Even Marantz used the black cat capacitors in that era, but Dynaco sold ST-70s using them until
near the end of production. I suppose Dyna had 10k or more boards made at one time. At least
at one point in the early 70s they switched from phenolic to fiberglass boards. They must have
bought well over a million black cats of several values to use in their tube amps and preamps,
especially lots of the 0.1uF and the 0.05uF ones.
near the end of production. I suppose Dyna had 10k or more boards made at one time. At least
at one point in the early 70s they switched from phenolic to fiberglass boards. They must have
bought well over a million black cats of several values to use in their tube amps and preamps,
especially lots of the 0.1uF and the 0.05uF ones.
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See post #1: all caps have been replaced, with new modern caps. Coupling 630v.
Amp was designed back in the 115-117 Ac days, now it’s getting 120+.
What about adding a CL-80 thermistor? Or a pair of them at the ac input after the power switch? No variacs here, and I’ve read about bucking transformers, but would rather start with something cheaper first if it would help. It’s not like it’s getting ready to set the wall on fire behind it, but it’s running pretty warm. Heater voltage about 6.6, and transformers will get up to 140 degrees and a little higher after several hours. I’m making a small parts list and am really considering these thermistors, but I really don’t consider these amps to be “overheating” , but if I can get them to run a little cooler, I will.
Amp was designed back in the 115-117 Ac days, now it’s getting 120+.
What about adding a CL-80 thermistor? Or a pair of them at the ac input after the power switch? No variacs here, and I’ve read about bucking transformers, but would rather start with something cheaper first if it would help. It’s not like it’s getting ready to set the wall on fire behind it, but it’s running pretty warm. Heater voltage about 6.6, and transformers will get up to 140 degrees and a little higher after several hours. I’m making a small parts list and am really considering these thermistors, but I really don’t consider these amps to be “overheating” , but if I can get them to run a little cooler, I will.
And 6a3, I definitely plan on trying the 4ohm tap on the vandersteen ‘s, just to see what they sound like. Gonna order the parts too to hear these amps in triode.
You can use a bucking transformer. This thread from another forum has some good information on tackling this issue. ( I hope it is okay to post links from other discussion boards.)
Dynaco ST-70: Base Line Testing | Audiokarma Home Audio Stereo Discussion Forums
Regards,
Tried the 4 ohm taps, and I don’t think I would call it an improvement. Big difference, tightened up the bass for sure, but at the expense of tightening everything else up also -
Sounded like I had run everything through a compressor. Now compression certainly has its benefits, but this sounded like less dynamics, smaller soundstage. Some areas were positive, but it just effects too much. Put it back on 8 and preferred that. Bass is looser,
But the “air” is back with this setting.
I think the negative feedback is the thing I want to adjust , although just very slightly.
The dynaco has 20db nf, and moving the nfb connection to the 8 ohm tap supposedly
Lowers it by 3db. I tried that and it’s too much. I’d like to be able to adjust about 0-3 dB worth. With a pot, but am not sure if this wouldn’t have some adverse effects to the circuit-
Don’t want to damage anything.
Sounded like I had run everything through a compressor. Now compression certainly has its benefits, but this sounded like less dynamics, smaller soundstage. Some areas were positive, but it just effects too much. Put it back on 8 and preferred that. Bass is looser,
But the “air” is back with this setting.
I think the negative feedback is the thing I want to adjust , although just very slightly.
The dynaco has 20db nf, and moving the nfb connection to the 8 ohm tap supposedly
Lowers it by 3db. I tried that and it’s too much. I’d like to be able to adjust about 0-3 dB worth. With a pot, but am not sure if this wouldn’t have some adverse effects to the circuit-
Don’t want to damage anything.
I've run Vandersteen 2C's on modded Dynaco's, here's what I learned.
Dynaco 70 with VTA old style 12at7 board: I never could get this to
sound right. Better 12at7's helped, but I think the problem was this
circuit uses less feedback than stock or the other mods I tried, and
the upper midrange was too harsh (there's a bit of a treble peak in
the 2C's)
70 with stock circuit with caps replaced, main power cap at 200uF,
resistors checked and matched: Not bad, but the bass and high treble
lose something. Midrange very nice.
