Wiring Question (Gibson GA-19RVT cap job)

[Leonc]

I recently bought an old GA-19RVT, and while it sounds like it's got a lot of potential, it's humming pretty loud. I just opened it up and I see what looks like a pretty shabby filter cap job and I'm wondering if this is the problem.

Here's the schematic

http://www.PatchWerkz.com/LeonsGear/GibsonGA19RVT/schematic.jpg


Here are a couple pics of the same area from different angles. I added a couple labels to make things a bit clearer.

http://www.PatchWerkz.com/LeonsGear/GibsonGA19RVT/existing_wiring1.jpg

http://www.PatchWerkz.com/LeonsGear/GibsonGA19RVT/existing_wiring2.jpg

I'm the first person to admit that I'm no expert (not even close). But those don't look like the original electrolytic caps do they (the 2 big 20mF C9A&B)?

V5 and V6 are the two 6V6 power tubes. V7 is the 5Y3 rectifier.

Note that on V6, there is a Pin 1 and several components attached to it. However, Pin 1 is not used in 6V6s...right? So they're using this as a point to connect several components. (On V5, there is no Pin 1, i.e., they used a slightly different tube socket.)

The C9B's + side and R12 start there where the power comes in (V7, pin 2). Then C9B (-) connects to C9A on V6 pin 1 (you'll also see the yellow/red cloth wire wired there too). You can also see a white insulated wire going to ground on the back of the Depth pot.

The + side of C9A is connected to R12 on V6, pin 4 and pin 4 is connected to pin 4 on V5.

Some ugly soldering and loose ends but so far, so good, I think.

The bias part of the circuit thru me a bit. Check out C8 (.02 mF) and R13 (270 ohm). C8 is going from shared ground on V6 (Pin 1) to V5 pin8. And then R13 is going from V5 pin 8 to ground on the back of the Depth pot...but that's where that shared ground on V6 Pin 1 is going so I guess they are indeed both going to ground in parallel.

So...outside of the not so great soldering job, anyone have any close as to why it's humming?

Like I said, I'm guessing these blue electrolytics are not original...but they could still be 15 years old for all I know...I think the first thing I'll do is replace them and clean up that mess on V6, Pin 1. Ugh.

[Wakarusa]

You are correct in assuming C9A/B are replacements. The work is ugly, but looks correct. Assuming that all the tubes are in good shape, some other possible causes of your hum might be:
- Is C10A/B still original? If so, it's probably in desperate need of replacement.
- The white wire from the cathode (pin 8) of the power tube socket to the back of the potentiometer may be creating a ground loop giving rise to hum. For reasons known only to Gibson, the power tube cathode resistor and bypass cap are taken to different grounds (Ck to the tube socket, Rk to the pot). Adding the wire to tie these ground points may have done more harm than good. If you're replacing C9 anyway, it might help to take Rk and Ck both to the same ground point on the tube socket where the center tap of the PT primary is also grounded (the red/yellow wire).

[Leonc]

Thanks for the info Todd. I finished replacing C9A & B before reading this. Still humming like a mofo. If I understand what they're doing, the pin 1 on V6 doesn't ground anything itself...it is taken to the back of the pot via the white wire and that's the grounding point for everything attached to pin 1. The cathode resistor, R13, goes there too, but it has a direct route. The bypass cap, C8, goes there too...but via the V6 pin and wire business.

What I wonder is if I shouldn't just get rid of the white wire and ground all that stuff directly to a point on the chassis? Maybe put in a metal screw and a terminal, scrape the chassis up real good there and ground all those parts to it (i.e., the r/y lead from the PT, the bypass cap, the cathode resistor, the two filter caps), then solder it real good. What do you think?

BTW, the original C10 A & B is still there. It's one of those big old brown paper double caps. Two pos leads and shared neg (black) lead. I guess that needs to go too...

[Wakarusa]

Thanks for the info Todd. I finished replacing C9A & B before reading this. Still humming like a mofo. If I understand what they're doing, the pin 1 on V6 doesn't ground anything itself...it is taken to the back of the pot via the white wire and that's the grounding point for everything attached to pin 1. The cathode resistor, R13, goes there too, but it has a direct route. The bypass cap, C8, goes there too...but via the V6 pin and wire business.

What I wonder is if I shouldn't just get rid of the white wire and ground all that stuff directly to a point on the chassis? Maybe put in a metal screw and a terminal, scrape the chassis up real good there and ground all those parts to it (i.e., the r/y lead from the PT, the bypass cap, the cathode resistor, the two filter caps), then solder it real good. What do you think?

