Mullard ECL82 stereo - where has the voltage gone?

[Silicon]

The two red wires seem to be connected the HT AC windings on the mains transformer.
The tag in between them is probably the 'zero' volts HT (effectively the negative HT) connection.

The C15 negative could be connected to that 0V tag.

The thin Blue wires seem to be the 'not ground' heater connection.
The thin Black wires seem to be the 'grounded' heater connections.

Since one end of the heater winding is already grounded, I can't see how you can create a centre tap which you can then ground.

[qualityten]

Chris Williams, thank you for the Mullard book link, which contains the 7W Stereophonic Amplifier chapter. (And thanks Restoration73 for a circuit showing how to supply the screen grid of the ECL82s if no UL taps are present.)

The layout diagram on page 109 shows an arrow to earth from C108. This is on the opposite side of the chassis from the input socket. Does this go to the earth tag of C15 and C12/112?

There is a series of earth connections for the electrolytic capacitors on the top of the tagboard in the same diagram. What is the correct place to earth this series?

And where is the 0V reference point for the circuit? To solve my original 'missing voltage' problem, I linked the C15 earth to the chassis ground, near the IEC socket.

[qualityten]

Silicon, your description of the situation is correct. So, I should just not try to create an artificial centre tap?

With that removed, is it okay to supply the rectifier valve and all the others from the same 6.3V supply? I pondered replacing the EZ81 with silicon diodes, but the whole 6.3V supply came from the one winding before.

[qualityten]

By the way, this is the link to my earlier quest for the circuit diagram for the 1964 Dynatron Savoy RG38 radiogram that the transformers came from: https://www.vintage-radio.net/forum/...ighlight=ECL82.

[snowman_al]

Thanks for the diagram.
If it really is wired like that, there is a problem. The 0 volt HT line has to pass through the 6.3 volt AC heater wiring to get back to the negative of C15.
That imposes an AC voltage onto the smoothing circuit...

Move the centre tap black wire straight to the -ve of C15.
Remove the heater wires off the transformer screen connection, bottom right. You can connect it, the screen, to the centre tap or straight to your earth wire.

The EZ80/81 is designed to use a single 6.3 volt transformer. Ie share with the rest of the valves.. As long as it is a good one there are no problems.

[qualityten]

Thanks Al. I clearly didn't understand the PT connections.

Can I confirm, as per the markings on my picture of it, that I:
disconnect the wire from the HT CT to the transformer screen tag (orange circle)
disconnect the black 6.3V connection from the screen tag (green circle)?
relocate those wires to the HT CT tag (yellow line)?

There is bare wire on the right hand side of the transformer, as pictured, connecting that screen tag to a terminal at the top right hand side. I don't know what that tag is for. This connection was there and assume I should leave that as is?

Thanks.

[snowman_al]

Not quite...
This might be easier to understand / do.
Leave the black wire marked as orange at the centre tap.
Fit a new wire from the centre tap straight to the -ve connection of C15.
Now remove everything except the black wire, marked orange, and bare wire from the 'screen'.
Re work the 6.3 volts so one terminal has your heater blue wires and the other has the heater black wires, It is not possible to see from the pictures how it is now. Just make sure none of the black wires are a ground or 0 volt wire...

So I think you should have top left to right: 1 red wire HT, 2 black wires at the centre tap, 1 red wire HT, 2 blue wires 6.3v, 2 black wires 6.3v.
Bottom left to right, 1 brown mains, 1 blue mains, 1 black (from centre tap) and 1 bare wire both on the 'screen'. Does that make sense?

One other thing is the bulb holder. Are both connections isolated from the chassis bracket? If not you will connect one side of the heater wiring to the chassis and defeat the faux centre tap again.

[Silicon]

I assumed that the two tags on the top right are the 6.3V AC heater connections.

One of the heater connections is then connected to the screen and ground/chassis by the thick bare wire.

Initially I think you should remove the resistors that you used to create an artificial heater centre tap.

Connect the negative of C15 to the HT centre tap.

If the screen tag is the main grounding point - leave that as it is.

Then verify that the HT voltage is OK. Sort that out first.

[snowman_al]

Out of interest again, where does the black wire now marked with pink, go to / from and do?

Please try Silicons idea first. We are both saying the same thing - the HT centre tap has to go straight to C15 negative. No need removing the 2 x 220 ohms though, if you have one side of the 6.3 volt heater supply grounded (0 volts) it just places a single resistor across the supply and shorts out the other...

[PJL]

Certainly explains the hum as the heater circuit will be sitting on 25V or so non-sinusoidal AC. Your HT voltage will also increase after the rewire.

