Tag: gyrator PCB

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”

01a Gen2 Preamp Build from Barry French

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.”

01a Preamp (Gen2) Universal HT Supply

Introduction

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.

Design

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:

  • 6BY5G: a nice double-diode damper with lower filament requirements than the commonly used.  This is my first choice
  • EZ-80: the famous rectifier. This is what Andy used.
  • EY-91: you will need 2 valves as there is one diode per bottle.
  • 6C4P-EV:  a nice Russian small double kenotron rectifier. Very cheap alternative for European builders 
  • AZ1 / AZ11 mesh valves. Their sound is unique, however they are very expensive these days. You will have to add a pair of voltage dropping resistors to accommodate the lower filament voltage requirement. 

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!

 

 

 

 

 

CX-301a DHT Pre-amp Build from Malaysia

A fantastic build of the 01a preamp using the gyrator PCB from Cheah:

I have just completed my 01a preamp with STP3NK60ZFP output follower. The preamp sounded great !!

A great looking preamp using Rod Coleman’s regulators, output follower and V-Cap capacitors. The jFET is BF862. 

Well done Cheah!

SiC MOSFET Follower Driver

How many more times

Led Zeppelin wrote a fantastic song on their first album: how many more times. You may not be a rock fan, but hey: what a great song. How many more times do I want to get back to this “slew rate” theme? I don’t know, as much as I have to. Plenty of comments out there of bad designs with wimpy drivers attempting to take the 300B/2A3 or even 45 valves to full tilt with disappointing results. Either way, they always blame the valves.

I came back to revisit the driving of capacitive loads effectively as I’m working on a new 4P1L PSE amplifier. Slowly, but getting there. Previously I looked at adding a buffer to the 01a preamp as a result of slew rate limitations found in Tony’s implementation of this preamp.

buffer

 

 

The circuit design

Continue reading “SiC MOSFET Follower Driver”

Tony’s 01a Preamp

IMG_3291I 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.

Continue reading “Tony’s 01a Preamp”

Abusing the Gyrator Load

On my previous post, I covered my initial build work on the gyrator test mule using the gyrator PCB. I did all the lovely soldering work (which I do enjoy not like milling or drilling) and proceeded to do several tests.

Some interesting observations based on my abuse of the gyrator which yielded on several MOSFETs and JFET damaged as a result:

  • CCS reference: I used an external multi turn 5KΩ potentiometer via lead cables. I wired it incorrectly and that contributed to one of the initial faults. Be sure you look carefully on this if you use an external pot. if you use the on-board trimpot, this is not an issue.
  • JFET: this is the interesting one. If you want to run the lower JFET at very low biasing current for a larger jFET (e.g. J310) you will find that the JFET needs to operate close to cut-off voltage (somewhere between -2 and -6V). This VGS required will definitely forward bias the Zener protection diode D1 and prevent from reaching lower bias current (I found it about 10mA for J310). To resolve this you just need to add a back to back zener as shown below. This isn’t a problem for an BF862 or a 2SK170 as their cut-off voltages are quite small.
  • Failure: if you abuse the FETs, they will die. And if they die you will get a nice short across them and you will measure nearly HT at the mu output. Just replace the MOSFET and JFET (probably both are damaged)

Continue reading “Abusing the Gyrator Load”

45 SE Amplifier

IMG_8261Introduction

More than 5 years ago, I built a fantastic single-ended amp with the unique 45. The 45 has a distinguished tone and personality despite its mere 2W of output power. If you have high efficiency speakers, then it’s a great amplifier to build. With 2W you can enjoy music in a mid-sized room. You don’t need more, this amplifier performs at its best at low output levels and in particular when playing jazz or classical music.

The 45 Amp design

There are plenty of design circuits out there. I settled for a simple triode driver using a gyrator load. The choice was down to the 6J5 and 7193 (a military version of the 2C22). Both triodes are medium mu and sound really nice. Depending your needs, you may opt for a different driver (even pentode). However, they need to be able to drive the large voltage swing required by the 45. I’d go for a 6J52P, 6e5P, 6e6P, D3a or C3g these days. It all depends on your needs and available valves. The driver is biased at 7mA to provide enough grid current to avoid slew rate issues. An improved version would be to add a MOSFET follower to provide better performance under grid current. An example of a follower implementation can be found here.

45 SE AmpThe 45 is biased hot at 34mA/300V. The anode can handle 10W and this operating point provided best sound in my view. The OT is crucial, so invest as much money as you can afford. Rod Coleman regulators are needed to implement this amp without hum and the unwanted  inter-modulation effects.

IMG_2401I carried out several tests on the driver to find the sweet spot for minimum distortion and full swing. The driver is a hybrid mu-follower composed by the gyrator and the 6J5/7193 triode. The valve is biased by a set of 5 red LEDs to about 8V. I think I had a combination of a white LED and LED to provide 8V in my implementation. The dynamic resistance is minimum and won’t impact the performance of the stage.

I used the Sylvania metal-base 6J5 but then settled for the 7193 valves. They sounded better and I was quite pleased with the overall performance of the amplifier.

The amplifier design is very simple. With the gyrator PCB you can simplify the driver build and also use different valves to experiment with them. I originally didn’t have a PCB so I built my gyrator in a prototype board.
IMG_2403

45SE Amplifier upgraded with the 7193 drivers
45SE Amplifier upgraded with the 7193 drivers

I’d highly recommend you building this amplifier. If you want to experience the single-ended sound, then this is one of the amps to build. Of course you can go for higher power levels with a 2A3 or 300B, however, the sound of the 45 is unique. Worth trying