July 14, 2020

EL36 / 6CM5 TESLA tube tubes store. TESLA EL36 / 6CM5 high power amp tube. Customers who bought this product also purchased. Class, Va, Vg2, Vg1, Ia, Ig2, Zout, Pout, THD. Line output, , , , B ( idle), , , , 36, 1, B, , , , , 38, 3,, , He offers a 6CM5/EL36 based 2 x 22 Watt Stereo Class AB1 triode amplifier, the fuss was about tube sound, while their solid-state amp’s are being repaired.

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Ratings for use in Class Rube 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. Canberra based design engineer Patrick Turner is one who has investigated audio applications for the 6CM5.

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. The amp is 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 tuve solid state amp’s.

In each channel, the 6CM5 TV line output output valves are connected as triodes and fl36 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.

The pale coloured aluminium central box is a channel with solid-state components for regulating the anode supply instead of using a choke. The sound is clean, accurate, detailed, emotionally involving, with the typical smooth treble and precise bass with a warm midrange.

There is a perforated steel cover which fastens down over the top of the amp to prevent anyone coming into contact with high voltage terminals”. I am most grateful for John’s contribution of time, experience and knowledge as evidenced below.

John contacted me in August – it quickly became evident that he had a much much greater knowledge of this interesting valve than I, and therefore I am most appreciative of his willingness to allow me to repeat his advice to me here. For someone who grew up in the ‘s, it’s ironic that I learnt about valves before solid state. I’m into collecting, restoring and building monochrome valve TV sets. As I build a lot of valve projects requiring an audio stage the 6CM5 has been of interest, especially as I have a huge quantity of them!

I’ve built numerous other AM and FM radios using only TV valves rather than comparatively rare and expensive GT and G style radio valves, which I’d rather keep for equipment made in that era. There must be at least 6BL8’s in my valve collection, and what a useful valve that is, for example.

It’s usefulness goes way beyond the mixer oscillator use mentioned in the data books. The other thing with TV valves, particularly the European ones, is they are good performers with low voltage anode and screen.

This is an advantage when you want to build up a power supply with modern transformers. For someone who wants to get into valve technology these days, TV valves are ideal! Outside Australia the 6CM5 valve is virtually unknown.

US, UK and European data books often do not show it. As a point of interest, one of my colleagues did the curves for the 6CM5 at AWA when they started making the valve.

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. New Zealand followed the series heater designs to a much greater degree than here in Australia.


The 6CM5 is not a plug in equivalent, although it might have the same or similar connections. What is unique about the 6CM5 is the low anode voltage. Again, the series heater TV design was responsible for this.

So, any line output valve is going to need a very small voltage drop from anode to rube when it is turned hard on. This means that for Class A audio, the 6CM5 is one of the worst valves you could use.

6P31S = EL36 = 6CM5 tube. Beam tetrode

Not to be deterred, I have experimented just to see what I could get out of a single-ended Class A output stage. Actually I’ve got a car radio project coming up. I tried three topologies: The conventional pentode connection was the worst! I couldn’t really get good linearity with decent output power. Screen grid drive was a lot better, even though about 42V pp was required to drive it. Click on the image to enlarge and view the circuit properly.

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

The best was triode connected. Although sensitivity was low, again requiring 42V ppthe output was the greatest and distortion the lowest. Maximum output was about 2. These tests were done with a regulated valve power supply, a BWD model The results aren’t really surprising, and after looking at the STC manufacturer data on the 6CD6 when used as a single-ended Class A output stage, it just confirms what I suspected.

For triode el63 the output is 1. So, yes it can be done, but it’s terribly inefficient when you consider that the heater current is 1.

A 6AQ5 or 6V6 will give nearly double the output power for less than half the heater current! It only gave out 7W! 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. But there are better valves for the job, and it’s only because of my quantity of 6CM5’s, and wanting to be differentthat I’ll use it.

