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Old 12th Jul 2019, 7:45 pm   #41
PJL
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Default Re: Voltages on ECC83 in BlackBox

Don't use it like this as it sounds like one valve is not conducting.

How did you test the cathode current as both cathodes share the same bias resistor?

Can you also check continuity of the output transformer primary. Not sure if the transformer is the Ultra-Linear (5 connections) or standard (3 connections)?
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Old 12th Jul 2019, 7:47 pm   #42
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Default Re: Voltages on ECC83 in BlackBox

Swap the EL42 valves over and see if the same valve is still hotter.
I assume you have changed the coupling caps.
It is possible that the output transformer is at fault.
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Old 12th Jul 2019, 8:35 pm   #43
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Default Re: Voltages on ECC83 in BlackBox

Thanks for your replies.
This is why I have raised the question, as I said it puzzles me.
The two valves both have a cathode current of 29 mA, I disconnected each cathode.
The coupling capacitors have been changed.
The continuity on transformers has been checked. Five connections.
I will carry out further in depth test over the next few days.
Cheers
John
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Old 13th Jul 2019, 11:31 am   #44
PJL
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Default Re: Voltages on ECC83 in BlackBox

According to Trader 1360, each EL42 should take 17.6mA cathode current. This would make the cathode voltage .0352*470 = 16.5V.

Don't use the player until the problem has been identified. Just a heads up that the first area to get right on a valve amplifier is the output stage as, due to the large currents involved, this is where a fault can cause expensive damage. Also, when working on an amplifier with a common cathode bias resistor as here, you should never operate the amplifier with just one output valve as the valve will self bias at a very high anode current.

I would guess you are accidentally measuring the combined current of both valves.

Another method of getting a rough estimate of current is to measure the DC voltage across each half of the output transformer primary, switch off and wait for a few minutes for the valves to stop conducting, then measure the resistance of each half of the output transformer. Apply ohms law I=V/R to get the approximate current. It is rough as the winding resistance changes with temperature and BE CAREFUL as the meter will be at HT potential.

PS: It's also worth checking the grid resistors (R12 & R13) are still 1M as carbon resistors tend to age by going high in value. Then switch on and check the grid voltages (with your DMM) are less than 1V as some valves develop internal faults.

Last edited by PJL; 13th Jul 2019 at 11:58 am.
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Old 13th Jul 2019, 12:24 pm   #45
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Default Re: Voltages on ECC83 in BlackBox

Morning PJL, it looks like I need to get my brain into gear! You are right I was incorrectly measuring the cathode current, must do better!
Iíve just been in the shed and carried out some more tests, Iíll update some time later.
Cheers
John
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Old 13th Jul 2019, 3:31 pm   #46
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Default Re: Voltages on ECC83 in BlackBox

Update.
Measured the resistance of the output transformer secondary.
Measured wrt centre tap.
One side. 680 ohms ( 370 + 310 ohms ).
Other side. 915 ohms ( 500 + 390 ohms).
Voltage measurements,dc, wrt chassis.
V2. (Hot 80*C.)
Pin 2, anode = 288v.
Pin 3 cathode = 15v.
Pin 5 screen = 297v.
Pin 6 control grid = 0v.
V3. (Cool 28*C.)
Pin 2 anode. = 306v.
Pin 3 cathode. = 15v.
Pin 5 screen. = 307v.
Pin 6 control grid = 0v.

I carried out several current measurements with my two Digital meter and one analog meter, but I stared to get very conflicting and variable results which is confusing, so I decided not to quote any until further investigations.
I will also probably order an EL42.
I hoping Ed Dinning can let me know if the output transformer readings are suspect or if any member can do so.
Cheers
John
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Old 13th Jul 2019, 3:48 pm   #47
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Default Re: Voltages on ECC83 in BlackBox

Many thanks to Ed Dinning who provided me with resistance readings, it looks like the transformer is ok, so Iíll order a new EL42 and carry out further tests in due course.
Cheers
John
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Old 13th Jul 2019, 3:52 pm   #48
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Default Re: Voltages on ECC83 in BlackBox

Looks like V3 isn't pulling its weight, duff valve or poor connection between valve holder tags and valve pins?

