AF Amplifier

OK - I've got the output transformer, I've got 250V, now its time to make the AF stage of my new valve receiver project.

Here is Harry sm0vpo's circuit...

Fortunately I am a kleptomaniac - I found an EL84 amidst my mountain of junk (along with the other tubes Harry used in his receiver) and some B9 valve bases. Talking of bases, here's the "pin-out" of the EL84...

At this point, things start to get tricky - you can't slap a valve circuit together quite as easily as a transistor equivalent - there is much more "mechanical engineering" involved.

The first stage of mechanical engineering yielded a working amplifier soon enough...

Here's a close up of the EL84 in action for all you tube fetishists out there...

Now for the second stage of mechanical engineering - which requires tedious chain drilling and filing [Dear Santa, please can I have a 3/4 inch chassis punch for Christmas? Your elves might like to know that a suitable model is available at CPC (order code ME18313) at the very reasonable price of £6.57. If you want to get me a 25mm one too (order code ME18317) I would be very pleased. p.s. I have been a very good boy all year.].

Here's the result...

The AF amplifier looks lonely at the far end of the Aluminium Chassis (Maplin code: XB68Y) with those empty valve bases waiting for more development on the project...

The amplifier works well enough - I'm using it to listen to the Advent Carol Service from St John's Cambridge on Radio 3 as I write this.

Next step will be the AF pre-amplifier and BFO - watch this space!

...-.- de m0xpd

HT Supply for Valve Projects

Disclaimer: This blog mentions dangerously high voltages. Bear in mind that you might get frightened or worse if you are incompetent, careless or unlucky - consider yourself warned!


I spent last week on a business trip to HB-Land. Took along my Eton E5, hoping to listen to some CW as "background music" but both 80 and 40m were silent. Don't know if my hotel was working as a Faraday Cage or if Swiss, French, Italian and other hams were on strike!

Instead of listening, thoughts turned to planning what I could build on returning home...

The next item on my list for the "Valve receiver" Project is the HT supply. I have a conventional power supply for my Paraset (made of an old Tektronic 'scope transformer) - but this kicks out nearly 400V with no load and I wanted something tamer.

I had also experimented with an inverter (after the circuit published by Harry sm0vpo and others). This worked well enough, but not with all transformers tried. I guess the transformer has to present inductance in the right "ball park" to get the astable to run in such a way as to make the entire system efficient.

The original inverter is shown below, along with its "favourite" transformer...

Whilst sniffing around the 'net, I found a circuit for an inverter in which the transformer is not a part of the timing of the astable - it should make the circuit less fussy about the transformer it is used with!

My circuit is seen below (I claim no novelty - all I've done is added the inverted stages to sharpen up the voltages driving the power transistors switching the transformer secondaries).

The (very) ugly version, made using "Dremel PCB" methods on a pre-owned piece of copper-clad, is shown here...

It works - with any transformer in my junk box - so now it is time to play with the audio stage... I wonder if I can find an EL84.

...-.- de m0xpd

Poor Man's Valve Output Transformer

I’m sure you know how it is. I’m sure you’ve been there. You’ve had an idea and – despite all indications to the contrary, despite KNOWING it is a foolish scheme – you just can’t stop yourself. So it is with me…

Guess it started with talking to Ian g3roo at Rishworth and watching him operate his B2. It wasn’t helped by talking to Johnny sm7ucz and drooling over his beautiful work on (another) replica Paraset. Things got serious when I joined the B2Spy yahoo group and saw photos and schematics of the B2. It got much worse last week when I started chatting with Dom, m1kta, and even downloaded some schematics.

It wasn’t that intended to build a B2 – no – I have enough instinct for self-preservation left to avoid that! I am, however, inexorably drawn towards another VALVE PROJECT!*?*!*! Like a beginner on a ski slope, I’m slipping towards the madness and inconvenience of those charming anachronisms – tubes!

Sure, I’ve built a replica Paraset (indeed, seeing Ian’s B2 has prompted me to have a few more QSOs on my Paraset). Sure, I have a nice collection of valve audio power amplifiers (including the inevitable Quads and – my personal favourite – the Beam Echo / Avantic DL7-35s). Sure, I run an old Fender Pro Reverb amplifier for my guitars and Fender-Rhodes piano. But – to be honest – I’m not really on home turf with tubes. Despite being a classmate of Morgan Jones for a while, I’ve only ever dabbled. So, now, it is time to really get my feet wet…

Whilst toying with the idea of (NOT) building a B2, I asked myself what it was about that foolish notion that really interested me. I realized quickly that it is the idea of playing with the receiver side, rather than the transmitter (the B2 has a much more sophisticated Rx than the Paraset). So, rather than condemn myself to the task of building a B2 Rx, I looked around for something more practical that would satisfy my cravings…

Harry Lythall, sm0vpo, has a nice design for an "Old School" General Coverage receiver that would suit my needs – most obviously by being valve-based. Also, importantly, it is a superhet.

