• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

freaky new phonostage

Hi all,

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:

circuit: http://www.vinylsavor.de/phono6hs5.gif
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
and LL1623/100mA.

Hope you have some fun examining the schematics.

Thomas
 
Dicey, Dave,

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.

Thomas
 
Vinylsavor said:
Dave, there are other tubes which require way lower B+
and which are suitable for such transformercoupled circuits.
For example the 6C45.

Good morning in Germany, I guess it is time to go to bed here on the left coast.

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...

dave
 
Dave,

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.

Thomas
 
Vinylsavor said:
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 bought some tubes from Gintaras. he is good -- he shipped the same day i put he money in the mail (i guess based on my good feedback).

dave
 
Hi!

The thread title is a bit misleading now. 'nichoch46' resurrected this thread from 2002.


Interesting. I have often wondered why inductors weren't used in phono preamps more often. I have a couple of questions. Why did you choose to use step down IST? Are the ISTs just providing the coupling function or is their reactance a part of the filtering functions.

The first IT drives the 600 Ohm LCR RIAA. The second transformer provides a low Zout.

All the RIAA is done in the LCR network.

Best regards

Thomas
 
Hi!

1200v is far too many for someone as inexperienced as me.

I absolutely agree. This design is not recommended for inexperienced builders. The same principle can be implemented with other tubes with minor adaptions. I've built similar phono stages with EC8020, WE437A and also more cheaply available tubes like 6AM4. All those operate around 200V B+

best regards

Thomas
 
Hi Kevin,

I used Lundahl chokes and transformers. They are very well insulated and rated for 2kV coils to core and 4kV coil to coil. And that is conservatively rated. No issue with insulation problems there at all! Such a phonostage is already in operation almost 10 years

Best regards

Thomas

This is good, I put much more credence in the fact that you have run one of these for nearly 10yrs without problems than in the voltage breakdown specifications because my experience with high voltage apps in semiconductor ATE and elsewhere has taught me to be very suspicious. We had transformers that were rated to 4kV core to winding and winding to winding that failed fairly quickly due to continuous electro-static stress, and in applications where the supply slewed rapidly insulation failures occurred below 1.2kV. I suspect moderate hours of usage and low dv/dt in this application probably reduce the stress on the insulation. I know Lundahl makes good transformers, and I use them in some of my own designs.

Still IMHO, a very cool and original design.. :D
 
Last edited:
Hi Kevin,

Per Lundahl made a presentation at last years ETF and specifically talked about the insulation measures they take. I can assure you their rating is very conservative. That's the least I'd woory about. Special care needs to be taken in the wiring and socket connections of such a design. I drilled out the neighbour pins besides the plates of the sockets of the 6HS5s to provide more creepage distances there.

I did implement a differential version of this design which I even ran at up to 2kV.

best regards

Thomas
 
Thomas,

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).

dave

I remember my first go at making a valve pre amp.
Of course I switched it off and grabbed it and got a shock.

So the next time i got smart and discharged the capacitor first and touched it and got a shock again, I had forgot to switch it off !

I now stick to sub 200 volts systems so hopefully i will live to a ripe old age.
 
Hi!

I may try one of these when funds are more available but my first one will have to be RC passive.

A while back I shared a design of a full function preamp with RC RIAA phonosection on AA. I call it the Octal preamp. It uses 6SL7/6N7/6SN7 tubes.
If there is interest, I'll open a thread about it and post the schematics

Best regards

Thomas