In need of advice on adding a ground

[bluemkiii]

I lucked into a ptp wired tube amp out of a 1960s magnavox tv. I hooked my guitar up to it and it sounded great! The only thing is it is not grounded. My roommate was using it and got a bad shock from it when he touched another piece of grounded equipment.

So far i have added a fuse, power switch and attempted to add a chassis ground. The thing immediately trips the gfci its plugged into unless the new ground is unhooked. Any advice on what im missing out on?

Here are some pictures.

Before modifications




after modifications "ground unhooked"




Thanks!

[Jazz P Bass]

Chassis Grounding

The first thing is to find out if the chassis is at an elevated voltage.
Measure the potential from the chassis to a known earth ground.
Measure Vac & Vdc.
If there is a failure in the amp circuitry, grounding the chassis will just blow a fuse or mains breaker.
I cannot tell from the pictures, but if there is a capacitor from one mains lead to chassis, remove it.

[Steve Conner]

I think that might be the mustard-coloured disk that says "RMC .01 1400V" on it... One leg seems to be connected to the line, but I can't see where the other one goes. If it goes to chassis, snip it out and see if that keeps your GFCI happy.

If that fixes the problem, then that capacitor was probably shorted, and that was the reason for the shocks. If the GFCI still pops, then the big black transformer may be faulty and you should probably stop using the thing.

[bluemkiii]

Thanks! ill check that when i get some new batteries for my vom tonight. The RMC .01 1400V goes from the power in to a wire that was jumped to another wire that goes to the chassis... dont ask me why... an electrical engineer told me too "the one that got shock" Seemed like a bad idea to me all along. I removed that jumper and i was able to touch the ground to the chassis with out tripping the GFCI.

[jakehop]

If the amp is a stereo P-P, which it looks like to me, you can convert it to mono. That might be more usefull as an instrument amplifier.

Kind regards, Jake

[Steve Conner]

I removed that jumper and i was able to touch the ground to the chassis with out tripping the GFCI.

(Edit: Having seen the schematic, nope, leave the jumper off)

I'd leave the thing stereo if you have two suitable speakers. It would sound great with pedals.

[RWood]

I have that same amp and use it for my "workbench stereo". It is really quite nice and I would encourage you to keep it as a stereo power amp.

I agree with all the other comments, and in specific with the one that Steve cited - that is the so-called Death Cap, named for reasons that your roommate will understand. That cap probably has a short. Since you have converted this to a three wire power cord, you do not need or want that cap at all, so simply cut it out of there.

Once you do that, by all means measure the chassis for any voltage and let us know, but ground should be at zero volts and I suspect it will be when you remove that cap.

Now, some of these amp versions had a weird and ineffective balance control that was mounted on the preamp of the console. Using the power amp in a standalone setup, no balance control, the amp will develop a motorboating oscillation after a few minutes. If yours does, too, it is an easy two resistor fix and I can help you with that. I have also attached a schematic in case you need/want to refer to it.

Nice job cleaning the chassis, btw, mine is nowhere near that shiny. Mine did have two Mullard EL84s, though, and two RCAs! Not bad for $35.

RWood

[Steve Conner]

Thanks for posting the schematic, RWood.

According to that schem, the cap is just between the two line wires. So if removing that jumper made the chassis not be live any more, then it seems as if the jumper was actually connecting the chassis to the line, and you should leave it off?

Or the schematic is wrong or something. Anyway now you have the schematic, you can rip out whatever mess is in there and connect the incoming power straight to the black and black/red wires on the transformer.

[bluemkiii]

Thanks for posting the schematic!

I removed the "death cap" and jumper and it came on just fine with the ground hooked up. I tested for DC and AC on the chassis and got nothing. Thanks for all your help! I plan to keep it a stereo amp so that it is more versatile. Could i piggy back a guitar input off each of the RCA inputs? or would i get distortion from that?

Oh, Rwood, mine is not that shiny. That is a picture that i stole from another site about the same amp. I had uploaded the other pics i posted and forgotten one of the thing from the top side and didnt feel like messing with flikr again. I do plan to clean it up once it is in good working order though.

[RWood]

How do i go about fixing this?

Easy. Look at the sockets for the 6EU7s. Do you see how each pin 9 (cathode) goes to a resistor on a terminal strip and from there to a colored wire that goes 'way across to the Molex plug?

Without the preamp plugged in those cathodes are floating, ungrounded. So we need to ground them with some additional resistance, and thankfully a grounding post is right next to those terminal strips.

Where each resistor is tied to its terminal, use a new resistor of at least 390Ω to connect it to the ground terminal. I used an 820Ω on each of them, and it sounds great. You can actually get more perceived volume (up to a point) by using a higher value here. So feel free to experiment.

In case you are wondering, yes, you may try a potentiometer here, as in the schem and the original design, but as I said it is ineffective. The volume is okay at either extreme but anywhere in the middle it is severely suppressed. It was a quirky design that works off the feedback circuit.

If you are sure you'll never again use the preamp/tuner unit, you can go ahead and remove the leads that run from the Molex plug to those terminals.

Here is a picture I snagged from Audio Karma. It is from an 8800 amp, vaguely similar circuit to yours but using 6V6s and 6CA4 power plant. It has the same driver/splitter with 6EU7s, the same feedback circuit and same balance deal. Your amp might have different color wires, though. If you haven't done it yet, do yourself a favor and replace the four coupling capacitors: two .047s and two .0047s.

RWood

[JHow]

...that is the so-called Death Cap, named for reasons that your roommate will understand...

Hmm. There have been several recent threads poo-pooh-ing the removal of these caps, and how rarely they fail, yet here is apparently a real second-hand (roommate) account of shock-by-death-cap.

Admittedly the roommate is okay, but like the kid Bill Murray's experiment in Ghostbusters, how many times do you want to get shocked?

[bluemkiii]

Thanks RWood! it stopped "motorboating" after it had been turned off for a while. Im not exactly sure why. If it did in fact have that balance feature would there been a knob at the hole one the chassis where it says bal? Mine has nothing there and hasn't made that noise but for that one time earlier to day.

[RWood]

You're welcome!

Magnavox used that basic chassis for many different versions and models, some of which did have a balance pot mounted there. Yours likely did not, unless you think your amp might have been modded by a previous owner. The 9300 series had at least four versions; mine is a 93-04. The 8800 series uses a lot of the same tooling, but has the different power tube and rectifier sizes, as I mentioned, has no choke, and although the transformer fits in the same holes, it is shorter as it has no 5V winding. Those, too, have an empty hole for the Bal pot, and an empty hole for a third 6EU7.

I'm glad the motorboating in yours stopped, but expect it to return after the amp has been on for a while. For some reason it is slow to develop. It really is a straightforward fix though.

I was listening to mine tonight, as a matter of fact, while working on an Eico integrated amp, an HF-20. Nice sound from the Maggie.

Good luck with yours.