London Audio Circle Meeting – June 2013

I arrived late due to some home duties I couldn’t get away from, but just in time for a proper listening session to Tom Browne’s “Sunrise” digital phono project. A superb achievement so far. Jon’s setup is pleasant as fantastic is to see him re-routing cables as his system were a maze (which I personally think it is!). I had a great time listening to different setups of RIAA stages, step up transformers, ADC/DACs and the joy of music and friends.

Better to read (un-biased?) opinions from other members of the group:

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Improving the curve tracer

After nearly 18 months of using the curve tracer, I guess that everyone would find ways of improving any testing equipment. I personally traced so many valves on this curve tracer that I already know all its limitations, advantages, what it likes, what it doesn’t like, and all. Just like a pet (or a partner) or whatever analogy you would like to make.

Tracing anode current has always been a real challenge. I opted for sensing the cathode current as it’s easier to implement. For low current valves (or high gm ones) I have a set of cathode resistors to avoid any significant impact on the bias point.

Obviously sensing anode is the right way to go, but that is a hard challenge if you are looking at just using sand. A transformer couple version is fine, but not as easy as the op amp in the cathode.
Generally you would like to sense screen current and also grid current if you are looking to create some SPICE models for the valves you are planning to use.

Grid current in A2 mode plots are scarce. They are only available for transmitting valves. If you want to use 2A3, 300B or 45 in A2 you won’t find those traces around. Hence, your SPICE model will lack of accurate response in A2, unless you are able to plot the A2 grid current of course.

Here is the new circuit based on the AD8479 which will allow 600V in common mode. This op amp will help us to easily sense not just anode, but also screen and even grid current if necessary.
The AD8470 stage will convert the current across the sense resistor. The gain stage is provided by the INA122. A set of preset can be arranged to provide x10 or x100 or whatever gain you want to implement that will fit your oscilloscope Y input.

I will need to breadboard this and report some results…
Ale

Lenco GL75 plinth

Last week I received a fantastic job made by Carl for my Lenco turntable project. A lovely plinth designed to fit a Lenco GL75 top plate and a Rega-like arm mounted on a hardwood tone arm board. The plinth is made to order and Carl made a very neat work as you can see on the pictures.

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307a with schade feedback (Part II)

Some time ago I traced the 307a that kindly Vegard Winge sent me from Norway. As suggested by Vegard, I re-traced them with lower screen voltage and increased feedback to improve the linearity of the 307a in this mode.

The modification to the curve tracer is simple. A simple resistor divider (R1, R2) provides the feedback to the grid. The grid driver provides a constant impedance so works fine. With a potentiometer you can adjust the feedback ratio and look at the impact on the curves. Very handy!

With 15.5% feedback and 125V screen voltage I got the best curves as shown below:

Nice to see the anode resistance coming down to 1kΩ.I tried matching a triode model to these curves with a good result

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Transconductance tester

Introduction

For some time I’ve been postponing the conclusion of a half-finished project. This is one of the many projects that I have around as many of you, but it was time to complete it as just some minor bits were outstanding.

My interest in measuring valve transconductance was very high since my early days of involvement with hollow state technology. The old valve tester I acquired didn’t measure it, I tried many ways to measuring it with different methods until I settled with using a CCS load and an AC meter as described here.

The problem I found though was that my true RMS AC meter in low scale (i.e. 100mV AC) didn’t like a significant DC voltage drop across the sensing resistor. Not sure why, but either way I wasn’t happy either without decoupling the anode to the sensing part of the circuit when using high voltages.

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Fixed-bias supply

Playing with various SE designs recently I got into refining a bit the fixed bias circuit. The above version, which is still under revision, is a good improvement to what I used before in my 45SE. The raw supply doesn’t need a large filtering stage. In fact, the capacitor multiplier (formed by Q1, R1 and C1) provides about 62dB noise rejection at 100Hz. This is about x1000 reduction factor!. If you consider that bias current is less than 2mA, you can arrive at your own conclusions regarding the filtering required. The bias voltage is set by the divider formed by R2, R3 and the potentiometer. I like using the 5T/10T ones. In this design, about 6V of bias span is provided. D2 provides a way to avoid C2 to discharge in case of an interruption of power and expose the output valves to current surges (and potential damage). C2 helps stabilising the FET voltage. With the BS170 biased at 1mA approx gives an output impedance of 50Ω. If higher transconductance FETs are used, then impedance can be lowered down to 5-8Ω (e.g. if using the IRF610) but you need to pump up the bias current.

4P1L Push-pull Amplifier

Shiny Eyes by Radu C. Tarta

This is a fantastic 4P1L push pull incarnation done by Radu. I hope you enjoy reading this post as much as I did. Cheers, Ale

Introduction

“Shiny Eyes” Push-Pull tube amplifier is the result of 6 months of research, experimentation and lots of music listening. It started in the summer of 2012 when life opportunity re-opened the door of an old passion: electron tubes. However, this time my goals were directed towards audio. I understood from the beginning that after 30 years of interruption I have to dedicate a great amount of time and energy to catch up with technology and ideas. I want to thank my wife Paula for her emotional support and understanding. I am also grateful to the fanatics from the DIYaudio community for their contribution to the world of tube audio experimentation. Personal thanks to Kevin Carter of K&K Audio for his advises and help, to Ale Moglia of Bartola Valves for his inspiring work and to Rod Coleman for his excellent filament regulators.

Initial tests

Here are few images taken during design, experimentation and final building: Early stages…

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4-65a SE Amp: Building the OTs

More work done this afternoon. Built the LL9202/100mA OTs in their supporting structure and fitted the crowbar protection boards for the primary windings. Also added the speaker posts. A heavy part of the amp!

LL9202/100mA with crowbars

Crowbar protection for primary winding

Crowbar fitted on the back of the OTs

Module completed

Speaker binding posts

Crowbars and LL9202/100mA

Crowbar protection

Protecting your valuable output transformer

When using fixed-bias you don’t want to take any risks and damage your output transformer. What happens if your output valve (e.g. a transmitting power valve) makes false contact at the grid or the bias supply fails to start for whatever reason. Well, your valve will conduct fully and probably melting the anode and if not damaging the primary winding of your OT. Both things are catastrophic and you don’t want to try it…too expensive!

You may think there is a simple solution: a fuse. We all know fuses are inaccurate and can be dangerous if they don’t blow properly. Also HT fuses are expensive and they don’t come up with a wide range of values.

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