Sunday, 25 January 2015

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

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