For more than 10 years I’ve been experimenting, designing and enjoying DHT pre-amps. Without over-dwelling on this subject, to me DHT preamps bring the unique sound transparency and detail we all seek for. They can make night or day difference in any system. And is not about gain, hence I’m focusing on sharing this blog entry.
Tag: DHT preamplifier
01a Preamp from South Korea
Time ago I made a set of gyrator boards for Chul in South Korea. His build skills are amazing and look at his work below. I love the structure and design, it’s simply beautiful.
ER801a Driver Stage
Received the ER801a today back from Thomas. They were working fine, it was a faulty UX-4 socket pin of my tracer which annoyingly made the grid to appear faulty and open at certain voltage level. I need to replace it.
Either way, they are now in my system and I can only say how much I was surprised with the sound of these. They take the detail and timbre of the VT-25 to a next level. Amazing result, happy to have these as part of my driver stage. The preamp is the 201a feeding the ER801a into the 4P1L PSE.
Highly recommended valves, worth the price indeed.
VT-25 DHT Preamplifer
VT-25 in action
Now I’m back from our long trip, I found some time to play with the “Mule“. I wanted to revisit my old VT-25 preamplifier. Many years ago I had my first VT-25/10 preamplifier which was based on a gyrator load. Then it morphed to a transformer coupled (LL1660/40mA) version to drive my TVC before I settled into the 4P1L for some long time.
The circuit design
The VT-25 has always been on my list of favourite DHTs. It’s gone ridiculously expensive these days and is hard to get. I have a couple of pairs in very good shape luckily.
C-299/CX-299 DHT Preamplifier
The start of a different DHT experience with the Mule
I built the “Mule” to provide enough flexibility to test other DHTs as pre-amplifier / line stage. Using the gyrator board, the flexibility is fantastic. Can share same HT and dial the right anode voltage. The LT supply can also be shared amongst many DHTs and Rod Coleman provided me with a set of different resistors to test the list of 9 or 10 DHTs I have in mind which haven’t listed carefully on this design.
The C-299
01a Preamp Gen2: Build Complete
Yesterday I started with the build of a new 01a preamp Gen2. I made some component changes during the build process thanks to Andy Evans who reminded me of the Russian FT-2 teflon capacitors. I had a pair left of 220nF FT-3 caps in stock!
The circuit is the same as the original preamp but with some component changes:
- C1 is 100nF/630V ClarityCap polypropylene
- MOSFET is DN2540 and jFET is BF862
- Rmu is 330Ω Kiwame
- Filament resistors are NOS Russian wirewound 51Ω/20W. I use a pair of them in parallel. Bias is about 5V.
- Filament bias using Rod Coleman v7 regulators. Set starved to 200mA
- The output caps are Russian NOS teflon FT-3 220nF / 600V. You can use a pair of FT-2 100nF alternatively.
The bias point is changed slightly up to 5V so the anode voltage is increased to 115V to get the 3mA of anode current. This time I’m using the BF862 which can be soldered in the gyrator PCB instead of the 2SK170. I preferred the sound and higher bandwidth as well as lower output impedance. The BF862 is a real winner as lower FET.
Here is a view of the preamp inside:
The heavy FT-3 caps are mounted on top of the gyrator PCB boards. The top anodised plate is 4mm thick and anodised. The teflon UX-4 sockets from Luciano Bandozzi (Jakeband) are mounted with silent blocks and Rod’s regulators are bolted to the top plate. you don’t have to as they dissipate very little power in this case.
How does it sound? Well, just played it for a couple of hours and I’m amazed with the subtle differences that the Russian wirewound resistors and output cap + BF862 can bring to this preamp. We did some listening tests recently with Andy Evans comparing filament resistors and these ones were real winners for both of us.
I hope it improves with time after breaks in a bit more.
01a Preamp Gen2: New Build
Yet, another 01a preamp. This time with a beautiful machined top plate and wooden frame:
Will try the Russian filament resistor which we tested at Andy Evans’ and sounded amazing. Just need to build a new pair of Rod Coleman Boards and do the wiring:
4P1L: pump up the current!
Background
I’m a firm believer than sharing knowledge and experience is the best way forward to continue learning yourself. It always pay pack at some point. This time Paul Prinz, a fellow implementer of the 3B7 DHT Preamp using the gyrator PCB, came back with a great suggestion. He found a MOSFET which could do high drain currents, it has high transconductance and most importantly the parasitic capacitances were low even close to the BF862. Hooray, I thought. We may have a great solution here to use the gyrator load for currents above 25mA and with similar performance to the great BF862. There are some other depletion MOSFETs that can do high currents, however they all have relatively high capacitances and low transconductances when VDS is low, like in the cascoded gyrator circuit.
The BSH111BK is an enhancement MOSFET, so doesn’t have a “depletion” behaviour like the jFETs. This isn’t a problem as the bias voltage can be set by the reference CCS.
For comparison, here is a brief summary of the key characteristics of these three devices:
| BF862 | BSH111BK | MMBFJ310L | |
| Ptot (W) | 0.3 | 0.3 | 0.225 |
| VDSmax (V) | 20 | 55 | 25 |
| VGS off (V) | -1.2 | -4 | |
| IDSS (mA) | 25 | 210 | 60 |
| Gfs (mS) | 45 | 640 | 18 |
| Ciss (pF) | 10 | 19.1 | 5 |
| Crss (pF) | 1.9 | 1.5 | 2.5 |
| Coss (pF) | 2.7 |
Tony’s 01a Preamp
I went to see my friend Tony today and helped him to fix his 01a preamp implementation. Time ago Tony used a prototype version of my gyrator PCB to build the Gen2 preamp with the addition of an output follower to address the slew rate limitations he had on his system due to the larger capacitive load.
Luckily we found the fault easily and it was a bad solder in one of the smoothing HT chokes. Once fault was rectified, we proceeded to take some measurements of this preamp.
2P29L – Preamp and driver for 4P1L PSE Amp
A very interesting Russian directly-heated pentode related to 4P1L is the 2P29L. It has a similar mu (μ=9), much higher anode resistance 2.8-3KΩ and transconductance of 3mA/V when triode-strapped. The filament requirements are much lower at 120mA. I picked one valve from my collection to submit it to the mercy of the curve tracer:
The triode curves are really nice:
This valve is as linear as the 4P1L (hooray). As a preamp it can be easily implemented like the 4P1L Gen2 preamp using a gyrator PCB which simplifies the building process:
Running it at 15mA and slightly above the recommended 160V achieve its lowest distortion.
We could also use this valve as a driver for a 4P1L preamp, which comes very handy for filament bias:
