Russian DHT preamp

mred

Senior Member
I have been reading about DHT preamps for a while.
After finally making a decision to give one a try, I started digging in the audio closet and simming a power supply using PSUD.
I have almost everything execpt a few resistors and the tranformers for the Coleman regs.
I have decided to build with the Russian 2p29l, mostly because of cost, but also because of reports they are least microphonic ot the popular tubes vintage or Russian.
I have a layout for.the power supply chassis.
LCLCLCRC filter, Coleman filament regs.
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I have 4 tubes ordered for ebay seller,, they come encased in aluminum, I have freed one of them.
I will try both canned and de canned just for fun.
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I hope to make progress on this in next few weeks, as my 2A3 amps have stalled for a while.... and as we know an idle mind is the devil's work shop.
 
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gable

Senior Member
Definitely following along. I've often wondered about a 2p29l, or 4p1L, etc. pre.
 

rogerfederer

Junior Member
i built the bottlehead smash which uses the 4p1L. microphonics is a huge problem with that design. had to treat it like a fussy turntable and use lots of isolation tricks.
 

mred

Senior Member
i built the bottlehead smash which uses the 4p1L. microphonics is a huge problem with that design. had to treat it like a fussy turntable and use lots of isolation tricks.
Well, do tell your experience and how you like it
I had been thinking about this since a Lenco member had built the 300b pre and reported on it about a year ago
 

rogerfederer

Junior Member
it ended up with way too much gain for any of my systems. instead of trying to deal with that and the microphonics issue, it has been sitting idle. if you want to borrow it to check out the sound i could send it your way for a listen.
 

mred

Senior Member
it ended up with way too much gain for any of my systems. instead of trying to deal with that and the microphonics issue, it has been sitting idle. if you want to borrow it to check out the sound i could send it your way for a listen.
Gain is a concern, I think the 2p29l has the same amount as the 4p1l.
I built a 6sn7 cathode follower pre and it was just not quite enough gain. Perfect for everyday listening, but if I got the urge to rock the casbah it didnt do it.
I converted it to 12b4a and it is just about right , I think a factor of 6 vs 9 for the 2p29l .
If I like the amp when done I would spring for transformers to put on the output and reduce gain.
Very kind offer to loan yours, I would be afraid of damage in shipping.
Wish you were in the Houston Texas area.
 

mred

Senior Member
Today's update.
I have laid out the power supply and have it ready for drilling and milling.
This morning I laid out the preamp section.
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I hope to have time at work to use a small mill to complete the larger holes and can do the small fansters by hand at home.
This is a project of scrounged up stuff and parts in stash so its a bit out of the normal maybe.
The top plates are 1/8 inch steel that were part of a switch console on a machine of some sort. They have a nice blue hammer finish. Rear panels are aluminium angle, no sides or bottom. They will have some sort of long bolts or tubes for support / feet.
I hope to have a kind of industrial / artsie kind of look.
A Sowter TVC will be used on the output, I am choke loading the 2p29l and using V reg tubes to recude the final B+ to chokes.
 
Very nice looking build. I do have a couple of questions/comments.

  1. Are the two chokes labeled L and R loads for the 2p29l tubes? If so then you might consider having them separated a bit more. Perhaps have each choke towards the outside of the chassis with the tube sockets in between. That would ensure minimal magnetic couple between the two chokes.
  2. Are you putting the TVC/AVC volume controls (can't tell which they are although they look like S&B TVCs) on the input or the output. I've always used mine on the input but some people report a preference for having them on the output. Would be interesting to hear if you or others have experimented with this option.
  3. Are you going to use grid stop resistors on the 2p29l? It's worth playing with those if you have the patience to experiment. Right now I'm listening to a transformer coupled 417a preamp based on a design called the Euridice published in Sound Practices long ago. I ended up using ferrite beads right at the tube socket pins instead of resistors and think that it helped open up the sound a bit. The transconductance of the 2p29l is quite a bit less than the 417a, so you should be able to get away with this without having trouble with oscillations. If there's interest I can write more about the 417a preamp but don't want to distract from this discussion.

