Gibson GA-15rvt suggestions

[willowhaus]

Hi! I ran across this place searching for info on the GA-15rvt, one of which has just come into my possession. I saw a recent thread here, which echoed some cap mods & such I'd seen elsewhere, but being a bit of a n00b at amp tinkering I wanted to ask if someone could give me a little more detail.

For instance, safety mods were mentioned but not detailed. (WTF is a "death cap"?!? ) Someone has put a 3-wire plug on this amp, but I'd like to verify it was done correctly. Just FYI, I am aware of draining the filter caps before mucking around and am planning to replace these as part of my initial cleanup. I also am ordering some OD caps to make the mods I've read about for this amp to improve the tone, as well as some new tubes.

Anyway, I'd appreciate any specific guidance y'all might offer on this particular beast. If I run into issues I'll post more specific questions. Thanks!

[selloutrr]

The Death cap refers to the capacitor soldered on to the ground switch. It was installed on amps that used 2 prong power cords. It allowed the user to toggle between shunting the hot or the nuetral to the chassis ground to minimize the hum.
It got the name death cap because when the cap fails it can short the voltage to ground and shock the heck out of you.


if you look at this schematic you can see how the hot wires are bridged by a 600v cap before they hit the fuse/power switch on to the transformer that is the death cap.
http://www.drtube.com/schematics/fender ... ematic.gif

The correct way by current standards is to omit the ground switch and cap all together. using a 3 prong grounded power cord. the white goes to the power switch, the black to fuse. the green (ground) is crimped or soldered to a terminal and placed against the chassis under a transformer bolt.

[willowhaus]

Excellent info - thanks!

[mcrracer]

The Death cap refers to the capacitor soldered on to the ground switch. It was installed on amps that used 2 prong power cords. It allowed the user to toggle between shunting the hot or the nuetral to the chassis ground to minimize the hum.
It got the name death cap because when the cap fails it can short the voltage to ground and shock the heck out of you.


if you look at this schematic you can see how the hot wires are bridged by a 600v cap before they hit the fuse/power switch on to the transformer that is the death cap.
http://www.drtube.com/schematics/fender ... ematic.gif

The correct way by current standards is to omit the ground switch and cap all together. using a 3 prong grounded power cord. the white goes to the power switch, the black to fuse. the green (ground) is crimped or soldered to a terminal and placed against the chassis under a transformer bolt.

Not to disagree with you but I had always learned black to fuse tip, side of fuse to the switch then to the PT. White straight to the PT.

[tubeswell]

I had always learned black to fuse tip, side of fuse to the switch then to the PT. White straight to the PT.

That's absolutely right. You don't want the mains fuse sleeve to be connected to the mains active under any circumstances

[selloutrr]

I don't base it on color to be honest I did until a couple years back I got a batch of IEC power cords that were reversed. It's always best to test continuity so you know what is what before you just solder by color.

My earlier color code suggestion is the same way the express layout is wired.

[mcrracer]

I don't base it on color to be honest I did until a couple years back I got a batch of IEC power cords that were reversed. It's always best to test continuity so you know what is what before you just solder by color.

My earlier color code suggestion is the same way the express layout is wired.

Point taken.
I will say it this way then. The HOT wire from the IEC to the fuse tip, etc.

[Phil_S]

Whether you fuse the hot to the tip of the fuse holder depends on the type of fuse holder. If you have the type where the cap grips the fuse, so that opening it immediately pulls the fuse away from the tip, that is the right way to wire it. If you have the kind where the cap does not grip the fuse and where the cap is what makes the final contact, then I think you want the side terminal connected to the hot lead. The latter is fairly uncommon these days. I wanted to point this out, because the principle is what matters here. When you remove the fuse cap, you want the mains circuit to open. So, use your head!

[Structo]

Yes, I guess you have to figure the average Joe doesn't unplug the gear before replacing the fuse.

At least we agree that the fuse should come before the switch.

[Phil_S]

What is it that you are trying to get from thei amp, tonewise? What don't you like? You're question is too broad to give a good answer. Also, don't get lulled into thinking that OD caps are what you ought to be using. Depending on where it's used, it could be wonderful or sterile. Don't be so quick to throw away an of those old tone caps.

IMO, the biggest problem with all of the Gibson designs of this series is the notch filters. They are just wrong for what most guitar players expect today. They may have been just right back in the day, as tastes change. Look along the top of the schematic and you'll see two of these: C3 R9 R10 and C4 R13 R14. The good/bad news is that caps in parallel are additive. If you want to lower the cap value, you'll have to replace it, but you can easily tack in another to see the effect of a larger value.

Does yours have an undocumented coupling cap between R1 and R4?

[willowhaus]

Tonewise the amp is too bright. It was helped a bit with some newer preamplifier tubes - seems to have a stronger midrange now. I plan to do some simple mods first and see where that gets me.

Yes, it has the .0047 mystery cap. I'm going to raise it up to .015. I picked OD caps because the guy who did the mod used them and since I was following his lead I thought I'd use the same parts. I don't know his motivation for using them, although it seems to be full of them already.

This is the page I'm following: http://www.axecentral.com/gibson-ga-15r ... 48789.html

I opened it up this evening to clean the pots & sockets. I had also ordered a pair of 20uf filter caps, but I see that somebody had already replaced them...with dual 20uf caps wired together. Not sure why they would do that - any idea?

[willowhaus]

I'm also planning on replacing the speaker at some point - probably going to stick with the 10", as I already have a 12" combo. Nothing is set in stone, though...

[willowhaus]

double-post'd...

[Phil_S]

Yes, I know the individual who posted that "kinda long" in your link. Not personally, but I've corresponded with him directly and it goes back at least 7-8 years. I think his experiments are worthy of the post. In the end, I think some of his observations are on-spot and others may be a little off.

The input coupling cap mod can be done by tacking in another cap in parallel to see if you like it. That one's a no brainer on account of that. Adding/changing the preamp cathode bypass caps is also highly recommended. Putting in a 20u bypass cap will let more bass through, which is something this amp needs badly, IMO.

The best thing I think you can do is fool with the notch filters you see along the top of the schematic -- two resistors with a small cap -- maybe a pair of 220K and a 500pf cap. There are two of these. I am weak on theory, so I'm not going to say what's to be done, but I think maybe the cap value is more of a problem than the resistors. In any case, you can place a jumper across the two 220K caps to bypass the notch filter. I'd bypass both of them (one at a time, then both) and see what happens.

The ice pick tone is a common observation and complaint about this series of amps. You can either figure out how to mod it, or this makes a decent platform for dropping in an 18W board. Heavens, did I really say that?

[willowhaus]

Yeah, I was pretty sure you were familiar with that page from your other posts.

I also have a couple caps for the cathode bypass - same 25uf value he suggested.

I'm interested in the tone stack thoughts too - I will try bypassing the filters per your suggestion & see what I have, and then try fiddling with some of the values. I've been running the amp with the treble way down, which actually doesn't sound that bad, although it could be better...

So any thoughts about the dual filter caps? I ran across some at the Torres Engineering site who was selling the 20uf caps, as well as 30uf that they were pitching as "better"...I thought this was a little dubious (his site's pretty heavy on the sales pitches ) and figured I should stick with what's in the amp, at least to start with. (Actually got my parts from thetubestore.com, where I've gotten a few things fro in the past.)

Is there anything to be gained by raising that value above 20 uf (what would now be 40uf in this amp, I guess?)