Your voltage is still too high.
With cathode bias, you shouldn't put more than ca 320V. Datasheet says 300V is the max.
With quality EL84s you can go higher, but only running fixed bias and staying away from max dissipation (12W).
Is your power transformer designed for hollow state rectification and you are using solid state diodes?
That can cause the very high voltage.
With cathode bias, you shouldn't put more than ca 320V. Datasheet says 300V is the max.
With quality EL84s you can go higher, but only running fixed bias and staying away from max dissipation (12W).
Is your power transformer designed for hollow state rectification and you are using solid state diodes?
That can cause the very high voltage.
That´s the point.
300 to 600 ohm DC resistance in series with a plate is "nothing", compared to expected kilo ohm impedances there.
And even less in a Pentode tube, where the internal impedance is way higher.
Pentode plate current vs. applied voltage is "almost" constant, thanks to screen grid, the main point of making it a "pentode".
Thanks JM, I'm thinking of them as triodes, when they're not. Screen voltage is 385V, versus 396 on the plates. And PARAFEED - Yes, my B+ is too high...The amp hums, and the whole thing is microphonic! Fortunately, FedEx just dropped off a PT I'd ordered, with two HT voltage choices, AND a 45 volt tap (for a negative bias supply, if I want to go that way). I AM using a SS bridge on my current pwr supply. I was using a Variac, but lowering it too far reduces my filament 6.3 VAC also (don't want that).
Oh, and a question on OT impedances - I'd heard that when you connect a speaker load to the wrong secondary wire (say 4 ohm speaker load to the 8 ohm tap), you significantly affect how the OT primary "appears" to the circuit. Like it may "appear" to the power section to be much higher or lower than it really is, and then affect power, bias, and who knows what else? Thoughts?
About OT impedance and connected loads: Impedance is "AC resistance". When there is no signal, there is no AC, only DC. In that situation it doesn't matter if you put a 4, 8 or 16 ohm load on whichever speaker tap. There's no voltage there anyways. There's no effect on voltage, current or bias of the output tubes.
Things are different when there is signal. When you have a OT with a 8k primary and 8R secondary, that means that when you connect an 8 ohm load, this is reflected as 8k. In other words, the output tubes 'see' an 8k load (high voltage, low current Vs low voltage, high current on the speaker side). Change the load on that 8R tap to 4 ohm, and the tubes will see 4k. Put 16 ohm and it will reflect as 16k. Working into a lower impedance, the output tubes will try to dump more AC current into the load. In a class AB amp, this means the current through the tubes goes up, increasing the DC voltage across the kathode resistor and with that increasing bias voltage. Again, this is only with signal. The louder you play, the stronger the effect. In an amp with a tube rectifier, this change in DC current also causes the famous HT 'sag'.
Why is the impedance important? With lower impedance you get a bit more power and HT sag is nice for classic rock, right? Well, that extra power and current is passing through the output tubes and they will have to be able to handle that. There is only so much current the cathodes can deliver and so much heat the anodes can dissipate. Stay within these limits (under idle, low power, full power and 'set to eleven') and you're fine. The engineers of old said that with a 300V supply, 8k is a good load for a pair of EL84, considering the physical limitations, output power and distortion. So that's a good starting point for many.
(I hope is a clear story: still early here an before my first coffee
)
Things are different when there is signal. When you have a OT with a 8k primary and 8R secondary, that means that when you connect an 8 ohm load, this is reflected as 8k. In other words, the output tubes 'see' an 8k load (high voltage, low current Vs low voltage, high current on the speaker side). Change the load on that 8R tap to 4 ohm, and the tubes will see 4k. Put 16 ohm and it will reflect as 16k. Working into a lower impedance, the output tubes will try to dump more AC current into the load. In a class AB amp, this means the current through the tubes goes up, increasing the DC voltage across the kathode resistor and with that increasing bias voltage. Again, this is only with signal. The louder you play, the stronger the effect. In an amp with a tube rectifier, this change in DC current also causes the famous HT 'sag'.
Why is the impedance important? With lower impedance you get a bit more power and HT sag is nice for classic rock, right? Well, that extra power and current is passing through the output tubes and they will have to be able to handle that. There is only so much current the cathodes can deliver and so much heat the anodes can dissipate. Stay within these limits (under idle, low power, full power and 'set to eleven') and you're fine. The engineers of old said that with a 300V supply, 8k is a good load for a pair of EL84, considering the physical limitations, output power and distortion. So that's a good starting point for many.
(I hope is a clear story: still early here an before my first coffee
)
@LATUBEGEEK - please do not start multiple threads on the same project, your threads have been merged and some posts deleted.Some clarification for this thread is needed and JMFahey brings up a good point that I forgot to ask about which is what is your screen voltage at before and after the OPT swap?
I think we all agree on is that the actual AC impedance of the OPT will not change the DC quiescent bias current. Something else had to have changed. The DCR in the OPT is doubled which doesn't change much in the way of AC impedance but you will get double the DC voltage drop across it now. What else has changed?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.