---Gary
 

mred

Senior Member
Thanks for the input Gary.
I did end up thinking about the cokes and now have repositioned them at 90 degrees to one anther. I may move them agian.
The TVC is going on the output, yes.
Currently planning grid stopper resistors. I have not tried ferrites before , certainly not opposed to trying them, any info would appreciated )
Feel free to share about the 417 pre.
 
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My 417a preamp is inspired by the Euridice preamp published in volume 10 of Sound Practices. Member J-rob here on the haven was the publisher of that magazine and still sells CDs containing PDFs of all issues. Highly recommended.

For the cheapskates among us (probably most of us) here are some web links showing more less the gist of the article.
Thorsten Loesch published his version here Loesch Euridice.
I built my version a long time ago - in the early 2000s and I also sent a few details to Thorsten who put a few pictures up on his website, which you can still find Gary's Euridice. Compared to what I wrote back then, I've tweaked things a little bit. The B+ dropped to 150v through the use of 0A2 regulator tubes. Power supply is now CLCLCR to the 0A2s - one for each channel. The 0A2s don't like a lot of capacitance so there is only a Russian 10nf teflon cap after the 0A2 and it's located close to the 417a.
With the lower B+, I dropped the cathode resistor to 75 ohms to run the tubes at ~ 10ma. The filaments are heated with constant current sources built around LM317s.
Regarding the ferrite beads, I used a very generic one. I can't remember where it's from. It looks like a Fair-rite 43 shield bead which you can buy from Digikey part # 1934-1523-ND. I looped the wire going to the tube socket through the bead once, i.e. the wire makes a loop around the bead and goes through it twice with the ferrite bead hanging right from the tube socket pin. Seems to do the trick.
---Gary
 

paulbottlehead

Active Member
If you rotate one of those chokes 90 degrees the way they are sitting, the coils are still on the same axis. I wouldn't be too worried about magnetic coupling between these parts.

Having the level control at the output will help dramatically with the performance of the preamp.
 

mred

Senior Member
I made more progress on this project.
I soldered up two raw supply boards for the Coleman filament regulators.
Got the power supply wired.
Tested the regulator supplies and they are on voltage at 15.6 v.
Tested my HT supply and it is about 100 v low no load, and almost 1/2 of my PSUD sim when loaded...
I scratched my head on this one last night for I while.
This morning while drinking coffee and getting myself going, I decided to look one more time at the data sheet for the EZ81 rectifier....
Picture Homer Simpson slapping his forehead "DOH!!"
Its a half wave retifier.
I wired it like a full wave.

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mred

Senior Member
The mailman brought me two diodes yesterday. I foolishly wired up a half wave EZ81 as a full wave so I now have a hybrid bridge using two NF4007 diodes forming a center tap return to ground, ( white wire running rectifier to ground bus).

I made snubber with two 10R resistors and two small caps to keep the transformer from reacting to the switching pulses of the diodes, or ringing as it is referred to.
I now have adequate voltage.
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The mailman brought me two diodes yesterday. I foolishly wired up a half wave EZ81 as a full wave so I now have a hybrid bridge using two NF4007 diodes forming a center tap return to ground, ( white wire running rectifier to ground bus).

I made snubber with two 10R resistors and two small caps to keep the transformer from reacting to the switching pulses of the diodes, or ringing as it is referred to.
I now have adequate voltage.
View attachment 11431
Why not build a Quasimodo snubber or its little brother the Cheapomodo and do the snubbing properly? There are looong threads in the power supply section on DIY Audio on this subject.
 

paulbottlehead

Active Member
If this is something you are really worried about, I would recommend simply using Schottky diodes, especially if you are unable or not interested in simulating or measuring the specific power supply you are working on, as each power transformer/diode combination needs different values for snubber parts.
 
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