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4P1L DHT Line Stage

iko

Ex-Moderator
Joined 2008
Lateral maybe, or power jfet like the new semisouth devices Nelson is using.

As for the gyrator vs the ccs idea. This was an old idea and I particularly wanted to make it a low voltage tube amp. The jfet as ccs gives nice performance. As soon as you make it a gyrator you add a capacitor and then you have to worry about the quality of that capacitor. I'd rather use a choke like the Hammond 156c (150H) than a gyrator.
 
Anatoliy,
Surprised with my first test results. Measured a simple 12P17L stage with a CCS load.
I played around to find the best operating point at Ia=15mA (or higher current) and biased at Vgk=-12.5V (LED array). Va=178V
Stage gain is about 10.5.

Interesting to find was that THD wasn't as good as a 4P1L. Got minimum of 0.17% (other DHTs perform far better than this)

I may need to do further testing to find a better sweet spot for this tube...

Because it is not a DHT, it has much bigger cathode area than 4P1L, that means on low currents / high negative control grid voltages it looses linearity.
 
Having listened my 26 pre-amplifier for a while now, I must say that there is something unique on these DHT pre-amplifiers. Andy Evans was right indeed and it’s clear to me now, why he spent so many years refining this circuit.

“The Siberian” (4P1L DHT Pre-amp) suffered an amputation during last Christmas, as needed the gyrator board for my 45SET amplifier. In the end managed to build a couple of cascoded mosfet gyrators and decided to bring back to life the russian pre-amp. But curiosity took me through a different path yesterday, and removed the 4P1L and tried a couple of nice Ken-Rad 71a triodes.


After playing a while in my workbench, I found a low distortion operating point at about:

  • Anode voltage: 108V
  • Anode current: 18mA
  • Bias voltage: -10V

The pre-amp circuit has not changed from my previous designs. In fact, the gyrator now is composed of a cascoded depletion mosfet using the BSP129. These are easy to find here in the UK, and my SMD soldering skills have improved so why not use them? Their characteristics are quite similar to the DN2540s (VDS=240V, IDSS=350mA, PD=1.8W, Gfs=0.36S, Ciss=82pF and Coss=12pF).

As filament resistors I used NOS russian power wire-wound 51 ohm resistors which probably can handle 10W or more.
The pre-amp performs beautifully and distortion is only 0.02% at Vi=1Vrms.

M1 gets really hot dissipating more than 1W, but so far is performing well. You would probably want to use a DN2540 in TO-220 case here instead.

Last night I hooked up the pre-amp and listened to some classical music.

Impressions so far? Well, it sounds superb. Another good DHT valve to be added to the top ten chart. Bass is good, treble is clear and overall tone is warm and pleasant.

I added more pictures in my website here:

My Blog

Cheers,
Ale
 

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Having listened my 26 pre-amplifier for a while now, I must say that there is something unique on these DHT pre-amplifiers. Andy Evans was right indeed and it’s clear to me now, why he spent so many years refining this circuit.

“The Siberian” (4P1L DHT Pre-amp) suffered an amputation during last Christmas, as needed the gyrator board for my 45SET amplifier. In the end managed to build a couple of cascoded mosfet gyrators and decided to bring back to life the russian pre-amp. But curiosity took me through a different path yesterday, and removed the 4P1L and tried a couple of nice Ken-Rad 71a triodes.


After playing a while in my workbench, I found a low distortion operating point at about:

  • Anode voltage: 108V
  • Anode current: 18mA
  • Bias voltage: -10V

The pre-amp circuit has not changed from my previous designs. In fact, the gyrator now is composed of a cascoded depletion mosfet using the BSP129. These are easy to find here in the UK, and my SMD soldering skills have improved so why not use them? Their characteristics are quite similar to the DN2540s (VDS=240V, IDSS=350mA, PD=1.8W, Gfs=0.36S, Ciss=82pF and Coss=12pF).

As filament resistors I used NOS russian power wire-wound 51 ohm resistors which probably can handle 10W or more.
The pre-amp performs beautifully and distortion is only 0.02% at Vi=1Vrms.

M1 gets really hot dissipating more than 1W, but so far is performing well. You would probably want to use a DN2540 in TO-220 case here instead.

Last night I hooked up the pre-amp and listened to some classical music.

Impressions so far? Well, it sounds superb. Another good DHT valve to be added to the top ten chart. Bass is good, treble is clear and overall tone is warm and pleasant.

I added more pictures in my website here:

My Blog

Cheers,
Ale

Thanks for the update, seems we tread similiar waters, my original plan was a 71A output pre/headamp. The issue is the 71A has low gain and transductance compared to the 4p1L so I think with the 71A one would need a gain stage if using an opt. The 4p1l could be a spud SET head amp as it has just enough gain to drive grados. But the advantage is the lower voltage of the filament voltage with the 71A should lead to less heater noise in the signal. The 4p1l has much more power than the 71A, lower RA and higher gain. But some have told me the sound of the 71A is very good, I think it is the original output tube. Understand that finding speimens without damaged filaments can be difficult. I have wondered how much degradation there would be by adding a gain stage thinking #26-71A-SEOPT vs 4P1l-SEOPT.

Of course substiuting the mofset stage for the opt that issue goes away, still on the fence with little time to test these ideas.

Have you measured the THD of the 71A , curious if it is as low as the 4p1l?
 
Hi Regal,
I was surprised with the 71a myself. Andy Evans told me that he tried it without any preferences amongst the other DHTs.
THD figure is with soundcard input load, so will need to retake measurement with a 47/100K load for a more realistic measure. Either way, it's really good so far.

How much power do you need on a GRADO headphone? They are really efficient. I must look at my notes, I remember that it was loud for me with less than 20mW.

