Dynaco stereo 70 series 2

I have one of these series 2 models that was found in a warehouse, looked to be unused, and has some odd features.

1st there is no series 2 label on the chassis
2nd binding posts are all black
3rd capacitors are black, most pictures I find online they are blue
4th circuit board cover on had two screws out of eight, no other holes showed any sign that there was ever a screw there

This amp had mildew on it all over, including the tubes. I'm thinking it was a first run of the series 2, maybe put together for testing purposes but forgotten about. When I powered it up slowly on the variac I could smell the fresh paint on the tubes burning, no sign that this amp was ever used. The cord appeared to have the original zip tie on it, and there isn't a scratch anywhere on this amp.

Anybody with more knowledge of this model please chime in.

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well I guess if it is running fine with no issues...great!
The color of the capacitor does not mean anything apart form that different manufacturers use different body colors.
As to all speaker posts being black, well.......
Its a nice 'clean' look, and as I've said above, if it is working and sounding fine, well done!
How about some inside shots? Would be very interested to see the innards!

MontanaWay wrote:well I guess if it is running fine with no issues...great!
The color of the capacitor does not mean anything apart form that different manufacturers use different body colors.
As to all speaker posts being black, well.......
Its a nice 'clean' look, and as I've said above, if it is working and sounding fine, well done!
How about some inside shots? Would be very interested to see the innards!

I added screen stability resistors since it is documented that these units were hard on their tubes and resistors. I am just curious to find out as much as I can about its history, to me it is odd that it doesn't resemble any others I pull up online. Here is an inside photo, these had a complete circuit board.

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MontanaWay wrote:well I guess if it is running fine with no issues...great!
The color of the capacitor does not mean anything apart form that different manufacturers use different body colors.
As to all speaker posts being black, well.......
Its a nice 'clean' look, and as I've said above, if it is working and sounding fine, well done!
How about some inside shots? Would be very interested to see the innards!

Here is the top with the panel removed.

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have a look on this page and scroll down, a Chinese based forum, you will see two units that just about look identical to yours. Kind of makes me think this is a Chinese made unit?
http://www.hifi33.com/avforum/index.php?topic=81121102190819&item=&page=1

oh...and this thread on this forum last year from a member located in Germany

https://dynacotubeaudio.forumotion.com/t2580-stereo-70-series-ii-capacitor-question

MontanaWay wrote:oh...and this thread on this forum last year from a member located in Germany

https://dynacotubeaudio.forumotion.com/t2580-stereo-70-series-ii-capacitor-question

Here is a good link to information on the series II, but like I said, mine does not say series II and also has no serial # anywhere. The warehouse was in Pennsylvania where it was found.

http://www.tnt-audio.com/ampli/dynaco1e.html

Bob Latino wrote:About 12 years ago Stereophile wrote a review of the Dynaco ST-70 Series II and were not especially kind on the Series II version in a comparison with the ORIGINAL Dynaco ST-70. This review is probably one of the reason's why the Series II version never sold well. Link below ..

Stereophile review of the Dynaco ST-70 Series II

This mediocre review prompted one of the harshest manufacturer's comment in Stereophile I have ever seen! Read below what Dynaco's Bob Rapoport had to say about Corey Greenberg's review of the ST-70 Series II .. (lol)

Dynaco's (Panor) manufacturer's comment about The Stereophile review of the ST-70 Series II

Bob

Thanks Bob, I have read about all that already. Audio politics at its finest.

I have an original ST-70 I have compared with, the original is a bit more open and detailed with the highs, but changing the tubes out to a quad of Svetlana winged EL's makes the series II catch up, plus have better bass. this was paired with a set of Cornwall's, and PAS-3X.

looking at the bottom of the power transformer, it does look like an Asian made unit, very typical on how they are finished. Unless of course the bottom bell cover has been removed......

MontanaWay wrote:looking at the bottom of the power transformer, it does look like an Asian made unit, very typical on how they are finished. Unless of course the bottom bell cover has been removed......

This is some of the info online.

I wanted to compare both the physical features, circuitry, and sound of the Series II, and the original ST-70. I happen to have both. It would make a good listening contest! The basic difference in circuitry between the Series II and the original ST-70 is in the power supply. The Series II used solid state rectifiers (diodes), whereas the original ST-70 used vacuum tube (Mullard 5AR4/GZ34).
The debate over these two types of rectification continues even today, but each has its advantages and disadvantages. There is no doubt that solid state rectification gives fast response to transient highs and lows. The vacuum tube rectifier is slower to respond.

The power supply filter capacitors in the original ST-70 totalled 90mfd. In the Series II the capacitance amounted to more than triple that. Greater capacitance in the power supply provides more bass and energy to musical passages. Also to consider is that the choke in the power supply of the Series II was lower than that in the original ST-70. This could be done because of the increased capacitance. What it allowed was a slightly greater power output from the power supply.

