Klemt Echolette Grounding Question

[Kazooman]

I am restoring several Klemt Echolette S tape echos that were manufactured in the mid-sixties. Here is a picture of my stable of echos:



These came to me in various states of repair from one that had been worked over by Bobby Staedel (the German Echolette guru) to one I bought from a guy in Holland that arrived with a thick layer of condensed cigarette smoke on every surface. The two shown with their guts exposed arrived in partial working condition. They both suffered from lots of distortion, even with bypassing the echo circuitry, and had very poor echos. I decided to replace as many capacitors on both of them as I could and this is no easy task. Here's a picture of a portion of the innards of one of them as I received it.



Not much room to work and you can't even see the caps that are really buried! The chassis is constructed in such a way that you can't open it up except on the two ends.

I have managed to get both of them working pretty well and the next step is to replace the two pronged cords with three prong properly grounded ones. I have done this chore previously on several amps.

Here's the question: As you can see, all of the grounds are tied together with a common bus throughout the unit. I have measured the voltage between this "chassis" ground and the actual ground on the mains supply. What sort of voltage reading might I expect?

With the two pronged cord in correctly (hot to hot and neutral to ground) I get 2 or 3 volts with the power switch off, but almost 50 v if I turn the power switch on. This seemed odd to me, so I checked the one unit with a three pronged cord. As anticipated, I read no voltage from the chassis to the ground at the wall outlet, but if I temporarily unsolder the internal ground connection I get the 50 volts. I have connected a wire from the chassis ground to the outlet ground and there is no measurable current.

I do want to install the three-pronged plugs, but I want to be certain I am not creating any new problems. Are these voltage readings "normal"?

Here's a picture of the schematic.



Sorry for the long post, but I wanted to provide as much information as possible without a long series of exchanges. Oh, BTW..the unit on the right came with a complete set of original Telefunkins. A nice surprise!

[Jazz P Bass]

I do not know what you are asking.
Why measure chassis to true earth ground?
There is no connection.
It is it's own system.
The Reason that you want the chassis AT earth potential is for personel protection.
If something goes wrong in the equipment & the chassis becomes "Live" (ie: mains potential) , then you want the breaker that the equipment is plugged into, to trip.

[Kazooman]

Thanks for the response. I understand that the chassis ground "is what it is". I actually did the measurement inadvertently while I was adjusting the voltages on the recording heads. I couldn't achieve appropriate values for one of the heads. I was surprised by the high voltage reading I got for the ground and by the lower voltage when the power switch was off. At first I thought I had the plug in backwards. When I flipped it around things got even worse. I was actually grounding the unit (through a ground loop) when I connected it to the amp. I just wanted to be certain that there wasn't some other problem with this old piece of gear such as having a short in one of the numerous PT windings. There is also a source of AC (albeit at a high frequency) well downstream of the rectifier. The ECC 82 circuit is an oscillator that feeds the recording heads. The voltages you measure when adjusting the heads is AC. Since I was having problems getting the correct voltage for one head I was concerned that there might be a faulty component in that portion of the circuit.

I'll just replace the plug and get on with it. I am still left with the problem of adjusting that head.

Thanks again.

[Steve Conner]

This is the age old grounding dilemma.

For safety reasons, you want the unit's internal ground bus tied to the chassis, which is connected to the third prong of the mains plug.

To prevent ground loops when it's connected to other equipment, you want the internal ground bus floating. But this presents a safety hazard, because the ground bus could go to a dangerous voltage in the event of an internal fault.

Electrical safety regulations have got stricter over the years, so the modern approach is to tie the grounds. Back in the 60s it was common to float either, or both. Wall outlets in many European countries didn't even have a ground prong, and some still don't.

One option that wasn't available in the 60s is to connect the internal ground bus to the chassis through a couple of beefy diodes. A pair of 1N5408s, maybe even a bridge rectifier with the DC terminals shorted. So it's floating for signals less than 0.7V, tied for large signals. That might be a good solution in this case.

If the ground bus is already tied to the chassis (couldn't tell from your description) then the place to put the diodes is in between the incoming green wire and the chassis. But you only need to do this if you get plagued by ground loop hum on installing the new 3-prong cord.

Digital multimeters will happily indicate a voltage even if there is only a minuscule current behind it. Some people call these "ghost voltages".

[Kazooman]

Thanks for the great suggestion. I will have to see if I have a ground loop hum when I am finished. I still need to find a source for a 3-pronged connector that will fit the hole in the chassis.

A while back I built a clone of my '63 Fender Reverb unit. One difference between the original circuit and the updated reissue model is exactly what you have described. The chassis ground is tied to the mains ground through the diodes. I did include that in my clone. I can easily replicate that here.

These old tape echos fascinate me. I desperately wanted one back when I was a budding rock star in the late 60's, but they cost $500 even then. Now that I have the time and some disposable cash I got sucked into looking for the "magic sound". It is very different than other types of delays or echos. The problem is that whenever you find one of these for sale it always comes with the disclaimer that "I haven't got any tape loops so I can't test it". "The tubes light up so it must be good"! I have learned that in addition to restoring the electrical components the mechanical aspects of the echos are also the source of potential problems. Well, it is just a hobby for me, not a business and I have enjoyed untold hours trying to get the echos back in working order.

[Toynbee]

Does anyone have copies of Kazooman's pics from this thread? I found one of his from another thread using archive.org.

[Kazooman]

I got an email alert that someone had responded to my eight year old post so I logged on for a look.

I have to admit that I drifted away from my attempts to restore the echos because I could never achieve the good, clean sound they were capable of fifty years ago. I moved on to other things.

I might have some pictures you can use. What are yo looking for and for what model. The Dynacords and Echolettes are very different. There are several versions of each, including later models that have printed circuits.

What are you seeking?

[Toynbee]

The pictures that were in this thread.

[Kazooman]

I believe that these are the original images that went with that post.

[Toynbee]

OK, thanks, Kazooman. My Echolette looks like this (attached). I put in a silicon type bridge rectifier (KBPC50-10) in place of the selenium bridge rectifier with a 250 ohm resistor in series with it.