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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Fri 17, 2019 1:47 am 
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Bones007 wrote:
Not to steal the thread... but what are these amps worth? I found one sitting in storage for years... have not tried it... audio nuts tell me these go for anywhere from 300 to 1000.... is that true?... What make them so special?
Thanks


If its been sitting for years, don't just try it... it will be worth less...... check ebay for internet market value.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Sat 18, 2019 6:46 pm 
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Posts: 4597
Location: Sunnyvale CA
Bones007 wrote:
Not to steal the thread... but what are these amps worth? I found one sitting in storage for years... have not tried it... audio nuts tell me these go for anywhere from 300 to 1000.... is that true?... What make them so special?
Thanks


$400-ish is about the middle of the range. This is not expensive- this is extremely cheap, compared to new manufacture tube power amps. There are plenty of people selling $4 power IEC power cords for $5000, $1000 for a tube amp (and the ability to sneer at your friends because you are a "pure tube amplification" guy, buying "credibility", which is what most audio people are in it for), is nothing.

There are literally hundreds of thousands of them around. It's probably the cheapest way to get a tube power amp. I have rebuilt a few dozen of them for other people, and one for myself, after a few of them, you can almost do it without looking at the instructions - which were also easily available since it was originally a kit.

It's also extremely simple to repair/refurbish, the parts are easily available. In fact, you can still get a complete kit for it, or any of the individual parts, generally of much high quality than the originals. There are also about a million (or so it seems) "modifications" to supposedly improve the performance, also very inexpensive. You can even get a complete, new-manufacture, ready-to-operate, version with the best of the modifications already included and a redesigned chassis for a mere $1600, which is *extremely inexpensive* for the performance compared to the potential competitors. It's durable and forgiving, at least as reliable as any other tube power amp.

With the best of the modifications, the performance is as good as any pure tube amp ever made, it generates about as much power as you can (~30-33 watts/channel, depending on which power transformer you get) from a tube amp without creating other essentially insurmountable problems. The only downside are the tendency of the front-end board to self-destruct, and the cost/availability of the 7199 tube. Almost all the modification kits come with a far higher quality PWB and replace the 7199 with various other more available tubes, like the 6GH8a, or 12AU7.

Basically, you can get about as good a tube amp as it is possible to make (which is not to say it is near the best of any power amp...) for under $1000 with some work. So, get the chassis and transformers from a working unit for $300-400, a modification kit for a few hundred, and in a few evenings you hae about as good a tube amp as anyone else (and better than most).


Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Fri 07, 2019 10:34 pm 
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Location: Beaver Falls, PA. USA
The replacement PC board is the way to go; anyone who has worked on the early boards knows that they are junk, which is why some manufacturers wouldn't use them.

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Tim KA3JRT


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Wed 12, 2019 1:50 pm 
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Part 3 has been posted in the original post on the first page


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Thu 13, 2019 6:48 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4278
Location: Boston, MA USA
You didn't really re-install the old selenium rectifier, did you?

-David


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Thu 13, 2019 8:07 pm 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
dberman51 wrote:
You didn't really re-install the old selenium rectifier, did you?

-David



Really did, and while it works fine on lots of different gear i have without issue ever, I've noticed it seems to drive people on message boards crazy for some odd reason.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Thu 13, 2019 8:29 pm 
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Location: Boston, MA USA
That old thing provides bias to the output tubes. It is arguably the most important part of the amplifier. If it fails and you lose bias, it will likely cost you all four output tubes and both output transformers.

I would replace it with a silicon rectifier immediately.

-David


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Thu 13, 2019 8:52 pm 
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Joined: Aug Sat 01, 2015 8:54 pm
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dberman51 wrote:
That old thing provides bias to the output tubes. It is arguably the most important part of the amplifier. If it fails and you lose bias, it will likely cost you all four output tubes and both output transformers.

I would replace it with a silicon rectifier immediately.

-David



Did you watch the video?


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Fri 14, 2019 2:36 am 
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Location: Boston, MA USA
The rectifier sections of videos 2 and 3. Why?

You may have never had one fail. I have never had one short out and smell bad. But I have had several develop high forward resistance and fail to deliver proper output voltage (down by 50%). Not a dangerous failure in a high voltage B+ circuit, but a terrible, terrible thing in a bias circuit.

Cathode-bias power amplifiers are self-stabilizing, but fixed-bias amplifiers are tricky and require great care with bias and screen grid circuits.

Spending all that time to line up screw heads and then risk costly tubes and output transformers to a 50 yr old selenium rectifier, where for $.01 you can have a silicon rectifier, is more than unwise.

-David


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Fri 14, 2019 3:36 am 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
dberman51 wrote:
The rectifier sections of videos 2 and 3. Why?

You may have never had one fail. I have never had one short out and smell bad. But I have had several develop high forward resistance and fail to deliver proper output voltage (down by 50%). Not a dangerous failure in a high voltage B+ circuit, but a terrible, terrible thing in a bias circuit.

Cathode-bias power amplifiers are self-stabilizing, but fixed-bias amplifiers are tricky and require great care with bias and screen grid circuits.

Spending all that time to line up screw heads and then risk costly tubes and output transformers to a 50 yr old selenium rectifier, where for $.01 you can have a silicon rectifier, is more than unwise.

