London DIY HiFi 

An 813 Amplifier


Paul Leclercq

last updated August 1999 

WARNING! This article concerns push-pull amplification and may, therefore, cause distress to certain sections of the readership 

July 1999

It has to be said: it's fascinating reading accounts of individuals designing and building valve amplifiers. I'm sure they do it properly. You know the sort of thing: days of calculus and other brain-torturing processes, followed by careful drawings etc., etc. And jolly good too I say. 

Well I'm afraid if that's what you like STOP here! You'll find none of that. 

Unfortunately this methodology is denied to me by a complete lack of understanding of 99.99% of mathematics and the associated logical processes which seem to be essential for electronics. 

I suppose I should explain why I ever got involved in this project. You see, I'm in it because I love valves - not because they provide better sound (this may well be true [or at least I'd like to think so]) but because I just love them. I would never dream of building a solid-state amplifier - it probably wouldn't work anyway as I understand nothing of transistors or integrated circuits - they're just black boxes to me. Now a valve…Take a ECC91 or 6AN4 (or 12B4A for that matter), with these you can see the glowing cathode actually working between the plates; what solid-state device could hope to compete eh? [I'm getting round to the point, honest]. 

Now I first heard of the 813 valve 28 years ago when working for a Government department. I told our engineers that I had built a kit amplifier which featured 807s. Their response was: "You want 813s - THAT's a valve!". I never forgot this, and so, when, just over two years ago, I was contacted by an old friend (now sadly dead) who was a keen amateur electronics enthusiast and he lent me a copy of Morgan Jones's book., the old bug bit again -. 

"Marvellous" I thought, especially when I read in the introduction that complex mathematics were specifically excluded. Hmm….. It's really a very useful book; my project would have definitely failed without it. 

Anyway, I let ambition get the better of me, read a few articles and decided (some would say foolishly) to ignore all sensible safety advice and build an 813 push-pull amplifier. 

The initial design was even more naïve than the present (mark umpteen) version (see colour diagram 153kB or this black & white one 206kB): it featured a 6SN7 input/phase-splitter, a pair of triode-connected 6V6 as drivers and the two 813s arranged in "Ultra-Linear" mode. The output transformer was supplied by Brian Sowter (who was very helpful). The contraption was a complete failure, although it did work after a fashion, and despite the 1200V or so on the output anodes, displayed no inclination to kill me. 

After this disappointment, vast expenditure on dozens of different valves ensued, most of which have been tried - and in different configurations (remember, the overriding feature is the VALVE) 

The next approach, which was much better, used a SRPP 6072a a la Audio Note and used a 6SN7 as driver (believe it or not, in non-differential mode). By this time I had begun to realise that performance was handicapped by lack of feedback. Since this represents about four pages of "sums" in Morgan Jones's book, I made one empirical attempt with a resistor or two; the results can be imagined. So much for feedback. 

At this point I weakened, and purchased a pair of 845s (triodes apparently being relatively happy without feedback, or so we are told). By this time I had developed a differential SRPP driver stage using initially a pair of E182CC. These are NOT a nice-sounding valve (a pity because they promise so much). 5687WBs sounded better, but 6SN7s sounded better still. This was a bit of a shock, because quite early on, I decided that low Ra is the thing (= low output resistance) and 6SN7's Ra is about 7.7k. I still believe in low Ra however, and if it were not such a fag, I would have cathode followers everywhere. 

The problem was to get the 300+v pk-pk needed to drive the 845s. Gradually this became an obsession, and, having eight 813s in stock I decided to try a pair connected as triodes. 

First problem was that I had no triode data at all - only RCA's beam tetrode data; at least this told me that I could run the screen grids at up to 1100V. To the best of my knowledge, 813 is the only medium-sized (100W pa) tetrode that can cope with this voltage. So I had no idea how much current the 813s would pass when over 1kV was applied in triode configuration. With the bias set as for the 845s I switched on whilst nervously watching my milliammeter. Surprise: at 1080V and -150V, the valve was turned off! After a period of fiddling, I found that about -110V was about right on average causing an 813 to pass about 60mA. 

The amplifier sounded better. There was greater "delicacy" (bearing in mind my 1979 'speakers and my 1945 ears) and power was up too - nearly 55W were available. 

A chap in Sweden contacted me as a result of a letter I had written to GA. He has 813s and inspired perhaps by my enthusiasm, went to the trouble of producing the triode curves you see here (103kB). Those who have read Morgan Jones's book will see that Ra is in the order of 2k or so with mu of 8; not a bad deal and much cheaper than American 845s… 

As the circuit shows the present, if not final (!) configuration incorporates an EC86 input stage with a 5842 as phase splitter, this latter being there simply because it's convenient - I have 7-pin types (e.g. EC 91) which would make better concertinas. I do not like the Raytheon 5842 as an input valve because it is so noisy, and with my amplifier that really is saying something. These input stages are followed by a differential pair (no current sink though) of G.E.C. A2134s connected as triodes. The A2134 is a 7-pin output pentode, designed to yield up to 9W in a pentode push-pull pair. I discovered it in the indispensable Vade-Mecum, having noticed the beautiful triode curves (and Ra at about 800 ohms!). It can stand a good voltage and with the cathode at only 20V or so above ground can swing plenty of volts. Mu is about 8. Also (very important) it's nice-looking, being tall and thin: as the Duchess of Windsor said, you can never be too rich or too thin! 

