Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

4P1L PSE Amp Finished!

I’ve been travelling a lot lately so haven’t had the chance to update on this project. A couple of weeks ago I finished the 4P1L PSE Amp:

4P1L PSE Amp

The amp is outstanding, just like previous incarnations and tests I have conducted over the years.  The level of detail and tone is unique. This is what I always loved from the 4P1L. I’m running it very hot (70mA per pair) and the output transformer is Amorphous Core 3K2 (more detail to be shared soon). It’s a simple stage with filament bias, so no cathode capacitor. The filaments are wired in series to reduce the heat dissipation. Despite this adds a bit more on the output impedance, the bass is powerful. I’m very surprised with the bass, but the level of treble is amazing. It goes over 40-50kHz, I will still need to undertake the classic measurements but so far is great!

It’s absolutely dead quiet. No traces of hum. 

Some more pictures below:

Glowing 4P1L and filament resistor stack!

And the full system below:

Gyrator PCB board updated (Rev06)

After some further testing and prototyping, I’ve updated the gyrator board PCB to provide additional protection to the lower FET device with:

  1. Protection Zener (D3) between drain and source (through-hole)
  2. Back to back protection Zeners (D1 and D2) between gate and source to ensure positive gate bias for higher currents on jFETs and use of enhancement MOSFET

Layout was carefully adapted to ensure track separation due to HV in place. Result is that the new gyrator board provides all protection needed on the lower device and simplifies the build process

 

 

Here is an example of a completed board tested:

Gyrator Board Rev06

 

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:

  1. C1 is 100nF/630V ClarityCap polypropylene 
  2. MOSFET is DN2540 and jFET is BF862
  3. Rmu is 330Ω Kiwame
  4. Filament resistors are NOS Russian wirewound 51Ω/20W. I use a pair of them in parallel. Bias is about 5V. 
  5. Filament bias using Rod Coleman v7 regulators. Set starved to 200mA
  6. 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.

 

4P1L PSE Amp: assembling top plates

The additional machined top plates arrived yesterday. I assembled them this morning. Just need to assemble the chassis now before soldering!

HT and LT connectors
Rod Coleman’s boards
Filament resistors
Filament resistors
Speaker posts
Amorphous OPT and plate

DHT preamp “The Mule”

The birth of the Mule

The name I guess says it all. This is yet another DHT preamp with the gyrator PCB. So what’s different? Simply, a breadboard DHT preamp module ready to be abused.  I’m planning to mod this to death and try a long list of other DHTs with the gyrator load. 

I will only need to change the valve sockets (or build an adaptor) as well as the filament resistors and Rod Coleman filament regulators. Simple changes which can be done fast, will open the door to quick tests on my system.

In order to make this simple and a rapid build, I opted to use an IKEA chopping board. These are made of a laminated hardwood and are dirt cheap. A couple of hours are required to drill all the board like this:

Job done. You only need to do this once. Here is another look at the half-build Mule:

The initial sockets are NOS short pin UX-4/UV-4. I will play around with the 01a before I move to other DHTs. I still need to add the tag strips for filament resistors, output capacitors and the filament regulators. 

Wiring will take a couple of hours and we should have another DHT amp to play with 🙂

 

46 driving 45 – SE Amp

My favourite valves together

Recently I revisited a beloved amp, the SE 45. This time I will share a more orthodox design without sand in play. Surprised? Well, I love lots of iron as well and here is a design I’ve been playing around for some time as I have all the components at hand. 

Driving the 45

Continue reading “46 driving 45 – SE Amp”

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  

Continue reading “4P1L: pump up the current!”

3B7 DHT Preamplifier

An enthusiast blog reader (Paul Prinz) implemented a nice version of the DHT preamplifier using the gyrator PCB but for the 3B7 triode. The 3B7 triode has a pair of DHT triodes on the same bottle. It has a high mu for a DHT (about 20) but with that it comes the higher anode resistance. This was a drawback when implementing a choke or transformer loaded stage due to the high anode resistance (there is no free lunch am afraid). However, with the mu-stage, this doesn’t become an issue and we can get the most out of this valve using the gyrator load. 

Although I tried the 3B7 in the past, I proceeded to get it out from my valve stash and trace it again. Here is a nice set of a Sylvania military NOS one:

Continue reading “3B7 DHT Preamplifier”

4P1L PSE project started

 A new amplifier build has begun. The 4P1L PSE is coming to life. First top plate developed and arrived this morning. A succesful layout for being the first one. Need to submit for machining the other 2 panels. It will be modular so I can make changes and experiment with different output transformers I have at hand (Monolith Magnetics, Lundahls, etc.). The HT power supply is ready and I’ve been fitting the filament supplies in child-proof boxes over the Christmas holidays. It will take me some time though to get this one up and running. Not much time to work on it am afraid.