55VAC on Chassis?

[chipaudette]

Hi All,

I'm resurrecting an old Fisher HiFi amp from the late 50's. Eventually, I may turn it into a guitar or bass amp. Right now, I'm just trying to get it working. The schematic is below:

Fischer CA-40 Schematic.pdf

To get started, I put in all the tubes and plugged the "speaker out" into a THD Hotplate as a dummy load. Then I plugged it in (two prong plug) to a power strip. I turned it on.

All the tubes seemed to warm up just fine. The power tubes had a pretty good glow to them. I checked the voltages on the main capacitor can and they were a touch low but probably OK (for example, B+ was ~380 instead of 415). So far, so good.

The problem is that I also measured the voltage on the chassis to make sure that I wasn't going to shock myself. Taking my DMM, I clipped one lead to the chassis of this amp and clipped the other lead to the chassis of a 2nd amp (Fender Deluxe Reverb Reissue guitar amp from early 2000s).

It read 55VAC and ~0.5VDC. The 55VAC number scared me a lot! I hadn't touched the chassis up to that point and, seeing that value, I wasn't going to start.

This part of my old house doesn't have proper grounding available. So, the chassis of the 2nd amplifier was floating as well. Is a 55VAC normal? Is it really just a measurement artifact and I'm actually safe? Doesn't seem right.

Any thoughts on what could be the problem?

Chip

[MWJB]

Well, you have already identified the problem...you have no grounded chassis connection at the AC cord and no wall AC ground reference to terminate it to. You would be reasonably safe as long as you didn't use yourself to connect the Fisher chassis (or anything connected to that chassis like instrument cable plug barrels, conductive mic/instrument housing that is connected to the chassis) to another ground reference that was at a different potential to the Fisher's chassis (as you ARE doing by connecting to the DR chassis, or if you touched something else with a path to ground like a water pipe/stood barefoot on a concrete floor etc).

Instrumentalist/singers using amps with 2 prong plugs run the risk of becoming a path to ground via the PA etc...it's not likely to kill you, but once you have been shocked like this, it's not an experience that most would repeat by choice.

Getting shocked in the face can quite throw you off your stride in a performance...imagine the scene, "our harp player's going to sing you a song now...", the band bring the song in and the guy just eats 3" sparks & jumps up & down shouting, "Shit! shit! Ooow!..."

So either, always wear rubber soled shoes, insulate exposed metal 1/4" jack barrels & ground metal mic/instrument casings and avoid touching ANYTHING that might possiby provide a path to ground, or do the sensible thing and fit a 3 prong cord that terminates to ground.

If you don't have a ground reference for your AC wall outlets in that part of your house, then your DR (or any other amp) is no safer than the Fisher.

[Enzo]

Your whole setup is unsafe.

First, turn the two prong plug over, and see if that reduces the difference between the two amp chassis.

Look on your schematic, right where the AC mains come in to the power transformer. C32 links the mains to chassis at the fuse. That is most likely where the voltage is coming from. They refer to that here as the "death cap."

[Diablo]

Not to hijack this thread, but I saw a Mojo video on the internet where they are plugging one of their stand alone reverb units into another amp and recommend to disconnect the ground (three prong power cord adapter with no ground connection) on the reverb unit so you don't get hum from ground loops. I assume the reverb unit chassis is safely grounded through the instrument cord to the other amp chassis and then through it's 3-prong AC cord to earth. Is this safe??

[chipaudette]

Ok, so I took off the two-prong plug and put on a new power cord with a three prong plug. I tied the ground prong to the chassis. Then, I plugged both the Fender and the Fisher into a long heavy-duty extension cord that was stretched across the house into known-to-be-grounded GFI circuit in the bathroom. Now I feel safe.

With safe power and grounding, I plugged the Fisher (via the Hotplate) into the Fender's speaker and fired it up.

On a couple of the inputs (mic, tape, and phono), I was able to get some outuput from the speaker. The other inputs made no sound at all. On the working inputs, the sound was super ratty sounding and was really non-linear in volume. The attack of the note would be out nice and loud (in a splatty blatty kind of way) and then the sustain portion of the note would just disappear. Ick.

Minipulating the front panel controls caused the sound to cut in and out intermittently. They've probably taken some hits during the many lives in other people's basements. Bummer.

So now I have to decide whether to try to track down the source of the blatty sound and fix it, or, gut the thing and rebuild the circuit based on some other (probably Marshall) schematic. Any thoughts from the audience?

Also, Enzo, I assume that you are recommending that I remove the "death cap". Am I right?

Thanks,

Chip

[Enzo]

Yes.

The death cap was probably leaky to start with. The death cap is not necessary with a three wire powr cord.

You quite literally probably have to replace every cap in the thing - they are 50 years old for God's sake. I don;t doubt it doesn;t work well. Whether those new caps are to restore the existing circuit or are part of something new is up to you.

[MWJB]

Hi Diablo,

Only one chasis needs to be grounded if you are daisy chaining amps/reverb tanks, if all devices have the chassis grounded you may get ground loop hum.

You would have to remember to reconnect the grounds when using any of these devices on their own, as they would no longer be grounded via another amp's chassis.

A safer alternative would be to leave all amps/reverb tanks grounded at the AC cord and use audio ground lift adaptors in-line with the interconnect leads.

[Diablo]

Hi Diablo,

Only one chasis needs to be grounded if you are daisy chaining amps/reverb tanks, if all devices have the chassis grounded you may get ground loop hum.

You would have to remember to reconnect the grounds when using any of these devices on their own, as they would no longer be grounded via another amp's chassis.

A safer alternative would be to leave all amps/reverb tanks grounded at the AC cord and use audio ground lift adaptors in-line with the interconnect leads.

Thanks MWJB for answer and explanation...