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In Reply to: RE: 437A to drive a 300B posted by xcortes on September 23, 2015 at 08:44:55
Hello,at -74V bias of your 300b tube, your amplifier will result in a very low input sensitivity of 1.8V RMS for full power.
The 437A tube has a mu of 41. I drew a loadline for a 5k resistive load and 350 B+. The gain goes to ~38 judging from the loadline.
I'm choosing a -2.5V bias. It's including some driver headroom and grid current. Also the voltage swing still looks linear on the loadline from 0 to -5V. 300B has some input capacitance of around 70-100pF, so she will adore the large current from the driver.Results taken from the loadline:
Vgk=-2.5V
Ia=37mA
Ua=170V
Pa=6,3WBut you need ~1.8V RMS for full power.
If you need more input sensitivity, I recommend D3A or EL802 as driver.
Edits: 09/23/15 09/23/15 09/23/15 09/23/15Follow Ups:
With a CCS plate load he would have more gain and the stage would be more linear.
Don't most CD players output 2vac RMS?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre,
Most CDPs/DACs will output a maximum 2VAC; many will output more. Are "quiet" recordings going to be produced with that output level? Maybe not an issue with most modern recordings, but worth considering.
Also, some prefer the sound of these high-gm, high-gain IDHT tubes run at modest current. For example, Gordon and Thorsten preferring ~10mA through the 417A (related but different tube).
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
"Are "quiet" recordings going to be produced with that output level?"
I assume by "quiet recordings" you mean recordings with a lot of dynamics?
Most files are "normalized" so yes, the peaks will be at a digital "0" and in those moments the player will be outputting it's max signal.
If the system has more gain than is required to drive the output tube to clipping then when the CD player is outputting it's max (on these digital zero peaks) the signal will need to be attenuated.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
At high plate currents these tubes sound steely, harsh and thin to my ears. Lower the plate currents down to 10 madc and the sound becomes much richer, smoother, and non-fatiguing.
MSL
Builder of MagneQuest & Peerless transformers since 1989
The moments of harsh sounding can appear. Although, I'm very happy with my EL802 at 35mA driving the 6P45S, while at 11mA it sounds slow and boring. I believe it has to do with the high input of the power tube capacitance.Here's a new loadline at 10k if one wishes lower driver current. It still looks linear to me:
Ugk= -2.5V
Ua= 150V
Ia= 20mA
Pa= 3W
gain ~ 39Yes Tre' ,
One could use a CCS for more gain and linearity, although my personal taste has disliked solid state CCS in the past.
Edits: 09/24/15 09/24/15
I think the low current through the single stage driver tube has to do with setting the current so it produces being similar but inverted by 180 distortion spectra of a full power'output tube. These two distortions sum to make the even order (dominant) distortions appear to go down at the cost of increasing higher order artifacts.
I remember running some simulations on this a number of years back and with a little patience I was able to get a distortions spectra like that of a PP amp from a SE300B amp by lowering the driver current.
dave
I did this on my GM70 project, I remember you saying it might be a good thing in lieu of the traditional currents. Distortion figures looked good, we'll see how it sounds in a few months.
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