You are ok.Hi,
As you know, cathode at AC ground and you get Rp according to the datasheet.
But the penalty you pay for cathode at AC ground .. I guess you work it out for yourself and how you did that.. Ck, or battery in the grid circuit - all second rate IMO. Stacked supplies for low level stages, probably not. Split rail supplies - yep.. l'd like to see more.
So, if raised AC impedance cathode - higher Rp, more Lpri, more phase shift at LF, the higher the LF pole and also C of the primary.. and BW shrinks quickly from both ends.
So far as the measurements.. my application is 150mVpp to the grid, horisontal load line. If you can run it again with those parameters, great.
So far as 200Vpp levels of drive, I do understand, and each to their own, but I would use a low Cin and low mu DHT, something like 71A. Stacked supplies without cathode (fil) bypass and <2kRp. Takes an additional stage, but so long as it is a good stage, IME it sounds just as direct, okay maybe some slight loss, but more different than 'loss' but very much more effortless and less strained on peaks.
Apples for horses
With other stage , you do not need preamplifier.
Today my amp is D3a , RC copling , 45 , Lundahl 1671 coupling , 300B , Lundahl 1664 OPT.
The best of all my 300B amps.
The peaks , they look like lots of Watts amp
Santiago
No, not more than 200vpp
I have many of these types 6688, 7721, 7722 and agree they are all good.
Larger plates, grid that biases at -40V, I'm not sure.. but something like 71A sounds better as a driver to me, even if it is two stage - as before
Trioded E280F (7722) is almost similar than D3a.
I used it in VAS stages substituting the D3a, in the similar operating point.
The tone characteristic of E280F is more "refined", against it, the D3a is playing as a rocker.
E55L is 600 ohm and would also work, but if you go too low with the drive impedance you have resonance issues, which some winders describe as 'benign'. Always struck me as an interesting word choice.
As to reason why 2 stage DHT may be prefereable to single stage high Gm, high mu, low Rp tube, I dont think it is Rp - drive impedance of D3a and 71A are close enough. Thoriated tungsten fil vs oxide coated cathode and subtstancially larger radiating area, and perhaps quality of the materials used?.
As to reason why 2 stage DHT may be prefereable to single stage high Gm, high mu, low Rp tube, I dont think it is Rp - drive impedance of D3a and 71A are close enough. Thoriated tungsten fil vs oxide coated cathode and subtstancially larger radiating area, and perhaps quality of the materials used?.
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With horisontal load line, I always thought the 7722 was better suited to larger AC volt swing than 7721.
Transfer curves seem to support this - take 15 or 20mA. I did find 7721 more 'raw' (which I like) and agree with your subjective comments on that.
6688 is amazing value, sample to sample variation is large enough to buy more than you think you need, 20pcs for $100?.
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