Hello all, just picked up a pair of mk iii’s, and looking for advice. I’m new to working on tube amps, but I know what sounds good to me. I’m using these guys, through a Tad-150 preamp (pretty decent Chinese/slightly modded by a guy that used to sell them in Chicago) , and vandersteen 2ce sig II speakers.
Pretty revealing speakers. I listen mainly to rock, so I want it to sound right-
Full and driving. Don’t care for classical, or being able to determine the orchestra
Hall size. Do listen to plenty of acoustic music, jazz, whatnot- and I’ve been a musician for 30 years- so I know exactly how certain instruments sound.
First impression of these dynaco’s- they are good amps. Very musical, with great separation. Not much to complain about. They are stock- rebuilt with new caps, selenium rectifier changed. 550 filter caps.
However- they are a little lacking- a bit thin sounding. Reverb stands out way too much. They’ve got decent low end, and plenty of highs, decent mids, but seems like something is missing. And that reverb area- really sticks out.
I’ve read tons about them, and am not sure what to attack. I thought maybe negative feedback- and I disconnected the 680 resistor and added a 250k pot after it, and it did make a useful control, but the range I got from the pot was way too much, slightly moving it up made a huge difference- mids popped out huge, and gain went up big time. This would be a nice control to have, but this pot was way overkill. I also tried the feedback at the 8 ohm tap, but that was a little too much as well. Anyhow- like I said, I’m new to this and have no idea really- this feedback system is beyond me, and it might not be the right area to mess with.
These mk iii’s replaced a TAD-60 power amp, fairly similar- 60 wpc from 4 kt88’s. It did have a feedback control, and it was quite useful. The tad might have a bad opt, is the only reason I switched.
So what do you guys suggest? I’ve done speaker placement , and my room is small, so nowhere else to go there. Acoustic guitars are missing their full sound. Too thin. Something is missing, or maybe something is standing out too much. I’m guessing missing.
I chose these amps, from what I read about them, good solid amps, USA made, every component easily replaceable, with good transformers, simple- and probably a good intro to messing with tube amps. And I needed something to do locked in the house! Which they have delivered already in spades!
Pretty revealing speakers. I listen mainly to rock, so I want it to sound right-
Full and driving. Don’t care for classical, or being able to determine the orchestra
Hall size. Do listen to plenty of acoustic music, jazz, whatnot- and I’ve been a musician for 30 years- so I know exactly how certain instruments sound.
First impression of these dynaco’s- they are good amps. Very musical, with great separation. Not much to complain about. They are stock- rebuilt with new caps, selenium rectifier changed. 550 filter caps.
However- they are a little lacking- a bit thin sounding. Reverb stands out way too much. They’ve got decent low end, and plenty of highs, decent mids, but seems like something is missing. And that reverb area- really sticks out.
I’ve read tons about them, and am not sure what to attack. I thought maybe negative feedback- and I disconnected the 680 resistor and added a 250k pot after it, and it did make a useful control, but the range I got from the pot was way too much, slightly moving it up made a huge difference- mids popped out huge, and gain went up big time. This would be a nice control to have, but this pot was way overkill. I also tried the feedback at the 8 ohm tap, but that was a little too much as well. Anyhow- like I said, I’m new to this and have no idea really- this feedback system is beyond me, and it might not be the right area to mess with.
These mk iii’s replaced a TAD-60 power amp, fairly similar- 60 wpc from 4 kt88’s. It did have a feedback control, and it was quite useful. The tad might have a bad opt, is the only reason I switched.
So what do you guys suggest? I’ve done speaker placement , and my room is small, so nowhere else to go there. Acoustic guitars are missing their full sound. Too thin. Something is missing, or maybe something is standing out too much. I’m guessing missing.
I chose these amps, from what I read about them, good solid amps, USA made, every component easily replaceable, with good transformers, simple- and probably a good intro to messing with tube amps. And I needed something to do locked in the house! Which they have delivered already in spades!
I suggest you make sure they are in stock condition, that is where they
sound best. But do get new matched 6550/kt88 from a dealer that matches
tubes at real voltages. McShine is one that comes to my mind.
If you think sound is "thin" turn up you bass control a bit, thats what
tonecontrols does best.
sound best. But do get new matched 6550/kt88 from a dealer that matches
tubes at real voltages. McShine is one that comes to my mind.
If you think sound is "thin" turn up you bass control a bit, thats what
tonecontrols does best.
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I am sure that will change as you listen more. All those old parts — and a design from days when modern parts were not available — show themselves.
Unlike most your speakers these have a fairly flat impedance:
That means you don’t have to worry abou tthe output impedance.
There are good bones there, i’d look to something that will utilize the voltage from the trafo, and is appropriate OPT impedance.
IIRC even something like 300Bs or a triple of EL84s (ie 6 per monoBloc) will work, althou using the holes that are there would preclude that last.
The entire front end needs to be turfed. I got a DynaMutt) (http://www.classicvalve.ca/docs/DynaMutt_octal.pdf) in my ST70 (the 9-pin version), it is a superb front end but i don’t know how adaptable to mono it is. There are others for the ST70 and I am sure there are some for Mk3 if you don’t want to futz around.
Better input terminals and proper speaker posts are a must IMO.
There is lots of potential.
dave
I would not put much more money into these amplifiers, they were at best only passable.
The power supply is inadequate, and their reliability is only fair due to the heat and HV.
If you don't like them (while they are still working) sell them and get something else.
The power supply is inadequate, and their reliability is only fair due to the heat and HV.
If you don't like them (while they are still working) sell them and get something else.
