Confused by ECL86s in Murphy A851G!

[Clydeuk]

Are you sure about there being a 3K3 resistor in series with the rectifier? That value seems far too high, I very much doubt that's on the HT rail.

[Nuvistor]

Is the 3.3k the smoothing resistor between the reservoir and smoothing capacitors.
You will still need the surge limiter resistor or the reservoir capacitor may get damaged.

Frank

[Station X]

The 3k3 resistor is indeed between the reservoir and smoothing capacitors.

What DC voltage are you seeing at the output of the new diode (marked end)? If it's in excess of 215VC you'll need a series resistor.

[Techman]

As always - try to get yourself into a 'logical' way of thinking when working on these things. I guess you have the circuit in front of you with the voltage readings. You've just replaced the rectifier with a new diode, so the obvious thing to immediately do is to measure the HT voltage and compare your reading with that of the circuit diagram and note any difference. It will then be obvious as to whether a series resistor is required.

You're getting there. In a couple of years time you'll be an expert advising others of these things - just as you do as regards to old Hi-Fi systems and high end cartridges and arms etc. where I note that you have a lot more knowledge on such things than I do.

[RojDW48]

Thankfully I only ran it for a couple of mins before asking the question. It was over 250V at the diode - so I inserted the 3K3 resistor - now it's 110V

[Nuvistor]

Why insert a 3.3k resistor? 330 ohm would be somewhat nearer the required value. I presuming the 3.3k smoothing resistor is still in place as it should be.
Frank

[Techman]

That's why you need a small stock of resistors to experiment with and adjust the value to obtain the correct voltage - the value will be in the low hundreds of ohms.

[Herald1360]

A surge limiter between rectifier and reservoir I would expect to be hundreds not thousands of ohms. I expect that only the anodes of the output valves are fed from the reservoir, the rest of the set from the smoother. The output anodes will be the main consumers of HT current.

[Herald1360]

Tell 'em wot you're going to say, tell it to 'em, then tell 'em wot you said

[Station X]

Until you get fed up with telling 'em!

[Techman]

Originally Posted by Herald1360

Tell 'em wot you're going to say, tell it to 'em, then tell 'em wot you said

You'll have to explain that post and how it relates to this thread, Chris

Edit: Sorry, Graham just explained it.

[RojDW48]

Are/were you a teacher by any chance, Chris? I used to be a very good one - sadly I am obviously not such a good student . I have some 7W 47R resistors - I shall try lashing them up in series to see if I can get close to the desired DCV. Don't despair of me yet!

[Station X]

Make sure the resistor goes between the marked end of the diode and C16, or to avoid all confusion between the mains transformer secondary and the unmarked end of the diode. ie as shown in the picture linked to in post #17.

[Radio tirana]

Hi
No do not despair, you could use the 3W resistor you mentioned to establish if 330 ohm is the correct value. It will get hot very quickly and may even smoke a little but should enable you to measure the HT.
It won't be far off. I will be surprised if the value you need is less than 270 or more than 360 ohms. I used a 25W metal clad resistor which I know is overkill but fixed to the chassis where the original rectifier used to be any heat is dumped to the metalwork.
Don't get to hung up on achieving exactly 215V. 5V either way will not do any harm unless you want to start adding resistors in parallel you will have to settle for a preferred value anyway
Kind regards RT

[RojDW48]

Sorry for the confusion - I read 330R in RT's post then, for some inexplicable reason, went out and bought a 3K3 resistor! I shall try to redress this error tomorrow. You are quite right Techman - I could save myself a lot of unnecessary fuss by being more methodical

[Nuvistor]

I would get a small selection say, 220, 330 and 390 ohm has suggested by Techman.

[Jimmyhaflinger]

Originally Posted by Edward Huggins

I have never known of an ECL86 to fail. But there were 4 in my Tripletone "Stereo 8-8" and after and after 43 years of use, 2 did measure as being "down". Edward

i had to replace no less than 4 of them in different sets instead...

one had a badly microphonic triode section, the others were in two separate record players and had leaky pentode sections with control grids rising spontaneously up to as much as 12v in the worst of them, their plates glowed red hot pretty quickly even with the coupling capacitor disconnected

[Herald1360]

Originally Posted by RojDW48

Are/were you a teacher by any chance, Chris?

No, I think I picked up the saying from someone teaching me about doing presentations. Not that different from lecturing, I guess.

It just popped into my head when I saw that three of us had responded nearly simultaneously with the same thought about 330R being a more likely value than 3K3 for a metal rectifier's forward resistance.

[RojDW48]

I have tracked down the appropriate 10W items to RS components - they are raising the order number and getting the chitty countersigned and stamped as I write (I haven't bought anything from RS for 30 years or more and it's reassuring to know they haven't changed a bit!). To be fair - they haven't got any in Bristol today but they will have them there for me tomorrow morning. More soon.

[RojDW48]

Just to double check my understanding of Graham's post 33 - I can connect the resistor in series anywhere between the transformer secondary and C16 - ie, either side of the diode? I'm thinking about available space when the chassis goes back in its box.