Cathode & screen resistor value in 6L6 driven 5F2A?

[martin manning]

Pentode connection is suppressor (g3) tied to the cathode, and that is the basis of the data sheets. No reason it can't be connected to ground (and so to a lower potential), but it'd be interesting to see if it makes any audible difference.

[funkgang49]

Measure the voltage across the cathode resistor and divide by the value to get the cathode current. (that will include the screen current)

The 470R cathode may be too much so you could start lowering it until you come up with around 50ma.
That should be about 20 watts.

Ok, measured voltage across 500 ohm 10 watt cathode resistor (with 394v at JJ 6L6GC anode) = 85V divided by 500 ohm = .17 cathode current?
How do you come up with the ma value and then determine watts?

[Firestorm]

Measure the voltage across the cathode resistor and divide by the value to get the cathode current. (that will include the screen current)

The 470R cathode may be too much so you could start lowering it until you come up with around 50ma.
That should be about 20 watts.

Ok, measured voltage across 500 ohm 10 watt cathode resistor (with 394v at JJ 6L6GC anode) = 85V divided by 500 ohm = .17 cathode current?
How do you come up with the ma value and then determine watts?

That's very hot. 0.17 is the mA value (170 mA) combined plate and g2 current. Effective plate voltage is 309 (394-85). Static dissipation for plate and screen, then, is 0.17 X 309 =52.5. Plate and screen should both be unhappy. Rk should be getting awfully hot, too. The tube does not like that amp, unless something else is wrong.

[gingertube]

That 85V is just way too high, as firestorm says.

You need to determine if that 85 V reading is real or not.

first make sure you are reading with no signal.

second make sure that your multimeter does not have a battery which is going flat (that causes multimeters to read high as the internal reference voltage drops).

If you are reading with no signal AND with good battery in the meter then check the voltage at the grid (pin 5) is 0V. An open circuit grid leak resistor allowing the grid voltage to float positive could cause the problem you are seeing.

If that is all OK then my guess would be that the output tube is oscillating and that causes the very high reading. Add a 15K grid stop to pin 5 of the output tube.

If it is still oscillating disconnect the 22K feedback resistor and see if it stops, if so you need to swap the primary wires on the output transformer.

Cheers,
Ian

[Structo]

Agreed.

Make sure you are measuring DC voltage and connect the black probe to the chassis near the output.
Then touch the red probe on the tube side of the 10W resistor, pin 8.

That current shouldn't be anywhere near 170ma with a 500R cathode bypass resistor.

Just so you know, 1 milliamp or ma is notated like this. .001
A milliamp is one thousandth of an ampere.

So .17 amps is 170ma.

[tubeswell]

Ok, measured voltage across 500 ohm 10 watt cathode resistor (with 394v at JJ 6L6GC anode) = 85V divided by 500 ohm = .17 cathode current?
How do you come up with the ma value and then determine watts?

Oops - did you measure the actual resistance of the resistor? (This needs to be done with the cathode bypass cap removed)

[martin manning]

Oops - did you measure the actual resistance of the resistor? (This needs to be done with the cathode bypass cap removed)

Nah, the bypass cap is going to be order-of-magnitude 100k DC resistance so it won't matter if it's there or not.

[tubeswell]

Yes Sorry I forgot about the effect of the relatively small Rk in this case. What I was more hinting at was whether the cathode resistor was 500R or some other '5' number.

[Structo]

Yes, like 50 ohms?

[martin manning]

An Rk of 50 ohms would result in a low (not high) cathode voltage, and the tube would be long gone by now. 5k would all but shut it off with Vk ~40.

[tubeswell]

Well when reading 85V on the cathode with 394V on the plate, one has to wonder about the cathode resistor resistance, or whether the meter battery has been changed yet? (I take it the output tube's not red-plating?)

[Structo]

I was referring to the cathode current.

The 85 volt reading can't be right.

[martin manning]

I'm with you there Tom, if it's wired correctly the cathode voltage just can't get that high, no matter what the value of the Rk is.

[Firestorm]

I'm with you there Tom, if it's wired correctly the cathode voltage just can't get that high, no matter what the value of the Rk is.

True that. Tube would be cut off before it could get there. Maybe a pic of socket wiring is in order.

[tubeswell]

Maybe a pic of socket wiring is in order.

+1