• 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.

4P1L DHT Line Stage

Hello,

Is the schematic correct in principle please?
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Regards
 
Hello,
I have two questions regarding polarization:
1) What are the advantages and disadvantages of using a series of zener diodes at the cathode. adding a small capacitor (1nf for example could help remove any noise)

2) I saw that we could also connect a battery to the entrance of the grid. What do you think please?

Regards,

Marco.
 
1. Zener diodes for the kind of voltage for biasing preamp tubes are not noisy, but they have too much effective dynamic (1kHz) impedance: as much as 25Ω for a 5V6, or 85Ω for a 3V9. Around 6V8 the impedance is probably low enough, but varies with current, at least a little, so there will be added distortion. 6V8s are noisier than lower voltage zeners (2V7 and 3V3 are quiet enough for other supplies in preamps).

2. Battery bias works well, if precautions against unwanted charging are undertaken, there are lots of posts on diyAudio you can look up.
 
Good evening,

here is the progress of my 4p1l project Everything is handmade with salvaged things. (the boxes are offcuts of Jatoba flooring). For now I'm building it with a variable cathode resistor and a bypass capacitor, I'll see if I change later. I will be able to find a working point in this way. Transformers: 5k / 600ohm. primary resistance 450 ohm Supply voltage 150v. I think the OPt will be around 35ma. Critics are welcome.

Regards

Marco


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Slightly OT but since I mentioned it.... I got curious and made a "simple" 2P29L preamp using some Hammond 126C interstages I wasn't using. These are used as plate chokes with a couple of 0.1uF Russian teflon FT-2 output caps in parallel. Usual stuff for me.

Well, guys, this is a serious alternative to a 4P1L line stage. It's lovely - lots of detail, loads of treble and bass. In short, exactly what you would expect from a top DHT. And useful gain as well - should be a mu of around 9. It sounds surprisingly good in this simple implementation. Food for thought - another cheap Russian military tube that sounds great. No microphonic for me either.

Early days - I've only been listening for 15 minutes, but I like what I hear so far.

Hi Andy,

Slightly late reply to your post #1381 ...this actually looks like something I could build...got 2P29Ls, got Hammond 126Cs, got FT-3 220nFs, have a variable preamp power supply...need to know:

1. What's that decoupling cap in the HV supply line?

2. Is this filament bias?

3. What is R1 on Rod's board?

4. What else do I need to know?

Tough to keep up with your experiments/changes, but they are definitely inspiring!

Just trying to get into the mix. Hope my questions are not too *OLD*.

Thanks and Best,

Robert
 
Hi Andy,

1. What's that decoupling cap in the HV supply line?

2. Is this filament bias?

3. What is R1 on Rod's board?

4. What else do I need to know?

Thanks and Best, Robert

I've switched to DC Link caps for HV and especially cathode bypasses. Kemet C4AQ. No bypass here - it's in filament bias. For Rod's board, best to ask him - he updates regularly. I looked up my old files and found this schematic, which is probably what I built.
 

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En ce moment j'ai 2 projet de préampli dht en cours (désolé, mais je me disperse beaucoup):un WOT 4P1L preamp et un 2p29l avec mu follower. Je cogite également sur un phono dont j'ai construit la boîte. Si vous le voulez, je peux vous informer de mes avancées sur mes projets.
 
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En ce moment j'ai 2 projet de préampli dht en cours (désolé, mais je me disperse beaucoup):un WOT 4P1L preamp et un 2p29l avec mu follower. Je cogite également sur un phono dont j'ai construit la boîte. Si vous le voulez, je peux vous informer de mes avancées sur mes projets.

1. I would build a 26 preamp not a 4P1L

2. Why do you want a mu follower with a 2P29L? Just put 15mA current through it. The Ri is low enough at around 3K. Keep it simple and the sound pure.