How can I tell if my Dynaco Stereo 70 is all original? I have searched
the internet but I have not found specific things to look for. I have
found plenty of mods that I am not interested in though.

On 8 Mar 2006 18:00:17 -0800, wrote:

How can I tell if my Dynaco Stereo 70 is all original? I have searched
the internet but I have not found specific things to look for. I have
found plenty of mods that I am not interested in though.

Post pictures top and bottom (NOT on this newsgroup
please; we're text only) and any number of geezers,
including me, will indentify an St70 to its component
atoms.

Why you'd want an original must remain your secret.
Good fortune,

Chris Hornbeck

Both of the above replies are dead-on.

But, what parts concern you as to 'originality'. Let's go down the list
from 'real' to 'silly':

Transformers: These are about the _only_ parts other than the chassis
and cover that should concern you in any way, shape or form as to their
originality. There are several schools of opinion on the quality of OEM
Dynaco Iron. Mine is that it is pretty good, if the rest of the system
is pretty good. But (especially) the power transformer is marginal if
other components are failed or failing.

Tubes: Well, gee whiz.... this would be a function of use and/or
abuse. Low use & proper bias = good tubes (if good originally). They
would decline from there.

Boards: Phenolic boards Bad. Glass boards Good. Phenolic boards Bad.
Glass boards Good. Phenolic boards can be rebuilt and made reliable.
But OEM even with light use are NOT trustworthy. Period. Depending on
what your time is worth, replacement glass boards are cheap.

Selenium Diode: Good, bad or indifferent, get rid of it. No
ifs/and/buts.

Main Filter Cap: Moving target. If the unit runs without even a trace
of hum and if the cap stays cool (relatively), then it is likely OK. My
direct experience with 40 year old electrolytic caps used in this
manner is that they are about to fail if they have not already.
Consequences of failure could be anything from a bad smell to destroyed
iron....

Bias Supply Caps: As above. Failure here could eat expensive output
tubes and conceivably output transformers, a bit of a stretch.

Switches and plugs: Your call.

All other components on the boards: resistors, caps, etc. Replace the
caps. Check _each_ resistor. The caps fail and the resistors drift.

Peter Wieck
Wyncote, PA

':

Transformers: These are about the _only_ parts other than the chassis
and cover that should concern you in any way, shape or form as to their
originality. There are several schools of opinion on the quality of OEM
Dynaco Iron. Mine is that it is pretty good, if the rest of the system
is pretty good. But (especially) the power transformer is marginal if
other components are failed or failing.

The transformers are of good materials and workmanship but the power
transformer is underrated and unless the amp is modified for much lower
load it will die in a few years of serious use. It can be removed and
repurposed.

The opt's are assymetrical for DC resistance on the primaries and thus
will never be really good in performance.

For what misguided yupsters are paying for ST70s on ebay one can have
a chassis built and buy a good power and two good output transformers.
Just a thought. (Power transformer for late Twin Reverb works great and
can be had for $50 if you have dealer account, easy to get, from guitar
vendors). If you could get one for $50 it would be good for education
but....

Tubes: Well, gee whiz.... this would be a function of use and/or
abuse. Low use & proper bias = good tubes (if good originally). They
would decline from there.

Boards: Phenolic boards Bad. Glass boards Good. Phenolic boards Bad.
Glass boards Good. Phenolic boards can be rebuilt and made reliable.
But OEM even with light use are NOT trustworthy. Period. Depending on
what your time is worth, replacement glass boards are cheap.

Suspend the OEM board from a tree or stand and hit it with a 28 gauge
shotgun from ten feet. It shatters beautifully.


Selenium Diode: Good, bad or indifferent, get rid of it. No
ifs/and/buts.

Agreed, but it is toxic. Store or dispose carefully.

Main Filter Cap: Moving target. If the unit runs without even a trace
of hum and if the cap stays cool (relatively), then it is likely OK. My
direct experience with 40 year old electrolytic caps used in this
manner is that they are about to fail if they have not already.
Consequences of failure could be anything from a bad smell to destroyed
iron....

The telco grade ones in Marantz sometimes reform beautifully (but the
consequences of failure are also much higher). The old ones in Dyna
must be summarily executed without mercy or hesitation lest other parts
die. But since they are all obtainable...

I'm concerned with its value. Wouldn't it be worth more in all
original condition? What can be changed without hurting the resale
value?

For what misguided yupsters are paying for ST70s on ebay...

I guess that's me. I was told the ST70 would be a good first tube amp.
I hope they are right.

The transformers are of good materials and workmanship but the power
transformer is underrated and unless the amp is modified for much lower
load it will die in a few years of serious use.

Bret, you keep trotting out this tired old mule at any excuse,
sometimes even spontaneously. Define 'a few years' and 'serious use'.

I know of one of these beasts that has been run (and is still being
run) about 12 hours/day, six (6) days per week, lo these last 8 years
of my knowledge, and some more-than-a-few years before that I am told
reliably. 4 years ago, I retubed, cleaned up, recapped and rebiased one
for a friend that had been going about 3 hours/day, 7 days a week, for
10 years, and is still going strong. And he never adjusted anything at
all since he got it from the original owner/builder.

Purely anecdotal, I admit... but sure not what you suggest or imply.

Put another way, I have yet to see a bad power-transformer on an ST-70.
Several fried OPTs, due to user-idiocy, but never a bad
power-transformer.

Anyone else here?

Peter Wieck
Wyncote, PA

CamMan wrote:
I'm concerned with its value. Wouldn't it be worth more in all
original condition? What can be changed without hurting the resale
value?

