The Legacy of Tube Preamplifier Designs – The Driving Output Stage III

Hello to everybody interested to read on,

today I want to report about my unexpected experiences which I made through my evaluations with inductive coupled stages within preamplifiers. As I said at the beginning of these series of articles, the red based RCA tubes have alway played in my tube amplified life a exceptional role of superiority. I came across to them by an article in Sound Practice Magazine in the early 1990ties. Shortly after that event I did find a handful used red tubes at a hamfest in these days in Germany.
The RCA Red Based Tubes 5690, 5691, 5692, 5693 are a set of specially made octal tubes as highly  mechanically rugged design variants (three mica spacers, three ss supports, strengthened and volume reduced glass envelope) of the common 5AR4, 6SL7GT, 6SN7GT and  6J7GT tubes made by several brands. From my experience these tubes are by far the inferior best sounding indirect heated small signal tubes made ever. The high mu triodes 5691 and the medium mu variant 5692 have been incorporated into my phono and line stages since I have used them the first time. They impressed me from the beginning with exceptional audible superiority and I never found something close to them. Their built quality and their exclusive metal materials are chosen without any restriction to form the perfect operating devices for the us cruise missiles with a shock resistance preference of 500 g and long life service of 10000 hours. I know in theory there are a lot of other tubes matching these specifications written down to paper, but in real I never found other tubes able to compete in all aspects of comparable applications as audio tubes, by far.

A equivalent to the white Alba truffles with tubes are the red RCA's: Here the RCA 5691 und 5692 with typical black plates and extra strong ss-columns and triple mica spacers with typical red octal sockets made and stamped by RCA. The same tubes are sold by GE, other brands made them with brown base and different mechanical design.

Within this evaluation I did use several indirect heated tubes with perfect theoretical preferences for the planned implantation as output tube with transformer coupling. I did test the famous and once exceptional expensive german government tube for telephony, the C3g, a highly praised contender strapped down to triode use it will show up with theoretical perfect internal resistance for transformer coupling. But in practice with Lundahl LL660/18mA it could not convince me with a above average performance. May be a problem that it is a penthode and not a real triode, but it could not follow on the better real triodes.  Further I did use the 6SN7GT/5692 as single system (8.5 K) with choke load, as I did test them with their loctal variant 7N7 in parallel with OPT and in single implementation with choke. Both designs are as well tested with 5687 and with ECL85, tested as two step amplifier with the pentode system strapped as triode with 3.5-4 k as partner for the output transformer coupling. I did listen for a couple of days to each set up before I changed the design for the next value of tubes. The C3g showed up with the worst performance, maybe a result as being not real triode, together with the 7N7, which showed the typical spills known from several brands of 6SN7 tubes.

My evaluation enclosure to test different set ups. here seen with Lundal Ll1667 plate chokes and german post sockets serving as stand up contacts for the adapter boards mounted on the top cover. 

It ended completely (if it ever will end once) different than expected and it is a good sign of such open ended research. I made a complete new bundle of experiences (thank you Sakuma San and again Ken Shindo), which are a lot more detailed, contradictory and expressive than I expected before. I am almost unable to describe here the complete evaluation process and its results. These comparisons brought me a complete new lesson about the role of impedances between tube stages and their effect to audible frequencies. Their physical hand over improves when low enough, comparable with a supported sort transport of natural energies and resolving finest textures of harmonics. Just pure ease.

The Ease of Listening

In general it can be said (again) that inductive loads bring a general refinement and linearity into each tubes performance, which is a tremendous advancement comparing it with classical rc-coupling designs. In 99% of commercially made tube preamplifier stages tubes are rc-coupled or show up with with SRPP designs, inductive coupling seems to be to expensive for profitable ventures. Almost 99% of all small signal tubes have a higher internal impedance than 4k, which means without transformer it is impossible to get it under the 1k output impedance line, this means all here described benefits will not be available within the most of all tube amplifiers. Tubes with high current heaters systems (RCA Reds pl: 600mA/20mA, 5687: 900mA/18mA) and relatively high internal current are better sounding than their lc counterparts.
The aimed low output impedance of less than 1 k acts as a perfect server for the dynamic and energetic attributes of the audio signal without destroying the tender micro informations and keep the harmonic structure in a natural balance. Both achievements reassemble a tonal and dynamic completeness of a audio performance as sort of ease, which hardly can be obtained with other design issues.

Any of the named tubes matches more or less in a almost perfect way the preferences for such implementation with a Lundahl LL1660 output transformer. Alternatively all these tubes have been used with a plate load choke LL1667 coupled with a pio capacitor. Both choke and OPT have been used with different air gaps to provide optimal induction before saturation (I used them in variants between 5mA to 20mA).

