Amp mods... Maybe I'm not so good at this.

[misterwhizzy]

So I decided to screw with things because I can't ever stop tinkering and just learn to play better. After a little consultation, I decided I was going for two mods, both attached to the depth pot on the back of my amp. The first was going to add a cathode bypass cap to the second gain stage, and the second was going to add a bass boost.

So first I had to get my workspace prepared.


At that point, I removed the existing depth pot from the back of the amp and started soldering caps onto the new depth pot, which is of the push-pull variety.


Then I affixed that to the back of the chassis.


I soldered the connections onto the board and the output jack where necessary.


And replaced the board in the headshell. The pot just to the left of the 8-ohm output jack is not stock, obviously. If you've gotten this far into this post, you already know that.


Well, things were not well in modification land. I shorted something out, I had the ground wire running to the output jack, and I had the output wire running to ground. And my beer was empty.

Over the last day or two, I realized that I had the caps backwards. The deep cap was acting as the cathode bypass, and the cathode bypass as the deep. I finally got frustrated and decided to just pull the deep cap out altogether. Then when I went to plug in and see how the voicing mod sounded, I only got what sounded like some Line6 BS. I think I had bypassed the power amp somehow.

Anyways, as of today, I have the cathode bypass engaged when I pull the switch and the stock configuration when I push it back in. It's definitely got a bit of a gain boost, but I really don't have a full report yet, because I haven't been able to get any real volume out of it yet.

I guess the moral of the story is not to do this unless you're willing to screw up, and don't do it on an expensive amp at all.

[GoldenVulture]

Re: Amp mods... Maybe I'm not so good at this.

So I decided to screw with things because I can't ever stop tinkering and just learn to play better. After a little consultation, I decided I was going for two mods, both attached to the depth pot on the back of my amp. The first was going to add a cathode bypass cap to the second gain stage, and the second was going to add a bass boost.

So first I had to get my workspace prepared.
[ATTACH]30579[/ATTACH]

At that point, I removed the existing depth pot from the back of the amp and started soldering caps onto the new depth pot, which is of the push-pull variety.
[ATTACH]30580[/ATTACH]

Then I affixed that to the back of the chassis.
[ATTACH]30581[/ATTACH]

I soldered the connections onto the board and the output jack where necessary.
[ATTACH]30582[/ATTACH]

And replaced the board in the headshell. The pot just to the left of the 8-ohm output jack is not stock, obviously. If you've gotten this far into this post, you already know that.
[ATTACH]30583[/ATTACH]

Well, things were not well in modification land. I shorted something out, I had the ground wire running to the output jack, and I had the output wire running to ground. And my beer was empty.

Over the last day or two, I realized that I had the caps backwards. The deep cap was acting as the cathode bypass, and the cathode bypass as the deep. I finally got frustrated and decided to just pull the deep cap out altogether. Then when I went to plug in and see how the voicing mod sounded, I only got what sounded like some Line6 BS. I think I had bypassed the power amp somehow.

Anyways, as of today, I have the cathode bypass engaged when I pull the switch and the stock configuration when I push it back in. It's definitely got a bit of a gain boost, but I really don't have a full report yet, because I haven't been able to get any real volume out of it yet.

I guess the moral of the story is not to do this unless you're willing to screw up, and don't do it on an expensive amp at all.

Don't drink and mod, bad for the concentration. Not a good idea when playing with High Voltage circuitry. Clumsiness could be fatal for you and/or your amp.
Decades ago, I watched a pissed TV serviceman blow up a transistor in my tv [ not a cheap transistor ] and then charge me for it. That's when I figured I need to learn how to do it myself.

[USAPatriot]

Re: Amp mods... Maybe I'm not so good at this.

I agree with GoldenVulture...I saw the beer bottle in the first photo and pretty much knew the outcome wasn't going to be good. Look on the bright side: you're still alive. There's worse things than just screwing up your amp. -Rod-

[karpathion]

Re: Amp mods... Maybe I'm not so good at this.

Take it all out, put it back stock, and try again. This time do it without drinking.

You'll get it.

[Empty Pockets]

Re: Amp mods... Maybe I'm not so good at this.

Reminds me of a time somebody offered to show me how to change the filter caps on my 68 Bassman while we split a 12 pack.

[GoldenVulture]

Re: Amp mods... Maybe I'm not so good at this.

Reminds me of a time somebody offered to show me how to change the filter caps on my 68 Bassman while we split a 12 pack.

And?

[crguti]

Re: Amp mods... Maybe I'm not so good at this.

Nothing wrong of drinking beer while you're modifying your amp. You just need to plan it beforehand what to do.

[glassman]

Re: Amp mods... Maybe I'm not so good at this.

I'm still admiring the beer!

[GoldenVulture]

Re: Amp mods... Maybe I'm not so good at this.

Nothing wrong of drinking beer while you're modifying your amp. You just need to plan it beforehand what to do.

Judging by his first post , he did that. Planned and prepared.
It didn't work out though.
One down to pissed impairment.

[glassman]

Re: Amp mods... Maybe I'm not so good at this.

Will, I do have a few suggestions...

I'd move the cathode bypass boost off of that push-pull switch and a few inches away on a switch of its own. Maybe even put a 3 or 5 lug solder tree on the center-rear board mount to further isolate it. Also, route those cathode bypass boost leads away from the speaker leads.

The reason I would do this is that you have two signals of very different potentials in very close proximity...this invites capacitive coupling and potential instability. If the signals are out of phase, you get cancellation that becomes more prominent as you turn up the volume. If they are in-phase, you may end up with an amp that starts doing its own thing as you turn up and pretty soon it's out of control.

My rule of thumb in these situations is 1/2" spacing for perpendicular wires and 2" spacing for parallel wires.

[misterwhizzy]

Re: Amp mods... Maybe I'm not so good at this.

Thanks, Jerry. I'll do that. I need to figure out where to run them, but I'll do that.

By the way, the reason it turned out badly was not because I had a single beer but because my caps were backwards. I put the big cap where the small one should have gone and the small one where the big one should have gone.

I got rid of the deep switch for two reasons. First, because I wanted to be able to go back to stock, and second because the whole reason the depth pot is there in the first place is to adjust the bass response. It just kind of seemed redundant. So now I just have the cathode bypass boost.

This bumps the gain up significantly. I honestly had no idea adding a capacitor into the circuit could have this much effect. It really thickened up the mids and shaved off just a touch of high end. I really like the results, but I might want to activate a slight mid cut when I pull that boost. I need to figure out how to do that.

I understand now why this isn't in the stock configuration, because a clean sound is basically impossible with this engaged. It's a cool option to have, and I'm seriously surprised at how much a difference this one capacitor made.