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Old 29th January 2012, 10:06 PM   #1
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Default Output transistors for Dyna ST-120?...

Hi all...

I'm new to this forum, but far from new to DIY audio... (still have all my '70s vintage issues of Audio Amateur...)

Question... Recently I was given an old Dynaco ST-120. As expected, its non-functional and needs a complete re-build... and its an oldy... before the "TIP mod".

Now, I've brought several of these things back to life over the years, but its been at least 15 years since I've worked on my last one. So... what I'd like to know is what output transistors available today are people using to fix these?... I remember buying parts from an outfit in Ohio way back when that had all of Dynaco's old inventory, but they're loooong gone... and I also remember using 2N5630s (what Dyna put in their "400" if I remember correctly)... but those are now NLA (at least ones I would trust buying).

I've read of people using 2n3772s... but as with 2N3055s (which I've also seen people use) they don't seem to have the required ratings ( Dyna originally used a specially selected 3055s).

In reading on this forum and others, I've seen recommendations for MJ15003s, MJ802s and the previously mentioned 2n3772... but I'd love to hear input from anyone that's done a rebuild recently and hearing what they used and how they worked...

BTW... to ward off opinions as to whether this thing should be saved or not... Let me just say that I've built/owned/repaired several ST-120s over a 35 year span and know their pluses and minuses... this is just a nostalgia trip for me...

Thanks... jjw.
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Old 30th January 2012, 10:17 AM   #2
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The datasheet for the 2N5630 gives fT as 1MHz

For both MJ15003 and MJ802 it is 2MHz.

Close enough? Probably.
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Old 30th January 2012, 11:02 AM   #3
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I've not done a rebuild but the 2N3773 is the obvious choice in my opinion.
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Old 30th January 2012, 11:48 AM   #4
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Here's another option:

Update My Dynaco
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Old 30th January 2012, 01:12 PM   #5
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If you don't want to gut it, try MJ15015. It has the Vceo and an actual SOA rating . MJ802 and MJ15003 would work but cost about 3 times as much. All these fast transistors could oscillate, so do the part of the TIP mod that involves putting 10-20 pf capacitors from b to e on the driver transistors.
Running mine at 20W more than 3 hours got the whole case and output capacitor so hot it melted the solder on the snap in terminal of the output capacitor. The resulting fireball under the lectern really impressed the junior choir. So I put PCAT fans on a bracket to blow on the two output transistor flanges. They are outside the cover and look like mouse ears. I put covers over them to keep the cat out of the blade. I power the fans with a 9v V wall transformer so they don't run full speed. Now I run my ST120 18 hours a day at home without heat or problem. Another guy looped the wires to the new snap in output cap through a little insulator board so they don't want to jump off the output capacitor. I recommend this. Snap in output caps are so much cheaper than screw terminal ones. Mine has all new electrolytic caps, except no 20 uf non polars.
Eliminating the cold crossover distortion with 6 transistors/channel really peps up the sound, along with the faster output transistors. djoffe came up with a design for that before he started selling LM3886 kits. I installed it and love the result. See this thread www.diyaudio.com/forums/solid-state/156627-dynaco-stereo-120-can-beautiful.html I'm using ZTX653 transistors from diodesinc instead of the 2N3904 on the input pair because when I got a little heat sink compound on the ring terminal the .5 ohm resistor is mounted on, it put 80V on the input transistors and blew them up. I built the two bias boards on 1.5"x3" fiber boards I drilled holes in and mounted them on angle brackets above the PC15's. They fit under the cover.
Oh, and somebody did a simulation on a leak Delta 70 with similar geometry and found the distortion got lower with TIP3055 transistors instead of 2n3772, and even lower with something even faster. I'm using NTE60's actually (probably MJ802 equivalents, I bought them in 1985) and the sound is now good enough I can't tell the difference between this and the CS800s amp which has no capacitors in the sound path whatsoever. Both amps sound better than my recapped ST70 with new Output and rectifier tubes. One channel is still using the RCA TO5 driver transistors, so I don't think these are a limitation. Both channels have a 4.7 uf 50V aerovox gold ceramic input capacitor, which many people will say is stupid, but solved the problem of a frying pan noise I got with new tantalum input caps.
One further tip. I found a modern NTE60 for the power transistor on the current regulator PCB PC14 caused the output current to trip off at 2 amps total instead of the designed 6.5. Actually that is the way it was when I melted the output capacitor solder. To to replace that old low gain RCA transistor, you have to put a resistor between b & e until the trip point is correct. I'm using a 225 W 10 ohm adjustable ohmite resistor to set the trip point, just connecting it to the feed to the PC15's. PC14 is not really a useful safety device, but it does regulate the open circuit voltage to 80 from 100 the transformer puts out, so that part is useful. I installed two 5 amp fuses for the separate PC15's.
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Last edited by indianajo; 30th January 2012 at 01:28 PM.
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Old 30th January 2012, 01:51 PM   #6
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Hi,
I used 2N3442G in all my rebuilt and the sound is excellent. Never have a failure with them.
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Old 30th January 2012, 10:29 PM   #7
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Thanks everyone for the input... this is a great forum. My primary concern is with stability when using alternate transistors... as its unlikely there would be any sonic difference.

The mods suggested are definitely interesting and food for thought, but I think I'll leave this one as stock as possible...

Thanks again... jjw
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Old 30th January 2012, 11:53 PM   #8
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The original transistor were SOOOO slow that it is hard to not improve the sound by replacing them. They don't make anything that slow anymore. I've got one survivor 1970 TO3 transistor, I can't measure the ft but the gain is pitiful. It is acting as the DC regulator in PC14. Since modern O.T.s have ft of 2-4 mhz, I suggest the TIP mod with the resistor across the output jacks, and the 20 pf caps across driver b-e, even if you don't change the driver transistors to TIP's. I did those mods. RF oscillation is a nasty thing that heats things up, and these sure don't need any extra heat source as bad as the heat sinks are. I left the RCA to5 transistors on one side. The side with the avalanced driver transistors I used NTE49 & 50 , and I had to use an NTE 249 for the input transistor. Because all the old stuff was blown up on one channel. All except the 5.1V zener that everybody says blows every time. If I had to do it again I would use MPS8099 for the little transistor and TIP31C and TIP32C for the drivers. The pinout is different on the TO220's than TO5, you have to twist the legs around.
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Last edited by indianajo; 30th January 2012 at 11:57 PM.
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Old 31st January 2012, 01:09 PM   #9
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"The original transistor were SOOOO slow that it is hard to not improve the sound by replacing them."

Hence my concern for stability and wondering what people were using successfully these days... the last thing I want to do is go through the process and then wind up with a smoldering 60 wpc oscillator... ;-)

As stated earlier, the amp I'm working with now is an oldie... before the "TIP" update... so I'll be bringing it up to the last factory configuration. Also, there is evidence of previous (attempted) repair demonstrating a lack of experience with this designs quirks...

...if I had a quarter for every ST-120 I've seen like this....

Thanks again... JJW
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Old 5th February 2012, 07:59 AM   #10
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The MJ15015 is just as slow as the original, try it first.
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