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In Reply to: RE: PING: Dr. LoMu/Tube Wrangler- Re High Mu tubes vs. Hi gain circits. posted by Iczerman on May 20, 2015 at 10:42:09
Why on earth ask them rather than the whole forum ? You'll just get the same predictable answers every time with those two .
Pentode drivers can be made to sound good but may not be to everyone's taste . I built a 300B amp with a pentode driver which I thought sounded good .
The Ra of a pentode does not dictate the Rout of the stage . Chances are that a pentode driver will have lower Rout than a 'DrLowMu/Tube Wrangler' type wimpy triode driver for instance
Al
Follow Ups:
Could you guide me to a source explaining that?
My limited understanding is that the plate resistor in parallel with the grid resistor of the next stage determines the load that the driver sees.
With the WE 91A amplifier as an example..the 91A has a plate resistance of around 750,000 ohms in pentode mode. And according to the schematics available..I don't see plate load or grid resistors in the meg. ohm range.
That's why I'm asking DR. LowMu/Tube Wrangler why they don't use pentode drivers.
Or I may misunderstand what's going on.
The output impedance of a pentode driver stage is the loading resistor in parallel with the plate load resistor. As mentioned previously, gain is about Gm X loading impedance.
The last pentode driven DHT amp that I built used a pentode with a decent amount of transconductance, and I ended up using a 10K loading resistor to get the gain that I wanted, which also provided good muscle to drive the 300B.
The downside to doing stuff like this is complexity. I had ~450 or so volts of B+ and my pentode would only take 200V on the plate. I ended up using the plate load from plate to ground, then regulating the 450V down to 350V, then using a CCS to provide current to both the pentode and the plate load.
Everyone seems so intent on throwing way too complex theory or dissing Dr. Low to answer the question. The answer is, and please correct me if I'm wrong, that the plate resistance of a pentode is very very high (constant current) therefore, the effective plate resistance = the plate load resistor. If you do the computation for 10Meg in parallel with 50K, for example, the result is essentially 50k. So... not such a high resistance for the next stage after all. FWIW, I have put together a version of Thorsten's WE91 using a 6BQ5 driving a 300B (without feedback or the regulated power supply shown in Thorsten's design) and I find it to be a wonderful combination.
Live long and prosper!
What is the plate resistor value on the 6bq5?Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/20/15
12k.
http://www.diyparadiso.com/proj/legacy.htm
Thank you.
12K driving a 300b should be just fine.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
You misunderstand what's going on.
Pentodes are "constant" current devices. Look at the plate curves. The Norton equivalent circuit is convenient for studying pentodes. The gain of a pentode is (more or less) gm X net load Z. Typical loads for pentodes are much smaller than R P and those impedances are in parallel.
BTW, a 6BM8 triode/pentode can be configured as a mu follower that is very effective as a DHT driver.
Eli D.
They don't use pentode drivers as they have a belief that the extra grids do things to the timing of the musical signalI'm not familiar with the 91A , although I've heard a lot about it . Doesn't this use the 310A ? Rout is going to be
Al
Edits: 05/20/15
Belief is the key word. What they claim has no proof. Some folks still believe the world is flat, even though the ancient Greeks proved the world is round, before a Cesar ruled in Rome.
Eli D.
Occam's razor? Simple, gets the job done in a 2-stage 2A3 or 45. No additional fiddling required. Of course I heard this from a non-EE "experimentalist", but one who is well regarded.
I would ask Paul Joppa. He might not necessarily agree, but he is both an engineer and an experimentalist and speaks from a variety of experiences both with his head and his ears.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
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