My evaluation enclosure with adapter board for two octal tubes, here shown with two RCA 5692 in parallel mode and choke load.

The by far best combination till now (actually only indirectly heated small signal tubes have been compared) are the choke loaded paralleled 5692's in pure class a mode. It was a instructive lesson when I did parallel the internal systems, because I did use each side of the tubes for separate channels before with OPT. Originally each half of the tube will show up with 8 k Ri, a bit to high for transformer coupling. To get it made I did parallel the internal systems in order to half the resistance and to match the transformers preferences ideally. But again with LL1660/20mA I was not really impressed comparing the audible performance with the single half tube version in choke mode. In choke mode the tube seem to have a much better linear performance and improved coherent tonal harmony. Before I did use the 5687 with 4 k Ri at the OPT with a quite good results (as described in my last article), this gave me the push to try the 5692 in parallel in a almost identical set up as well. The 5692 were different, but still some sort of limited, a bit repressed, not the pure wide open natural performance I was looking for. So I did use this design with chokes. It took my breath away how much the soundstage improved. It did not even remind me a little to the OPT version and improved against the single half tube in a way I did not expect. Here now it was a perfect example to show how right low impedances are supporting audio frequencies and its physical character of delivery.

Everybody knows this effect from coupling speakers to a power amp. If the preferences of speakers are ideally matched by the driving ability of a power amp, the sound starts to shine with dynamic completeness and with liquid naturalness. This process is often compared with the physical parameters of a car, were the weight and its power are arguments, but can have different formations with two or eight cylinder engines, either with small or big cubic capacity. A perfect match of amp speaker/combination shows up with exceptional driveability, comparing it with the high torque of a big high power pump loaded diesel engine…

As well other tube stages will show the same importance of their deliver ability to prevent any losses of finest energy, liquidity and tonal structures to hand over from a source. In terms of a audio chain there are several transfer stations, were signals can loose a tremendous part of their completeness through wrong coupled impedances. It starts with the cartridge, as moving coil it might have to match the first stage, a step up transformer (a quite well known critical match were you can loose almost 70% of the cartridges signal), as next the step up transformer needs to match the grid of the voltage amplifier of the first stage of the RIAA-equalizer. If the the phono stage is a stand alone unit, its output stage will be the next critical point. Otherwise the commonly highly underrated line stage is the next critical step. For the most people the line stage is less important, because it does not make any high amplification, it's role is just to balance different levels of sources and to bring their output impedances to a lowest possible level, that the following amplifier can take the signal without any loss. (From my todays point of view with this new experiences this are nice little words for a exceptional important function).

My RIAA-equalizer with two 5691 in the voltage amplification stage and two 5692 choke loaded (Lundahl LL1667/20mA) under (stainless cover) for the output stage.

After the experience with the paralleled choke loaded 5692 and its superior drivability I did introduce this design into my RIAA-equalizer as output stage. As well here the coupling improved in a exceptional unforeseen way the audible informations. Together with my line stage it forms a train of musical informations realized never before. Both output stages are now high current low impedance mode in pure class a. Exceptional refinements without any hardening or compression effects, carried with a dynamic performance of pure liquidity and natural finesse. A completely new impression about ease of listenability. This system will be back upped for further evaluation.

As preview to one of the next articles about preamps and direct heated tubes I show here a rare delicacy, the Philips made TS20 eqv. of the STC4021A or the WE104D. A ideal low (1.8K) impedance triode from the 1930's from the telephone era shown with typical Johnson 210 bayonet sockets (like 300B).

In the next article I will try to find arguments for the use of the parafeed coupling. I am awaiting a well combination of benefits from choke loaded designs combined with even lower impedance figures, we will see.
Read on soon if interested, Volker

The Legacy of Tube Preamplifier Designs – The Driving Output Stage II

Hello to everybody for the first entry in 2015!

I ended my last entry about the output stage of tube preamplifiers with the words: "the role of the tube is highly overrated". In a way it is and in some other ways it is not. In terms known from classic tube preamplifiers where "tube rolling" is often practiced method for filter like compensations in order to equalize typical sonic errors which are quite normal in tube audio sets from mixed brands. If a tube ideally matches the preferences of a output transformer, the two parts are melting together to be seen as one functional unit and the typical effects from different tube makes are getting quite negligible.

