The M211 Se amp by Johan Cools

For those of you unfamiliar with the 211,please note that this beast puts out a meager 30 watts in normal single ended use just the same as the 845.

The main difference with its very close brother the 845 is the gain and therefore it is also much easier to drive.

Actually this is quite a significant difference as experience has allready proved that low Mu power tubes usually sound much finer and better.

But the 211 is an exception, its gain is not pushed to the limits and with a µ of 10 its still very reasonable..
This amp is simple and with more then enough gain thanks to the use of 3 stages the possibility to add feedback is left open.

Johan approached me last year for this project and i drew him the schematic and a month later he asked for another schematic without coupling capacitors so i suggested interstage transformers all the way, the added advantage of having more then enough gain is that the interstages can actually step down.

This is a very often forgotten benefit, they sound much better used that way.

The first triode the high mu 6SL7, which has excellent sound but had to much gain ( amp was hissing and to nervous ) and therefore it was very easy to go back to the 6SN7, the other limiting factor is that the LL1660 inter stage is very good but not the 10mA rated version wich is a bad match with high impedance tubes like the 6SL7 ( the LL1660/3mA is more suitable ).

As a driver many tubes are suitable, but Johan having build many PSE KT88 monoblocks and fitted them with the KT66 wanted to retain that tube as a driver especially since he is very cost minded, the 5881 is very cheap and was retained as the one to use, upgradable to KT66 without any problems.

The correct interstage is the high current LL1671/30mA version and 30mA is ok but not great.The interstages needed loading as otherwise there was quite some ringing.Wrongly connected windings really kill the frequency response as Johan found out.

Powersupply wise we stick to tube rectification for the high voltages and big capacity for ripple smoothing and a lot of chokes for the same reason as well. The Erea chokes are cheap and good so the bank didn't have to go bust on these.

Filaments are DC regulated to avoid hum and negative bias is the preferred way to go as well.( except for the input stage ).

The output tube is classicaly connected with a 5K6 load in order to have reasonable power output, the transformer is the limiting faktor here the LL1620/80mA connected alt C can handle 25 watts of output power, there is a new LL1688 that is better suited as there is more iron so more inductance available and a higher load possible as well, but all in all very good as it is..

Sound is excellent and the spektrum analyser shows this to be true.

Please note that 10dB of 3rd and 7dB of 5th harmonic must be discounted ( from generator)

A few pictures + complete schematic .

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