GEC DA41 SE A2 amplifier Part 2

Power supply

My long time favorite for smooth filtering is a LC supply. Not only the mains residual is almost a perfect sine wave, but transients response is better than with a capacitor input. With a large choke value, ripple can be very low. However, to keep first cell time constant around 15mS, the total series resistance must remains low. A well wound power transformer and equally good choke are mandatory. For that purpose, I use top quality Hashimoto’s irons for more than a decade with total satisfaction.

A first cell (25H/50µF) will reduce ripple to Vripple = Vout / 6π² f² LC √2, a second one (180 ohm/100µF) will floor down residual to negligible value.

Some voltage adjustment being necessary on CF anode to precisely set DA41 grid current, I splitted supply in two after the second cell. One arm to the 6K6/6V6, the second to the 6CG7.

Making of

Aluminum CNC chassis, epoxy coated, plus a few components

DA41 filaments supply on sub chassis

Amplifier completed, ready for measurements and listening test. It's a beefy unit weighing almost 40 pounds.

DA41 bright light is a pure enjoyment in the dark

Test setup. Max power, 100Hz_10KHz squares and THD

7,8 Vrms/8 ohm at clipping, 7,6W as expected

Despite a 18/20Kohm internal resistance the DA41 adapts quite well to the output transformer and low end roll off is not so obvious during listening test

Short reminder: transformer primary inductance has a direct effect on the low frequency response. The -3dB low frequency cutoff is determined by

F_Low = Z/2πL

Where Z is the primary impedance (reflected impedance in parallel with the tube internal resistance) and L the primary inductance. For a 5Kohm reflected load and 23H@70mA inductance (Hashimoto H20-7U)

F_Low = 28Hz

THD: 2,76% @ 1Watt, 6,06% @ 5 Watt. Nice harmonics distribution, mainly H2

I tried a close loop feedback between DA41 anode and 6K6/6V6 grid to get a flatter 100Hz square signal and it was disappointing. Sound became muddy, loosing all the life music could bring. Regardless of apparent poor scope results or distortion I prefer, by far, the amplifier without feedback. We must not lose sight that there is little to none relation between music, which is more in the transients domain, and squares.

Reason why it’s of prime importance to calculate accurately all time constants including power supply to respect signal ADSR.

Listening report

Garrard 301, FR24, Entré EC30, C3g phono preamplifier, 6J5 line preamplifier

On my homebrewed Klipsch Altec system the 6V6 gives the best balance with a great sense of refinement and a silky smooth sound. Each record I spin on my Garrard plays with ease and life. Dynamic is excellent, nervous or soft depending upon music. I love the way a trumpet or a sax is rendered and human voices are really addictive. Low end is firm, ample with a convincing restitution even if it doesn't extend deeply. Music is played without the somewhat harshness heard with transmitting triodes like 809 or 811. It was easy to make a comparison, the PT260 transformer can supply 6,3 or 7,5V. Is it the fact the DA41 was developed as a pure audio tube or the materials used to build it, I couldn’t say. Anyway the GEC sounds much more natural and involving than its counterparts and it blends very well with a 6V6 (Mazda or Westinghouse). The 6K6 is surprisingly more punchy with deeper bass extension, but restitution appears less natural with a forward presentation and could be a better choice on Rock music.

I am somewhat confused and surprised. To this day I don’t know which of the Visseaux A710 or the GEC DA41 is the best of my amplifiers. In any case I listen to both with great pleasure and that’s the most important. Music, music toujours…

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...

LCR Phono preamplifier revisited

A few month ago I sold my E88C/E86C LCR Phono preamp, playing the recently build C3g/E182CC RC one. I soon realized I couldn’t live without. Not to say the C3g is inferior, it is the most enjoyable preamp I have today but I miss the absolute delicacy and sweetness the LCR had. My wish would be a preamp combining all qualities, even if everyone knows that the search for the ultimate phono, the one that brings pure enjoyment on any kind of music, is a never ending story.

Whatsoever, it didn’t take a long time before I decide to undertake a new one. New project, new interrogation. Is the final result a matter of network or tube choice, what has the most significant impact on sound reproduction ?

I personally think that active components, the way they are used, the dynamic behavior and the distortion figure under use conditions, have a much bigger influence upon sound than the EQ itself and decided to keep the same approach than in the RC phono, a first stage pentode instead of a triode.

Fortunately, when cost was in par with my wallet, I stocked a few transformers and can build a complete unit with the same irons previously used in the E88C LCR phono. Just had to buy new RIAA EQ sets.

Temptation was great to use the same tubes with the benefit of an A/B comparison, but the C3g gain would have been too high especially on a 20K load. I had to find a substitute with equal qualities and not too exotic. The search for the (almost) perfect equivalent took some time and I finally stopped on the E83F. A high Gm pentode, easy to source, apparently low distortion, low noise and primarily developed for class A audio applications in telephone repeaters. Surfing the Web I discovered it has been used in the last stage of the iconic TAB Telefunken V76 microphone amplifier. Choke loaded and capacitor coupled to the output transformer (full shematic here). This preamp has a reputation of excellence. A serious reference and a great encouragement for my project.