Van Alstine 70 "Ultimate" board: This sounded good, clean to the
high/low extremes. This is a pentode-triode circuit similar to the
original but with simple regulation of the input and driver stages,
and all the time constants redone.
KTA 6SL7/6SN7 circuit with Mark II/III output transformers, and large
brute-force power supply. This was very nice, it had a sense of calm
in the presentation, absence of listening fatigue? This could have
been due to the power supply (several times higher capacity than my
other builds, big choke and power transformer) and/or the input
circuit itself (regulated 380V supply too.)
I'd reckon the Van Alstine and KTA were close, but each with different
strengths. Difficult to really compare because of the power
difference, and I never got fresh power tubes in the KTA circuit. When
I moved I tore that down because it was massive, and the power
transformer had a bad mechanical hum. May still rebuild as monos.
I also tried 4/8 ohm taps, and slightly preferred 8 for similar
reasons as 60cycle.
I expect any of the "Geek/Classic valve" circuits would be good
candidates too.
For a tube amp for these speakers I would want these things:
Power: at least push-pull KT88s
At least 20db feedback for damping and low distortion
Improved power supply significantly over the original Dynaco
-jgf
Dynaco 70 with VTA old style 12at7 board: I never could get this to
sound right. Better 12at7's helped, but I think the problem was this
circuit uses less feedback than stock or the other mods I tried, and
the upper midrange was too harsh (there's a bit of a treble peak in
the 2C's)
70 with stock circuit with caps replaced, main power cap at 200uF,
resistors checked and matched: Not bad, but the bass and high treble
lose something. Midrange very nice.
Van Alstine 70 "Ultimate" board: This sounded good, clean to the
high/low extremes. This is a pentode-triode circuit similar to the
original but with simple regulation of the input and driver stages,
and all the time constants redone.
KTA 6SL7/6SN7 circuit with Mark II/III output transformers, and large
brute-force power supply. This was very nice, it had a sense of calm
in the presentation, absence of listening fatigue? This could have
been due to the power supply (several times higher capacity than my
other builds, big choke and power transformer) and/or the input
circuit itself (regulated 380V supply too.)
I'd reckon the Van Alstine and KTA were close, but each with different
strengths. Difficult to really compare because of the power
difference, and I never got fresh power tubes in the KTA circuit. When
I moved I tore that down because it was massive, and the power
transformer had a bad mechanical hum. May still rebuild as monos.
I also tried 4/8 ohm taps, and slightly preferred 8 for similar
reasons as 60cycle.
I expect any of the "Geek/Classic valve" circuits would be good
candidates too.
For a tube amp for these speakers I would want these things:
Power: at least push-pull KT88s
At least 20db feedback for damping and low distortion
Improved power supply significantly over the original Dynaco
-jgf
Yeah the st-70 is probably not quite optimal for the 2c speakers. I think minimum vandersteen recommends is 40wpc. Although, that’s close enough it should still sound decent, but not as good as more wpc would sound. Did you keep the 2c’s? And what are you running through them if you did? The mk iii should be fine with them , 60wpc. My other power amp was 50-55wpc with kt88’s and it sounded really good with them.
Thanks for the driver board comparison with them, I’ll look up the kta and van alstine and see what they’ve done. I read van alstine talking about the feedback circuit and he was adding an input bandwidth and changing a feedback resistor value in one his mods to the dyna, saying it really helped.
This mk iii is stock and sounds pretty good, great separation , but just a little thin in a few places. Most of the resistors have been changed on the inside, and some on the board , but some of the ones for the nfb circuit are original. I haven’t disconnected them, but have tested them with power off of course, and they check out ok. I may change them anyhow .
I’ve read a lot about the nfb circuit, but haven’t found any real good instruction or tests on it. Most of the replacement boards seem to lower it, but they mostly seem pretty radical in changing the whole driver circuit, so lots of variables, hard to nail down any one thing that effects the sound. I very well may change out the entire board, that’s a whole different can of worms there, with so many to choose from. But first I’m going to work with the stock circuit and see if I can’t figure out these small changes I’d like to get.