[qualityten]

Thanks for the clarification Al and Silicon. Al, the black wire marked in pink goes to the neg of the C15.

I have a bit of time off tomorrow and will rebuild the reservoir/filter capacitor arrangement, ensure that the voltage to the ECL82 pentode G2 comes after C15, and then do the grounding arrangements as per the three posts before this one.

[Diabolical Artificer]

That fuse holder on the left is connected the wrong way round, as it is it's a shock hazard.

Andy.

[qualityten]

Thanks Andy.

[qualityten]

I've completed the rewiring thus far, but have three final earthing questions. I think the answer is yes to these, but I'd be grateful for confirmation.

1. Where is the best place to connect the circuit to the chassis: should this be at the input RCAs (meaning I should not have their grounds isolated from the chassis) and thence to the C15 ground?

2. Should the 'busbar' earth connections, which are linked all along one side of the tagboard, connect to the C15 ground too?

3. And should the IEC power socket earth wire be connected to the chassis next to the IEC socket?

[Chris55000]

Hi!

1) The chassis–metalwork should be connected to the H.T.–/earthy busbar at ONE point only immediately adjacent to the input sockets.

2) The Centre–tap lead from the h.t. secondary of the mains transformer should be returned to the negative tag on C15 and the free end of the H.T.–/earth busbar connected to the same point. if you use a "humdinger" preset or two resistors across the 6.3V supply, make sure the "wiper" of the "humdinger" (or mid–point of the two resistors) has it's own separate lead back to the C15(–)/H.T.–/busbar connection.

Remove all the valves, and check with an ohmmeter to make sure each side of the heater circuit on each valve measures approximately half of the "humdinger" preset value, or the value of each of the two resistors used across the heater line, as the case may be.

All intermediate connections to H.T.–/earth should be made at the nearest point along the bus–bar to the components/valveholder tag needing an earth connection, which should be as short and direct as possible.

If you are using a can–type electrolytic capacitor for C15, this should be insulated from the mounting.clamp by an insulating sleeve or strip.

3) The mains–earth should go to the chassis as close to the I.E.C. input as possible – this is a safety earth connection, not a functional one.

Chris Williams

[Diabolical Artificer]

Wrote this via PM to the OP re fuse holder shock hazard... I could be wrong as the fuse holder is a bit different to one's I've used but mostly fuse holders have a bottom connect and side connect, this side connect is exposed when you take the cap off, if you have live connected to it, therefore you have live that can be touched. Usually the L is connected to the bottom lug & can't be accessed.

From your pic, L is on the side connect, amps plugged in, amp goes quiet, tum te tumpty tum, best have a look at the fuse, ehm, looks ok, best just rest my hand here on the side of chassis whilst i have a think, ouch, ya b*gger! I've done this, but not with a fuse holder.

Re grounding, see here - http://www.valvewizard.co.uk/Grounding.html but in a nutshell you can ground your ground anywhere you like and it can be grounded with a wire or low value resistor of diodes or a combination. If your using a star ground, the IP isn't a convenient place to put it.

Re grounding, all grounds go back to your main smoothing cap, with the big thirsty high current gnds going on first, EG OP stage, OPT FB return. Like in life when Qing to get into the disco, the big lads with muscles mouthing off at the bounces get to the front of the Q, the wimps and weido's hang back, at the back of the Q, in that dark bit by the dry cleaners : ) Humorous (hopefully) metaphors aside the idea is you don't want big current's anywhere near sensitive, high gain IP stages say, just like in life; the sensitive creative arty types sit at one table, the shouty rugby lads at another, hopefully far away..

So, design/layout your gnd bus like a nightclub Q, the entrance being neg tab of the smoothing can cap. Make sure this caps can is isolated from chassis/gnd.

Hope that helps, Andy

[qualityten]

Chris and Andy, thanks to you both for these answers. Chris, I’ve not forgotten the resistance checks you mentioned previously.

[qualityten]

With all the above attended to, I’m pleased to report that the amplifier is working well.

But there is a softer but still very audible 50Hz hum. I’ll try and get a large 6v battery today to see if it disappears with a DC heater supply. A humbucking arrangement is ruled out because one side of the 6.3VAC is at 0V. I wish I knew how Dynatron did it!

[Herald1360]

Is it physically impossible to disconnect that one side of the 6.3V from ground and fit a humdinger instead? (ie is that 6.3 - 0V link internal to the TX?)

[snowman_al]

Originally Posted by qualityten

... A humbucking arrangement is ruled out because one side of the 6.3VAC is at 0V. I wish I knew how Dynatron did it!

Did you see my mention of the pilot light bulb holder? If you remove it from the chassis does the 6.3 volt supply still have a connection to 0 volts?