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In Reply to: RE: Valve preamplifier biasing posted by Will G. on February 25, 2021 at 17:02:55
Eli is right. The 300b is cathode biased (self biased) and the control is a hum pot. The 300b is being used as a cathode follower to provide a low output impedance and should not affect the sound of the preamp. There is also negative feedback in that circuit and that will also limit the difference between the sound of different tubes.
What gets me is the value of the final coupling capacitor. They went to the trouble of creating a low output impedance and then didn't make the output coupling capacitor large enough for the user to take full advantage of it.
For example, if you wanted to use this preamp to drive a SS amplifier with a 10k ohm input impedance the output coupling cap of the preamp would need to be about 8uf to pass all the bass. Instead it's 1uf.
Oh well.
P.S. the schematic above shows two 47 ohms resistors but if the company is telling you to adjust a pot for minimum hum then it is clear that they replaced those two 47 ohm resistors with one 100 ohm hum pot at some point in the production run.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Very interesting read, thanks Tre' for the explanations.
"The 300b is being used as a cathode follower to provide a low output impedance and should not affect the sound of the preamp. There is also negative feedback in that circuit and that will also limit the difference between the sound of different tubes."
What's the point of using such expensive tubes as cathode followers? Is this just a marketing ploy designed to capitalize on the reputation of the 300B as an audiophile-approved tube?
Assuming that the 300B could possibly sound good in a preamp, wouldn't it be better to build it with the 300B as the input and use the 6SN7 as the cathode follower?
Agree but the plate resistance of the 300b is low (700 ohms) so a 300b (all by itself) as a gain stage would make a nice preamp. Very linear, low output impedance, just about the right amount of gain (almost all systems have way too much gain).
You are right, this is just a marketing ploy. They could have used just one 6sn7 for each channel. A gain stage followed by a cathode follower. The 300b serves no real purpose in this design.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Not sure where you're getting your numbers from, but the 1ufd cap into a 10k load would yield about 16Hz cut-off. Could be better, but at 8ufd as you suggest, you're down to 2Hz.
Pretty meaningless.
Dan Santoni
I'm not sure how important it is but if you want 20Hz to be phase shift free and at 100% amplitude, then the -3db point of the high pass filter created by the coupling cap and the impedance that it sees needs to be at 2Hz.
A -3db point of 16Hz means the phase is disturbed all the way up to 160Hz.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Some folks say F 3 needs be a decade (2 Hz.) away from the passband, while other people think 2 octaves (5 Hz.) away is OK. That makes the absolute minimum value for working into that blasted IHF "standard" 10 Kohm load 3.3 μF.
My price/performance suggestion is a 4.7 μF. metalized polypropylene part bypassed by a 0.47 μF. 716P "orange drop". An improvement is a 0.33 μF. PPFX Multi-Cap as the bypass, but costs escalate.
Eli D.
In some rooms, with some speakers, the higher F3 can be better but if the room is tamed properly and the woofers (and woofer amps) are up to it, then the lower F3 can be better.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The 7.5 uF, 330V ASC are not the worst linestage coupling caps. Not even close. They'll tolerate any reasonable B+ required, are easy to get and are not very big... :)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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