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I have some of those european Valvo AA post-tubes (NIB) in my storage, that I never seem to get the propper use for.
This is one of the most beautiful DHT tubes i´ve seen, but the high-isch Ra (30k) medium Mu (30) never seem to fit anyware.
Anyone had any luck with such type of tube, and in what topplogy?
Follow Ups:
Hi,
I'm using the sister of the AA, the Sif Ba with great succes as a voltage stage in my amp. Ra is 25K. I'm using 480V B+ and 2x 220K anode resistors, and an unbypassed cathode resistor.
Sounds fab!
More inspiration here: http://www.dhtrob.com/projecten/ba_vv.php
NC
Since I have a quad of these tubes,
How about a PP line amp using a LL1660PP in 2,25+2,25:1 coniguration?
(Zout will still be rather high i know..)
Since the need for voltage swing is so low in a line amp, perhaps a PSE connected to LL1660/10mA (4,5:1 configuration) could be a better choise?
how about a picture of this beautiful tube?
The RFT data sheet shows 250v max plate voltage, so perfectly capable as a driver. Try it with a 45 which has a low Miller capacitance - I get -3dB at 150kHz.
The AA is one of the oldest tubes still out there, and as Al Noakes pointed out, it was a repeater tube, hense has a really long Life and very low distorsion.
Here in Europé these (often German made) DHT´s are sometime easyer to find than the US made #26 DHT´s.
I bought a quad of these in UK several ears ago. Still in unopend boxes, but has never came up with any idea how to best use them.
Thanks for the pic. You are right, it is a nice looking tube. Good luck with your efforts to use it.
Another one..
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
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.
--------------------------
Buy Chinese. Bury freedom.
You are right pix - you can't use them because of the high Ra. Send them to me for appropriate disposal... in one of my amps.
Seriously though, If I owned them I give them a try just driving one of the usual DHTs. Sure, it might be a few dB down in the higher frequencies, but some speakers and rooms likely benefit from that... and it won't necessarily sound flat, warm, dull etc as you might expect.
Go crazy. Have some fun.
Regards,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
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