All about electronic valves and hi-fi
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:
The additional machined top plates arrived yesterday. I assembled them this morning. Just need to assemble the chassis now before soldering!
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 🙂
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.
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 |
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:
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.
Barry French has recently build his version of the 01a Preamp Gen2. Here are a couple of pictures:
And Barry’s impressions:
“The 01a Amplifier is a stunner, personally I feel it leaves the 26 out in the cold, better top & bottom by a Country Mile, this was built using the Russian FT-3 Caps on the Output, Russian PIO Caps on the Boards with Jupiter 0.1 μf Wax/Oil Caps from B+ to Ground, the Power Supplies for both Filaments & B+ are from my original 26.”
I received many requests throughout this year of folks building the 01a preamp (Gen2) asking for advice on the HT supply design. Well, I have my own incarnation which serves multiple purposes as is a shared supply, therefore isn’t useful to anyone.
My friend Andy Evans came up with a supply using available components. This is exactly what you are looking for the HT supply. I made some tweaks to Andy’s design, so all credit should go to him.
The HT supply design is very simple. It’s mainly a choke-input valve rectifier supply. It has an additional LC smoothing stage. Here is the high-level circuit, see some notes
The transformer is a 250-0-250V / 50VA with an output current of 60mA. It has two windings for 6.3V AC heaters, but you only need one. Here is a great choice from a recommended seller.
The valve rectifier is a double-diode damper. Of course you can use some other options, but I like the sound of them. Here are some you might want to consider in your build:
For the indirectly heated diodes, it will be better to connect one end of the filaments to the cathode.
The chokes are commonly available. These are from Hammond (155J) and have 15H @ 30mA. The downside is the high resistance, over 1kΩ. This isn’t an issue here as the current consumption is low so the voltage drop is minimal. With a choke input supply you need a minimum current to operate. In this case is about 15-16mA so a bleeder resistor is needed (29kΩ 5W wire wound) .
C1 serves to equalise the output voltage. It pushes the supply to operate a bit more like cap-input (hybrid) by increasing the output voltage. I use it to tune the output voltage to 200V.
C2 and C3 are classic motor run capacitors. I personally use ASC Oil ones 450VAC rated. You can choose what you can get hold. You can use any good quality film capacitors. I like the WIMA DC-LINK ones, they are great.
The output ripple is about 7mV. This isn’t a problem as the gyrator load has a very high supply rejection (PSRR) so no need to go crazy on this. If you
The design is so simple that anyone should be able to build this easily.
Hope it works for you.
Merry Christmas!
I’m now back in business. Building a new 4P1L PSE output stage so will reuse the 300B/4P1L Flexible PSU. I never managed to post an update on the troubleshooting I had to do to get this HT PSU to its optimal state.
The output voltage was lower than expected and the 50Hz component extremely high. Something was wrong. So I traced the issue down. I found a bad solder in one of the rectifier’s cathode. The supply was operating in half-wave mode.
The supply is choke-input with 6AU7 rectifiers (hybrid bridge with FRED rectifiers). The transformer is custom made and has multi-taps for 300-400 and 500V. The tuning capacitor for the choke input is 470μF, then choke is 2.5H into 50uF oil cap. The filtering stage per channel is 20H + 100μF Oil caps;
Here’s a test of the supply at 330V/60mA per channel. It’s very rewarding now to see no 50Hz component and that the ripple at 100Hz is just 4mV (ignore the mA typo on the image):
