Feb 19, 2016

MC1/60, E60M, TM100 SE amplifier - Part 1

In the DIY world of triodes amplifier lovers, most people know the best of the large power tubes like the 211/VT4C or the 845. These tubes are easy to source, still manufactured by Chinese, Czech or German. It's even possible to find some NOS ones from GE, RCA…
There are plenty of circuits to try for the hobbyist and parts like output transformers are easy to find.
This topic will make you discover some of the best European triodes and especially french ones. For audio use everyone, almost everyone, knows the tubes from Mullard, Philips or Mazda, but very few have had the opportunity to get on Neotron, Visseaux or SFR.

These manufacturers made tubes mostly for administrations and are rarely present on the tubes vendors stalls still active today. In fact, most of these tubes were bought by the Japanese in the 80's and are now selling at indecent prices. Understandable when you see manufacturing quality that may turn you pale. Philips MC1/60 or SFR E60M where built to a very very high standard even for the tube industry of that time.

Let me introduce our competitors.

Philips MC1/60

This Dutch power triode was specifically developed for audio, rated at 75W plate dissipation,
very close in specification to the 211/VT4C, but easier to use due to its lower trans-conductance.
The MC1/60 have a second advantage with its 4 V oxide coated filament rated at 3,3A (to compare with the 10 V/ 3,25A of the 211). We will talk latter on an other advantage of this oxide coated cathode in term of music sweetness. Sweetness and naturalness shared with the very best of the 4 V triodes like the PX4 or the AD1.


French military triode made by Société Française Radioélectrique in the 40's. Incredibly well made with cost no object materials, twice the weight of its Philips relative, these tubes can bear the 3X75B military code. Very close in specification to the MC1/60 which is very helpful due to the lack of information concerning the SFR company products.
Like the Philips it's a 4 V / 3,3 A oxide coated cathode, anode power rating 75W.


French military triode made by Mazda in the 30'/40',
also can bear the 3X75B military code.
Very different construction with glass rods to support filament springs.
Despite the old fashion and less impressive construction it is probably the best sounding triode of this group, and probably one of the best sounding ever.
Oxide coating cathode, 4 V / 3,3 A, power dissipation 75W (better not to go beyond 60W).
All these triodes can be switched as they have the same filament ratings, use the same socket and are very close in specifications.

Top view shows the very different construction.

The 6 watt monster amp

My first intention when I decided to build an amp around these tubes was to get a reasonable output power with the best possible sound. You will say this is a very personal and very subjective goal but during the past 20 years I built a lot of amp, preamp but only kept 3 ones after intensive listening test with golden ears friends (10 / VT25 SE and VT25 / VT25A AB1 PP amps, 6J5 / NP206 Tango transformer coupled line preamplifier). These materials serve as references to my future constructions.

My requirements are small in term of power. I mainly listen to Klipsch La Scala speakers upgraded with JBL 2470 drivers and Alk filters and at 105 dB I can shake hell with just 1 watt.
So I decided not to push my tubes too much and make them sing at 5/6 W output.
Looking at the curves I found a satisfying operating point at 600V / 80 mA with a 3.5K load,it may be weird at first sight but at that point internal resistance is mainly 1.6K (S=8mA/V and μ=12.5). This gives the maximum output with a 3.5K load (and I had the transformers on the shelf for a long time, it helped a bit to make a decision).

The driver was a more difficult choice. I needed very low distortion, large RMS output voltage, low output impedance, large bandwidth. Not easy to combine. After several tests with the most commonly used drivers and tubes I stuck to a 6SN7 SRPP feeding a transformer. In that way I had a well defined load for the SRPP, a very good bandwidth despite a coupling cap (no current through the transformer) and the possibility of less final distortion using a 10K/40K step-up transformer that double the driver voltage. Moreover I could use a negative voltage bias and the possibility to set the current depending upon tubes to be used.

The 6SN7 choice was not a choice. This tube is well known for its linearity, its sound qualities and I have plenty on hand.
Long time ago I have modelized the SRPP based on a very interesting thread by Merlin Blencowe from AudioXpress ( The optimized SRPP ) and I just have to fill some fields in a Excel file to have my operating points, resistor values, gain and output impedance.

Now some math considerations to see the different AC voltages involved in this project.

Power output should be 6W/8 ohm or 6.9 Vrms.
Output transformer have an impedance ratio of 2.28 10-3, a voltage ratio 4.78 10-2 (-26,4dB).
It means for the power tube a plate voltage of 144 Vrms or 408 V peak to peak to get my 6.9Vrms.
Looking at the MC1/60 curves this can be done with 48 V peak to peak on the grid (17Vrms).
Gain of power triode is 8.5 (+18.6dB), confirmed by calculation where

                                                  G = µ x Rload/Rload + ρ.

Now to get 17 Vrms for MC1/60 complete anode swing I just need 8.5 Vrms (24 V peak to peak) out of my 6SN7 SRPP due to a 10K/40K interstage transformer used in that circuit. Easily achieved, under 300V anode supply the SRPP circuit give a gain of 13.4 (+22.5dB),means 0,63Vrms at input (1.80 V peak to peak) and very low distortion.

The amplifier overall gain is :
+22.5dB (6SN7) + 6dB (Transformer) + 18.6dB (MC1/60) – 26.4dB (Transformer) = +20.7dB

To be continued......
full amplifier circuit, construction details, listening test, tubes to be used, mods and tweaks.

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