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I tried three topologies: For a higher anode voltage, a 22kOhm 2 Watt series resistor and two 75V Zeners in series to ground form a nice regulated screen supply. This has made the filter choke redundant, and many of these have been thrown out. Also, the sound was very bright. Very little is published on this, so take heed!

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. The results here were no better.

In this configuration with low Volts and low impedance anode loads you are fighting the dynamic internal anode resistance of the valve, making it almost impossible to match an anode load with reasonable output power and efficiency.

The one I have here actually produces 36 watts quite easily before clipping!


Changing the load to 4 ohms saw watts RMS, basically meaning that the plate load was more suited at this setting. This is an advantage when you want to build up a power supply with modern transformers. If rube thought of 4 mains transformers seems and sounds a bit off-putting, just bare this in mind – the transformer for the screens supply need only be about 50 ma this is enough even for 4 tubes and will be small.

I have spent a lot of time on this point, and can’t argue enough to add these to every valve amplifier you make. They don’t look bad at all!

I also tried screen drive, via transformer coupling, and also zero bias mode where you drive the tubr and control grid via a 22kOhm series resistor at the same time. The scope photo shows it just clipping at The big problem with these amplifiers is low efficiency due to fighting the internal anode resistance of the valve R a.


The amp el336 by no means a state-of-the-art high-end unit in terms of style and finish, but it is amazing how much better it is than most other more powerful solid state amp’s. The 6CM5 could obviously be replaced with a 25E5 if it’s convenient to obtain 25V mA for it’s heater.

The values aren’t critical, with the resistor being from about 2. The cathode follower has only current gain, so we need a grounded cathode stage to obtain some voltage gain.

6P31S / 6P31C = EL36 = 6CM5 tube. Beam tetrode

Their ratings are usually max volts plate and screen, but with a 7kv peak pulsed plate voltage. The anode dissipation limit of this valve also brings some interesting questions to mind.

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. These days people tend not to use filter chokes in power supplies as there are plenty of high capacity electrolytics available at reasonable prices. Conventional push-pull EL36 implementations are covered in detail elsewhere, so my comments in this article below are related to using the valve in a single-ended output stage.

In reality the actual power to the voice coil will be less due to transformer losses. It is important to remember that series heater TV sets were standard in the UK and Europe, whereas parallel heater designs were pretty much unique to Australia and about half of US sets.

My last method tried was to raise the HT to Volts with a 5kOhm anode load, and run the EL36 in grounded grid mode.

TDSL Tube data [EL36]

So, any line output valve is going to need a very small voltage drop from anode to cathode when it is turned hard on. 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!?

I am now ready to re-build all 4 of these amplifiers as and when, and then eventually display them on this website. This would do some nice things in single-ended configuration I guess, or in push-pull it would be quite efficient for triodes, while requiring a modest drive.


I acknowledge inspiration for my design came heavily from the following web pages; http: Click here to read Grant’s most informative article! I intend to go ahead and use the 6CM5 in single-ended Class A applications but only where heater current isn’t a problem and a couple of Watts is acceptable output.

A pair for watts, fixed bias with a volt stabilized screen supply, volts on the plates!! A pair of valves were used in Class AB1 push-pull, with most of the output coupled to a modulation transformer and some driving another 4 in Class C yes, you read it correctly. I tested all valves and they are 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.


Lots of secondary taps for various impedances, including 70V and V line driving windings. The one referred to in this text primarily, is that with tuhe top-cap anode connection. Feedback isn’t applied to the 6AV6 as it’s a triode handling low voltage and in my opinion it isn’t necessary. Several months later, John emailed me further, to announce that some significant headway had been made in this regard. Allowing for the DC across the speaker transformer, the total supply will be more than this hence the V in my circuit.

My circuit yields somewhat more output than say a 6BM8. Be aware of the fact, that the power transformer is wired rather oddly in these amp’s.

There is a perforated steel cover which fastens down over the top of the amp to prevent anyone coming into contact with high voltage rube. The operating conditions are most bizarre no bias and 25V for the screen gridbut it really does work, giving about 4W output.

They were of unusual construction and very compact.