Akiran Canberra based design engineer Patrick Turner is one who has investigated audio applications for the 6CM5. The second characteristic that I found problematic was that when using a fixed bias supply the grid voltage was critical within a few Volts, from trying to set an anode current of 50mA to being completely cut-off. What did become apparent was the less bias the 6CM5 had, the more power it would produce without distortion. And finally, keep in mind that the el current for this amplifier is 2. EL36 The Valve Museum Looking at how much power you need to get a cherry red anode around 40 Watts all I can conclude is that the 12 Watt rating is conservative. The anode dissipation limit of this valve also brings some interesting questions to mind.

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Having started at about the age of 9 and I am now You will be working with potentially lethal voltages! What follows may also not be a suitable undertaking for some people. Most companies that used the EL output tube the Belgian company Faylon and the German Dynacord being just two examples most probably chose this tube for its short, stubby, fairly beefy, relatively low voltage specifications enabling a smaller overall height of their amplifiers?

I have also not done any experimenting with them in self cathode bias mode. On the basis that the EL36 is not very linear for audio applications especially in single ended self bias mode , and even though in push-pull most of this linearity is cancelled out, I still cannot make any guarantees to their performance in the self bias mode.

Please also note important points at the end of the text. The one I have here actually produces 36 watts quite easily before clipping! A pair for watts, fixed bias with a volt stabilized screen supply, volts on the plates!! Geloso did in fact do 2 models and matching slaves using the EL output tubes. This was to the best of my knowledge the only Geloso model to use a quad of output tubes. I presently own a pair of each of these models. These had all cost me a fair bit of money and I still needed more!

They are getting very rare nowadays, though they turn up now and then on Ebay but the prices are extortionate.

A while ago I decided to spend some serious time on one of the smaller amps the model with just a pair and see if I could come up with some descent results using cheaper and or more readily available tubes! As I am able to design and wind my own transformers I decided to do the experiments with separate power supplies for now, so I could play around with voltages and try and achieve the desired results; the end idea being to rewind the mains transformers with the voltages I ended up with.

The amp itself had only a swing of volts for the negative bias, so my first move was to change this for a volts swing through two 20 turn pots; one for each tube. These were brilliant and actually were the best results of all I had tried from a power point of view.

I was seeing about 80 watts across the 8 ohm load. Changing the load to 4 ohms saw watts RMS, basically meaning that the plate load was more suited at this setting. The problem now was that there was no way that the top cover would fit with these 2 tubes towering out of the chassis like a pair of sky scrapers! Next job was to rip out the Magnavol bases and replace with a pair of good old octals.

This left the field wide open for trying a multitude of output tubes. These are an old TV horizontal deflection tube with a plate dissipation of 11 watts. With volts on the screens, , , and volts on the plates respectively, they performed quite well. I was looking at about 50 watts with the volt plate option, and the plate load seemed to suit too.

Nevertheless, no matter what else I did they would give no more. I now wondered how hard I could push a pair or quad of these? Rewire number "? I left the screen supply at the previous volts and started off with a volt plate voltage, biasing the two tubes for zero crossover distortion and was quite pleased at seeing about 52 watts RMS 8ohms; plus the plate load seemed to suit as well as any impedance change either way 4 or 16 ohms showed less results.

This was starting to look good! And there it was: This is a whopping An impressive Pushing the EL36 family of tubes this hard may reduce their lifespan? They are still holding in there with no change in any way.

Who cares for now, anyway! One other point here. However with these tubes being slim, I can cut the tube base holes a little larger, mount the bases from underneath with some 12 mil ferrules, and then the tubes will drop through to give enough clearance.

One of the photos shows a re-wound power transformer with the extra screens grid 2 winding on two tags previously used for the 20 volt bias winding. I have brought the new 40 volt bias winding out on fly leads, no more tags left! Also, the very simple independent screen supply can be clearly seen. The photos show the actual quad mod running on completion. The scope photo shows it just clipping at The main smoothing capacitors are even keeping the clipping point flat with no ringing coming over the top of the caps whatsoever!

I used 2 x uf volts in parallel to achieve this, at this level the ripple on the HT line is about 1, ma! As with all electronic experimentation, there are components hanging out everywhere. I am now ready to re-build all 4 of these amplifiers as and when, and then eventually display them on this website. However, having the design knowledge and winding facilities for transformers myself, it is of course convenient for me to do this. Nevertheless it can still be done as follows.

Deduct this figure from and what you have left you will need to source a transformer of the remaining volts about — ma for a pair of tubes or — ma for a quad. In series with your original transformer this will give you the volts DC plate supply on load.

These may require a change too. Apart from possibly the HT caps down the line, everything else will be fine. With a simple small bridge rectifier and a cap of uf volts, remove the screen grids feed from the existing HT supply and place it here. In my case the bias winding on the power transformers were only 20 volts; not sufficient to achieve a 50 — 55 volts DC negative rail. I simply re-wound mine to 40 volts. A lot of fixed bias circuits are usually quite high and are deliberately reduced accordingly.

If you have 45 volts or more then that is fine. If not, you will need to re-think this; possibly with a very small, few milliamps 40 volt transformer to give the higher needed voltage. The other method is of a much higher impedance, and also tends to sag as the main HT winding drops with currant drawn.

You may notice that on the sketch of the 2 or 4 bias potentiometers, that one end of them is on ground!? In theory this is not a practical thing to do as this would allow the pot to completely shut down the bias of the connected tube! I have however done this on purpose, as it again keeps the bias impedance as low as possible. Needless to say after completion of the mod it should be powered first without the output tubes in place, by metering grid one negative volts on each tube base pin 5 and winding all the pots to maximum voltage first.

Then after putting the tubes in, work backwards checking cathode current at each tube cathode 1 ohm resistor pin 8. Set them all to 30 ma. You may need to re-do this a couple of times as the tubes settle.

If you feel uncomfortable with this, simply lift the ground of each pot through a 10k resistor per pot or a common grounded one of 2. The EL36 family of tubes were originally designed for TV line output usage although Philips did make an amplifier using them for audio in class AB1, even though their own data shows them as class B for audio!?

Their ratings are usually max volts plate and screen, but with a 7kv peak pulsed plate voltage. They normally rate in as a 12 watt plate wattage, but there is a lot of talk out there that they can and in fact do handle considerably more.

They seem to be quite happy at this, and in class AB1 too! I have sketched some schematic drawings of the necessary changes; to most people they are self explanatory.



Ratings for use in Class B push-pull are readily available from Philips datasheets available on the internet. An anode voltage up to V, screen voltage of V, and a relatively low anode to anode load of 3. You can visit his website article and pic of this amplifier. They are always sad to have to return it when their solid state amp is fixed. In each channel, the 6CM5 TV line output output valves are connected as triodes and are driven by a long-tail pair using a 6CG7. The input tube is a paralleled 12AT7. The output transformers were made in in Sydney and the power transformer and chassis is a new one by T.


EL34 / 6CA7 / KT77 Tubes



EL5 , EL6 , EL32 , EL33 , EL35 , EL36 , EL37 , EL39 , EL50 , EL51


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