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I will have to second this. There is a cult following for everything Dynaco. You can actually get the 70 to sound decent if you put enough money into it. If it were me I would clean them up and peddle them.
The trafo in my DynaMutt does get quite warm. It hasn’t dies yet.
For my SCA35/ST35 build i have 2 trafos so half the current.
You could install something that draws less current to move th eexisting trafo into a more comfortable range. Start with SS rectification.
dave
For my SCA35/ST35 build i have 2 trafos so half the current.
You could install something that draws less current to move th eexisting trafo into a more comfortable range. Start with SS rectification.
dave
I’m still getting used to them, but I like them. From what I’ve read, and there sure is a lot to read on these amps, is that the original circuit is a good design, and with the components being in spec, performs quite well. I see the tons of heavy mods available- the redesigned boards, but I think I’m going to stick with the stock circuit and get used to it maybe replacing just a few bits here and there and see if I get any good results.
My feedback experiment seemed to work well, just need to find the right pot to get a usable adjustment range.
Here’s a current series of testing on a Dynaco that got very good results- a stock unit:
http://www.audioregenesis.com/documents/ST-70 Base Line Testing.pdf
They do certainly put out some heat, but my other 4 kt88 amp cooked pretty hard too, I think kt88 amps just put out heat. I see where some have used thermistors to soften to initial start up,and that they may be useful for slightly lowering line vintage voltage , running things a little cooler. Anyone tried these with the dynacos and gotten them running a little cooler?
My feedback experiment seemed to work well, just need to find the right pot to get a usable adjustment range.
Here’s a current series of testing on a Dynaco that got very good results- a stock unit:
http://www.audioregenesis.com/documents/ST-70 Base Line Testing.pdf
They do certainly put out some heat, but my other 4 kt88 amp cooked pretty hard too, I think kt88 amps just put out heat. I see where some have used thermistors to soften to initial start up,and that they may be useful for slightly lowering line vintage voltage , running things a little cooler. Anyone tried these with the dynacos and gotten them running a little cooler?
Performance and reliability of unmodified MkIII is generally good and does not lack
modern followers.
Modified by hoarders is another case ..
modern followers.
Modified by hoarders is another case ..
You have a decidely different take on the amp than me. They are the kind of tube sound that gives tube sound a bad connotation. Loosy-goosy.
dave
Replacing the tube rectifier with solid state diodes will raise the Voltage of the B+.
The output tubes will get hotter.
The output transformers will warm up.
The power transformer will heat up.
Try connecting your Vandersteen to the 4 Ohm tap.
It has impedance at Bass frequencies of 4 Ohms.
See the curve in post # 4.
All just my opinions.
The output tubes will get hotter.
The output transformers will warm up.
The power transformer will heat up.
Try connecting your Vandersteen to the 4 Ohm tap.
It has impedance at Bass frequencies of 4 Ohms.
See the curve in post # 4.
All just my opinions.
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That's an idea - but
it will kill the can-cap during warm up. You will need to replace the cancap
with series connected caps or arrange some delay on the B+, both these will
complicate the device and still not add anything sonically.
MkIII is , like most Dynaco devices, optimized in more then one way.
If the amp has the original black cat paper coupling capacitors, they should be replaced soon,
as they always leak and cause unstable bias on the output tubes, if not outright failure.
They sound bad, too.
as they always leak and cause unstable bias on the output tubes, if not outright failure.
They sound bad, too.
Rayma,
Good point.
The best thing to do with those old coupling caps on Dyna amps is to hide them from your house and your memory (no museums here, someone might want to use them).
I had a stereo 70 that had 100mA on the left channel. One tube had 40mA, and the other tube had 60mA. Saturated output transformer, resulted in Bad bass, Distorted even in Midrange and Highs (intermods), and one very hot tube.
Be sure to replace with a modern 600V or 630V cap.
bOTL and petertub,
A correct solid state "substitute" rectifier would have to have enough resistance in it to drop the same voltage as the tube rectifier. In that case, the only reduction in heat would be the [missing] filament of the rectifier tube, and no current heating of the secondary winding that runs the rectifier filament. But the B+ heating would be identical.
Running that first B+ filter cap directly from solid state diodes will really heat up the B+ secondary windings, even if you replace the cap with a new one. And the current of the output tube screens will go up (higher screen voltage). And higher screen voltage means higher plate current (and at the new higher B+).
If you thought your amp was too hot, then with that modification it really will be hot.
Just biasing for the same current as before the mod, will still dissipate more screen watts, and more plate watts.
Good point.
The best thing to do with those old coupling caps on Dyna amps is to hide them from your house and your memory (no museums here, someone might want to use them).
I had a stereo 70 that had 100mA on the left channel. One tube had 40mA, and the other tube had 60mA. Saturated output transformer, resulted in Bad bass, Distorted even in Midrange and Highs (intermods), and one very hot tube.
Be sure to replace with a modern 600V or 630V cap.
bOTL and petertub,
A correct solid state "substitute" rectifier would have to have enough resistance in it to drop the same voltage as the tube rectifier. In that case, the only reduction in heat would be the [missing] filament of the rectifier tube, and no current heating of the secondary winding that runs the rectifier filament. But the B+ heating would be identical.
Running that first B+ filter cap directly from solid state diodes will really heat up the B+ secondary windings, even if you replace the cap with a new one. And the current of the output tube screens will go up (higher screen voltage). And higher screen voltage means higher plate current (and at the new higher B+).
If you thought your amp was too hot, then with that modification it really will be hot.
Just biasing for the same current as before the mod, will still dissipate more screen watts, and more plate watts.
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You need to do that anyway. and given modern caps are way better and way smaller, room for more of them.
dave