5E3 Mods

[rtaylor76]

Hello, I know I am a newbie here, and am just getting a hold of design of guitar amps. I have done some minor repair and built a kit, but I am trying to make some mods to the Fender 5E3 Weber design. Would someone be kind enough to look at my schematic and let me know if I should make any changes?

Basically it has a Blackface T/B front end with a 5E3 power section. It only is one input, so I used the other side of the preamp tube for makeup gain. My biggest question here is do I need to worry about impedance here going into the PI? If so, how could I make changes? Also, do I even need the 470K series resistor, or would it even be better to add a bypass cap to that resistor?

Other mods are a heftier first filter cap, bright switch, fixed bias PI, and NFL. Although I am considering taking the NF or make it switchable to give it more of a vintage feel.

Any suggestions? I would certainly appreciate the help.

5e3_schem -TB mod.pdf

[loudthud]

I see you have installed the so called paul c mod for the concertina. Check this thread: http://music-electronics-forum.com/t14923/ I show an experimental circuit where you can tweek the bias on the concertina to change the headroom.

The 470K might be a little on the high side, you could lose some treble. I suggest something lower,, maybe 47K.

[rtaylor76]

The 470K might be a little on the high side, you could lose some treble. I suggest something lower,, maybe 47K.

Would I even need that series resistor?

[Chuck H]

Not sure exactly where this series resistor is. I'm guessing it's between the BF type V1 setup and the next gain stage. If so it is part of a voltage divider. If you remove it you will change the gain of the amp. It will go up. Probably WAAAAY too high. You could change both resistors in the divider (that series resistor and the "load" resistor) to lower values and reduce series resistance while keeping the gain structure the same. I doubt you'll hear any significant difference though. The way I like to do it is with a split plate load. You can search here and Google for "split plate load" and learn a little about it, but it may be outside your skill level right now.

Since your amp is not actually a 5E3 it would help us to answer questions if you post a schematic.

[pdf64]

Chuck, check the link at the end of the OP.
rtaylor76, a resistor may be needed, or it may work fine without one. It's acting as a grid stopper and it's good practice to fit them, but like loudthud advises, 470k will lose treble; 10k to 22k should be sufficient as grid stoppers, unless the layout has problems. Pete.

[Chuck H]

Yeah, Those get past me sometimes cause they're small and subtle.

As a grid stopper I'll agree with the above observations. But that's a lot of gain going on so I do think you would benefit from some atenuation or the amp may become unstable. You could just decrease both the grid stopper and the load resistor (1M in that circuit). Something like 100k or 150k load and a 22k stopper might be worth a try.

[rtaylor76]

Thanks everyone for your help. It looks like I do have a few options. I think the best approach is to decrease the load resistor to 500K or so, as well as decrease the grid stopper to 22K as per the suggestions. It would be interesting to try the split plate load, but I will try that approach first before changing the design too much. I am also thinking of keeping V1 a 12AY7 or 12AU7 much like the original Tweed Fender's (I should have mentioned this in the first post). This I think will help keep the gain down going to the PI.

I appreciate the assistance. If anyone has other suggestions on this or other parts of the design I would love hearing them.

Thanks again.