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Old 8th March 2005, 01:46 PM   #1
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Default Dynaco ST-120

I was wondering if this amp would be worth saving or modifying? What are opinions on this amp?

Dave
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Old 8th March 2005, 07:56 PM   #2
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Chatty bunch around here, aren't they?

And I still think the answer is 'yes', but:

1. Turn it into a personal project...in other words, do it yourself.

2. Don't expect sonic bliss from an ST120...it was a stepping-stone on the road to high-quality SS amps, and has a strong place in history, but though a restored ST120 can sound pretty decent, just don't listen to a really good amp for a few hours after, else you are likely to discover the same thing that a lot of other ST120 owners did once really good SS designs began to appear and they relegated the Dynaco to a corner of the closet.
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Old 8th March 2005, 08:13 PM   #3
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OK, I will go out on a limb with nothing but my memory to back me up. Back in the early 70's I breifly owned a ST-120 with a Dynaco outboard capacitor upgrade, which alleged more power.

Several friends owned ST-70's at the same time..... the ST-120 wasn't even close. The ST-120 quickly made way for a SWTP, Stereo Tiger, which for some odd reason I liked more.

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Old 8th March 2005, 08:15 PM   #4
SY is offline SY  United States
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A rational approach is to just replace the guts with a chip amp. The issue will be the single ended supply; it will take a bit of ingenuity to get rid of the output coupling cap.
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Old 29th August 2011, 12:24 AM   #5
pwgtang is offline pwgtang  Armenia
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I recently pick up a dead Dynaco 120,after I fix it,I found the amp had some
crossover distortion,and those 300 Ohm resistors had cause PCB burned. I had
modify the circuit with good result,distortion lower, put out more power
before clip than original circuit.
I know this is old thread,but may be some one interesting to try it out too.
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Old 29th August 2011, 12:36 AM   #6
pwgtang is offline pwgtang  Armenia
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I recently pick up a dead Dynaco 120,after I fix it,I found the amp had some
crossover distortion,and those 300 Ohm resistors had cause PCB burned. I had
modify the circuit with good result,distortion lower, put out more power
before clip than original circuit.
I know this is old thread,but may be some one interesting to try it out too.
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Old 29th August 2011, 01:12 AM   #7
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Getting rid of the crossover distortion at very low volume is the goal of djoffe in this thread:Dynaco Stereo 120...can be beautiful
It appears you have tried to do the same with a much lower parts count. I presume the 30-40 ma you are talking about is the current across the .47 ohm resistor between the top and bottom output transistor.
Your circuit appears to have the disadvantage that if the pot wiper loses contact, something awful happens. I like the parts count, but not that effect.
My ST120 I always played loudly enough (1v PP average out), I never noticed the crossover distortion. But if you turn the input down enough, it is there.
The tremendous advantage of these is that if something goes wrong, the series cap protects the speaker very cheaply. The disadvantage of these is that the 1/8" aluminum flanges used as heat sinks heat up if you use more than 1 v pp for more than an hour or two.
Have fun. I've built and installed the djoffe transistor bias circuit. I'm not using mine now because there is a thermally sensitive joint in there on PC14, that the djoffe circuit I built physically covers up. I'm waiting until the rains of winter to dig into it again, when I will build new mount brackets just for debug. When both channels are working, it sounds very good at 1v PP, very similar to the peavey CS800s I bought last year. Both st120 with joffe bias and the cs800s sound better than the tube dynakit ST70 I am actually using, but as is usual, tubes are so **** easy to fix that I am using the ST70 this summer without thinking about it.
I did put PCAT fans on the heatsinks of the ST120 like micky mouse ears, so I can leave it on all day and not worry about it melting the solder off the output cap. When that melted and the lead sprang up, it cause a ball of fire under the lectern at church 10 years ago at the end of a 3 hour Christmas Cantata rehearsal. The junior choir was very entertained. The ST70 came to the rescue that time, too, worn out 6CA7's and all. Hint- if you use snap in output caps (3300) , glue them through a lexan board and loop the wires to it through the board for strain relief before soldering them to the snap in cap.
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Last edited by indianajo; 29th August 2011 at 01:38 AM.
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Old 29th August 2011, 01:28 AM   #8
djoffe is offline djoffe  United States
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Default Shameless commerce division...

with apologies to frick and frack, the tappet brothers, of cartalk fame...

I make a chipamp kit to replace the amplifier modules of the Stereo 120...

check it out at:

Update My Dynaco
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Old 13th September 2011, 03:26 AM   #9
pwgtang is offline pwgtang  Armenia
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Djoffe, I want to keep the original pcb with minimum components change.
Indaianajo, I had changed again, the update version I removed the diodes and
pot,I only used a red LED for the bias,the bias voltage on the LED
is around 1.835 volt one channel and 1.850 volt on other channel.
I also parallel the 35 uf cap to LED which I removed from the pcb
early.
I found out high idle current sounds better to me,with the red LED
as bias,the idle current are 250ma on one channel and 300ma on
the other, I had to add more heat sink for high idle current to avoid
over heat.
I may match the LEDs later.
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Old 13th September 2011, 04:22 AM   #10
djoffe is offline djoffe  United States
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Default High Idle current

I have no doubt that the high idle current would sound better, as I'm sure it dramatically reduces the inherent crossover distortion. I'd be a bit worried about thermal runaway, though...but I guess the right combination of larger heatsinks and appropriate tempcos in the bias LED, plus the right thermal coupling, might be able to stave off those problems.
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