But before I try to refer about the benefits and disadvantages of output transformers in a preamplifiers, it might make sense to describe the general technical advantages of such inductive coupling. Again, it can't be said with enough emphasis, the preamplifier is sound wise one of the most important electronic devices in a elaborated audio chain. In early small signal stages, even more with high gain phono stages, the preamp has a tremendous impact to the attitudes of the finalized sound stage of a complete system. In real life almost everybody takes a lot more care about power amplifiers and loudspeakers as to be the most decisive components for the sound, a common and well established position in the world of hifi since generations.
In history manufacturers have long time given preamplifiers a underrated position, just a functional necessity to equalize different sources, that was it. Almost every well named manufacturer in the golden age of audio did power their units from dedicated power amps. I do not know one example in history till the 1970s years that a preamp has got equipped with a complex power supply, i.e. with regulation. In these days cheap transistor regulators got introduced for voltage regulation and stabilization, since the benefits of tube rectification already had got completely forgotten. Back in these days the transistor has been seen as a universal remedy to any sort of disease and it was cheap. A inductive iron choke or even  better two have been hardly found in any commercial audio component designed in the western world after 1950, even in preamplifiers breaking the 15k $ pricing line like AR, CJ or MFA in the 1980s. But instead each tube got regulated alone by one own transistor in order to distinguish the last touch of typical tube life...
Within expensive government amplifiers custom made to order at that time period you can still observe all the goodies for perfect signal shaping, – chokes, tube rectification, stabilization and regulation. As in Japan inductive couplers in combination with tubes have always been involved more or less as standard solution from the beginning of the rediscovery of tubes for audio amplification around 1970.

Western Electric amplifier, all stages are transformer coupled

Today we have learned to use transformers and chokes again as indispensable parts within tube amplified devices, a development which is given way by the communication throughout the web. Their influence is a improved linear performance of tubes, i.e. their refinement of micro dynamic detail, their ability to open up for holographic performance, their positive effect to keep the music in a natural flow with unsurpassed sonic completeness and as well their ability of involvement, – all these attributes culminate in the small signal stages of a preamplifiers as to be seen as little sound machines.

Within this low impedance any sort of resistive loads like from different cables or their different length is a negligible factor. Without such low Z standardization, like common in the tube equipped hifi world, it is impossible to drive coming tubes stages without loss or it will come to very common difficulties between connecting impedances and as result inadequate dynamic losses. In particular with tube amplified audio components this topic is a major issue for the "never ending process of device exchange mania" in the audio world. It is as well a welcomed perfect instrument to keep the market in a permanent flow of search …?

One instrument to keep the losses as low as possible is a defined low output impedance within each amplifying stage. Almost my complete life with audio components it was said that transformer coupling within tube amplified components is a necessary must, but will show up with several inherent restrictions and limitations. The loss of bandwidth and typical adding effects like the increase of dynamics from bridging transformers (input - output: input - output: etc.) are the most well known limitations named. The arguments did sound logical to my understanding, if every unit will add two coupling transformers at the input and output stage into a chain of several components, it cannot be better like without them. It has to be seen that the omission makes additional components necessary, like dc blocking coupling capacitors. These are soundwise some of the most shaping parts within a amplifier design. As well resistors for the dedicated anode load of the tube will bring obvious effects into the soundstage like roughness and phase shifts. Simple RC-coupled circuits (anode or cathode followers) are much simpler design solutions with almost no extra costs comparing the result seen from a technical point of view, where the transformers is the most expensive part.

Within the rediscovery of tube amplifiers within audio in the late 1970ties and 1980ties, transformer coupling within small signal stages was almost forgotten and was seen as a expensive and as aged historical defeated solution in the western world in these years. Instead US american designers did work hardly to introduce new silicon devices to regulate each tube separately as brave hearts new invention. As in Europe, the british, french and german designers preferred simplest traditional rc-coupled designs from the RCA handbooks to skim the cream of profit from the new growing interest at tubes in the audio markets. So the advanced transformer coupling was reintroduced mainly in Japan in preamplifiers. In Japan inductive phono equalization was seen as superior already in the 1970ties years, a topology which make lowest possible impedancies a must. So such 15K:600Z preamplifier transformers haven been produced by all well known transformer brands (Tango, Tamura, Hashimoto) and have been widely used in advanced designs from that time on.


