metaruss

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An integrated push-pull beam tetrode power amplifier.

The Harman/Kardon A50K was the kit version of the their A500 integrated amplifier.  This one was originally built by my father-in-law, probably some time in the ’60s, and saw quite a bit of use.  He gave it to me while cleaning his garage, where it had spent the prior 30 years or so.  The chassis and transformers had some corrosion and was packed with bird seed (mice), but otherwise seemed to be in good condition.

I originally was going to salvage it for parts (HK had a reputation for making quality output transformers), but it appeared to be in reasonable condition, so I considered fixing it up instead.  I discussed it with Jim McShane (of Harman/Kardon Citation restoration fame) and was encouraged to see if it worked.  I connected the power supply to my bench supply and brought it up very slowly to attempt to reform the caps.  One of the caps in the voltage doubler was drawing current and appeared to be unwilling to recover, so both of those were replaced and the amp was brought up on a variac (no tubes, of course).  The results were encouraging, so I sprayed the switches and pots with DeoxIT, dropped the tubes back in and brought it back up on the variac.  Everything seemed fine, so on went the test speakers and the tunes.  It worked like a charm!

In the end I opted to restore it with a kit that Jim Mcshane can provide.  It replaces all of the power supply resistors and caps, coupling caps, and the caps in the RIAA section.  It also has replacement grid stopper and grid leak resistors for the output tubes.  Some of the tubes were weak while others tested very strong.  Initially I thought the 7355 output tubes were tired, so I got a set of Sovtek 7591XYZ tubes.  These can be substituted after a minor change to the socket wiring and seem to work fairly well.  Note that a “normal” 7591 tube will not operate properly in this amp without further modifications, due to the strange relationship between the output tubes and the preamp heater supply.  The “7591XYZ” is really a re-based Russian military 6P3S, which is closer 5881 than a 7591.

After the restoration was complete, testing revealed that the original 7335 output tubes were still strong.  Apparently this is because of the rather modest operating conditions for these tubes.  Two of the four Telefunken 12AX7 tubes still tested strong, so those are doing duty in the RIAA preamp section.  The preamp and amp input tubes were replaced with Sovtek 12AX7LPS’s.  The phase splitter is a 12AU7 that also tests OK (“made in Italy”, probably Philips).