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In Reply to: RE: round and round we go .......... posted by vinnie2 on January 23, 2016 at 17:54:45
Hi Vinnie,If I have missed something in your previous threads, just point me in the right direction - no need to repeat yourself here.
As you are aware, lowering the cathode R draws more current, which in turn drops more voltage across your PS resistances, which in turn affects your plate voltage, which also affects your operating points (less voltage, more current). With that out of the way...
Is the 45 loaded by a transformer providing a 5k load?
Does your PS (potentially) supply 375V, which is then dropped to 220V using an RC node? Is this to supply the 26s and 45 from the same node? If yes, I would use an alternative strategy: 375V, then add a RC node to tap off enough voltage to run the 45 at close to textbook points (say 300V or 331V), then add another RC node for the 26s (220V)
If that is not possible, you might feel comfortable knowing that triodes that are pretty tolerant with operating points - they can be run safely and still sound fine at a variety of operating points. They don't need to be super-finely tuned to work well. Their point will change with age anyhow. That said, tubes have limitations that should not be exceeded. Again, I am sure you know this. Just sayin' as a reality check.
Finally to your question: I'd choose either the 1200 or 1500 cathode R; the 1500 R may be more ideal, but choose what sounds best to you. A 5k load should work well with either operating point. I'd take Gordon Rankin's advice not to run the 45 over 35mA... I'd be looking for a couple of mA less, say no more than ~33.1mA. You will also get a touch more power with the higher voltages, but it probably will not be noticeable.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 01/24/16Follow Ups:
and they are from my ears. I tried going up to the classic operating points of a 1500 ohm cathode resistor, 275vdc plate volts, -53vdc grid volts and 35mA plate current, but it sounded a bit harsh to me. I then went immediately back to 1200 ohms, 223 volts, -41 and 34.2 mA and you could really hear the difference. It was much more effortless sounding, and very smooth without losing anything in the process. Still the detail and clarity you expect with a 45.
So, that is what I am going to go with for operating points on the deuce26/45. They are very close to what palustris recommended in his original post.
I still need to procure a few parts, and then I can get serious about buiding it. I may look at making a few tweaks to the psu too. First thing I have to do is determine what value the filter chokes are as it does not seem to be marked on them and I built this thing 15 years ago and used a schematic from SP magazine and I don't know if I followed it exactly. I want to run it through psud2 and see how it performs there.
As Porky would say, "ttthat's ttthat's all folks!"
At least for now : )
Edits: 01/24/16 01/24/16 01/24/16 01/24/16 01/24/16 01/24/16 01/24/16 01/24/16
Thanks for letting us know.
I am not surprised at your findings. Your operating point is a little hotter (more current vs. voltage), which lowers distortion and your current is less than 35mA, which seems to help with the 45. The 45 is also dissipating less power, which may work well with that tube type. It's wise, I think, to run tubes conservatively for longer life.
Well done!
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Hey 91
I am running it into a 125ese set at 5k, so yes, and I have set it up with two nodes, just like you suggested for the 300B. That seems to work well.
I had completely forgotten about Gordon's article in SP magizine #13. I built a parafeed version of the Bugle about 15 years ago and it was my main amp until I built the 91A clone (also from SP). I just sold it a while ago to finance some new projects, but the 45 is still one of my favorite tubes; that's why I am building this one. The 45 seems to really work well with the 26.
I just reread Gordon's article again, and while he does say the 45 is best with no more than 35mA plate current, 35mA is the operating point he chose for the Bugle, along with a plate voltage of 275 and a bias between -52 and-54. I guess I should try those operating points and see what happens. Gordon also used 1500 ohms for the cathode resistor.
I keep forgeting that tubes are a lot more friendly than we think they are and allow us some wiggle room. Being a civil engineer by training and profession for 30 some years, I got used to trying to match the numbers the specs asked for. It wan't until I switched over to the planning side of things that I started learning how to look at the big picture. By the time I retired a few years ago, I was finally starting to do that. I guess I should apply some of that to my audio projects.
Thanks for the reminders!
Edits: 01/24/16 01/24/16
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