Since I have the B+ voltmeter and plate current ammeters, it is easy to observe what my amp is doing under any given situation.
What I have noticed is that the limiting factor is my transformer.
I have come to this conclusion by observing my amp running with different rectifier tubes and, more importantly, different cathode resistor values.
The conclusion, based on my observations, is that lowering the cathode resistors to pull more current through the output tubes simply results in B+ sag.
For example: With a 5AR4 rectifier and 620ohm cathode resistors, my B+ sits at about 430V and ammeters indicate ~57mA of current through the tubes.
Same setup with 560ohm cathode resistors (simulation shows B+ remaining at 430V and tube current going up to 63mA) - my meters indicate B+ drops to ~410V and tube current is up to barely 59mA.
I could be wrong - but what this tells me is that trying to pull more current from my transformer is a lost cause. Also, the transformer runs considerably hotter under these conditions!
It's like when you run an engine at 4 thousand rpm and you are already getting all the torque and horsepower vs. redlining it and getting nothing more than extra heat and a much shorter life.
The sound? Zero difference to my ears. I'll be going back to the 620ohm cathode resistors.
Note: I've also learned that SEAmpCAD assumes a transformer with infinite current capability. I like how PSUD2 accounts for the load and adjusts B+ accordingly. It would be nice to have all that functionality wrapped up in one software package.
What I have noticed is that the limiting factor is my transformer.
I have come to this conclusion by observing my amp running with different rectifier tubes and, more importantly, different cathode resistor values.
The conclusion, based on my observations, is that lowering the cathode resistors to pull more current through the output tubes simply results in B+ sag.
For example: With a 5AR4 rectifier and 620ohm cathode resistors, my B+ sits at about 430V and ammeters indicate ~57mA of current through the tubes.
Same setup with 560ohm cathode resistors (simulation shows B+ remaining at 430V and tube current going up to 63mA) - my meters indicate B+ drops to ~410V and tube current is up to barely 59mA.
I could be wrong - but what this tells me is that trying to pull more current from my transformer is a lost cause. Also, the transformer runs considerably hotter under these conditions!
It's like when you run an engine at 4 thousand rpm and you are already getting all the torque and horsepower vs. redlining it and getting nothing more than extra heat and a much shorter life.
The sound? Zero difference to my ears. I'll be going back to the 620ohm cathode resistors.
Note: I've also learned that SEAmpCAD assumes a transformer with infinite current capability. I like how PSUD2 accounts for the load and adjusts B+ accordingly. It would be nice to have all that functionality wrapped up in one software package.
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Sounds like you have learned a lot from your previous travails and now are experimenting further. A clear case of the DIY addiction.
Indeed. I am one of those people who isn't really happy unless I am focused on fixing something or making it "better". I have plenty of failures and half-finished projects under my belt, but it is all for the sake of tinkering to keep my mind busy.
Regarding this current observation (pun intended); had I understood this better before building my amp, I would have chosen a beefier power transformer. It would be nice to have the option of pushing ~70mA through my tubes while maintaining 430V B+, especially since I am pretty sure there will be a pair of 6L6GC under the Christmas tree soon.
Regarding this current observation (pun intended); had I understood this better before building my amp, I would have chosen a beefier power transformer. It would be nice to have the option of pushing ~70mA through my tubes while maintaining 430V B+, especially since I am pretty sure there will be a pair of 6L6GC under the Christmas tree soon.
Hi Cogitech,
Can you shed some light on what voltmeter you used to measure the B+ and plate current?
Can you shed some light on what voltmeter you used to measure the B+ and plate current?
What would happen if you introduced a series connection of a second transformer? Say a 25V 1A secondary? Then when your main transformer collapses, the additional 25V wont - and you'll get to hear what it'd be like as if your main had the grunt to supply the full voltage at the additional current.
A temporary, experimental endeavor - but may confirm your wish to have chosen a better power tranny - or validate that you wouldnt have heard the difference anyway.
I once used this to do the opposite - lower the power tranny output so I could run the tubes at higher current, without killing them due to plate dissipation.
I wonder what would happen if you connected such a series transformer to a variac, with a DPST switch, arranged so you can boost or buck the tranny secondary voltage by swapping phase with the switch? Dial in a sweeter operating point than what the stock power tranny winding provides?
Put that with a 4 gang variable 1K, 5W resistor on your cathodes...if there is such a thing. "Dial" that sucker right in - a little more current here, a little more voltage there - oops too much current - a little less now! That'll keep you occupied getting just the right balance between everything going on - in an empirical way.
A temporary, experimental endeavor - but may confirm your wish to have chosen a better power tranny - or validate that you wouldnt have heard the difference anyway.
I once used this to do the opposite - lower the power tranny output so I could run the tubes at higher current, without killing them due to plate dissipation.
I wonder what would happen if you connected such a series transformer to a variac, with a DPST switch, arranged so you can boost or buck the tranny secondary voltage by swapping phase with the switch? Dial in a sweeter operating point than what the stock power tranny winding provides?
Put that with a 4 gang variable 1K, 5W resistor on your cathodes...if there is such a thing. "Dial" that sucker right in - a little more current here, a little more voltage there - oops too much current - a little less now! That'll keep you occupied getting just the right balance between everything going on - in an empirical way.
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