Gain Control in Amps

[Microdot]

How does a gain control work in a tube amp?

I’m asking because I am modding my Epiphone Valve Jr and none of the master volume/gain controls do what I’d expect.
For example in my 50W Ashton valve amp, there is the master volume, then a level or gain control for that channel. Keeping the same volume you can dial in the distortion with the gain knob.

Now in the VJ, you need the amp to be up past 12 o’clock before you get overdrive. I’d like to be able to push the amp’s breakup back, or be able to have it at low volumes.

Most amps have a master volume and a gain control which work in the way I’d like to, so I’d think it would be possible on these wee amps?
I have tried several gain/master volume mods on the VJ, I’ve replaced R5 with a 500K pot, which didn’t have much effect at all. Ive replaced R7 with a 500k pot, which allows you to slightly ease off gain – but it isn’t really helpful.
Ive added a negative feedback circuit which keeps it clean a bit longer, but it drops in volume so it is basically the same when it comes to volume/distortion levels.
I’ve done a bunch of component changes as documented online and it does sound great.

I love the overdrive this amp makes, but I’d like to have the cleans I get but at high volume since I use pedals often. A control which would allow me to get the desired overdrive at low volume, or the cleans at high volume would really put the icing on the cake.

I will start looking at amp schematics to see how they go about adding their gain controls, but I’d still like to know If it is possible.

Cheers!

[Microdot]

I have both a JJ 12AU7 and 12AT7 to try different gain amounts. Hopefully it should let me have more clean room.

But there must be a way of adding like a distortion control as there are on pretty much all other amps.

Please if anyone can let me know how they did it on ANY amp, I am willing to try it on my VJR.

Cheers!

[Steve Conner]

There are basically two kinds of amp. They are often referred to as MV (master volume) and non-MV designs.

Master volume amps, from the SLO and Mesa/Boogie onwards, are made with a huge surplus of gain in the preamp before the master volume. So you can turn the preamp gain up for screaming distortion, and then turn the master volume down for a comfortable volume.

Old-fashioned non-MV designs, such as the Fender Champ and the 5E3, don't have much preamp gain. To get them to break up, you need to crank the volume to overdrive the tubes in the power amp.

The Valve Jr. is a non-MV design, pretty much inspired by the Champ. So probably the easiest way to push the breakup of your amp earlier is to use a stomp box that provides a clean boost. This more or less converts you to a MV setup, where the gain control on this stomp box is the distortion control, and the volume on the amp is the master volume.

Otherwise, you'd probably need to add an extra preamp tube. I've seen plans for a DIY booster/distortion stomp box that actually has a tube in it. Something like that would get you more tube gain without having to hack your amp up too radically.

The other thing you asked for, getting the clean tones at high volume, is not possible at all. The VJr. only has something like 3 watts, so to get loud volume, its single tiny power tube has to be driven to the limit and it will ALWAYS get dirty. Big clean tones need a large powerful amp with plenty of "headroom".