BTW, the original C10 A & B is still there. It's one of those big old brown paper double caps. Two pos leads and shared neg (black) lead. I guess that needs to go too...

You're right about the grounding.. hadn't enough coffee in me when I first looked. The white wire's gotta stay.

For the hum try replacing C10 and see what you get. Once that's done if the hum persists and you have spare tubes try a swap, if you don't but have an o-scope you might be able to pull tubes and determine which stage is humming the most and then look for bad grounds.

Since the pot appears to be the main ground point, it wouldn't hurt to pull it out of the front panel and clean out any oxidation, replace (or install) the lockwasher and make sure it makes good contact.

If all of that fails it may be time to look for a new ground point as you describe.

[Swarty]

I'd say there is about a 99% chance that C10 is the culprit. Those big ole double ceegar caps do not hold up. I'd also recommend a couple terminal strips to mount the caps on.

[Leonc]

Okay, just ordered a replacement for C10 A & B. Antique Electronics (CE Dist) had some radial multi-caps that appear to be a good replacement.

[Leonc]

Replacement for C10 A & B showed up today. Got it in (man, that was a serious bugger...don't think I've done anything harder!) and guess what...





















...hum gone! :D

[Leonc]

Next on the agenda: put in a 3 prong AC cord. I've done this before...but maybe not correctly. It's been suggested that I use a dp/dt and replace the current switch. So if you could please confirm if I've got it right.

Green AC lead goes to ground on the chassis.

Black AC lead goes to one of the center terminals on the switch (the terminals that are connected to either of the other two sets of terminals depnding on switch position).

The white AC lead goes to other center 'throw' terminal.

One lead from the PT goes to one the end terminals on the switch. The end terminal across from it goes to the fuse tip. The other PT lead goes to the other terminal on fuse.

What I'm less clear on is what to do with the CAP that goes from the switch to ground. This is the "death cap", right? You can just get rid of this?

[TheAmpNerd]

I'd...





... never mind.

[Wakarusa]

Next on the agenda: put in a 3 prong AC cord. I've done this before...but maybe not correctly. It's been suggested that I use a dp/dt and replace the current switch. So if you could please confirm if I've got it right.

Green AC lead goes to ground on the chassis.

Black AC lead goes to one of the center terminals on the switch (the terminals that are connected to either of the other two sets of terminals depnding on switch position).

The white AC lead goes to other center 'throw' terminal.

One lead from the PT goes to one the end terminals on the switch. The end terminal across from it goes to the fuse tip. The other PT lead goes to the other terminal on fuse.

What I'm less clear on is what to do with the CAP that goes from the switch to ground. This is the "death cap", right? You can just get rid of this?

You can use the existing SPST switch ('least that's what shows in the schematic) with no problems. The correct wiring would be AC cord black to the tab at the back of the fuse holder, then a wire from the side tab of the fuse holder to one pole of the switch, then from the other pole of the switch to the PT primary. The other PT primary connects directly to the AC cord white (a nice westinghouse splice and some shrink tubing makes a nice finished appearance and is perfectly safe). The green AC cord goes to a ground lug on the chassis different from the main amplifier circuit grounds.

If you want a two pole switch, I'd suggest a DPST. One pair of poles equate to the poles of the SPST described above, the other pair of poles would get the white AC cord and the "other" PT primary lead.

Typically when you see a double throw (the "DT" part of the switch designation) it's so one position is standby and the other is operate/run or, occasionally, their used to switch between a low power and high power setup.

[Wakarusa]

Oh yeah.. the .022uF originally from the AC line to ground is indeed the "death cap". You can remove it or replace it. If you replace it use a cap rated for the AC working voltage (which will typically mean a 600WVDC cap).

[Leonc]

Todd, thanks a bunch for your input on this, bud. Very generous of you!

BTW, I think the DPDT was suggested because it enables you to switch both the black and white AC power on and off together. What's your take on that?

[Swarty]

A properly grounded amp should require no switch or cap.

[Leonc]

Got it all wired up and sounding good.

Had RCA blackplates in it. Ugh. Waaaay too bright! Put in some Raytheon micanol-based 6V6s and GE 6EU7s and a 6C4. Got a Raytheon 5Y3 too. This sounded very, very good. Much better than when I got it. But the old Jensen was sounding very weary and thin to me. Threw a Cannabis Rex in there--Wow, man...wow. Luscious cleans, really gorgeous and meaty.

Thanks all for your help!