The implementation

As already described a perfect internal resistance of the driving tube is essential for the best possible transformer matching. Values of 3 to 4 K are good, lower values are even better.  If you match a given tube to a output transformer it can be helpful to charge the secondary for improved frequency response. Transformers like the Lundahl LL1660 with a 4.5:1 ratio don't have a specific primary impedance, their primary is a reflection of the charge at the secondary from the following stage. With a resistor parallel to the secondary winding the output impedance will change the primary as its transmission. Together with a capacitor some unwanted peaks in the frequency response of the amplifying stage can be adjusted for perfect flatness.
Another main important question is the inductance of the primary winding of such a transformer. It is the important value for the low frequency extension of a tube. As you can see in the table, a low internal resistance is very helpful for a good match of low frequency response (there can be seen loads over 5 kilo ohms bring several problems into the design), were the transformer itself is the limiting factor. A air gapped transformer like the Lundahl is set for maximum dc in its primary winding within its necessary air gap. A Lundahl LL1660/18mA is set for 18 mA dc in primary and within this configuration at 100 henry inductance before the core saturation will limit its performance.
Now with such preferences we have already three values in mind, a maximum internal tube resistance for 100 H is around 3 kilo ohms with a maximum current of 18mA limit. A internal resistance of 3000 ohm divided through the transmission of the transformer will give the wanted output impedance.

From the RCA handbook

Now we do look for tubes with a internal resistance of almost 3 kilo ohms and a average current of 18mA at a class a working point. Paralleled 12AY7/ECC82/6SN7 are true candidates and have been historically used in such designs. But there are other and less known tubes like the 6S4, 6386, 5687. A very clever alternative are the multiple system tubes like ECL84/86 etc., they combine a complete amplifier system with perfect output parameters in one glass envelope. And there are the hard to get ancient direct heated triodes, like 3A/110B and WE equivalents WE101, the french, dutch and german post tubes, all with 4 kilo ohms internal resistance are promising candidates for the extra delicate soundstage only direct heated tungsten cathode filament tube are known for.

I made some tests with 6SN7GT/5692 and with 5687, the latter is for me a new tube in the field of experience, since I did always rely on the famous RCA red base types for decades. The 5687 is a typical space age tube developed for computers in 1950 but mainly used in military and industrial applications of the 1970ties.
One tube, two transformers, two resistors, two capacitors together with a stepped attenuator form a perfect line stage able to drive any tube amplifiers input stage. Working in pure class at 18mA current this tube (I had several different brands like Sylvania, Raytheon and RCA in test) sings with exceptional natural fluidity, micro detail and perfect dynamic attributes.  Homogenity, perfect phasing, a unknown depths and wide open soundstage show up with a balanced dynamic performance. All results are benefits from the inductive load of the tube instead resistive loads with more common designs. In theory it is said that the tube works more linear at a inductive load. The transformer if perfectly implemented, brings several benefits into the performance, which is a sort of sonic complete- and naturalness with refinements unknown from RC-coupled or SRPP designs.

Typical japanese designed preamplifier interior with transformer coupled tube stages and inductive RIAA equalisation, stepped attenuattor and massive pio capacitor use, here a design made by P+C company, Japan.

Only a few designers do have understand the lessons of history about the general improvements of transformer coupling and its alternative less improvements for the soundstage. Like Sakuma who did couple almost any tube at any stage with transformers in all his life, for him it seems to be a mandatory question. He brought this idea to a further point since he did built always the complete amplifying chain into one case, from cartridge step up transformer to the speaker out put transformer within three or four stages all coupled with interstage transformers. To the common theory such a system with five to six transformers in one chain will perform with a hardly limited bandwidth of a bakelite telephone, but in reality it seems to be very different?

Sakuma intergrated amplifier design from MC-input to speaker output in one monophonic designed chassis with some transformers on top.

As well Ken Shindo did always implement transformer coupling into his more elaborated amplifiers, since 15 years he did install them even into his simpler designs, a sign of indispensability? Or Kondo who did use widely a combination of SRPP (here one tube regulates the other, it can be seen as one tube acting as electronic choke) topology and the benefits of transformer coupling in mixed exchange within his high priced complex units. In general you will find in Japan loads of diy kits and custom built preamplifiers from the past 40 years were coupling output transformers are a indispensable standard. As already named the inductive equalized phono stage (LCR) is it a preferred topology since 40 years as the final word for RIAA deemphasis. Originally designed for the use with professional NBA (japanese broadcast) components, this topology got very famous and is highly preferred by japanese enthusiasts, even if the implementation of such unit without complications seems to be impossible. Here a low z output behind the gain stage it is a must to couple the inductive equalizing filters in between. A 600 Z transformer output is the only possible way to see the red sun again, but this topic will be discussed at its own in some future time.

Read on soon about alternative ways of inductive transformer coupling for output stages, like the parafeed or choke loaded design topology, Volker