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Earthing my neutral??

As the title says I want to know your views on this: I have a receiver which requires 220 volts DC HT for the valve anodes and 12.6 volts DC for the heaters from a seperate PSU  at the moment I have it run via a variac  fed from an isolation TX the unfortunate thing is its audio output is a bit on the low side so I want to run it through an audio amplifier the thing is the amp has the mains neutral straight to chassis  and one side of AF input also to the chassis  so if I link the 2 beasts together my receiver RF Earth will also be taking the amps chassis to earth I could run it all from the isolation TX  do you think that's the best option?
  • Earthing the neutral makes it a TN-C, even if your supply is TT ... if you're on a public LV supply, that would not comply with legislation (ESQCR).


    It also might cause mis-operation of RCDs (including, potentially in some circumstances, an RCD not detecting a fault that is should) - RCDs are not to be used upstream of TN-C circuits in the rare circumstances they are permitted in the UK.


    Neutral is a live conductor, and therefore your only choice with older equipment (or home-grown equipment) where the neutral is connected to chassis, is to use an isolating transformer
  • Kelly


    I agree with what Graham has said.


    The kit you are talking about sounds a bit vintage to me. I have a love and fascination of old amateur/ military kit and have owned a lot of it over the years before going all high tech with a modern transceiver.  What is it you are using?
  • Hi thanks for your replies it sort of confirmed what I thought  il just run the lot via an isolation TX. The receiver is a US Army unit from the flying corps originally it was run from a vibrator pack to turn 28 volts from the battery's into 220 DC for the anodes my set has its heaters wired for 12 volts but some are 28 volts which wouldn't be much good because I don't have that sort of voltage available.  The amplifier is an ex dansette record player amp  it has the turntable motor as a auto transformer to give 90 volts for its 2 heaters and the mains is half wave rectified for the HT. I love old kit it's easier to work on and tends to just work when asked to not like computer gear which always seems to find a thousand ways to not work 

  • Kelly


    If you want a vintage Rex then have a look on EBay for a Racal RA17. I had one for years before going all modern. Built like brick outhouse and weighs a ton.


    For a bit of fun there is a lot of military Clansman kit on EBay. The 321 HF TX/ Rx if you want to go mobile.
  • The usual fix in the old days was to have a double wound 1:1 audio transformer between the live chassis parts and the connectors for the audio to line inputs or in the case of radios also for external headphone sockets. Except on the HRO where the cans are at anode potential, which is not so good if your high impedance phones have exposed 6BA terminal bolts .

    Much lighter and cheaper than a mains voltage transformer. Do be very wary of direct rectified mains supplies  - you may think the chassis is neutral ,  but it will work and give no errors with the chassis live as well

    I hope you are treating all this with the respect it deserves - I have had a very nasty surprise off the grub screw holding the volume control knob onto an old radio of that era. Originally the grub screw was back filled with some sort of wax, but the plug of it had dropped out.

  • Hi all bit of a disaster here at Kelly towers I plugged my little receiver into the valve amp I had then plugged the amp and receivers 2 power supply's into my isolation TX switched on the amp and the receiver valve heaters all ok switched on the HT and fizz BANG the power supply for the heaters billowed out thick brown smoke accompanied by some sparks so I switched off bloody quick. I checked all my wiring  could find nothing wrong so substituted a different heater supply as the old one was now fried  carefully switched on and would you believe it the same thing again ANOTHER little PSU is now defunct. I don't understand what went wrong other than it being typical of my sort of luck. None of the mains earths on any of the gear were connected to anything just live and neutral the only earth was the RF Earth from the receiver remember all items were supplied via an isolation TX I really would appreciate your opinions on what went wrong  other than me having crossed 13 black cats at some point
  • Valve heaters were typically 6.3V. Sometimes series/parralled up for other voltages. Sometimes with vintage radios for domestic use they were supplied with a two core flex with the neutral connected to the chasis inside a wooden or plastic box, hopefully getting the flex terminated in the supply plug the correct way around. As Mike says plastic control knobs on the spindles poking out of the case. The valve heaters all wired in series with a wire wound dropper resistor to get the 6.3V across each valve. 


    I am thinking you have to much load on your valve heater supply for it's rating? 


    Mike


    The HRO war time receiver was a very good receiver considering it was a WW2 vintage. My school radio club ( junior anoraks this way please) had one with a range of the coil packs. I had a AR88 of the same vintage also built like a brick outhouse needing a fork lift to move. I saw one yesterday watching a documentary on the Dambusters also a T1154/R1155 inside the Lancaster. The senior management was not best pleased when I was pointing out all the different bits of kit ,I think I saw a T1509 in the background in one scene.
  • OK. so before you turned on the HT, all the heaters and the heater supply were fine ? If so , how long did you let it warm up ?

    If more than a minute or two it sounds as if  perhaps  the HT is getting back into the LT supply and it does not like it. So to make sense of this we need more info.

    How are HT and LT derived if you have two switched  supplies ?

    Traditionally the heater supply would just be a transformer down to 12 or 6V , maybe with one side or centre tap earthed. But it in a transformerless set, it may be a constant current string of really odd voltages - some  Valve series E (as in the ECC83, double triode or EL509 pentode ) are constant voltage devices, and heaters are in parallel

    Valve series P  (As in the PCC83 or the PL509 pentode) are all 0.3A, but the larger devices take more volts - these are used in the transformerless designs with all the heaters in series across the mains, maybe with some resistance to make it up to 240V.

    If the HT rectifier is a valve, then it  needs a seperate heater supply, as the kathode of that rectifier is at the HT voltage. Silicon has largely done away with that.

    So some more description, and we may be able to help.


    also is either  side of The HT or LT output earthed - a classic would be to earth both sides of the LT supply one at the PSU and the other at the rig. Result  a hot LT supply and little else.

    Is it possible one valve has developed  a heater to kathode short? This can give very nasty HT- LT interaction.
  • Hi mapj1 I gave it around half a minute warm up time  the set itself has no built in power supply when in its millatery role it had a vibrator pack which plugged into a socket at the back this is long gone and as it would of needed 28 VDC would of been unusable anyway.  Back to now the 12.6 volts for the heaters came from a perfectly standard  3 amp 12 volts PSU the heaters consume around 950 mA when running and are a nice healthy orange color. Now for the HT.this is derived from a bridge rectifier fed from a variac with about 170 volts  AC input it gives 220 DC  with a 100uF smoothing cap in place. The connection from the set to the power supply's is via 3 wires the green is the common negative for heaters and HT  black is heaters positive and red HT positive. All worked well until I tried to connect it to a valve audio amp with its chassis connected to mains neutral and the audio connected one half via a resistor to the valve and the audio common to chassis  all 3 items were plugged into the same isolation TX when it all went bang
  • John Peckham:

    My school radio club ( junior anoraks this way please) .... 

     


    Guilty as charged ?


    John Peckham:

    > I had a AR88 of the same vintage also built like a brick outhouse needing a fork lift to move.


    I also had an AR88D and with the speaker in its metal case. ?