loud before you got past 1 on your master volume, putting a lower gain valve in the PI position will let you have finer grained control over your volume (this usually will not affect your maximum volume too much as most amps saturate the power stage before you get to 10 on the dial).

JezWynd

I used a 12AU7 to tame my souped up Excelsior. It worked fine, whereas previously moving the volume past 1 would be far too loud, I could now turn up to 5 or 6 safely. It altered the sound somewhat, left the amp very clean sounding.

ICBM

Using a lower-gain valve also changes the tone - typically widening the bandwidth, ie increasing bass and treble at the expense of midrange, and making the sound 'clearer', so it's not the same as just turning the gain down.

Dave is right to warn that with some modern amps you need to be careful, because a lower-gain valve will always draw more current than a higher one, which will increase the power dissipation and voltage drop across the plate resistor, and may exceed its ratings. Older amps were generally built with larger resistors so it wasn't an issue. (A particular trap for the unwary is the Fender Bassman reissue - Fender reduced the size of the resistors compared to the 1950s amps, so using the original-spec 12AY7 valve in V1 can blow the resistors.)

Luckily, Jet Citys are also built with overspec resistors throughout and aren't at any risk.