First stage

Beyond the basic DC study from datasheets, my old HP dynamic analyzer was of great help to set the best operating point and get a similar harmonics distribution than the C3g. Once done, gain is close to 160 with very, very low distortion.

Second stage

RIAA network plus transformers (A8713 20K/600 and TKS50 600/50K) has a 0.158 @ 1KHz overall gain which commands an amplification of 15 to get +45/+50dB. What immediately comes to mind is the 6SN7/12AU7 family of tubes. Having a good supply of miniature double triodes, it was obvious to give them a try.

From the dozen references tested I only kept six. All have outstanding performance, with a personal preference for Mullard M8136/CV4003 and Tungsram E80CC. These two fine tubes are just followed by Siemens 5814A triple mica and RTC 5814A. The others (Sylvania 6072A & GE 12BH7A) being slightly less involving while keeping all other qualities like great tonal balance, wide soundstage and lifelike restitution. Distortion was a bit higher with the12BH7A and 6072A had too much gain and a forward presentation.

DC point is set according to manufacturer recommendations. Below the E80CC setup, identical for M8136 and 5814. The 12BH7A needs a lower bias resistor

Schematic is very similar to the full triodes preamp

Note: LCR network can be either feed or loaded by 600 ohm, in any case you have to remove one 600 ohm resistor ( R4 or R12 ), this ensures network to work properly. Thanks to Bruce B. who pointed out this issue in my schematic.

Supply, parts and making of

Wishing to keep this phono as close as possible to the first one, at least in design, I calculated a double choke C input supply. For basics refer to the EC88 LCR phono supply post.

R7 serves two purposes: draws the necessary current to set DC voltage and acts like a crude shunt regulator.

Just one capacitor in signal path and another in G₂, its choice will greatly influence final balance. Not really fan of mellow or colored restitution, I prefer a neutral sound, less appealing but more realistic. A few, very few caps have such ability and they are not super exotic, fancy, expensive…. ITT PMT/2R and relatives like LMT, Aerovox V161 (polyester, used in original Pultech equalizer) are close to that ideal. Finally choose the PMT/2R, a long time favorite. Thanks to Jean Hiraga.

During the first listening test I noticed some hum, about 2mV, a bit annoying. This is a recurrent problem with pentodes (partition noise & HV supply ripple) and to fix this issue I replaced Rg2 carbon resistor by a metal film one and added a hum cancellation capacitor in parallel. Calculation is tedious (RDH 4th Ed. p539) for precise cancellation and usually not a standard value. I used 0.15uF for 0.192 calculated and got 0.3mV residual noise. Inaudible at listening distance and barely discernible close to the speaker, I can now run intensive listening tests. Also added a 22K grid stopper.

inside PSU

Mullard 5Z4GY rectifier is the best performer in my system, great presence with deep bass.

Phono completed

E83F soft glow

Measurements

First observation, the preamp have a great dynamic potential and huge headroom. With 80mV input signal I got more than 30V peak/peak with no sign of clipping. Couldn’t go beyond because my scope was on the lowest selectable sensitivity.

Next step was a THD test. Just have a look at the pictures to see how good this preamp is. Considering signal generator + dynamic analyzer total harmonic distortion (~ 0,014%) this phono exhibit 0,021%@1V rms, mainly H2 (H3 being almost at background noise level).

At 5 Vrms distortion is only 0,18% with the same harmonics distribution. Not that bad…

Listenning report

Even if the E83F deserves an A+ whatever the brand, I found the Tungsram more attractive. As for the E80CC used in the SRPP line preamp, just two manufacturers for this fine pentode : Dutch group Philips ( Mullard, Valvo, RTC, Amperex …) and the Hungarian one. Not as glamorous as Mullard or Telefunken, Tungsram tubes are of excellent quality and completely underated.

Sound is well focused, tonally correct and ample. Less punchy than the C3g but still offering a very detailed and vivid sound. Most obvious qualities are clear (could be dry depending upon second stage tube - Sylvania 6072A) and clean, different of the E88C/E86C combo which, in my memory, was softer and less analytical. This is probably due to a more euphonic sound in a full triode setup. Pentodes are by far more neutral and I understand Telefunken choice for the V76 mic preamp when it is crucial to catch the sound during a performance with a total neutrality. The E83F is an uncompromising tube of exceptional transparency. When I compare to the C3g, the E83F appears slightly less seductive but certainly the most truthful.

After several hours of attentive listening this preamp is an excellent performer on philharmonic music, big band, human voice, cello solo and organ, but not only. It is the only phono that makes me hear or feel the recording studio acoustic. The 2nd stage tube that remains the best (balanced, detailed, neutral) on any kind of music is the Tungsram E80CC.

One drawback, you need top-notch drivers otherwise listening tends to be boring or tiring. Had the not so good idea to set the JBL 2470 in place of the Altec 288C in my homebrewed speakers. Will never do it again. Every improvement highlights mismatches and weaknesses.

vinyls, vinyls forever.....