Pye black box resistor overheating

[leslie5555]

I have a black box amp its a mark two amp with the ECC83 two EL42s and a EZ41 valve line up , the old hunt caps have been replaced with new ones.
And the smoothing can has been replaced with two new ones.
The problem is that R16 which is rated at 470ohms which I believe is the cathode bias resistor gets extremely hot, to the point that it has gone brown.
I fitted a new one after checking it was the correct value and it still overheated.
What should I be checking to find what is causing this to overheat?

[Station X]

Resistors get hot if they carry too much current. R16 carries the combined screen grid and anode currents of the output valves.

I assume you've changed the coupling capacitors C4 and C5 and checked that there's no positive voltage on the control grids?

What is the voltage across R16?

I'm looking at the manufacturer's service sheet.

[Boater Sam]

Is the resistor 2watt or more? I suspect you have a too low wattage resistor.
Measure the voltage across the resistor with the amp running and some volume on.
Use Ohms law to find the current through the resistor, I=V/R
Then watts=I*V and add at least 50%
Long winded but easy way.
Sorry if your gran is an egg sucker already.

[leslie5555]

Thank you both for your replies.
Graham C4 and C5 have both been replaced along with with C1 C3 and C6 C2 C7 .
I have checked all resistors and R4 R8 R12 and the said R16 have all been replaced.
Sam R16 is a 2W type and thanks for your advice , I have limited knowledge so any help is well received.
I have to work today and will make the checks tonight and get back with the results thanks.

[Station X]

R16 is specified by the manufacturer as being 470R 0.5W. The test voltage across it is specified as 17.6 V, so it is being slightly over run at 0.66W.

By all means replace it with a 2 watt component, but check the voltages first to make sure you don't have a fault.

[Boater Sam]

The OP said in the last post that R16 was already a 2w resistor, Graham, so there is a problem somewhere.
So we need to know the HT voltage and the voltage on both EL42 anodes and cathode please.
Do we have a model # so I can find the diagram?

[Station X]

Leslie.

I''ll leave you in Sam's capable hands. There's no point in us both giving you the same advice.

[Boater Sam]

Mk 2 is ultra linear so we need the EL42 screen voltages too please. What voltage is across the 470ohm R16?

[leslie5555]

Hi Sam thanks for your help, I checked voltage across R16 I wasn't sure if it should be ac or DC so I did both ac =68volts and DC=31volts the output valves both measure -00.5v
But its being a bit of tinker now and only getting intermittent voltage across R16
Also when there is voltage across it there is a loud hum from the speakers

[PJL]

That looks bad so don't power it up again except under controlled circumstances. You are using the manufacturers data and R16 is the common cathode resistor? 31V is almost twice the expected voltage of 17V so twice the current which will damage the transformers.

Is the mains selector on the 225-250V setting?
Has the gettering in either of the EL42's turned white?
Do you have a means of testing the EL42's?
Do you have any replacement valves?

[leslie5555]

Well I sorted the intermittent voltage, that was down to broken connectors in the valve base of the rectifier.
The voltage setting is correct and I have two other identical amps in working order so I know the el42 valves are working ok
Now its working again the voltage of output valves pin 5 to chassis is just rising all the time until I have to cut the power

[Boater Sam]

Pin 5 of the EL42s is screen grid, it will be nearly at HT volts. They feed off the output transformer.
What was that figure you gave -00.5v? Where? There should be no connections on pins 3&4, pin3 is also a cathode connection
All voltages after the rectifier are DC.

I've got Trader sheet 1360, the numbers are different, Cathode common resistor is R14 on mine. If you have 30 odd volts across it, there is serious trouble for the transformer.

[leslie5555]

The -.5v was obtained from pin 5 on the EL42s but that was when the rectifier base was playing up and no current was passing through R16.
As I said I am of limited experience with this, please can you explain which pins I should be using to check the anode and the cathode of the EL42s thanks.

[leslie5555]

Maybe good news I powered it up again to recheck the voltage and one of the EL42s started to glow bright orange, so I swapped it from another working amp and I am now getting 18V across R16.
Does that make sense to you and do you think it was a faulty valve?

[Peter.N.]

Could well be.

Peter

[Boater Sam]

Valve pins number clockwise from the gap looking at the wiring side of the socket.
To get which pin is which search on the National Valve Museum site for the valve number.
This is the EL42 page:
http://www.r-type.org/exhib/aaa1134.htm

That would explain the very high current, hope that the transformer has not suffered damage, if it's working looks like it's OK. Cathode current @17mA is about right.
Your dodgy rectifier contact could of been responsible for the valve failure.

I would doubt that you need to worry about the 2 output valves being matched unless it sounds bad. If you want to match you really need 2 identical milliamp meters, one in each cathode lead and select valves that give the same readings, or use someone with a good valve tester to find a pair from your stock.

Give it a good testing but watch it, first sign of any overheating of the output or mains transformers hit the off switch.

Sam.

[leslie5555]

Thanks for your help Sam

[Boater Sam]

It's fun, you done good.