The feedback has a huge effect- just slightly lowering it, brings out the mids tremendously. Gain goes up super fast too, which is why I just want to make a very small adjustment. Just not sure what would be the best part of the nfb loop to mess with . When I adjusted it right at the 16ohm tap, it had the huge changes I just mentioned, but there may be a more subtle place to attempt to adjust. I saw the 47r resistor going to the 6AN8 mentioned, but no idea which way to go on it , or how much.
Thanks for the driver board comparison with them, I’ll look up the kta and van alstine and see what they’ve done. I read van alstine talking about the feedback circuit and he was adding an input bandwidth and changing a feedback resistor value in one his mods to the dyna, saying it really helped.
This mk iii is stock and sounds pretty good, great separation , but just a little thin in a few places. Most of the resistors have been changed on the inside, and some on the board , but some of the ones for the nfb circuit are original. I haven’t disconnected them, but have tested them with power off of course, and they check out ok. I may change them anyhow .
I’ve read a lot about the nfb circuit, but haven’t found any real good instruction or tests on it. Most of the replacement boards seem to lower it, but they mostly seem pretty radical in changing the whole driver circuit, so lots of variables, hard to nail down any one thing that effects the sound. I very well may change out the entire board, that’s a whole different can of worms there, with so many to choose from. But first I’m going to work with the stock circuit and see if I can’t figure out these small changes I’d like to get.
The feedback has a huge effect- just slightly lowering it, brings out the mids tremendously. Gain goes up super fast too, which is why I just want to make a very small adjustment. Just not sure what would be the best part of the nfb loop to mess with . When I adjusted it right at the 16ohm tap, it had the huge changes I just mentioned, but there may be a more subtle place to attempt to adjust. I saw the 47r resistor going to the 6AN8 mentioned, but no idea which way to go on it , or how much.
I've since acquired Gallo speakers, so I didn't keep the
Vandersteens. Powering them with a stereo 70 was just barely acceptable in
a small room, but the compression effect of running out of power gets
noticable at moderately high volume. As the power supply is beefed up,
this is less of a problem until it craps out more suddenly on high
volume parts.
The Van Alstine mod available on the web is a really early version, I
did try that too early on, I thought it was an improvement over
stock. They're on at least a 3rd version now, but they don't give out
the schematics. The 2nd version was supposedly nearly identical to
the "Dynaco series 2".
I think lowering feedback can sound 'interesting' at first, but
ultimately I didn't like it. Generally messing around with feedback
requires a scope and a dummy load, or you risk damaging your
speakers. I did play around with a three triode circuit and got the
feedback to about 16db with just one compensating cap, but never got
to adding compensation to the input circuit, it takes a lot of
iterations, I never built up a safe setup where I could adjust a
powered up amp, so it was a lot of power cycling and waiting.
One thing I would check on the stock board is that the resistors that
are supposed to be matched are within 1% or better (the phase splitter
resistors and the power tube grid to ground resistors).
Vandersteens. Powering them with a stereo 70 was just barely acceptable in
a small room, but the compression effect of running out of power gets
noticable at moderately high volume. As the power supply is beefed up,
this is less of a problem until it craps out more suddenly on high
volume parts.
The Van Alstine mod available on the web is a really early version, I
did try that too early on, I thought it was an improvement over
stock. They're on at least a 3rd version now, but they don't give out
the schematics. The 2nd version was supposedly nearly identical to
the "Dynaco series 2".
I think lowering feedback can sound 'interesting' at first, but
ultimately I didn't like it. Generally messing around with feedback
requires a scope and a dummy load, or you risk damaging your
speakers. I did play around with a three triode circuit and got the
feedback to about 16db with just one compensating cap, but never got
to adding compensation to the input circuit, it takes a lot of
iterations, I never built up a safe setup where I could adjust a
powered up amp, so it was a lot of power cycling and waiting.
