Getting the most out of a 50C5 -- just because I can
Output: 5.0W / Channel; 8 Ohms
Frequency Range: (TBD)
Class A operation
Active Screen Regulation
Low Noise, Solid State Power Supply
If you remember the "good ol' days", you've seen 'em everywhere: the ubiquitious 50C5 -- in just about every "All American Five" AM BCB receiver, in those cheap record players that you gave to your kids so they would keep their paws off the really good stereo or HiFi, in many a guitar "practice amp", and a wide variety of other appliances such as home intercom systems, or general purpose, low power audio amps. All running with a transformerless power supply connected directly to the AC mains, usually with a 35W4 half wave rectifier diode.
If you remember those days, the memories are probably not accompanied with a great deal of fondness. After all, the 50C5 was designed specifically to operate from the resulting low DC voltage that these transformerless supplies provided (the 50C5 is specced for a VPP= 110VDC). Being that the DC ripple filters usually weren't all that good, and that these were almost always super cheap devices, the output transformers were very small with low frequency cutoffs of 100Hz or higher. The specced Q-Point was selected for power output, not low distortion. Even what should have been more "high end", such as low power audio amps (a certain kit introduced in 1959 that remained popular well into the 1970s comes to mind) didn't sound all that good. Therefore, just about everything that used the 50C5 as an audio final sounded, more or less, gawdawful.
If there is a failure, it isn't with the type itself, but rather with how it was almost universally used. Indeed, the spec sheet gives two sets of plate characteristics, one for the VSGSG= 110VDC, and another for VSGSG= 90VDC which is a good deal more linear. With some slight spec-busting, and this more linear set of plate characteristics, it is possible to get the distortion down to acceptable limits with a shallower loadline.
Given the comparatively enormous cathodes that allow for high plate currents at comparatively low VPK's, the 50C5 was made for Class A operation. Given that, the design of the final is a self-balancing, power LTP with active tail loading. This is possible since the finals never enter the plate current cutoff region. Active tail loading also serves to enforce Class A operation since the sum of the plate currents never changes. This eliminates the phase splitter to get one more active device out of the signal path. In the vintage push-pull designs, the phase splitter was almost always an inferior design anyway, as these amps were never intended to be "high end" so far as sonic quality was concerned. Class A certainly is not the "efficiency" mode of operation, though the Little Imp manages a serviceable 5.0W of output.
As with any other beam "tetrode", the 50C5 has improved linearity with active screen voltage regulation. These days, voltage regulation is easily implemented with a solid state voltage regulator. Active regulation also provides a very low impedance path to AC ground to get distortion increasing feedback voltages off the screen grids.
Of course, the OPT quality plays a major role in sonic performance. Being that 50C5 applications were not designed for sonic excellence, the vintage OPTs designed for the 50C5 are unacceptable. The Little Imp uses custom designed OPTs that do not represent a sonic compromise. They are not cheap, but, then again, decent sonic performance never is.
The first voltage amp is a trioded 6AU6 small signal pentode. As a pseudotriode, the 6AU6 offers outstanding linearity, and provides just the right amount of front end voltage gain. The 50C5 hardly represents a difficult load, so providing the necessary drive for good high frequency performance is not a formidable design problem. Enhanced drive and linearity is achieved with the use of an active plate load. Given the low VPP, active loading offers much improved sonics and drive than the more common passive plate loading. There is more than enough gain margin for the inclusion of gNFB with good input sensitivity.
The main power supply does not use the most common transformerless design from the "good ol' days". There isn't enough voltage reserve from that, and, most importantly, this type of power supply was a major end user safety compromise. You might be able to get away with that with a TV set or a BCB receiver, since the only thing an end user was expected to connect is an antenna that could easily be isolated from the rest of the circuit. However, can you guarantee that the end user would never attempt "surgery" on their equipment to add some feature not provided? Back in "the day" electrocutions and near electrocutions from devices with a hot chassis were distressingly common. Even if that did not occur, equpiment destruction due to cross connected AC mains short circuits also regularly occurred. There aren't very many computer sound cards, CD players, etc, that can survive 120Vrms. Therefore, the Little Imp includes an isolation power transformer. There really is no excuse, and there never was, for not including this most vital of components. Besides, electrocuted customers are not repeat customers.
The ripple filter used here is the "active decoupler" (sometimes called a "capacitance multiplier") type implemented with a power MOSFET. The MOSFET has very little sensitivity to variations of its drain voltage. This serves to isolate the AC ripple from the DC rail. Since the gate draws almost no current, it's an easy task to filter the ripple from the gate, and to set the output voltage. This ensures a quiet amplifier (unlike the "good ol' days" where ripple suppression was largely a function of an OPT that rolled off far above the 60Hz ripple frequency -- YUCK!).
There are a variety of types in this particular family:
50C5 -- 50V / 0.15A heater
25C5 -- 25V / 0.3A heater
17C5 -- 16.8V / 0.45A heater
12C5 -- 12.6V / 0.6A heater
(Note: these latter two have a higher VHK rating for TV useage.)
6CU5 -- 6.3V / 1.2A heater
The 35C5, despite the similar looking type designator, is a different, though related by application, type. It is definitely not a 35V "50C5", nor are the ratings the same. The 35C5 is not a 50C5 substitute at all. The 35C5 gives less power with more distortion. Despite the different type designator, the 6CU5 (they could not call this a "6C5" since that type is a small signal triode) is otherwise identical to the 50C5 & company.
There is another related type with the same ratings: the 50B5. This type had the "7BZ" pinout (same as the 6AQ5) but this proved problematic in that this pinout put the plate pin right next to heater pin #4. This caused problems, especially with the heaters connected as a series string across the AC mains voltage, and so the 50C5 was developed to get that plate pin far away from the heater connections. The 50B5 quickly dropped out of sight, and so you aren't likely to find very many of these.
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