I don't know. It well may be they are more valuable 100% original.
They sure as hell aren't listenable that way. I think some of the more
well known PCB conversions have reasonable resale and if you save the
original board and parts you could always put it back.

For what misguided yupsters are paying for ST70s on ebay...

I guess that's me. I was told the ST70 would be a good first tube amp.
I hope they are right.

Well, years ago when they were giveaway items they were right. Today
there is no real good old amp I know of you can buy cheap (at fair
market value) and learn on. I would build a popular design from
scratch for maximum learning value maybe even one of the little EL84
designs floating around. The Mullard book AudioXPress sells gives very
explicit construction details including the chassis size, tag board
wiring, so you _could not go wrong_ with that. Personally I hate EL84s
for hifi but they are small and inexpensive and okay for educational
purposes. A lot of people who like what I call "****slobber" or
"cow****ed sound" LOVE EL-84s. (I am always taken to task on that.)
You could even start out with using a pair of cheap guitar amp OPTs and
upgrade later to minimize your investment.

Ya gotta pay your dues to play the blues. Some people think I'm a
horrible jerk, but although I concede you could take umbrage with the
presentation, you know in essence I'm right as to the consequence. Of
course people knew Goldwater was right, too, and he got his ass beat
to all hell!

wrote:
The transformers are of good materials and workmanship but the power
transformer is underrated and unless the amp is modified for much lower
load it will die in a few years of serious use.

I saved four or five of them with burnt transformers from the
dumpster. I can only extrapolate on the numbers I wasn't around to
intercept. My guess, unscientific I concede, is that when a transformer
lets go the smell and WDF (wife disgust factor) leads the thing to be
thrown out without hesitation or remorse. So you don't see those.

I have seen more with bad OPTs than power transformers. I have seen
ones with open windings too, or OPT secondary shorts such as they will
actually work but perform lousy unless the amp is run at constant power
into a dummy load or used for guitar. Then it blows up, but in hi-fi
service it just sounds a little weird.

Back then a MINT ST70 was worth between ten and fifty dollars ACV
depending on who was offering. I really dodn't want them because the
OPTs were undesireable for guitar amp use in that the overdrive grind
was not what people wanted. Sunn used them and they did not sound the
way the local rockers wanted so were worth little. Fisher, HH Scott,
Lafayette OPTs put in a guitar amp would give the overloaded kind of
grind guitar players who wanted the sounds you hear on records by
****ty 70s bands like Foghat, Rush, Mahogany Rush (two different
bands), and Ted Nugent. And those units would. The Sunn worked fine for
country rock and easy rock and even new wave Elvis Costello, Cars,
Blondie, Police type music and were then a little more saleable. But
you could not ****in' GIVE them away then! Not in the Midwest.

Oh yeah, the profanity. Back then if you were talking to rock and
rollers you had to use the word "****" in each and every conversation
or they could not comprehend what you were saying. This has fallen out
of favor, i concede, but that was then so i'm not apologizing. Just the
flavor of the times along with Pot smoke. I hate pot personally but the
bong resin does make an okay substitute for RMA flux.

I guess that's me. I was told the ST70 would be a good first tube amp.
I hope they are right.

They are. But not if nothing has been done to it since it left the
factory. That is an inevitable and spectacular failure waiting to
happen. But with a modicum of basic soldering skills and a few bucks
worth of common and easily available parts, they can be made both fit
for polite society and also quite sweet-sounding. They also happen to
be quite forgiving of other than deliberate idiocy, despite Bret's
representations.

If you want to be walked through it (the ready-for-polite society
routine), Curcio Audio does a nice job of that.

Peter Wieck
Wyncote, PA

wrote:

How can I tell if my Dynaco Stereo 70 is all original? I have searched
the internet but I have not found specific things to look for. I have
found plenty of mods that I am not interested in though.

I had an orignal factory unit.
Tube sockets were pop rivited in place instead of screwed
Original can cap is obvious.
Selenium rectifier
black gate caps may be black cat can not remeber which ones
carbon comp. resistors.
disk cap was 2 in one no 2 individual caps
This should help to determine originality
also The name.
Dynaco was factory made, Dynakit was kit made
Mike Mueller

Sounds original... and the rivets indicate true factory wired. Dynaco and
Dynakit do not indicate factory wired... my factory wired says Dynakit.

The 2-in-1 bias cap is definitely original. Also, the 1% precision bias
resistors are white.

Another indicator of originality is that 2 of the caps near the middle of
the circuit board are the flat-square-red ones with colored dots to indicate
value.


"mike" wrote in message
ink.net...
wrote:

How can I tell if my Dynaco Stereo 70 is all original? I have searched
the internet but I have not found specific things to look for. I have
found plenty of mods that I am not interested in though.

I had an orignal factory unit.
Tube sockets were pop rivited in place instead of screwed
Original can cap is obvious.
Selenium rectifier
black gate caps may be black cat can not remeber which ones
carbon comp. resistors.
disk cap was 2 in one no 2 individual caps
This should help to determine originality
also The name.
Dynaco was factory made, Dynakit was kit made
Mike Mueller

My Stereo 70 is 45 years old and I am the original owner. I purchased
and assembled a kit in the spring of 1961, my freshman year in college.
The amp has been used for 45 years. Since retirement 6-7 years ago
the amp has been used 8-12 on most days. I'm listening as I write this
note. A few tubes have been replaced over the years as I have moved 7
times. About 8-9 years ago the amp was recaped (and maybe an output
tube replaced, I'm not sure); the boards were checked and any trouble
spots fixed. I'm hoping it will run another 45 years. The matching PS2
has on had the selenium rectifier fixed.

Gary Spear