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. :(
Brad, I found your blog post through a web search as I also have an A300 amp and am looking for replacement electrolytic cans. Can you tell me your source for the Panasonics? I cannot locate a suitable replacement for the three-section 100uf@200v/50uf@400v/200uf@50v. I have found acceptable replacements for the other two, but would prefer to get three of the same brand.
ReplyDeleteThanks for any guidance you can offer.
I had purchased all of my caps through Digikey (www.digikey.com). I had used the Panasonic EB Series caps which are rated up to a temperature range of 105 degree C. and are available in voltages up to 450V. I believe they are some of the best caps you can buy for the price.
ReplyDelete-Brad
hi. i see that this is just a tad outdated, but i came across your site while looking for info on my A300. I restored almost 2 years ago and have been on a constant hunt to gather the remaining HK a300 enthusiasts out there. I have all the info you may ever need on this amp. however, i have been having a problem with blowing out fuses as soon as i power the baby up, and i cant for the life of me track it down. I have tried everything and was just wondering if you had ever encountered this...just one day, i played it for about 2 hours and all was good, then suddenly fuse started to blow. hmmf...let me know if you are still using your amp and i apologize for the randomness of this message :)
ReplyDeleteHi Aleks,
ReplyDeleteYes, I am definitely still using my a300 and loving it. Strange you are blowing fuses like that, have you tried powering it on without any tubes in it to see if it still blows? If it does, then it may be an issue with the power transformer ( but hopefully not as i don't know where you would find a replacement). If it doesn't blow with the tubes removed, I would just say to test every resistor and make sure none are open / no shorts and inspect the wiring as it is possible some of the old original wiring may be frayed as well. I replaced most of the resistors on my amp anyway as the tolerances were off on them causing uneven volume between the two channels.
You will really need to use a variable ac transformer to test the amp as you can slowly provide power to the unit while watching it's current draw, this will allow you to isolate the bad component / short without blowing the amps fuse.
Let me know if you are able to isolate the issue,
-Brad
Hi Brad - I'm restoring my a300. I was wondering if you had the component list for the capacitors you replaced on hand. I'd like to order them now, but don't have the amp in front of me at the moment.
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