Having said all that, the low anode voltage is of interest, as it may permit a valve amplifier to operate on modern low voltage power supplies. This I have yet to play around with. I guess I should emphasise that my disappointment with this valve was with its application in single-ended mode. Several months later, John emailed me further, to announce that some significant headway had been made in this regard. The operating conditions are most bizarre no bias and 25V for the screen gridbut it really does work, giving about 4W output.

This is with V on the anode and a 5kOhm load, so it compares favourably with ‘proper’ audio valves like the 6AQ5 and 6V6. My circuit yields somewhat more output than say a 6BM8. The original bias setup was sensitive and critical, as the non-linearity of this valve is unbelievable. However, the DC stabilisation circuit I have subsequently used seems to have solved that problem. Driving via the control grid g1 is not the way to use this valve for Class A audio, as much as I have tried.

You can only get about half the output power that you can get from screen grid g2 drive. Remember we a forcing a switching valve to work as a linear amplifier. Triode mode is the best way to use this valve when conventional g1 drive is used. My circuit provides about the theoretical maximum audio power from a V anode supply and 5kOhm load.

Apart from the horrendous heater power, it probably works as well as a 6V6. I say ‘probably’ as I haven’t done any actual distortion tests but only visual inspection of distortion as seen on the CRO. At first it seemed to be a lost cause, trying to use this valve as an audio amplifier. The valve-data does include Class A operation, but that is for a V supply at mA.


This and the required Ohm speaker transformer don’t tend to match in well with other valve circuitry and parts availability i.


Alas, at this voltage the 6CM5 is very non linear! Designed for Class C work, this is hardly rl36. Looking at the data, we see the anode dissipation is about 10W max yes, it can be slightly higher depending on screen dissipation, but I prefer to be conservative.

This means about 40mA anode current at V which is similar to the likes of 6M5, 6V6, 42, etc. In reality the actual power to the voice coil will be less due to transformer losses. Additionally, the method of bias and voltage regulation also bears some importance for maximum e,36 output. This is why in a small mantle set or a car radio, a 6V6 may only give out 2W before distortion becomes evident.

As I’ve mentioned before, initial attempts at getting a 6CM5 working in tubee conventional triode or pentode circuits only provided about 2W. Screen grid drive seemed to be a better option, going on the theory that the gain of the valve would be less, and possibly more linear.

In practice it did seem to work better and a bit over 2W was available. My latest and successful experiments came from tune theory that RC coupling was probably not such a clever idea for screen grid drive. For ordinary control grid drive it’s fine as the grid is of infinite impedance.

However, screen grids draw current, which does not suit the relatively high impedance of RC coupling. Either transformer or cathode follower drive would have to be used. The practicalities of using an audio transformer meant that a cathode follower stage would be preferred, and so a test circuit was tried.

The results were looking promising but still less than the requisite 4W was forthcoming. At this point the 6CM5 had the screen grid at about V and the grid bias set to draw about 45mA anode current.

What did become apparent was the less bias the 6CM5 had, the tubf power it would produce without distortion. So, why not get rid of the grid bias and just control the anode current with the screen grid voltage?

That’s where the breakthrough came! With no bias and 25V on the screen, the anode was drawing 45mA at V. I couldn’t believe it when I was getting 3. Looking at the circuit now, to start with tue output stage, the supply used was V 50mA. The anode voltage should be V when the 6CM5 anode is drawing 45mA.

Allowing for the DC across the speaker transformer, the total supply will el336 more than this hence the V in my circuit. Note that the control-grid and cathode are connected together sl36 there is no bias; the cathode resistor is used for a DC stabilisation tubs.

The screen-grid voltage is set by the 6CG7 cathode follower; about 25V is what is required for correct 6CM5 anode current. Due to such a low cathode voltage, it is necessary to take the load resistor to a negative supply otherwise the waveform becomes clipped on the negative swing. The V supply tuube not critical at all; anything more negative than about V is adequate.