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Dynaco Stereo 70 ST-70 Series II Tube Amplifier


MechEngVic
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For sale on eBay, this amp was badly reviewed initially so few units were sold. It's RARE. Truth is though, this is an awesome sounding amp! It fixed the main problems with the original: Bigger power transformer, bigger capacitance, solid state rectification. I own one and have always been pleased with it. And it especially sings matched to Klipsch speakers. Only problem: No replacement/upgrade PC board is available like for the originals,  but I rebuilt mine with little trouble. 

 

https://www.ebay.com/itm/Dynaco-Stereo-70-ST-70-Series-II-2-Tube-Amplifier-Tested-Working-READ-/362764796499?_trksid=p2385738.m4383.l4275.c10

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5 hours ago, joessportster said:

They are a great sounding amp but 1100 is high I sold 1 here last year with ZERO issues for 500. Mine was infact near NOS with the screen resistor mod added

Yes I bought that one..

 

 

Here's the back story on it..

 

http://dynacotubeaudio.forumotion.com/t3035-dynaco-stereo-70-series-2#p26383

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On 10/2/2019 at 11:52 AM, joessportster said:

They are a great sounding amp but 1100 is high I sold 1 here last year with ZERO issues for 500. Mine was infact near NOS with the screen resistor mod added

 

On 10/2/2019 at 11:52 AM, joessportster said:

They are a great sounding amp but 1100 is high I sold 1 here last year with ZERO issues for 500. Mine was infact near NOS with the screen resistor mod added

Yes, I remember reading about your acquisition with great interest. I rebuilt mine several months ago, replacing everything except the diodes. Then I read your mention of Dave Gillespie's writings and recommendations. I have to thank you for giving out that information. I read through everything I could find from him and realized the series II is the perfect candidate for the screen resistors, and I made the modification. Blowing cathode resistors is the bane of this amp and I believe the screen mod is the cure. No problems since. I think you gave dirtmudd an incredible deal when you sold it. 

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  • 1 year later...

I recently acquired a Dynaco Stereo 70 Series II with one of the cathode resistors blown to smithereens - haven't done any further testing yet.

Questions for the Dynaco veterans - 

1 - Where can I get the parts, schematic, etc. to perform the screen resistor modification - and replacement cathode resistors?

2 - Is it safe to assume that if the cathode resistor is blown, the finals also need to be replaced on that channel?

3 - Why in the hell did I ever sell those Dynaco A25 speakers I had in the 70's?

 

Thanks in advance

Starship Trooper

 

 

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  • 11 months later...
On 4/18/2021 at 7:32 PM, Starship Trooper said:

I recently acquired a Dynaco Stereo 70 Series II with one of the cathode resistors blown to smithereens - haven't done any further testing yet.

Questions for the Dynaco veterans - 

1 - Where can I get the parts, schematic, etc. to perform the screen resistor modification - and replacement cathode resistors?

2 - Is it safe to assume that if the cathode resistor is blown, the finals also need to be replaced on that channel?

3 - Why in the hell did I ever sell those Dynaco A25 speakers I had in the 70's?

 

Thanks in advance

Starship Trooper

 

 

Trooper,

Don't know if you're still around, but if you are and still have these questions, I can answer them for you. I got the schematic and lots of assorted info. Sorry it took so long to see this post, this forum seems to stop notifying you of answers to threads if they are older.

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On 4/18/2021 at 10:32 PM, Starship Trooper said:

I recently acquired a Dynaco Stereo 70 Series II with one of the cathode resistors blown to smithereens - haven't done any further testing yet.

Questions for the Dynaco veterans - 

1 - Where can I get the parts, schematic, etc. to perform the screen resistor modification - and replacement cathode resistors?

2 - Is it safe to assume that if the cathode resistor is blown, the finals also need to be replaced on that channel?

3 - Why in the hell did I ever sell those Dynaco A25 speakers I had in the 70's?

 

Thanks in advance

Starship Trooper

 

 

 

If a cathode resistor was blown and you're asking the questions you are asking you may be in over your head, no offense.

 

The reason I am saying this is because the series II has an different bias setup and you are going to need to troubleshoot why the one cathode resistor was passing so much current. It could be a problem with the bias circuit, at best it may have just been a power tube failure as these amps are not nice to the screens of power tubes. One of the reasons I dislike ultralinear amps, Pentode done well is better.

 

The "mod" is simple, cut the trace close to pin 4 on the PCB and drill two holes on both sides of the cut trace, maybe 12mm-15mm apart depending on the type of resistor used and install the screen resistors, as for a value 150 ohms should be fine. I have seen people install the resistor directly where the screen wires enter the board but that's too far away from the pin to do much good, you want the resistor as close to pin 4 as possible. The picture above shows them soldering the resistor to the UL tap wire and bringing it directly to pin 4, that's not a bad option but I think it looks like an after thought. Cutting a small section of the trace is easily done with an x-acto knife, I like to drill holes to mount parts but you don't even have to, you can bend the resistor leads to a 45° angle and solder them flat to the trace on both sides of part of trace you cut. This at least puts the parts rigid on the PCB board and looks stock.

 

Before doing any mods I would go ahead and make sure the entire amp circuit is functioning correctly and output power under load vs distortion is to spec. Once it is operational to spec then move onto installing the screen resistors.

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1 hour ago, captainbeefheart said:

 

If a cathode resistor was blown and you're asking the questions you are asking you may be in over your head, no offense.

 

The reason I am saying this is because the series II has an different bias setup and you are going to need to troubleshoot why the one cathode resistor was passing so much current. It could be a problem with the bias circuit, at best it may have just been a power tube failure as these amps are not nice to the screens of power tubes. One of the reasons I dislike ultralinear amps, Pentode done well is better.

 

The "mod" is simple, cut the trace close to pin 4 on the PCB and drill two holes on both sides of the cut trace, maybe 12mm-15mm apart depending on the type of resistor used and install the screen resistors, as for a value 150 ohms should be fine. I have seen people install the resistor directly where the screen wires enter the board but that's too far away from the pin to do much good, you want the resistor as close to pin 4 as possible. The picture above shows them soldering the resistor to the UL tap wire and bringing it directly to pin 4, that's not a bad option but I think it looks like an after thought. Cutting a small section of the trace is easily done with an x-acto knife, I like to drill holes to mount parts but you don't even have to, you can bend the resistor leads to a 45° angle and solder them flat to the trace on both sides of part of trace you cut. This at least puts the parts rigid on the PCB board and looks stock.

 

Before doing any mods I would go ahead and make sure the entire amp circuit is functioning correctly and output power under load vs distortion is to spec. Once it is operational to spec then move onto installing the screen resistors.

 

The member you're replying to hasn't been on the forum for a year.

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21 minutes ago, wuzzzer said:

 

The member you're replying to hasn't been on the forum for a year.

 

Yes I did notice that. I didn't revive the thread but figured since someone did if the OP comes back here he might find the information. If not, somebody else searching online for information may stumble upon the thread and find the information. These amps are rough on power tubes, vintage specimens had better screen handling capabilities and so when looking for power tubes for high power ultra linear amps like these you need to choose wisely and get one designed specifically for ultralinear use. The Chinese KT88-98 specifically mentions this in their datasheet and spec's the screens at 600v and 8 watts. The original real winged C made in Russia were rated a little better at 610v and 10 watts g2, but they are tough to find and expensive now. I have seen the JJ datasheet claim the same; 600v and 8 watts but have had far too many fail or die early in these types of amps so I don't trust them or many other types.

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