Harmon Kardon A300 capacitor replacement

After almost a month of listening to my new tube amp there was one slight problem I began to notice. The right channel seemed to be a little bit louder than the left. The problem seemed to get worse the more I listened to it. Or maybe I never realized it and now that I have, I listen FOR it which makes it more apparent. Either way... after a lot of checking, swapping channels of signal sources, switching speakers, etc... the right channel was louder than the left.

I figured I had three possible causes of the problem. A single or several bad/weak tubes, an out of tolerance resistor, or a bad capacitor. I started with the tubes. I swapped the 6V6GT power pentodes to see if the louder channel swapped, which it did not. I moved the 12ax7 triodes around as well, with no difference in channels. There was really nothing more i could do without a tube tester (which I find myself needing). But for now I'll assume that the tubes are ok.

Next step will be the capacitors... which I wanted to replace anyway. Lots of browsing websites of tube amp builders and repairers turned up a ridiculous amount of information about the best capacitors to use for an amp. Lots of people recommend things called "orange drops" which provide the best 'classic' sound . Whether this was the technical name of the caps or a description of what they looked like, Digi-Key turned up zero results for 'orange drops'. Now on to the more scientific approach. This site provided me good information on the linearity of different types of capacitors. After looking at this data, it looks like most capacitors would be well suited replacements with the exception of a few(ceramic disc, tantalum, etc...). I purchased high quality Panasonic polyester film capacitors for the signal paths and ordered high quality Panasonic 105 degree C. electrolytics for the power supply filtering while I was ordering parts. The original filtering capacitors were introducing a slight 60hz hum into the audio, and I know a fresh set of electrolytics for the power supply would solve this issue. I have always used these Panasonic high temperature caps for replacements in other devices... always with positive results.

Before:



After:



The results? It sounded better... for sure! The 60hz hum was definitely gone. I always use the Panasonic 105 degree C. electrolytics in all my projects, the extra price is worth it. The overall sound with the new caps in the signal path? Better... definitely. The only problem was my ears were still annoyed at the louder right channel. Time to start checking resistor values. :(

Analog pathway

I have always wanted a real tube based home audio amplifier. The idea has always intrigued me. With today's high end audio equipment and all it's clean transistor based output power capable of producing hundreds or even thousands of watts per channel, why such a fascination with an old obsolete method of amplification? I had to find out. Just the idea of having an amplifier that is 100% analog based with no DSP and no A/D and D/A conversions going on seems so cool. To hear real tube compression and microphonics instead of the usual digital harsh sound we are so accustomed to with todays electronics. To have an amplifier that only produces 12 watts per channel play just as loud as a MOSFET based amp rated hundreds of watts per channel. To amplify such low level signals by using just these mysterious glowing glass tubes, a few large coils of wire, and a handful of passive components... I have to see what they are all about.

I began my search as usual at lots of audiophile pages with owners talking about such and such amp, why it is so awesome, and how much it cost. My search then turned to eBay (my preferred method of purchasing almost anything) to find one I wanted. After looking at a bunch and reading lots of user reviews on the ones available, I decided on a Harmon Kardon A300. It was clean looking and in good shape, inexpensive, and working with all tubes intact. This was important because a set of new tubes (new old stock that is, tubes that were made 40+ years ago that were never used) could easily cost more than the amp itself. Most tubes are still manufactured by a handful of companies today (mostly Chinese), but most people seem to say that they don't sound anywhere near as good as the old Russian, UK, and US tubes made forever ago.


The A300 is a stereo amp with approximately 10-15 watts of output power per channel. It has 9 tubes, 4 6v6 outputs, two per channel in a class A push-pull configuration, 4 12ax7 tubes and 1 12au7 tube in the preamp section. Upon receiving, I hooked it up to my system, replacing my current MOSFET based amp, turned it on and hoped for the best. As for any vintage electronics, there is always a chance for disaster or death with first power on. I looked it over really well, so luckily there was no smoke and there were no sparks... after about 30 seconds I could see all of the tubes glowing so it was time for a listen.

(what's a pcb?)

Now don't get me wrong, digital based equipment sounds good, really good (with the exception of mp3's. An mp3 compared to a well mastered cd on a good system lets us forget how lossy the compression of mp3's actually are). So when I started listening to music through this amp it was amazing. Did it sound better than a non tube amp? Yes... well maybe. It sounded different. So this is the 'tube warmth' everyone talks about, I can't really explain it but it sounded good.

So the next step is to begin upgrading the amp. Replacing all the 40+ year old caps will be a good start... it will be interesting to see if it changes the sound quality at all...