2P29L DHT Preamp

[pmcuk]

I just finished a 2P29L preamp and have it up and running and listening to it now. It's a directly heated pentode which is being used in triode. I can already hear that it sounds very good indeed. I've been using a 6SN7 as driver tube for my 2a3 amp, and this works fine with input straight from my DAC. So I put this preamp in front of the 6SN7 driver. The sound is maybe slightly improved - the 2P29L is clearly sounding better than the 6SN7, good though that is. I'm surprised that adding a stage could improve the sound.

So here is the schematic. I went for simplicity, so this time no filament bias. I could probably improve the DC filament supply. Larger capacitors and a dropper resistor between first and second cap. I could add a choke even though that might be overkill. So tomorrow I plan to have a good look at it and see what I can tweak. I'll certainly be building more of this design.

 

[pmcuk]

I converted the circuit to filament bias, so small cathode resistor and no bypass. It's a very lively yet refined sound, which reminds me a lot of the 10Y. It draws you in, definitely involving. Another of the great DHTs and the easiest of all to work with. It's a true high end kind of sound - makes you listen to all your favourite tracks all over again. I'll be building more for sure.

 

[richard mainwaring]

I would love to be able to do stuff like that.

 

[pmcuk]

I would love to be able to do stuff like that.

I'm completely self taught. Years and years of experimenting and reading loads of tutorials, and hanging out with clever people like Morgan Jones and Thorsten Loesch.

This 2P29L can be for sale. Just one input and no volume control at the moment, but space to add a nice volume control. PM me if interested.

 

[pmcuk]

I've made a breakthrough with the 2P29L which I'm very pleased with. I've always wanted an all-DHT amplifier and I think I've finally cracked the design. This is a single ended 2a3 amp with just the 2P29L as driver tube. It sounds great, but needs more gain like a 1:3 or 1:4 step up. So I've been looking at input transformers from the likes of Lundahl, Sowter, Jensen, Cinemag, UTC and Hammond. At present I am using a Hammond 124B in 1:3 step-up and it sounds much better than I expected from its low price. It's 49% nickel core, which some prefer to 80% nickel. I've been listening to this for 2 days now and love it - it's giving old and familiar tracks a new life. Lovely tone and transparency, very involving as DHTs can do so well in their special way.

Using a step-up input transformer in an all-DHT amp is something that both Thomas Mayer and Ale Moglia use, Thomas as 10Y into 45 and Ale as 47 into 300B. This is very high-end stuff. They both use the Lundahl LL7903 but I'm looking for cheaper and indeed better alternatives, so scouring long lists of audio transformers for some stand-out ones. The amplifier isn't simple to build, and requires very good power and filament supplies, but it's not too complex either and just 2 stages. It may just fit in one chassis and if not it will have an external power supply as it does at present.

Very pleased with this!

Andy

 

[dave]

I'm surprised that adding a stage could improve the sound.

Its strange how that happens.

 

[rabski]

Its strange how that happens.

Another case where practice trumps theory.

Exactly the same in my system. The 845 really has about enough gain to use on it's own, as it's got a Noble pot in it (now). However, sticking an active pre in front of it is a major improvement to my ears. Bigger and better defined soundstage and a more realistic presentation of instrument timbre. It just sounds basically more like the real thing with a decent active pre. Yet it shouldn't in theory. It's adding a link in the chain that technically ought to take something away. I have no idea why. It doesn't make sense, but it is what it is.

Anyway, it gives me the fun of building more preamps...

 

[smithy27]

I'm completely self taught. Years and years of experimenting and reading loads of tutorials, and hanging out with clever people like Morgan Jones and Thorsten Loesch.

This 2P29L can be for sale. Just one input and no volume control at the moment, but space to add a nice volume control. PM me if interested.

Interested

 

[pmcuk]

I've been looking at all-DHT amps - I started a thread on DIY Audio (All-DHT amplifiers) and got a large number of very interesting schematics. Some with a step-up transformer, and some with 3 DHT stages, the reason being that 2 DHT stages needs about 3x or 4x more gain. So I decided to do a test of the differences between these choices. So I did an A-B listening test with these alternatives. Same music, same 2A3 SE output stage, just changed the input stage.

1. SUT Hammond 124B 1:3 into 2P29L in filament bias.
2. 10Y stage replaces the SUT into the same 2P29L stage.

Well, my hunch was that the SUT (Step Up Transformer) would sound better, but I was unprepared for how much better. It's considerably better - no contest, and it's not subtle. Make no mistake - a 10Y stage is wonderful by itself and I'd be hard pressed to name a better DHT, and a SUT isn't needed for a preamp. But there was far more transparency and detail when it comes to an amplifier with just 2 stages. The 3 stages just thickens the sound. The tone is fine, that's not the issue. But the loss of clarity isn't acceptable to my ears. So I'm going straight back to 2 stages with the SUT. I could tell about 30 seconds into the first demo track that the 10Y stage wasn't nearly as transparent as the SUT, but I listened to 3 different tracks before I gave up. There was no point in continuing.

