It’s been a long since I last posted here some project stuff. I’ve been busy with work and family. On my spare time, spent mostly listening to music, developing some synthesiser modules and playing music as well!
I missed ETF this year, sad to say. Work commitments made it impossible to travel to France. Hope that’s not the case next year as am keen to get back. I’ve seen some pictures and feedback and seems it was a blast despite the COVID restrictions.
On my exchange with Rod Coleman regarding this post, I asked him to send me a pair of his latest DHT filament regulator (version 9) to test them on my 801a preamp with a cathode bias resistor (degenerated or un-bypassed):
These regulators are design with the objective in mind of having a larger impedance path in the cathode. The negative terminal in this version always need to connect to the cathode resistor.
I will modify the VT-25a/801a preamp I built back in 2016 to use these regs. I will need to remove the filament bias element and replace with the cathode resistors. I will also upgrade the gyrator boards and get these boards for the FT-3 caps I have in place:
Some new holes will have to be drilled I think on this nice aluminium top plate. Let’s see how it turns out.
The circuit has already been presented:
I won’t be adding initially the low gain attenuator network (R2 and R3) nor the source follower. The rest is the same as above
Pity you couldn’t make it to Bellême. Would have loved to put a face to the name and exchange some words with you. Nevertheless – freshly baked ETF pictures: https://flickr.com/photos/_hb/albums/72157720213882175
Cheers: Holger
Hey Holger, yes it would have been great to meet you and enjoy a good conversation. The work you have done with those speakers is superb.
I think you have captured on these pictures the essence of Belleme ETF, amazing job. Really admire your work.
See you next time.
Cheers, Ale
Hello
We are very interested in your opinion between the V8 version of Rod Coleman and the new V9 version you are testing.
A friend of mine and I built two preamps: mine with VT25 tubes and the other with 2P29L tubes, following your schematic and using Coleman’s DHT circuits.
I have to say they sound so good that I never thought they could improve further, as it seems from a review I read on Diyaudio.
Both preamps use Coleman V8 power supplies, with Schottky diodes in series to the cathode (13.5V for the VT25).
We would like to know:
If the difference in performance is such that it is worth making the change.
If it is possible to use the same cathode circuit described above.
Thanks for your precious “great” work that you share with everyone.
Greetings
Jhon and friends
Hello Giovanni,
Apologies for extremely belated reply, over a year, but I just saw the message. You can see my 801A build on Ale’s site. A little bit above this post.
Just read your querie regarding Rod’s V9 Filament Regs. I run a 801A pre-amp (also CX301A and a 26), very close to the VT 25 but much more powerful. It first ran with V8’s – but because I was writing often to Rod at that period, it happened that the V9’s came ou,t so I bought a few pairs to upgrade all of my DAC DHT pre-amps. Do they make a difference – indeed they do but on a very sensitive system. Clearer highs and mids, slightly more commanding bottom. Its a drop in replacement with Sic diodes filament bias – you will need 17V rectified supply. 6x1V SIC diodes + 7,5 filament voltage is 13.5V but the regulator needs at least 3V above for proper regulation. A little more is better. The V8 is more forgiving in some ways nevertheless V9 is definitely better. If you decide to go that way, I do not think you will be disapointed. Should you require more info you can reach me on [email protected]. All the best if this message reaches you. Lionel.