Fixed Bias Board

Long time ago I built a series variable voltage regulator for 600V. It worked flawlessly and survived all sorts of abuses as is on my bench HT supply.

With the same circuit design, I developed the final stackable PCB (see previous post here) with this regulator:

Looks more complicated than it is. The single-supply Op Amp (LM358) needs a low voltage supply. I derived this from a simple CCS (DN2540) and a pair of 12V Zener diodes. The voltage reference is the famous TL431A and with P1 you can adjust the output voltage. The feedback resistor pair (R14 and R15) senses the output. C6 is for frequency compensation. The MOSFETs used are ST3LN80K5 which have built in protection Zener diodes, so none of the ones shown in the diagram are actually needed. T4 provides current protection to the pass device T2.

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300B SE Amp: build part VII (D3a driver)

It didn’t take long (or at least as long as I thought it would) to finish the driver board. It has a D3a hybrid mu-follower with SiC cathode bias arrangement:

The board is mounted on top of this previous board.

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300B SE Amp – here we go

A Brave New World

Surely you’re as tired as I’m with COVID-19. One of the best things I can do to distract my mind is to keep myself away from social media. Every stone you turn, there is COVID or a statement about it. I won’t moan as I have a job for now and a healthy family. Some members of my family were infected but nothing major. I can only say is that the world has change. And so my day to day life looking after the young family whilst working is a real challenge. Starting my fifth week of lockdown, I have to distract somehow my mind at times, otherwise will go mad.

A New Concept

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300B Amps: an early breadboard

Browsing my schematic archive I found this early implementation from some years ago on the 300B:

The amp design is straight forward. Let’s start from the output stage. The 300B is run hot at 33W (376V/90mA) with a fixed bias of about -78V. I used a pair of LL1623/90mA OPTs which I had wired on 3K:8 mode.

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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

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Situbes digital panel meter review

I’m a heavy user of fixed-bias output stages. Yes, I do prefer them despite the additional complexity. However, I’m not looking to open a can of worms around this subject. On the contrary, I wanted to report a fantastic product developed by Situbes.

SiDPM Digital “Panel” Meter

Here is the brief description from the website. I suggest you take a look at the datasheet as well:

The SiTubes DPM is a digital “panel” meter packaged in a standard octal tube envelope.

It measures a DC input voltage from 0 to 2V full-scale. Several selectable legends (V, mA, A, etc.) can be selected by an external programming resistor. It is particularly useful to measure supply voltages (such as plate voltage) and tube bias levels (normally plate current) in tube amplifiers.

The DPM can be powered from an AC or DC voltage. The power input is isolated from the measurement input, so floating measurements can be made up to 1500V above or below the power input. This allows high-side current measurements – for example, sensing plate current directly at the tube plate – or the ability to be powered from a supply that is not referred to ground.

My review

Looking at their construction you will realise instantly the high-quality of this product. Impressive finish and presentation. The tube is made of glass and fits very tightly to the 8-pin plastic base.

I did a simple and basic test on my work bench to test this device and its accuracy. In 5 minutes I wired it on my curve tracer to access the pins easily without soldering a test rig. With a 1Ω 1% resistor I configured the device to current mode and placed my 5½ digit bench meter in series for reference. The refresh cycle is very good, more than what you’d need in normal operation. Accuracy with the reference resistor was great. It’s calibrated as provided by the seller and error was below 1mA up about 200mA which is the planned use case for me. The OLED display has the right brightness for day operation. It’s just great.

You can use them easily to measure anode/cathode current, grid bias or supply voltages in multiple configurations.

They are pricey, but worth every penny. A top quality product which I’m keen to use shortly in one of my next builds.

Fixed bias regulator

Testing the Rod Coleman Fixed Regulator

As part of my 300B SE project design, I looked into various fixed bias arrangements and regulators. Rod Coleman has developed another fantastic circuit after the success of his DHT filament regulator which is now the preferred filament supply kit within the DIY audio community. After many years of refining the DC filament regulator, Rod came up with a clever design for fixed bias using the same concept: a gyrator and a temperature-compensated CCS. Instead of feeding a current through the DHT filaments, in this case the current is used to generate a clean bias voltage across a “bias resistor”. The bias resistor is bypassed by a capacitor as the high impedance loop formed by the regulator and the bias resistor is sensitive to pick up HF noise which could be amplified by our system.

The kit is of the same quality you would expect from Rod’s boards and very easy to build. It takes less than an hour to build the boards:

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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.