What OT are you planning to use with the 4P1L? Interested in looking at options as may end up building a spud SET with filament bias and a 4P1L....

Cheers,
Ale
 
Today I did some experiments back again with the 6P21S. This little chap is not microphonic at all. As per my previous post, it shows to be really linear. When did a listening test a couple of months ago, I remember it to be very clean and warm.

Starving the filaments provides an interesting impact on the harmonic composition. When lowering the filament current it changes the amplitude of H3 in comparison to H2. See attached diagrams. The minimum distortion is achieved at 440mA (THD=0.01%), however when increasing filament current to further 410mA, distortion increases again due to mix of harmonics, becoming more pentode like. At 450mA you get 0.06% but a richer harmonic mix....

All tests were done with the following setup:
  • Vsupply=170V
  • Anode load: cascoded mosfet gyrator (mu output)
  • Filament bias: filament resistor 10 ohms
  • Vi=1Vrms / Vo=9Vrms

This may be my next candidate to test as pre-amp now :D
Cheers,
Ale
 

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Administrator
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What means did you take to compensate for the shift in effective bias since you are using filament bias? When you vary the filament current the operating point of the tube also shifts by virtue of the change in filament derived bias. (Less filament current means less bias voltage as well)

I suspect it is very difficult to separate the effects of filament starvation from shifts in the OP when using filament bias. It would be very interesting to apply fixed grid bias and repeat the experiment.

As designed IMHO it would be impossible to separate the effects of the two variables that change as a result of shifting the filament current.

The results you show are very good however, and I also suspect you meant 410ma, 440mA, and 450mA in that order for the filament currents? (Can you clarify?) :D
 
Hi Alberto,
30sp tested on my workbench, but never got it a full listening test. A great triode, I'm actually helping out a friend building his version with a cascoded MOSFET mu follower gyrator and with filament bias...should sound really nice.
So far: 26, 4P1L, 71a, 6P21s, 10Y and 46 are my favourites

Most of DHT on filament bias are great, with some exceptions due to microphonics etc. I didn't quite like 112a and 01a

30 is an easy valve from a filament point of view. I starved the filaments down to 60mA with very good results
Cheers
Ale
 
Right - my first listening test with the 4P1L. I have them as a differential pair driving PP 2a3s. Right now I have 170v on the anodes with a plate choke made from the 90K side of the Hammond 124b and FT-3 coupling caps. OPTs are O-netics 6.6K. 600 ohm shared cathode resistor unbypassed with 13.6v on cathode, so about 11mA each tube. I should do something with the cathode arrangement like a negative supply - I have -57v available. Or a CCS. Slightly starved filament - 3.8v. Screen and grid 3 connected to anode

Anyway listening tests. It's a very detailed sound with good bass. But I have what is a real problem for me. I'm all about timbre - woody clarinets and bassoons, breathy flutes, real sounding French horns. I'm getting quite poor timbre - the strings are OK though not very silky smooth, but the woodwinds are nothing like my 300b SET with 46 inputs. Sound is rather thin and unmusical - woodwinds are only half recognisable as the authentic acoustic sounds I'm used to with DHTs. I've seen that Klaus said the same, like "Curves are nice, although my experience is not as rosy: i tried it with 60..80V on plate, battery bias, and the low-current versions (3..6mA) were thin-sounding. Increasing Ia to 15mA (Vg=-3V) added 'body', I suspect the tube likes generous currents" Baby of Camel - All DHT PP

So my question - what am I hearing here? Can others confirm or deny that that this is a bit thin sounding and lacks timbre? Is it possible I'm getting too much 3rd harmonic from starving the filaments. Any ideas? I want to try and rescue this project, but disappointed right now. My experience with the 46 has been really excellent timbre.

What can I try here?

andy
 
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Hi!

I didn't quite like 112a and 01a

I'm surprised that you didn't like the 01A. The 01a and UX201A are very special tubes. They are not the transparentest tubes but provide a little bit of sound processing which can create marvelous results. Especially with digital sources they can do very well. Some can be a bit microphonic but it is easy to find quiet ones

Best regards

Thomas
 
Right - my first listening test with the 4P1L. I have them as a differential pair driving PP 2a3s. Right now I have 170v on the anodes with a plate choke made from the 90K side of the Hammond 124b and FT-3 coupling caps. OPTs are O-netics 6.6K. 600 ohm shared cathode resistor unbypassed with 13.6v on cathode, so about 11mA each tube.

Andy, I am the first to call out the thin sound you are describing, I still haven't had time to set up the 4p1l headamp, but your comments are concerning me.

One thing is I planned a much higher current draw than 11 mA, I mean triodes always sound best to me at their limits. Have you tried 30-40 mA ?
 
Hi!



I'm surprised that you didn't like the 01A. The 01a and UX201A are very special tubes. They are not the transparentest tubes but provide a little bit of sound processing which can create marvelous results. Especially with digital sources they can do very well. Some can be a bit microphonic but it is easy to find quiet ones

Best regards

Thomas

Agree. Initial test weren't great. Now I'd been running for over a moth a pair of rainbow 01a which are very quiet and the sound is very warm. Like playing from CD so agree with your comments. Need to add a cathode follower stage to improve bass in my current setup. That is my next test as per my posts in the 26 thread
 
Andy,
You need to run them at 20mA or higher. I will post the curves with filament starvation in the evening.
I don't think you need to starve filaments in your case unless microphony is a problem. You will be getting high level of 3rd harmonics for sure at that current level.

Difficult to judge as you are not doing an apples to apples comparison. How hard will it be to breadboard a 46 stage to swap and listen to the difference?