Where are you getting the -3.7 volts bias for the EML30A? A negative grid supply? Or a cathode resistor? What is the purpose of the resistor on the cathode? If drawn as built, the filament for that tube is grounded, is that correct?
The bias is from the resistor. This is called Filament Bias. Great primer from Thomas Mayer:
http://vinylsavor.blogspot.com/2012/03/filament-bias-part-1-concept.html
http://vinylsavor.blogspot.com/2012/03/filament-bias-part-1-concept.html
Oh, okay. The only thing I can think of is that the regulated filament supply on that tube was shunting more current to ground, or somehow causing a drop in the cathode resistor value, causing the tube to bias hotter. Maybe a leaky cap, or somehow creating some resistance in parallel with the cathode resistor? Did you measure the actual cathode resistance when that filament supply was in place?
Supplies can oscillate if the wire length from the output to the socket is too great, (ie. it doesn't like it).
It's all speculation though unless you're going to put it back in and investigate, which is worthwhile if you are interested.
It's all speculation though unless you're going to put it back in and investigate, which is worthwhile if you are interested.
I’m thinking leaky cap messing up the bias. If the supply for the input tube is off what do you get?
Different bias = different current (if Ua relatively constant).
Try to measure both driver tubes on left channel (Ua, Ia), where filament bias resistor is 3R (voltage on resistor is 3.65V, so estimated bias voltage is 3.65V+half of filament voltage, so about 6.15V).
BTW EML30A filament parameters: 5V, 1.4A (5% tolerance).
If the filament resistor is 3R, then voltage on it must be 4.2V. If it's only 3.65V, the filament is underheated (1.22A).
MUST to measure filament current too!!
If the Ua and Ia is similar (for example within 5%), then tubes are -almost- identical.
If one of these identical tube put to right channel (where filament bias resistor is 3.2R, so voltage on it 6.7% higher than another channel), the bias point is changing, the anode current will be lower, anode voltage higher.
If the filament current is decreasing -about 6% lesser- until filament bias voltage is identical than in other channel, the anode current and anode voltage differentia will be minimised. Then the filament current in this channel will be lower, but 1.316A is still tolerable.
If you put 1 ohm resistor to ground in series on each cathode you can monitor easily the current.
Walter
Walter
Bela - glad to hear from you. The measured filament voltage is 4.8V (measured at the filament pins). I like to run the filaments a bit starved.
You stated "bias voltage is 3.65V+half of filament voltage, so about 6.15V)". The Filament Bias voltage is the voltage across the resistor (which is measured at 3.65V). Not sure why you are adding "+half of filament voltage". Please explain.
Yes, I agree that the filament is at 1.22A. The EML tubes come with a tube specific measurement which states that my tubes should draw 1.3A which is within 6% of what I am experiencing.
You stated "bias voltage is 3.65V+half of filament voltage, so about 6.15V)". The Filament Bias voltage is the voltage across the resistor (which is measured at 3.65V). Not sure why you are adding "+half of filament voltage". Please explain.
Yes, I agree that the filament is at 1.22A. The EML tubes come with a tube specific measurement which states that my tubes should draw 1.3A which is within 6% of what I am experiencing.
Direct heated tubes has filament as cathode. The whole length of filament is emitting electrodes.
If you use "virtual cathode" (low equal value resistors from each end of filament tied together), the "cathode" voltage is concrete, so bias (grid voltage relative to cathode) measurable.
If you use only one end of filament as "cathode point" (for example in case of filament bias, or one end of filament grounded), the "cathode" voltage is indefinite. Good approach to consider half of filament as cathode point. "Cathode" voltage is unmeasurable, can only be estimated as filament voltage/2.
Proper bias voltage can be measured indirectly. If you measured the Ua, Ia in case of filament bias, and another fix biased virtual cathode point arrangement fulfils it, this bias voltage is the "real" value.
If you use "virtual cathode" (low equal value resistors from each end of filament tied together), the "cathode" voltage is concrete, so bias (grid voltage relative to cathode) measurable.
If you use only one end of filament as "cathode point" (for example in case of filament bias, or one end of filament grounded), the "cathode" voltage is indefinite. Good approach to consider half of filament as cathode point. "Cathode" voltage is unmeasurable, can only be estimated as filament voltage/2.
Proper bias voltage can be measured indirectly. If you measured the Ua, Ia in case of filament bias, and another fix biased virtual cathode point arrangement fulfils it, this bias voltage is the "real" value.
Pat, please check this:
disconnect the output of the filament regulator.
Measure OHMS (20k range) from the raw DC (+ and -) .. to the chassis and to the 0V at the cathode resistor.
If this is any measured resistance, find and remove what is causing the leakage. It will bring down the filament bias voltage, and cause high plate current.
disconnect the output of the filament regulator.
Measure OHMS (20k range) from the raw DC (+ and -) .. to the chassis and to the 0V at the cathode resistor.
If this is any measured resistance, find and remove what is causing the leakage. It will bring down the filament bias voltage, and cause high plate current.