Safety-conscious types will be horrified when I reveal that the power-supply is separate from the amplifier although I have been reasonably careful. There are PET plugs (rated at 6kV) and 25kV cable for the HT line, and special military three-way plugs for "middleT" and bias supplies. The filament and heater supplies are carried via high-current 8-way Admiralty plugs. The power supply carries a large meter marked kV which displays the available anode voltage for the 813s - and demonstrates how horribly variable the mains supply is. The only problem I have experienced with the power supply has been the HT rectification. The HT transformer is ex-military: 612-0-612 in bridged mode which equates to a peak voltage of 1.7kV, so large potentials can appear across the bridge. This resulted in tracking and lots of blue flashes and blown fuses until I replaced the board with a fibre-glass turret strip which now carries 16 BY 127s. I have had no problem since. 

More recently most of my work has been related to trying different valve types although this is quite the wrong approach since the amplifier will drive only transmission line loudspeakers; anything else sounds like a biscuit tin! Zero feedback….And I ought to experience stereo again, but of course have to finalise the design. Thorsten Loesch has suggested a current sink for the diff-pair driver stage. I have tried these in the past and they don't work; Thorsten thinks that perhaps I may succeed with his design; I hope it's very simple! Another suggestion has been to apply feedback by connecting the filaments of the 813s to the output side of the output transformer. I'm considering this at present; the reduction in gain may mean that I can use an extra valve at the front - lovely! 

The acquisition of a 'scope has been most helpful. Now I know that my S/N ratio is about -37dB don't laugh! - I have no idea where the noise comes from and anyway my 'speakers give 80dB for 1W so I can hardly hear it anyway. Frequency response when last tested was <15Hz - 30kHz +/- 1.5dB which I consider quite creditable for a lashup; and anyway the contraption makes lovely music - it really does.

And now a few words and views on valves I have tried. 

Great value is available off the fashionable beaten track: EC86 is a very good and cheap input triode with mu of 65, and gm of 13 or so. Ca-g is very low <1pf. 

If gain of 50 or so is enough, there are millions of super quality E180Fs about. This attractive Mullard industrial pentode was developed for TV cameras. It sounds very good configured as a triode - gm is 18 or so. They can be had for about three quid. 

In fact there are numerous small pentodes which can make good triodes - so long as you watch out for high Ca-g. However many such as E280F are expensive. One cheap one I did try was EF184; it sounded fine… I hear that 6AU6 makes a good triode, but Ra at 12k or thereabouts is too high for me! 

If you need a high-current driver with reasonable mu,. try 6S4a. This is a single triode of 8.5W pa. It can stand 550V, has mu of 16 and Ra is about 3.7k. It also has the advantage of being able to put up with 200V between its heater and cathode - though I wouldn't recommend it! Almost as robust (6W Pa) but with mu of 6 (and Ra c. 1k) is 12B4a. 6BX7 is an octal double triode with Ra of 1k and mu of 8 or so. It is good sounding (I think a bit like 6SN7) but rather greedy in terms of heater current (1.5A). In fact all of these sound pretty good to me. 

Another lovely-sounding valve is STC's 3A/167M, roughly equivalent to 437A but with a Loctal base. I have only six of these, all used, as I've never been offered any new ones. Mu is 47, and Ra 1k - therefore gm is 47. Pa is 7W and maximum anode voltage 350V. I'm very tempted to throw caution to the winds and push my luck running a pair over their top voltage as drivers. The fly in the ointment is the Ca-g which is 4pf - too high for a sensitive input stage, but perhaps alright as drivers. 

Critics may suggest that I direct my attention perhaps to improving the power supply, but I'm sorry to say that I find this area boring - even though it is the heart of an amplifier. It may be I'll never build the other half (even though I have the parts) but will continue to fiddle with valves for which there could hardly be a more enthusiastic ambassador than myself! 

Paul de Raymond Leclercq
July 1999

August 1999

Finally I decided to change things a bit. I opted to replace my A2134 drivers with a 6BX7 (mu is 10, Ra 1k), which I would drive from the 1kV supply. My selected operating point was 280V @ 20mA. My chosen valves for the input stages are 6GK5(mu 78, Ra 5k) and EC91(mu 100, Ra 12k) for the concertina phase-splitter. As the 6BX7 was arranged to draw 40mA (two anodes) to drop the voltage I used a pair of large wire wound resistors (ex-govt of course!) totaling about 8.5k. These were followed by a pair of Aerovox electrolytics - 220uf @ 400V in series (= 110uf @ 800V). This left me with a supply of about 700V. There were a few violent blue flashes when I powered up! I moved a couple of things and there was no recurrence. Amazingly the voltage on the anodes of the 6BX7 was 288V - not a bad guess. I also had to guess the value of the load resistor for the 6GK5 as I have no curves. With a 425V supply, I guessed 39k, with a 130 ohm cathode resistor. The load resistor was the only component I had to change - to 33k and all was then as planned - 136V on the anode. The EC91 installation is the same as with my 5842 - just 18k 2W resistors top and bottom; the anode voltage is 280 with 138 on the cathode. Both these valves are drawing about 8mA. Next, I would like to make a current sink (40mA) for the 6BX7. The trouble is that every time I've tried this, it DOES NOT WORK. 

It sounds quite good, particularly the bass. The problem with high-gain valves though is the tiny grid voltages they need: 6GK5 (or EC97 for that matter) functions with just -1V or so. At these levels it is too easy to introduce distortion. If one is committed to triodes, then the only way to get proper gain - except of course with a pre-amp - is to employ more stages; perhaps I'll try a pentode input next - say an E180F or EF184. That'll upset everyone will it not?

Paul de Raymond Leclercq
August 1999

mail Paul at Paul DOT Leclercq AT tnsofres DOT com 

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