Swop the valves over an measure the voltages on both valves again.

Lawrence.
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Old 13th Jul 2019, 4:08 pm   #49
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Default Re: Voltages on ECC83 in BlackBox

Yes Lawrence I think your right about the valve and possibly pin connections, I intend ordering a new valve.
This has been very frustrating and your comments about pin connections etc may well be true, thanks.
Cheers
John
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Old 13th Jul 2019, 4:12 pm   #50
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Default Re: Voltages on ECC83 in BlackBox

I should have said that I changed the position of the two valves and the low running temperature follows the valve.
Cheers
John
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Old 13th Jul 2019, 4:27 pm   #51
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Default Re: Voltages on ECC83 in BlackBox

To check the current you need to measure the voltage across each half of the output transformer primary. The measurements from chassis are not suitable as we can't estimate the current.

It's also important you check the grid resistors R12 and R13 and measure the EL42 grid voltages. Only when you have done this can we confirm the state of the valves.
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Old 13th Jul 2019, 5:47 pm   #52
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Default Re: Voltages on ECC83 in BlackBox

R12,13 both checked with a reading of 1.1 Meg and 1.2Meg. See previous post for other voltage readings. Iíll carry out further tests in due course.
Cheers
John
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Old 13th Jul 2019, 7:22 pm   #53
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Default Re: Voltages on ECC83 in BlackBox

Hi John, not unknown for the valveholders to have bad pins, but ir is normally confined to the ECC83 base

Ed
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Old 13th Jul 2019, 7:33 pm   #54
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Default Re: Voltages on ECC83 in BlackBox

Hi Ed, thanks for your pm regarding the output transformer.
I had to change the valve base of the ecc83 two pins were fractured.
The pins on the EL42 bases look ok, but I will of course double check.
Cheers
John
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Old 13th Jul 2019, 9:00 pm   #55
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Default Re: Voltages on ECC83 in BlackBox

Sorry, missed you had taken the grid readings. I would agree that one of the EL42 looks very low emissions. It would still be nice to see the voltage reading taken across the transformer primary halves as final confirmation.
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Old 14th Jul 2019, 4:30 pm   #56
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Default Re: Voltages on ECC83 in BlackBox

Just measured the voltage across transformer.
Hot valve = 25v
Cold valve = 0v.
So I think we can say the ďcoldĒ valve is faulty.
Apart from the actual fault this has been both frustrating and interesting.
To explain very briefly.
I decided to measure the Ia and the Iκ, this is were the frustrating set in, I was getting ďoddĒ results, for example, I was getting an Ia of about 24mA and and Iκ of 3mA?
Cheers
John
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Old 14th Jul 2019, 4:57 pm   #57
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Default Re: Voltages on ECC83 in BlackBox

Don't run it for long like that or the good valve will be wrecked.

I am a bit concerned that the faulty valve is not conducting at all. Is the heater lit? Did you clean all the pins carefully?
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Old 14th Jul 2019, 7:44 pm   #58
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Default Re: Voltages on ECC83 in BlackBox

I intend getting two new valves.
I gave them a quick clean, i intend to revisit and give them a thorough clean, Iím wondering if I have intermittent contacts, this has been mentioned before, maybe thatís why Iím getting some odd results.
Thanks for your helpful comments.
Cheers
John
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Old 14th Jul 2019, 8:08 pm   #59
John10b
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Default Re: Voltages on ECC83 in BlackBox

Forgot to ask for any comments on the Ia and Iκ?
Cheers
John
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Old 14th Jul 2019, 8:27 pm   #60
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Default Re: Voltages on ECC83 in BlackBox

Could it be that Ik was 30mA, eg: Ia+Ig2.

Lawrence.
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