So that's it for me - in a SE amplifier circuit it's SUT all the way. The Hammond 124B is much better than it ought to be. It's actually pretty good. 49% nickel. But I'll be looking at several others. The sound with this setup is a revelation to my ears. Nothing sounds like an all-DHT amp for clarity and tone. And for sure very few on this forum will have heard one. They are not commercial products.

 

[rabski]

And.... bang goes my revised idea for the second 845. Thanks old pal

I was going to revert to the sadly-late Jon's original design of 2a3 drivers for the 845s, but pentode (c3g) first stage. The current running one uses Pentode - pentode - 845 and actually works very well (6AC7, KT66, 845), as the pentode is capable of enough clean voltage swing for the 845s. However, the drawback is that the overall sound is far more dependent on the first stage (and coupling) than I'd like. I like using interstage, but for 845s, they're hard to implement.

The step-up, 2a3, 845 may just be the ideal solution.

 

[pmcuk]

And.... bang goes my revised idea for the second 845. Thanks old pal

I was going to revert to the sadly-late Jon's original design of 2a3 drivers for the 845s, but pentode (c3g) first stage. The current running one uses Pentode - pentode - 845 and actually works very well (6AC7, KT66, 845), as the pentode is capable of enough clean voltage swing for the 845s. However, the drawback is that the overall sound is far more dependent on the first stage (and coupling) than I'd like. I like using interstage, but for 845s, they're hard to implement.

The step-up, 2a3, 845 may just be the ideal solution.

Yes - Jon was much loved, a larger than life character.

I'd go with the step-up 2a3 > 845. Ale is using the LL7093, the one that Per Lundahl recommends for input to a tube stage, and Thomas Mayer has used it in his 10Y > 45 amp. I don't spend that kind of money so for now I'm using a Hammond 124B which is surprisingly good. I've ordered a Hammond 1140-LN-C, 4:1 36K:2.2K, 10K:600 from KGA. One of their broadcast series.

UTM are now making SUTs in Poland - https://utmindustry.com/

UTM2546
2+2:1+1 €36.95 +30dbu
Impedance: 10k:2.4k, 10k:600Ω, 2.4k:2.4k, 2.4k:600
Primary Inductance: 70H+70H @100Hz
Secondary Inductance: 8H+8H @100Hz
Primary 240R Secondary 100R
Mu-Metal, High Nickel Core, 79% Nickel Steel lamination

UTM2581
tube input 1+1:2.5+2.5 €39.95 +30dbu
Impedance: 1.2k:7.5k, 1.2k:1.8k, 600:1.8k, 600:15k
Primary Inductance: 2.2H+2.2H @100Hz
Secondary Inductance: 130H+130 @100Hz
Total DC Resistance PRI: 20Ω
Total DC Resistance SEC: 830Ω
Max Level: @50Hz: +30 dBu
Mu-Metal, High Nickel Core, 79% Nickel Steel lamination

 

[rabski]

So many options, so little time...

I hadn't known about UTM. Interestingly, there are a couple there that look suitable for IV conversion. I'm just about to email them for some more info. There's a 10K:10K that may be the answer to a prayer here without the wallet damage that experimenting with Lundahl entails.

 

[zeta4]

Yes - Jon was much loved, a larger than life character.

I'd go with the step-up 2a3 > 845. Ale is using the LL7093, the one that Per Lundahl recommends for input to a tube stage, and Thomas Mayer has used it in his 10Y > 45 amp. I don't spend that kind of money so for now I'm using a Hammond 124B which is surprisingly good. I've ordered a Hammond 1140-LN-C, 4:1 36K:2.2K, 10K:600 from KGA. One of their broadcast series.

UTM are now making SUTs in Poland - https://utmindustry.com/

UTM2546
2+2:1+1 €36.95 +30dbu
Impedance: 10k:2.4k, 10k:600Ω, 2.4k:2.4k, 2.4k:600
Primary Inductance: 70H+70H @100Hz
Secondary Inductance: 8H+8H @100Hz
Primary 240R Secondary 100R
Mu-Metal, High Nickel Core, 79% Nickel Steel lamination

UTM2581
tube input 1+1:2.5+2.5 €39.95 +30dbu
Impedance: 1.2k:7.5k, 1.2k:1.8k, 600:1.8k, 600:15k
Primary Inductance: 2.2H+2.2H @100Hz
Secondary Inductance: 130H+130 @100Hz
Total DC Resistance PRI: 20Ω
Total DC Resistance SEC: 830Ω
Max Level: @50Hz: +30 dBu
Mu-Metal, High Nickel Core, 79% Nickel Steel lamination

I think those should be very good. Primary inductance is a little low but ok if your driving the SUT from a low source impedance. Peak signal handling of +30dbu is high so this should mean low LF distortion. Prices are very good too.