freaky new phonostage
didn't build much this year, but last week I felt the
urge to heat up the soldering iron again and do something...
Several friends of mine are intereseted in a similar phonostage
like my LCR EQed 'vinylsavor' which uses the EC8020. Since that
tube is nearly unobtainable, I looked for suitable alternatives with
a similar transconductance. I once stumbled over the freaky 6HS5
a pulse regulator beam power triode for color TV applications.
Mu=300, gm= 65000, rp =4600 Ohms
A first experimental lash up which I built last year, showed that
it is usable for audio. Unfortunately it does not really come to life
below 1000V B+.
Last weekend I completed a two stage transformercoupled phonostage with it. Of course with 600 Ohm LCR EQ and
ultrapath. B+ is a whopping 1250V (probably the phonostage
with the highest B+ in the world ?).
After some minor tweaking it worked fine and sounds excellent!
Heaps of resolution and dynamics, nice colors and timbre. It
has more microphony problems than more common preamps,
due to the use of those large power triodes. But microphonics
are managable by using appropriate shock mounting.
I currently use the S&B TX103 1:10 MC step ups which I have on
loan. Not as good as my favorite Jensens, but still excellent
performers. Overall the phonostage has a gain of 72dB. This can be boosted to 78dB by using a 1:20 step up, or even to 90dB by
introducing an additional 1:4 step up transformer after the RIAA
EQ. Zout is a low 200 Ohms, so even 600 Ohm attenuators or
TVCs can be driven. No active linestage needed.
Schematics can be found on my website:
LCR EQ: http://www.vinylsavor.de/lcreq.gif
I'll write up a more detailed description later this week.
ITs are Lundahl LL1660/25mA, chokes are LL1668/25ma
Hope you have some fun examining the schematics.
impressed. Bet some folks get a twist in their mind, looking at the schematics :)
Mmmmhh, always was a KISS fan!
Bravo... that circuit will make people think for awhile (me included). 1250V is beyond my self-imposed B+ safety limit (subject to change as i get more practise).
thanks for your compliments.
I tried to draw the schematics in a way that makes the
signal loops more clear. May be I failed ? You think the
circuit is confusing ? Well, if you have any questions about,
just shoot them at me :)
Dave, there are other tubes which require way lower B+
and which are suitable for such transformercoupled circuits.
For example the 6C45.
The 6C45 has quite a rep. Hard to find thou? From trying to folow some of the threads on the JoeList, this likes iron a lot, Grid-chokes. Plate-chokes. I guess transformer coupling would serve this...
the 6C45 ist not that difficult to source as far as I know.
There is a guy from Lithuania who occasionally
sells some on ebay. he has a web page: kwtubes.com.
I never dealt with that guy myself, but heard from others
who had successful transactions with him.
I haven't tried the 6C45 myself yet. What I dislike about them
is their large variation in mu and gm from tube to tube. The
datasheet gives a mu of 52 + or - 16. So they definitely
need to be matched.
I'm wrong or you are the only that use IT and plate choke at the same time? ;)
Although these chokes are suitable as plate chokes, they are not used as plate loads here. This is the ultrapath scheme. I like to use high inductance chokes in the B+. The way ultrapath is implemented here, the B+ needs to have very low ripple..
My only concern with the design is how good the insulation is in the magnetic components over some period of time..
The schematics look perfectly logical to me, and very easy to follow.
Interesting and original, can't wait to see what effect it has on demand for heretofore largely ignored HV beam triodes.. :D
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