-David


Had you watched,

1. You wouldn't have been asking me if I installed it, unless you were being coy.
2. You would not have brought this up for debate. This was explained in detail


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Fri 14, 2019 7:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4278
Location: Boston, MA USA
jcrubin wrote:
dberman51 wrote:
The rectifier sections of videos 2 and 3. Why?

You may have never had one fail. I have never had one short out and smell bad. But I have had several develop high forward resistance and fail to deliver proper output voltage (down by 50%). Not a dangerous failure in a high voltage B+ circuit, but a terrible, terrible thing in a bias circuit.

Cathode-bias power amplifiers are self-stabilizing, but fixed-bias amplifiers are tricky and require great care with bias and screen grid circuits.

Spending all that time to line up screw heads and then risk costly tubes and output transformers to a 50 yr old selenium rectifier, where for $.01 you can have a silicon rectifier, is more than unwise.

-David


Had you watched,

1. You wouldn't have been asking me if I installed it, unless you were being coy.
2. You would not have brought this up for debate. This was explained in detail

I saw that. I ignored it because the remarks were stupid. Consider yourself warned.....

-David


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jun Fri 14, 2019 7:08 pm 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
dberman51 wrote:
jcrubin wrote:
dberman51 wrote:
The rectifier sections of videos 2 and 3. Why?

You may have never had one fail. I have never had one short out and smell bad. But I have had several develop high forward resistance and fail to deliver proper output voltage (down by 50%). Not a dangerous failure in a high voltage B+ circuit, but a terrible, terrible thing in a bias circuit.

Cathode-bias power amplifiers are self-stabilizing, but fixed-bias amplifiers are tricky and require great care with bias and screen grid circuits.

Spending all that time to line up screw heads and then risk costly tubes and output transformers to a 50 yr old selenium rectifier, where for $.01 you can have a silicon rectifier, is more than unwise.

-David


Had you watched,

1. You wouldn't have been asking me if I installed it, unless you were being coy.
2. You would not have brought this up for debate. This was explained in detail

I saw that. I ignored it because the remarks were stupid. Consider yourself warned.....

-David



You feel better now?


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jul Mon 08, 2019 1:17 pm 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
Part 4 posted to OP


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jul Mon 08, 2019 10:28 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4597
Location: Sunnyvale CA
jcrubin wrote:
dberman51 wrote:
Had you watched,

jcrubin wrote:
1. You wouldn't have been asking me if I installed it, unless you were being coy.
2. You would not have brought this up for debate. This was explained in detail

I saw that. I ignored it because the remarks were stupid. Consider yourself warned.....

-David



You feel better now?


This was less-than-charitable, but most people really *do* feel better when they offer up sound information to beginners and the less-experienced, and it usually does irritate them to get blown off.

You video "explanation" notwithstanding, you probably should change that rectifier as suggested. But given your attitude to date, I am super-sure that you know better than everybody else. Those people at SERV-PRO have to eat, too. Maybe they can post a video!

Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jul Thu 11, 2019 8:02 pm 
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Joined: Apr Sun 23, 2017 11:22 pm
Posts: 528
Location: 44035 (Near Cleveland Ohio)
+ 1 regarding the bias rectifier. Just because you managed to drive for 40 years without wearing a seat belt, doesn't mean you shouldn't use one. . . . :lol:

What surprises me - for someone as meticulous as you repeatedly demonstrate - is why you didn't upgrade to ceramic tube sockets. It's sorta like wearing Wal-Mart socks with Gucci shoes 8)

If "The Amazing Kreskin" was still around, I'm sure he'd foretell your eminent repair of that crispy PC board as well.

Dan


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jul Mon 15, 2019 6:08 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4794
Location: Freehold NJ 07728
I won't go into the debate about using the selenium rectifier as a guy who probably restored 75 ST-70's I have seen that selenium fail. On one example, I was testing a ST-70 before I restored it for a customer, I set the bias, all good, I had my Heathkit IP-5220 that has a AC current meter and with the bias set, it pulls around 1.6 amps went upstairs for a few minutes, went back to the basement and saw the amp meter pegged at 3 amps... The selenium failed....

But on another note, excellent job on the restoration! I am the guy who "designed" the bias balance for Kevin at Dynakit.

Sal

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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Jul Tue 23, 2019 3:36 pm 
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Joined: Sep Wed 10, 2014 7:13 pm
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Location: Urbana, Illinois
I’m another guy with 40+ years of Dynaco tube amp experience.

I always replaced the selenium bias rectifier with silicon. In fact, all the way back into the early 1970’s when I assembled more than 20 ST-70 kits for a local stereo store to supplement my finances as a starving university student, I always discarded the factory selenium rectifier and used a 1N4007 instead.

Today I own one ST70. Mine also has a solid-state main rectifier. It has a 1990’s vintage power transformer from Triode Electronics that was specifically designed for a solid-state rectifier. The B+ is very close to the original factory power transformer/GZ34 setup. This replacement power transformer barely gets warm.

IIRC, Dynaco started using glass/epoxy PC boards in production during the early 1970’s.
Those old brown phenolic PC boards might still “work” but they sure are ugly. New PC boards are readily available at reasonable prices, both duplicates of the original PC boards and boards with improved/updated circuit designs. So I would be inclined to replace the old brown phenolic PC boards myself.

-EB


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Aug Mon 12, 2019 1:18 pm 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
Part 5 posted to OP


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