Yes -- but hardly to the extent that it occurs in the ST-70. Under a worst case scenario, the two power amplifiers in the 70 could pull as much as .5 A from the rectifier tube if it and the power transformer were capable. Under the same scenario in the MK III (full power 20 Hz sine waves), a stock unit draws just over 300 ma, producing about 55 watts RMS. Also, where as the stock ST-70 pulls about 220 ma just waiting for the needle to drop on the record, the MK III only pulls about 150 ma in the same scenario. Other than the higher PIV the rectifier tube must withstand in the MK III, this all adds up to a much kinder environment for the tube to operate in in that unit.
The performance of the MK III would no doubt improve as rectifier capacity increases. However, it does not have the history of rectifier tube failure that the ST-70 does, and in my experience, stands up to the worst conditions quite admirably. Back in the 60s, a near exact copy of the Dynaco MK III circuit was used as the basis of the power amplifier section in a particular model Sunn guitar amplifier (including the use of authentic Dynaco transformers as well). It was known to be a dependable model even under the rather extreme conditions that kind of service requires, such that I have always considered the MK III a "band rated" amp (my term) dependability wise. I was in a pretty good garage band during those times, and it was well known throughout the bands what worked, and what didn't. I am rather certain that given the same scenario, a stock ST-70 would not have garnered the same reputation.
Dave
The performance of the MK III would no doubt improve as rectifier capacity increases. However, it does not have the history of rectifier tube failure that the ST-70 does, and in my experience, stands up to the worst conditions quite admirably. Back in the 60s, a near exact copy of the Dynaco MK III circuit was used as the basis of the power amplifier section in a particular model Sunn guitar amplifier (including the use of authentic Dynaco transformers as well). It was known to be a dependable model even under the rather extreme conditions that kind of service requires, such that I have always considered the MK III a "band rated" amp (my term) dependability wise. I was in a pretty good garage band during those times, and it was well known throughout the bands what worked, and what didn't. I am rather certain that given the same scenario, a stock ST-70 would not have garnered the same reputation.
Dave