As well as being a bit of a tyro with valves, I have absolutely no practical experience of superhets. Sure, I own some old radios which are superhets, including an AR88 to which I moor my GP14 dinghy (HI HI). However, all my HB efforts to date have been direct conversion devices (discounting the perverse goings on inside the computer in my SDR radios). It is time to enter the murky world of IF transformers (I can’t wait to make a “Wobbulator”) and Harry’s design looks like a good learning opportunity. I made the first steps this afternoon…

Starting with the AF stage, I hit on an immediate problem – the output transformer. I’m going to follow Harry’s advice and use a little mains transformer, having one on hand (which I got from Maplin, along with its smaller cousin – the smaller cousin is seeing service as the AF output choke in my Paraset and the donor transformer in question (seen below) was lying around in a junk box, not realizing it was about to be transformed into an AF output device).

Harry explains how it is desirable to strip down the transformer to break open the magnetic circuit, in order to avoid the d.c. current through the primary (the anode current) saturating the transformer. I stripped down the donor transformer – you can see immediately why the core laminations are called “E” and “I”.

Harry counsels using a cheap transformer, in which the laminations won’t be lacquered together. Unfortunately (and totally out-of-character) I had paid way too much money for mine and so disassembly involved a Stanley knife. Anybody foolish enough to follow me down this path will have ample opportunity for cutting themselves as preparation for the nasty HT shocks to follow HI HI. You can see the donor device stripped down to component parts below…

Putting all the “E”s into the “coil” from one side is easy and what will be the top of the frame (once the device is the right way up) holds the “I”s in place while you get ready for the next stage.

The Es and Is are isolated by what Harry calls a “thin sheet of typing paper” – you can see my paper in the frame in the picture below. Also, I’ve put some insulating tape around the Es to guard against mechanical rattles.

The re-assembled device is seen here…

I even left the QC sticker on – just in case I need to take it back to Maplin’s!

Now let’s see what this puppy does…

The aim is to use an ordinary loudspeaker, with its low impedance, as the anode load in a common-cathode valve stage. To do this, the transformer must transform the impedance of the speaker to a much higher value.

I measured the impedance, looking into the primary of the new transformer, with the loudspeaker loading the secondary. The set-up was just a power amplifier driving the transformer in series with a 100 Ohm resistor. I could measure the voltage over the transformer primary and the voltage over the resistor (and so calculate the [magnitude of the] impedance).

The measured impedance is shown in the graph – three traces are visible; one for the loudspeaker driver directly, one for the loudspeaker connected over all the (center-tapped) secondary and one for the loudspeaker connected over half the secondary.

The loudspeaker alone shows the expected 8 Ohm nominal impedance, a peak at its mechanical resonance and the beginnings of a lift due to coil inductance at high frequency. The transformer coupled impedances are much higher – but the resonance peak of the loudspeaker is still visible in both cases.

I seem to have here the beginnings of a viable output coupling transformer for the AF stage – I’ll let you know how I get on with it!

…-.- de m0xpd

Step Attenuators for Cheapskates

I always had a soft spot for step attenuators. They are, however, notoriously expensive and my pockets are notoriously shallow!

Imagine my surprise when I was fortunate to pick up a nice unit made by Advance on eBay last year for a song…

OK – so the operating impedance is 600 Ohms – but I work in acoustics and audio and a 600 Ohm unit is still useful for AF work. What's more, it is a precision instrument and it is built like a battleship - in fact it is painted like a battleship!

Modern equipment tends to operate at higher input impedances than the “old school” 600 Ohms, so I set to work making a 10k unit, which became known affectionately as “dial-a-dB”…

Those awfully nice people at Maplin only stocked 2 pole 6-way rotary switches, so “dial-a-dB” was built in “Base 6” with six unit dB steps 0 – 5 and six dB steps 0 – 30., but it has served well in the lab. This attenuator also had a switchable internal 10k terminating resistor (just in case it was operated into a load much higher than 10k, in which case the dBs wouldn’t be as dialed).

My new-found interest in RF was crying out for a 50 Ohm version – but I was reluctant to make a copy of the “dial-a-dB” concept as the rotary switches weren’t going to work (interested readers can learn about the layout implications of step attenuators at RF by reading the ARRL Handbook (2009 edition, page 19.44) etc).

I was stuck – I am just too tight-fisted to shell out a pound a throw for some 2-pole change over switches, as I would need at least 6. Imagine my delight when I stumbled across a bag of 18 suitable push-button switches at a recent rally at the entirely reasonable price of a pound!

A moment’s work on Eagle produced a repeating pattern of the fundamental “pi” attenuator network...

