by dcgillespie » Wed Apr 28, 2010 8:59 am
Hifi --
It's good you checked for that condition as when it occurs like that, it robs power, causes wild IM distortion, and is not very kind to the output tubes either. At the very least, it is an excellent example of how stability in a NFB amplifier can be upset by simple seemingly inocent changes to the design. In your case, I don't know what output transformer you used, but it also plays a huge roll in both the low and high frequency stability issue. Hafler's original transformers were excellent in this regard, and while there are other good transformers out there, changing them into the circuit would still very likely require changes in the stability networks to maintain the stability of the original design. It may very well be that (possibly) using other "similar" transformers along with the change in coupling cap values is what caused the instability -- where as either change by itself may not have been a problem.
Hafler was king of the stability issue when his Dynaco amps first came along -- at a time when some famous Heath and even an Eico or two were rather famous for burning out tweeters due to their instability, or had the breathing woofers as I previously mentioned. A properly running Dynaco ST-35 is stable into any kind of load -- even capacitive only loads.
Negative Feedback is simple in concept, but very complex to apply correctly. It is at the root of your issue, as if you disconnected the NFB loop, the breathing would go away. But then you would also lose all the benefits it produces of low distortion, high damping, and improved frequency response. In fact, it is these very kinds of situations that cause so many people to dismiss NFB as a bad thing, which is a shame. Applied correctly, it produces so many benefits, and few concerns. But so many who either don't understand the issues involved, or tried to make it work and couldn't, just dismiss it all together, and give it a bad name that in concept it doesn't deserve.
Installing the original coupling cap values is the first step. Then, regardless of what test equipment you have available (although it's extremely helpful), there are some simple tests you can do to get an idea of how stable your amplifier really is. Keep us posted!
Dave