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Old 17th Jul 2019, 3:08 pm   #1
John10b
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Default EL42 in Pye Blackbox puzzle.

I have just finished refurbishing the Pye Blackbox Amplifier.
The type with 5 connections on the output transformer and two EL42,s.
I thought I would share something that puzzled me for a while when carrying out current measurements after I had fitted two nos EL42’s.
On both valves the Ia was about 14mA. I disconnected the lead on output transformer.
The total Iκ about 35mA. Meter in series with cathode resistor.
I then decided to measure each valves Iκ.
So I disconnected the wire going to pin and put meter in series.
What do you think my reading was?
I’ll give you time to think about it, one clue is to know the layout and wiring of the amp!
Cheers
John
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Old 17th Jul 2019, 3:53 pm   #2
kalee20
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Default Re: EL42 in Pye Blackbox puzzle.

Ia for each valve of 14mA means total 28mA for the pair.

Then 3.5mA for Ig2 is reasonable, accounting for 7mA for the pair and giving total Ik of 35mA.

If you disconnect each cathode in turn and insert an ammeter, you will introduce a bit of extra resistance for that valve alone, which will unbalance things a bit - I'd say 16mA Ik for each valve, measured one at a time.

You say something about the Black Box wiring - I am totally unfamiliar so the above is just theory of basic P-P with a common cathode resistor!
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Old 17th Jul 2019, 4:23 pm   #3
John10b
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Default Re: EL42 in Pye Blackbox puzzle.

You have applied the correct logic, it’s not what I actually measured.
Your last sentence is the clue, in that you need to know the amp well.
Cheers
John
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Old 17th Jul 2019, 4:38 pm   #4
TonyDuell
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Default Re: EL42 in Pye Blackbox puzzle.

I notice that the EL42 has the cathode brought out to 2 pins. Are the cathodes of the 2 output valves linked by linking one of the pins on one valve to one of the pins on the other valve, with the resistor linked from the other pin on one valve to chassis? If so you could have ended up measuring the total cathode current of the pair of valves again.
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Old 17th Jul 2019, 4:49 pm   #5
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Default Re: EL42 in Pye Blackbox puzzle.

I may be wrong on this, but something from the distant past tells me that this is the amp where there's a mistake in the circuit diagram drawing in the service sheet regarding the output valves and transformer. I'd be careful that you don't damage those new valves.
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Old 17th Jul 2019, 5:04 pm   #6
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Default Re: EL42 in Pye Blackbox puzzle.

In a photo I'm looking at both pin 7's are connected together, the cathode resistor is connected to pin 3 on one valve only (Pin 3 is i.c. according to the valve data specs. I'm looking at)

Lawrence.
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Old 17th Jul 2019, 5:10 pm   #7
John10b
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Default Re: EL42 in Pye Blackbox puzzle.

Tony you are correct about the valve bases being linked, and the internal connections of g3 going to a pin which is also linked to each valve base.
Techman the output transformer seems to be connected correctly.
So when I disconnected the single wire of the cathode of one valve base and inserted my meter I go a reading of zero!
Cheers
John
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Old 17th Jul 2019, 5:31 pm   #8
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Default Re: EL42 in Pye Blackbox puzzle.

Your actual amplifier will be correct, it may be the circuit drawing that's wrong. I seem to remember that the anode and screen connections were reversed, but don't take my word for this as I could be confusing this amp with some other one I had dealings with at some time.

Can't understand why you're getting zero cathode current - there WILL be a very simple answer.
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Old 17th Jul 2019, 5:40 pm   #9
TonyDuell
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Default Re: EL42 in Pye Blackbox puzzle.

As I understand it there are 2 connections in parallel between the cathode of the output valves, one for each of the pins the cathode is brought out to. So if you open one connection and connect the milliammeter there, it's effectively shorted out by the other connection and will read zero.
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Old 17th Jul 2019, 5:45 pm   #10
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Default Re: EL42 in Pye Blackbox puzzle.

And I would think that that's the simple answer, Tony.

Edit to say - this shows that you need to study the circuit VERY carefully when doing tests like this.
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Old 17th Jul 2019, 6:02 pm   #11
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Default Re: EL42 in Pye Blackbox puzzle.

Not wishing to worry the OP, as I think the reason for no cathode current has been identified, but I had a brand new valve with an open circuit cathode, ie, no internal connection due to a manufacturing fault. It was faulty new old stock and I suspect that some of these old stock valves being sold are original manufacturers returns that didn't get returned. I looked carefully through the glass and could see the disconnection to the pin on the inside of the valve. I was sent another valve and all was good.
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Old 17th Jul 2019, 9:18 pm   #12
John10b
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Default Re: EL42 in Pye Blackbox puzzle.

Yes Tony that is the reason, and I should have spotted it, but there you go
Thank you Techman and all, and this was only because I wanted to carry out all the measurements, it has been a lesson learned.
Cheers
John
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Old 17th Jul 2019, 10:05 pm   #13
Chris55000
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Default Re: EL42 in Pye Blackbox puzzle.

Hi!

I think I can give a bit of background to this!

For some reason, both the EL41 and EL42 had inconsistent base specifications for pins 3, 4 and 7 and this must have influenced the chassis–wiring instructions developed in Pye's factory at the time these Black Boxes were being made!

There were at least two independent valve data–tables from reference books in exsistence at the time that gave the following incorrectly specified base connections:–

EL41:– 1 h, 2 a, 3 g3,k, 4 g3,k 5 g2, 6 g1, 7 k, 8 h
EL42:– 1 h, 2 a, 3 k,g3 4 – 5, g2, 6 g1, 7 k,g3 8 h

The Mullard data for both valves gives:–

1 h, 2 a, 3 i.c., 4 i.c., 5 g2, 6 g1, 7 k,g3 8 h

I suspect Pye probably used one of the incorrectly specified valve base connection tables that stated 3 and/or 4 were connected internally to pin 7 (which is the correct cathode pin), and planned the layout of the cathode–resistor wiring to use pin 3 which was easier!

Mullard and BS CP1005 "Code Of Practice for Electronic Valves" have both stated categorically that pins annotated "i.c" (internal connection) must NOT be used as anchor–points for wiring or alternative electrode connections simply to make wiring more straightforward!

I have certainly come across EL41/EL42 valves where pins 3 and 4 are simply wire–stubs projecting above the button base of the valve with no internal connection made inside the valve, and trying EL42s made like this in a B.B. wired as Member Lawrence describes would result in failure to operate!

What Pye should have done, of course, was to link pin 7 on each valve–base together and connect the cathode–bias resistor from one of the pin 7's to chassis, leaving both 3 and 4 blank on both holders, then it wouldn't have mattered how exactly the valve was manufactured!

Chris Williams
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Last edited by Chris55000; 17th Jul 2019 at 10:35 pm.
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Old 27th Jul 2019, 6:13 pm   #14
John10b
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Default Re: EL42 in Pye Blackbox puzzle.

Thanks for the information Chris, apologise for late reply just got back of our hols.
Cheers
John
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