You can either copy a set of resistor values from the schematic in the ARRL Handbook (op.cit.) or you can brew up your own values for custom attenuations (and operating impedances) using an online calculator.

Alan Yates has an ideal calculator on his excellent website here (look for “Resistive pads”) . I chose to build a six-section attenuator with 1, 1, 3, 5, 10 & 20 dB pads, giving every integer dB attenuation between 0 and 40. Of course the attenuation is only as good as the resistor accuracy (I chose nearest E12 value – which Alan’s calculate automatically generates) but – hey – this is only supposed to be a hobby!

I’m now the proud father of a pair of switched attenuators - all that remains is to encapsulate them inside a box with some appropriate connectors at either end.

The "dial-a-dB" name was both descriptive and alliterative - I'm looking for a similarly potent tag for the new model. Best I've come up with to date is "punch-a-pad". If you can do better, please let me know!

...-.- de m0xpd

The m0xpd keyer

I guess it all started late in 2008, when my wife decided (/agreed/was persuaded) to buy me a Bencher paddle for Christmas. I had to make a keyer (which, for those who may know as little as I did at the time, is a piece of electronics to interpret the state of the paddles’ two push-to-make switches into a useful series of dits and dahs).

Fortunately, Eamon “Ed” Skelton, ei9gq, had just published a simple design using a PIC microcontroller in RadCom (Vol 84, No 12), so I was laughing. The same design is featured on Ed’s own pages. All I had to do was make a few mods (I wanted, for example, to be able to interface both a paddle and a straight key to the same device) and I was ready to go.

My schematic is shown below…

Fortunately, I know PICs pretty well and had the ability to modify Ed’s code and burn it into a chip (using my PICStart programmer).

The keyer works just fine (thanks Ed) and I boxed it up with an internal piezo sounder to use with rigs without sidetone and a switching relay to key valve rigs for which the open collector transistor wasn’t going to work. That little piezo sounder got me thinking…

We all spend a lot of time worrying about the spectral purity of our RF emissions and thinking about the way in which we key the output stage of our CW rigs. A sudden switch-on of a pure tone generates other frequency components. That is just as true at audio frequency as it is at RF. So – when we switch a sidetone generator, we really ought to apply a carefully designed envelope to the signal, rather than just switch it on and off. This same method is used in audiometers used to test hearing – pure tones are presented to the patient at different levels and they are gated by an exponential envelope, rather than crudely switched on and off, in order that hearing is tested strictly one frequency at a time!

A little search on the ‘net soon tracked down a sidetone generator circuit by G Forrest Cook, wb0rio, which is presented as a “Smooth Tone Clickless CW Sidetone Generator” – exactly what I wanted! You can find the original here .

The circuit works by making a pretty pure sinusoidal tone (produced by filtering a triangular wave) which is then passed through a voltage-controlled amplifier. Rather than crudely switching the waveform “on” and “off”, the VCA scales the tone with a nice exponential “attack” and “release”. I made a Spice simulation of the relevant part of the circuit to understand how it operates – you can see the result below.

Now I couldn’t resist building a keyer with “proper” integral sidetone oscillator and so I pulled together a PCB in Eagle – you can see my prototype below.


I now use the keyer / sidetone combination in all my H/B rigs. It is built into my “Funster PLUS” (post to follow) and it is particularly useful in keying my SDR rig – as there is an 80msec delay between pressing the key and hearing the sidetone on my SDR set-up otherwise. 80msec might not sound much to you – but it is enough to mess up even my lousy CW!

…-.- de m0xpd

USB Synthesizer

Well, this is my first outing into the Blogging game (I am a notorious late-adopter) so apologies for all errors, omissions, etc...

I got my USB synth kit from Jan, G0BBL, at the G-QRP Rishworth mini-convention.

The kit flew together without any hitches, thanks in part to my illuminated magnifier, a nice new Weller soldering station (birthday present) and some 0.5mm solder.

Here's an image of my completed module...


Honestly, if somebody as cack-handed as I can put one of these together, anybody can!

After a few quick tests, I hooked up the module to my SoftRock RxTx. I can't figure why the instructions indicate a connection between the PTT lines - seems to work fine without it!

Within moments I had first QSO with Rog, gw3uep, with the rig in a hell of a mess on the bench...

Those with 20/20 vision will see (from right to left) one of the soundcards necessary to get the whole show on the road, the USB Synth module, the SoftRock (in box with lid removed) and a piece of chipboard with my keyer, a HexFET PA and a bandpass filter.

The PA was producing about 10 obscenely QRO Watts and the keyer will be the subject of future post (always assuming my enthusiasm for this project endures).

Apple Dynamics was my old company - and the nameplate on the shelf is a piece of nostalgia!

You can get more information about the USB Synthesizer kit from the SDR-Kits website.

...-.- de m0xpd