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In Reply to: RE: High Ra DHT tubes posted by pix on September 03, 2015 at 01:19:50
30k is fine for a front end tube , most of these medium mu DHT were used for repeaters and have a long life , but have low Va max so are not really suitable for swinging a lot of volts . I have used 4020 which are similar as input stage on a push pull amp driving a concertina . Currently have a 3A/108A input stage driving 4P1L output stage via cathode followers . This has a very good sound
Al
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Acutally I have never used CFs.
What Zout could be expected from these type of tubes in such configuration?
You would use a cathode follower as buffer stage . Cathode follower output impedence (approximated) is 1/gm so for example a gm of 2mA/V is approx. 500 ohms Rout . I would not recommend using a DH tube as cathode follower . IDH types are more suitable and have much higher mu and gm . You would use the AA as a front end in front of the cathode follower .
Al
But do you considder the DHT/CF combo to keep the very attractive sound-signature of the DHT, or does the CF ad some greyness that CFs are often bespoken of?
I really like DC coupled cathode followers into a DHT output stage . The other way to use such valves is as front end for a concertine phase splitter . I did this scheme with Triotron SD1 and PP 2E22 output stage
Al
It's been my experience that a follower should be run near its dissipation rating using plenty of anode voltage. I also have never designed-in a CF unless it could be inductively loaded (choke loaded or transformer coupled). Some people won't use them under any circumstances, but I think that's a narrow approach to amplifier design that isn't justified.
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