One thing I would check on the stock board is that the resistors that
are supposed to be matched are within 1% or better (the phase splitter
resistors and the power tube grid to ground resistors).
I don't like the original Dynaco circuit one bit: it's way too compromised in order to make something cheap.
The smartest mod I've seen is from Kara Chaffee of deHavilland, published in the AudioXpress Mar 2001. I misplaced my copy, but IIRC, she skilfully turned the circuit into a modern version of the Mullard circuit, while retaining the original PCB. It's for the experienced DIY'ers only.
The smartest mod I've seen is from Kara Chaffee of deHavilland, published in the AudioXpress Mar 2001. I misplaced my copy, but IIRC, she skilfully turned the circuit into a modern version of the Mullard circuit, while retaining the original PCB. It's for the experienced DIY'ers only.
Planet 10,
It is not that easy to change from a tube rectifier and cap input filter;
to a solid state rectifier and choke input.
You will not get the same B+ voltage.
Original:
Example, take a 350-0-350 secondary
Use a tube rectifier with 50V drop
Cap input filter 350 x 1.414 = 495V
495V - 50V = 445V
445V using tube rectifier and cap input filter
Solid state rectifiers and Choke input:
Take a 350-0-350 secondary
Use solid state rectifiers with 1V drop
Choke input filter 350 x 0.9 = 315V
315V - 1V = 314V
314V using solid state rectifiers and choke input filter
445V versus 314V
Does not work.
Given the same power transformer, when switching from tube rectifier to solid state rectifiers:
Use a power resistor in series with each diode, so the resistor voltage drop gives the same B+.
But the only power and heat savings when you do that is:
There is no 5V rectifier secondary heating, and no rectifier filament heating.
With the required voltage dropping resistors in the circuit, the B+ is no stiffer than it was with a tube rectifier.
Not much is gained.
And yes, I do like solid state rectifiers and choke input filters.
You just have to start with a higher voltage B+ secondary.
And you are right! . . .
When you change to choke input filter, you usually need one more RC filter stage in the B+ filter to get the ripple down to the level it was with a cap input filter.
It is not that easy to change from a tube rectifier and cap input filter;
to a solid state rectifier and choke input.
You will not get the same B+ voltage.
Original:
Example, take a 350-0-350 secondary
Use a tube rectifier with 50V drop
Cap input filter 350 x 1.414 = 495V
495V - 50V = 445V
445V using tube rectifier and cap input filter
Solid state rectifiers and Choke input:
Take a 350-0-350 secondary
Use solid state rectifiers with 1V drop
Choke input filter 350 x 0.9 = 315V
315V - 1V = 314V
314V using solid state rectifiers and choke input filter
445V versus 314V
Does not work.
Given the same power transformer, when switching from tube rectifier to solid state rectifiers:
Use a power resistor in series with each diode, so the resistor voltage drop gives the same B+.
But the only power and heat savings when you do that is:
There is no 5V rectifier secondary heating, and no rectifier filament heating.
With the required voltage dropping resistors in the circuit, the B+ is no stiffer than it was with a tube rectifier.
Not much is gained.
And yes, I do like solid state rectifiers and choke input filters.
You just have to start with a higher voltage B+ secondary.
And you are right! . . .
When you change to choke input filter, you usually need one more RC filter stage in the B+ filter to get the ripple down to the level it was with a cap input filter.
Problem solved- added a 1k linear pot and a 220k resistor in series at the feedback opt 16 ohm tap. It’s a minor adjustment that the feedback does, but it’s perfect for my speakers, room and of course ears. These amps sound great- I think they might have overdone the feedback back in the 50’s, trying to get perfect test specs, and everyone was using vinyl- and I would guess they tried to minimize poor condition vinyl noise . Anyhow , with today’s super clean digital- the amps sound a bit “cold and clinical”- this feedback trick fixes that , and it’s adjustable, so if you want stock feedback, listening to vinyl or whatever, just turn the knob.
@60cyclehum A little late here. I'd like to try this. Did you do it without drilling holes? I'm pretty green on amp repair, could you ELI5 or have a picture?