gain control in express amp

[Reeltarded]

Well, yeah. It can be. I had a Rivera amp with a matching control for hot and not so hot guitars. I always left it wide open.

You could take maybe a 500kL pot with a 500k resistor on the output leg so you have a total 1M and it never goes to zero there. You would have 500k of trim.

I am not sure about what this does to the 1M ground reference. I think it stays as it is, but I have been wrong before.

[phsyconoodler]

I'm thinking a type 4 master might be the best for this amp.

[Reeltarded]

Why did you choose that?

If you are trying to dump gain, look elsewhere! A master of any kind is going to do the opposite. Lower volume=less headroom. T-IV? That's a ppimv? You are going to totally add gain you can't imagine. The PI starts breaking up really hard at lowered volumes. You lose the NFB. Sterile.

Where is your NFB circuit routed, btw? Increase NFB for a little headroom in overall sound.

[phsyconoodler]

Not looking for headroom,just trying to get that beautiful gain at a lower volume and possible a little less distortion.
I'm not opposed to a PPIMV like the Lar-Mar but I wanted a preamp gain control so I can control how much gain there is for the sake of versatility.
So I'm going to try a voltage divider to V2 and see how much bottom end gets lost by mucking with the filter.
I now some guys even use a 12DW7 in V1 to get a little less ooomph.
these amps really pack a wallop!But by just cranking the amp and using the guitar volume the dynamics can be out of whack and volumes all over the place.
Some songs don't need massive gain and in your face ripping distortion.
So some kind of a gain dump would be nice sometimes.
I know some schematics show a 56k divider resistor before V2 and that must dump some gain.I also don't need to have the fattest bottom end all the time either.
Versatility is what I'm after,nothing more,nothing less.

After all,two knobs are more control that one volume knob,am I not right?

[Firestorm]

How much are trying to lose?

If you take the 220K output tube grid loads down to 150K (even 100K) it'll clean up a lot. You'll lose a little "something."

[JoeCon]

Firestorm

You're refering to resisitors R16 and R17 where the bias comes in?

[Tillydog]

Not looking for headroom,just trying to get that beautiful gain at a lower volume and possible a little less distortion...

The output stage distorts first in the Express, so it happens at pretty much a fixed volume. It doesn't matter where you dump signal in the preamp - you may as well just use the volume pot on the amp. If you want the distortion at lower volume, then voltage scaling (the whole amp) is a good way to go IMHO. (Or use an attenuator).

Andy

[phsyconoodler]

I don't want to use an attenuator and without losing the original 'killer' tone of this circuit,I am heading toward just using a master volume of some kind.
I gutted a Marshall class 5 recently and hand-wired it into a 'wreck circuit and it made the original circuit sound really lame.That amp is ok at 5 watts,any more and it needs some control over the volume.
Lets face it:most guitar players don't like rolling off the volume control on their guitars.They would rather have either a foot operated volume pedal or have the amp give them what they want at a reasonable level.
When Mr. Fisher designed this amp people understood that rock and roll was loud.Now soundmen all over are asking us to '"turn down!" and that sucks.
But we all have been there in a live setting.
I'm going to try both types of master volumes and a different voltage divider to v2A just to see what happens.
I did build a 6V6 version with a master volume and it rocks,it's just not quite there in terms of control.Fussy? Not me so much,but guys I build amps for complain about everything these days,so you do what you have to to make people happy

[Clyde]

I've done a little messing with this circuit and it seems remarkably balanced in it's design to achieve Mr. Fischer's aims. Unfortunately when you change very little, you wind up with something else. May or may not be a bad thing depending on what you're after.

[Gibsonman63]

I find that an attenuator works great for mine, but I usually run it at about -9 or -12 dB maximum. Combine that with less effecient and/or fewer speakers and you can probably get it down to tolerable.

6V6s are cool in this amp and will take it down a notch or two in volume. I ran mine that way for a year, but the clean to mean is much more pronounced with the EL34s, so I ended up going back. If you run the guitar wide open all the time, there is no reason not to try it, 6V6s are relatively cheap.

I bought a VVR to put in mine, but never got around to it after getting the attenuator working properly. The VVR works great in my Liverpool. I have heard mixed results reported trying to put it in an Express. I would imagine that it is the same issue as the attenuator; you just can't take too much away from this circuit without losing some Mojo so you may have a limited useful range.

If you aren't working the crap out of the volume control on your guitar with this amp, you are missing a lot of the fun. Lot's of different sounds and they are all great.

[Firestorm]

Firestorm

You're refering to resisitors R16 and R17 where the bias comes in?

I don't see any numbers on the layout, but yes the resistors people sometimes call "bias feed" or "grid return." Used to do this a lot on Deluxe Reverbs where wall voltage creep had taken them over the top. You play those stock and you can watch the screens get brighter in response to your playing. Cool, but not good. Back those resistors down and everything starts to behave again. Cleans it up a little, too.

[Firestorm]

Not looking for headroom,just trying to get that beautiful gain at a lower volume and possible a little less distortion...

The output stage distorts first in the Express, so it happens at pretty much a fixed volume. It doesn't matter where you dump signal in the preamp - you may as well just use the volume pot on the amp. If you want the distortion at lower volume, then voltage scaling (the whole amp) is a good way to go IMHO. (Or use an attenuator).

Andy

Just a difference of opinion here, but I wouldn't scale the whole amp. Voltage reduction in preamps doesn't much change their gain, but kills their headroom so you could wind with an amp where the stages distort in a different order. I only scale the output. YMMV.
Requires some kind of MV to control how much signal hits the output tubes. MVs screw with the nfb, so that might have to be variable, too. IMO.

[Tillydog]

Just a difference of opinion here, but I wouldn't scale the whole amp. Voltage reduction in preamps doesn't much change their gain, but kills their headroom so you could wind with an amp where the stages distort in a different order. I only scale the output. YMMV.
Requires some kind of MV to control how much signal hits the output tubes. MVs screw with the nfb, so that might have to be variable, too. IMO.

I understand what you're saying...

From my limited experience (sample size=1) scaling the whole amp avoids having to introduce a MV of any sort (which I don't think is a good idea in this design, IMHO, for the reasons you state), and doesn't really change he gain structure. Sure, you lose some headroom, but IME it's still the output stage that goes first (this is with JJ 6V6s). My B+ goes down to ~60V and the essential character of the amp is still there IMHO.

As ever, YMMV!

Andy

[sepulchre]

If you aren't working the crap out of the volume control on your guitar with this amp, you are missing a lot of the fun. Lot's of different sounds and they are all great.

+1 to that! Same goes for most amps IMHO.

[sepulchre]

(Oops. Accidentally posted twice.)