Not really .. The Dynaco Mark VI was a 120 watt monoblock amp from the mid '70's. The only thing about the M-125 that was similar to the Mark VI was the fact that both amps used a parallel push pull output circuit. There are many differences between the two amps.
1. The M-125 can use 6550, KT88, KT90, KT100 and KT120 output tubes whereas the Dynaco Mark VI driver circuit was designed for an 8417 output tube. Dynaco mentions in the Mark VI manual that there is no substitute for the 8417 tube.
2. The driver circuit on the Mark VI used a single 7199 driver tube for both the voltage amplifier/inverter sections. This is similar to the drive circuit on the ST-70. The M-125 uses two 12AU7 or 5963 or 5814 or 6189 or 12BH7 tubes. The front driver tube is the initial voltage amplifier and the rear driver tube is the phase splitter/phase inverter tube.
3. The Mark VI had only 75 uF of DC power storage to power the output transformer. (25 uF before the choke and 50 uF after the choke) The M-125 has 336 uF of DC power storage for just the output transformer which is about 4 1/2 times what the Mark VI had. The M-125 also has another 167 uF for just the driver board. This is not said as an indictment of the Mark VI. Back in the mid '70's the state of tube amplifier development was such that it was difficult to get a lot of DC power storage at a reasonable price. Today's electrolytic caps are smaller in relation to how much DC storage they have and are also less expensive than what was available back then.
4. The Mark VI had a built in solid state rectifier and a delay circuit. The delay circuit, however, only delayed B+ high voltage to the DRIVER circuit. There was no B+ delay on the Mark VI to either the output tubes or the output transformer. When you turned the Mark VI on, the four output tubes and the output transformer looked at 500 VDC instantly. Some say this is bad for the output tubes and you will get "cathode stripping" on the output tubes but as I mentioned in another post I don't think it's a major issue. The Dynaco SCA-35 and ST35 both have solid state rectifiers with no delay and thousands of these amps are still out there working fine. On the M-125 I recommend the Weber WZ68 which has a short built in delay that Weber says is about 3 seconds but I have measured maybe 5 seconds before B+ high voltage is all the way up. There is also a tube rectifier option for the M-125. If you use a tube rectifier (GZ34) with the M-125 using four output tubes (125 watt mode), the amp will play fine but will not produce full power with a GZ34 tube rectifier in there. I have, however, used a GZ33 tube rectifier in these amps. A GZ33 tube rectifier with a choke input can flow up to 300 milliamps continuous. I haven't measured the power output with a GZ33 tube rectifier but I feel it does a better job than a GZ34 in driving this amp. For full power with four output tubes a solid state rectifier is a better choice. I have also use the T-SSR01 solid state plug in rectifier from Antique Electronic Supply and, although it has no delay, it also works fine in the M-125.
One final note here about GZ34 rectifiers in Dynaco amps that has been talked about in a couple of posts. You should never "short cycle" any Dynaco tube amp. Short cycling is when you turn the amp OFF after it has been ON for a period of time and then turn it back ON quickly (within maybe 10 or 15 seconds). This short cycling can take out a rectifier tube - the usual sign being "fireworks" inside the rectifier tube. Why does this happen? When you turn the amp OFF the voltage on the rectifier tube drops almost instantly but the quad cap holds the voltage for a longer period of time. If you turn the amp right back on and the voltage on the quad cap (has not as yet bled away and) is substantially higher than the voltage inside the rectifier then current will flow backwards from the quad cap into the rectifier and sometimes this reverse flow will take out the rectifier in an internal display of "fireworks" inside the rectifier tube. If you shut a Dynaco amp OFF I recommend leaving it off for at least one minute before turning the amp back on again. In that one minute the voltage on the quad cap will bleed down to almost nothing and when that amp is turned on again the current will flow from the rectifier tube TO the quad cap and not in the reverse direction. You can also put diodes in line with power transformer's secondaries to prevent this backward flow of current.
There was a pretty good discussion of this on a couple of threads from June and July of this year on the DIYAudio tube forum at the two links below.
Last edited by Bob Latino on Thu Jan 01, 2015 10:57 pm; edited 1 time in total