GEC DA41 SE A2 amplifier Part 1

Marconi Osram company, a GEC subsidiary, made some of the finest and thought after valves, from the scarce and super expensive PX family to the world famous KT one. These magnificent tubes were implemented for decades in high quality audio gears and remain today a standard in quality and reliability.

Almost every diy’er knows the KT66 and KT88, but very few the industrial DA tubes intended for high power public address amplifiers. Among these one deserves a special attention because of availability (at least for some time…), price (compared to a DA30, 60 or 100) and stunning sound quality. The DA41/CV1076.

A very fine directly heated audio triode with thoriated tungsten cathode and stamped anode (unlike the graphite one of its American relative TZ40). A pair of this powerful tube could deliver up to 175W with 5% distortion in class B push pull.

Of course such a power can’t be reached without some amount of grid current and this is probably why we don’t find much literature or schematic from experimenters. Class A2 appears to be a major obstacle for many amateurs. Implementation is not more complex than A1 if one understands the driver basics requirements.

When a grid becomes positive it acts like an anode and a few electrons emitted by the cathode are attracted by the grid. These electrons return to ground through driver impedance Zout and create a voltage drop in this load. If Zout is too high the driver is unable to raise grid to the desired voltage. Imagine a tube with 1K Zout intended to provide 10mA at +10V, it will only be able of 10V – (1K x 10mA) = 0V ! Ohms law.

Different arrangements provide current under low impedance. Shishido's way is a well known one but at the price of an expensive step down transformer, same for a choke loaded cathode follower. An interesting setup is used by Tossie Yamanaka. Very similar to a MOSFET drive, it is a power triode directly feeding the following tube grid.

This circuit is a reissue of a fully documented study by Charles F. Stromeyer (Proceedings of the Institute of Radio Engineers 24 (7), 1007-1026, 1936) and called

"General theory and application of dynamic coupling in power tube design"

...a simplified method of driving a power tube without the need of coupling devices and grid-biasing means. The power section is one whose useful plate-current versus grid-voltage characteristic is realized only with positive values of grid voltage. Its low input impedance is in series with the cathode-ground circuit of the driver tube. This impedance, also, automatically provides a negative bias for the grid of the driver, thereby eliminating external biasing. Since the electronic coupling of the two tubes varies with signal excursions, this method of amplification is termed "dynamic coupling." Practical considerations show immediately that the driver must operate into an impedance which is considerably lower than its own plate impedance. It is shown that the distortion which is produced when working with such ratios is minimized partly by making the driver circuit degenerative in order to nullify the varying effect of the driver's mu...

It’s simple, smart and works flawless. To determine operating point all we need to know is the grid current at the desired grid voltage. This is usually found in tubes datasheet.

GEC DA41 datasheet don’t give much information about grid current operation. I used the DA42 curves, a very close relative with indirect heated cathode. At Vg +25V/Va +350V, grid draws about 8mA which gives a 3K input impedance.

At that bias point, a 5K anode load appears to be the best power/distortion compromise and I can expect about 8W of great quality.

From the above statement the driving impedance must be at least 10 time lower than Zin, 20 time better. Only a cathode follower achieves this. Additionally it must be capable of some power. The simplest way could be a resistor loaded stage, this commands to raise the final tube cathode voltage with a resistor too, fully decoupled to prevent degenerative feedback, introducing a time constant in circuit and inevitably a phase shift.

The Yamanaka setup avoid this problem and collects electrons directly from the grid. Input impedance of power tube is at the same time the CF load and bias. The DA41 is directly tied to ground, no phase shift in such circuit. The main difficulty is to find a suitable tube that accommodates a 3K load, rather low, while supplying the necessary current to the final stage.

I spent some time consulting my books to find a good contender. To my disappointment very few triodes are usable and the only one that could match, a 6CK4, is hard to find in quantity (I usually pair two tubes from a batch of twelve). Best solution is a triode wired pentode. From half a dozen tested I only kept two, 6V6 and 6K6. The last one is almost unknown, darn cheap and can be considered a scale down 6F6, much better sounding. Despite its small size it’s a linear and powerful tube. Perfect for the task with 150 ohm Zout in triode mode.

Tung Sol and Visseaux, both excellent

Drawing the operating lines on Ip_Up curves seems to place the working point in a non linear region. Not the case. A cathode follower works under 100% feedback and computed characteristics are almost verticals with little curvature. Distortion remains very low. AC swing about +/-25V, enough to completely swing DA41 grid.

Really easy to implement, final bias is adjusted through anode voltage.

Such setup has a gain inferior to 1 and have to be be paired to a high gain, low distortion stage. Low distortion means high DC voltage and high load. I used the power horse 6CG7 double triode. A 150K anode resistor sets gain and a 0,025uF/200K coupling network insures good frequency transmission to the CF stage. Capacitor value can be small with a CF as input impedance is very high. In fact the grid resistor appears to be boostrapped, it’s value being multiplicated by μ+1.

Once working points, loads and DC voltages calculated the amplifier appears very simple. Three tubes, two capacitors and a few resistors.

More to come...