Both amplifiers used cathode bias. Series II bias was controlled by a solid state circuit using LEDs to achieve a correct bias. This bias circuit had a solid state rectifier. The original ST-70 used a variable potentiometer to control the bias. It was set by connecting a voltmeter to the circuit and then adjusting the potentiometer until the correct bias voltage was achieved. The bias circuit utilized a selenium rectifier.

This addition will eliminate tube arc and resistors blowing, two pin 4's were feeding capacitors so jumpers were added and theirs traces interrupted. I am told the addition of the more modern power supply and extra capacitance can cause havoc, this should help the tubes last longer.

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Hi,
    What you have is a early version of the Panor ST-70 Series II that was first produced by Sound Values of Ohio. Sound Values & Panor eventually parted ways at which point most of the parts were made & assembled in Asia. All of the parts such as the circuit board, transformers and metal work on your unit were actually made here in the USA. As you say...this may be one of the first production units produced by Sound Values.

It appears to be in nice condition...Enjoy!

Kevin @ Dynakit

Bob,

Thanks for posting the info from Stereophile. That was a pretty funny letter from the outraged rep from Dynaco / Panor! I'm sure we've all written a letter, report, email, text message etc that could have benefitted from waiting one more day's reflection before sending. Mr. Rapoport might have waited a week longer. It was amusing, however.

DynakitParts wrote:Hi,
    What you have is a early version of the Panor ST-70 Series II that was first produced by Sound Values of Ohio. Sound Values & Panor eventually parted ways at which point most of the parts were made & assembled in Asia. All of the parts such as the circuit board, transformers and metal work on your unit were actually made here in the USA. As you say...this may be one of the first production units produced by Sound Values.

It appears to be in nice condition...Enjoy!

Kevin @ Dynakit

Thanks for the information Kevin, it is very much like having a brand new amp, and I'm very happy to have it in my Dynaco collection. list below...

I'm very impressed with its sound since putting my winged Svetlana tubes in it, looks like one of the original tubes is very poor or defective, causing it to sound very thin.

This all black amp should look right at home next to my sweet MK-II's done in black.

ST-70
ST-70 II
MK-II (4)
MK-III (4)
SCA-35
SCA-80Q
PAS-3X
PAS-3

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Good lookin' unit ya got there ...

Be interesting to see how accurate that LED biasing system really is. I see the New! Improved?? current generation "dynaco" uses the same setup.

Also be interesting to compare other similarities between the Panor and new production Radials. From what little I've seen, they may have just dusted off the Panor design with a new case? If that's the case, wonder if Stereophile will just dust off their old review?

sKiZo wrote:Good lookin' unit ya got there ...
Also be interesting to compare other similarities between the Panor and new production Radials. From what little I've seen, they may have just dusted off the Panor design with a new case? If that's the case, wonder if Stereophile will just dust off their old review?

As always in audio journalism, it will depend on the level of payola involved.

Since I have both, a very nice original example of the ST-70, and this series 2 version, my review is unbiased and without $ involved.

My ears tell me the series 2 has better bass, but highs are slightly rolled off a bit. Since I have them paired with horns, I think the original ST-70 is a bit bright anyway when playing CD's.

Having read and talked to a few series 2 owners that had cathode bias resistors fail pretty often, I feel adding screen stability resistors will eliminate this problem. Dave Gillespie's writings on this subject points to the more modern power supplies causing the tube arcing, and resistor failures.

My opinion on this series 2 is that it probably was never sold, and was either pulled out of line because of a blemish, or maybe for testing. The fact that it does not have a series two logo, or serial number on it anywhere, makes me think it might of been from the very first prototypes. A production unit put out for sale would of had a serial number, and the series 2 on the chassis. A bit of a mystery, but I'm happy to own a part of the Dynaco (Panor) history.

Although it's not anything over the top special, just having what could be a factory prototype, or NOS early series 2 Stereo 70 has a bit of coolness to it.
Thanks to all that have commented!

Steve,
if you didn't get a manual with the amp, let me know, I've got it on my computer, it's 1 Mb.
Roy

tubes4hifi wrote:Steve,
if you didn't get a manual with the amp, let me know, I've got it on my computer, it's 1 Mb.
Roy

Thanks Roy, I was able to print one from online, so I'm good to go.

There's one on evilbay that closes tomorrow. It's only $215 now but I'm not so sure I'd trust the transformers.

daveshel wrote:There's one on evilbay that closes tomorrow. It's only $215 now but I'm not so sure I'd trust the transformers.

I couldn't find it after doing a search.

I couldn't find it either, do you have an auction ## ?

Ebay has it listed as "Dynaco Stereo 70 Tube Amplifier"     (without the quotes)

The pictures show the quad cap replacements as being burned and the main circuit board has several burn areas on it.

stewdan wrote:Ebay has it listed as "Dynaco Stereo 70 Tube Amplifier"     (without the quotes)

The pictures show the quad cap replacements as being burned and the main circuit board has several burn areas on it.

They don't have it listed in vintage so that's why it doesn't come up in my search.

I believe it probably had a tube short to cause that damage, could of taken out the OPT also. Screen stability resistors would of helped protect everything.

I believe mine was starting to have a similar meltdown, the modern power supply is the culprit.