Building gyrator boards

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I’ve been on some business travel so haven’t had much time to work on stuff, however I did get a set of gyrator boards for a friend and a customer:

  1. BF862 configured for 4P1L preamp
  2. 2SK170 configured for 01a preamp

4P1L preamp with BF862 gyrator

Many have asked me about this preamp with gyrator load. Here is the latest implementation which I preferred most in terms of sound. The mu resistor is 470Ω which is a nice compromise between BF862 transconductance and distortion. I adjusted it on test. I use a 100nF for C1 so R6 is 10MΩ. R4 can be either 300KΩ, 330KΩ or even 390KΩ. Difference would be only on the voltage range for the CCS. I found running it at 25mA to be perfectly fine, some BF862 can even do J310. I prefer this SMD compared to the J310. It performs much better even at high frequency:

4p1l-preamp-gen1b-gyrator-pcb-detailed

 

4P1L gyrator boards

4P1L gyrator boards

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Tony’s 01a Preamp

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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.

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27/56 Preamp from Jose Martins

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Jose Martins sent me an email with his recent built on the 27/56 preamplifier using the gyrator load and these PCBs. I recently posted an idea using the lovely 27 IHT valve here.

Here is a picture of the finished preamplifier:

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Abusing the Gyrator Load

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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)

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Gyrator load – test mule

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I’ve done several tests using a simple gyrator PCB test mule. It was time to build a proper and flexible test mule for extreme abuse:

  1. 2 Boards for current flexibility
  2. Board 1: BF862
  3. Board 2: J310
  4. Top FET is IXTP08N100D2 for 1000V operation
  5. ZIF socket pins for CCS reference resistor and RMu. This will give the necessary flexibility to try any combination in the gyrator depending on the triode and or the power supply
  6. External pots for ease regulation of anode voltage

test muleThe top MOSFETs are bolted on the aluminium case which will act as heatsink. For tests this should be sufficient.

The top plate of the case looks like this:

test mule1

There are 4mm posts are for HT supply, GND, mu-output, Anode. There are also a pair of 2mm posts for current sensing per board.

Some further soldering to do and job done!

DvB (IN1166) input transformer test

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IN1166 DUT

IN1166 DUT

This is a long overdue post, as I have carried out these tests time ago but never got around to publish the results. Dorin from DvB transformers sent me a pair of his transformers for testing. These are great devices for amp input stages, particularly for a nice Spud amp using medium mu output stages with 6P15P or 6e5P for example.  Either PP or SE output, this will give you a great solution for a minimalistic design: Continue reading

45 SE Amplifier

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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