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Dynaco Mark III?

Blackdog61

Blackdog61

AK Subscriber
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I’ve really enjoyed the ST70 I built for the last ten years or so, but……..
A friend has a pair of IIIs for sale which were modded by his father who passed a few years ago. I haven’t seen them and my friend says they should probably be stripped and rebuilt. Price is a nice lunch which sounds like a definite yes, but I like my ST70. The midrange on the 70 is just so good, I don’t need more power and don’t know if the project is worth it.
Tell me, what would I gain and lose?
 
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The MK III's are not even close to the 70's for transparency and HF extension plus you hardly even notice the extra 20 watts.

Out of the hundreds of tube amplifiers I've owned the only one I kept is a modded ST-70.
 
That’s what I expected. I listen to jazz other acoustic music in a small room and am very fond of the EL34 sound. I would jump on Mark IVs.
 
Ya, you wouldn't even notice the extra 20 watts output. Even if you doubled the output of the ST-70 thats only around a 3db difference in volume.
There isn't even a huge difference in volume between my VTA ST-70 and my
M-125 125 watt mono blocks.
 
My first thought was to try to set them up for 34s but with almost 500v on the plates that would be tough, and to what end? Probably more heat too.
The ST70 provides all the power I need and has been dead reliable for going on a decade.
 
Repaired a few MK III's that were running Trioded connected EL34's.

Sound was greatly improved over the UL 6550's, But I'd still take ST-70's over them.
 
nerdorama said:
The Mark 2’s used EL34 with the same power and output transformers. If they are biased properly no problem.
Click to expand...
From my limited research it looks like there were changes made to the Mark II during its run. Some sites claim the OPT was similar, omitting the 4 ohm tap but an upgrade to the OPTs of the III were available. Triode Electronics offers a new board for use on both the II and III. So what gives with the much higher B+? How different are they and could a Mark III be made to run EL34s?
 
FWIW, I had the Mk III amp way back when it was a pretty new model, I think. The power supply can capacitor that would blow periodically and need to be replaced. Only amp I had with that problem. My take is that the Stereo 70 is more reliable.
 
Blackdog61 said:
I’ve really enjoyed the ST70 I built for the last ten years or so, but……..
A friend has a pair of IIIs for sale which were modded by his father who passed a few years ago. I haven’t seen them and my friend says they should probably be stripped and rebuilt. Price is a nice lunch which sounds like a definite yes, but I like my ST70. The midrange on the 70 is just so good, I don’t need more power and don’t know if the project is worth it.
Tell me, what would I gain and lose?
Click to expand...
At that price you cannot resist.
Contrary to others in this thread : MkIII in original shape does what the ST-70 is good at with more power and at better reliability.
 
Not having owned either, original ST-70 or MK111's I cannot comment on sound difference but it sure would be fun owing and playing with such desirable amplifiers. I would love owning a pair just because but I am always buying, building electronics I don't need.
 
nerdorama said:
The Mk2’s used a 5U4 rectifier which has a higher voltage drop than a 5AR4 as used in the MK3.
Click to expand...
also a drop resistor vs a choke, which possibly drops some extra voltage as well.

Its the same power transformer though, and if you look at the Mark II schematic it lists two transformers, one of which is the same used on the Mark III. I think the only difference is the 4 ohm tap, so it should be no issue running an EL34 in a Mark III, it will have the benefit of better filtering and might make an extra couple of watts. To get the same bias current setting though, the voltage will be different. Mark II is 1.56v / 12 ohms = 130mA total. In a Mark III, its .13A * 11.2 ohm = 1.46 volts.

Mark II

1790595249746.png
Mark III

1790595362861.png
 
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