• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Dynaco 70 restore

E

Eddie hy fi

Active Member
Hy im doing a restore on a dynaco 70 tube amp that has be sitting for many years , --going slow but i have now done a rca input jack kit and a speaker jack kit and bias test jack upgrade kit , and im part way threw a restore kit too , waiting for moor parts to get here for the PCB board and i do not even have the out put tubes yet , for now my questions are what should the choke resistance read ? should i just go with a new choke transformer ? --- and is replacing the can cap ok even if its a new one with the same specs 30 ,20, 20 , 20 at 525 volt ? and please help on this dumb question but i want to wire in a run lite or pilot lamp , where should i tap into for a 6 volt source or should i go with a 110 volt---i have some here that are 6 volt leds and i may have some 110s too , or do i need to get a12 volt led pilot lamp? --- the restore kit is a waste of money , these parts i could have gotten for way less and its limited to me all ready buying moor parts , my mistake on that one , would not do that again
 
Register to hide this ad
The choke should be ok, even if it looks like some wax oozed out. You could unsolder one end and put an ohm meter across the choke leads. It should read around 62 ohms I believe. If your adding a pilot light I would go with a 6.3v.
I have seen where a few people have upped the cap value on a section or two of the cap can and even upped the voltage some. But I'll let someone else chime in on that one. I have one of Tubes4HiFi's ST-70 kits I built, but it has a different driver board and driver tubes. The cap can in mine is a 20/30/40/80, 525v.
It also has the bigger power transformer.
 
The original can is a high-failure item that can take out the rectifier, so I recommend replacement as a precaution at 525 or 550V rating. The choke is probably fine - failures are very rare IME.

You'll need another rectifier and a dropping resistor if you plan to use the indicator on 6.3V. I would not use a 120V indicator because of possible hum increase.
 
Thanks , and ok , but can some one tell me where i can fine the 6.3 tap on the original board , and or do i need a drop resistor from a 12 tap and what value and whers the tap , there can be two pilot lamps if we need to even out the sides . --- yes the choke reads 62 and a half ohms -------------my question on the can cap is , the new one that i have is just 30 ,20, 20 ,20 at 525 v , if i wanted to later could i just add a 10 or 22 uf cap to one or moor of the sections , like the choke section , some run that higher , and i could add a bit to the 30 , -- but most capacitors only go to 450 volt , is that enough ?
 
ive all so replaced power cord and added a basic CL-70 thermistor , are you guys ok with this , it has served me well on other builds like maggies , etc
 

Attachments

  • 100_4827.JPG
    100_4827.JPG
    159.7 KB · Views: 58
Eddie hy fi said:
Hy im doing a restore on a dynaco 70 tube amp that has be sitting for many years , --going slow but i have now done a rca input jack kit and a speaker jack kit and bias test jack upgrade kit , and im part way threw a restore kit too , waiting for moor parts to get here for the PCB board and i do not even have the out put tubes yet , for now my questions are what should the choke resistance read ? should i just go with a new choke transformer ? --- and is replacing the can cap ok even if its a new one with the same specs 30 ,20, 20 , 20 at 525 volt ? and please help on this dumb question but i want to wire in a run lite or pilot lamp , where should i tap into for a 6 volt source or should i go with a 110 volt---i have some here that are 6 volt leds and i may have some 110s too , or do i need to get a12 volt led pilot lamp? --- the restore kit is a waste of money , these parts i could have gotten for way less and its limited to me all ready buying moor parts , my mistake on that one , would not do that again
Click to expand...
I think the choke is ok. The originals are dropping wax, that's normal.
What you should look for is matched EL34 tubes. Or even better a matched quad from a reputible dealer, if any of the
tubes break this dealer could send a new one with the same parameters.

As for pilote lamp, a 12 Volt micro lamp could be installed in the center of the octals at the front and feeded from
the 6.3V AC that powers the filament on the board.

The original PC board and 7199 is most likely ok, if you have not removed it give it a chance .
 
Eddie hy fi said:
is a mullard rectifier tube worth the xtra cost ? --or is a new JJ ok here ?
Click to expand...
No, imo, there are two ways to deal with it. One is the rectifier diodes socket mod that improves reliability with any other 5AR4, and the other the Gillespie silicon rectifier substitute.
 
So far i just stayed with original specs and the old pcb with some new parts and now the new can cap is in , all the old parts where still with in spec except the can cap it red high and at 80 in one section , i can now see that the out put tubes are going to cost me with limited choices , for driver tubes i have adapters and 6GHA8 tubes , for now the JJ rectifier is all i have , not going to do solid state , going to replace the silver micas on the pcb , they will be here today but then im not doing any moor to this old pcb , no pilot lamp yet ------may go for a tung sol rectifier tube but not yet , thats another 60 bucks
 

Attachments

  • 100_4831.JPG
    100_4831.JPG
    143.6 KB · Views: 52
  • 100_4829.JPG
    100_4829.JPG
    176.9 KB · Views: 51
Why limited choices for output tubes? There are lots of EL-34's, 6CA7's, and
KT-77's to choose from.
 
The supply of Russian-made ones is a bit thin but you should be able to get JJ or Shugang versions. I have both in an EL34 and think they sound fine. Of my small collection the JJ actually perform the most like my old production Mullard tubes do in my Pilot SA-260 at least.

Are the originals bad or missing? If not maybe give them a run and see what you get.
 
When it comes to the Russian rectifier tubes I would skip the ones with the fancy names. I compared all of them Tung Sol, Mullard, Gold Lion etc. and they all look to be the same tube with variations in basing and different screenings on the bottles. The cheaper Sovtek will work just fine in the ST70 and you will save money. The Chinese rectifiers sold by TAD also work well.

There's a bit of history involved with the JJ 5AR4 as a lot of folks have had problems with them in the ST70. That's why you'll get some push back against using them.
 
The so-called "yellow sheet" rectifier socket mod is highly recommended for improved tube and amp reliability, I use two 1N4007 diodes in series on each leg for increased piv margin, and jumper tab 4 with tab 6 to double current capacity of the tube now acting as a time delay ramping series resistance.
aeM6gt.jpg
 
Last edited:
JohnT123 said:
When it comes to the Russian rectifier tubes I would skip the ones with the fancy names. I compared all of them Tung Sol, Mullard, Gold Lion etc. and they all look to be the same tube with variations in basing and different screenings on the bottles. The cheaper Sovtek will work just fine in the ST70 and you will save money. The Chinese rectifiers sold by TAD also work well.

There's a bit of history involved with the JJ 5AR4 as a lot of folks have had problems with them in the ST70. That's why you'll get some push back against using them.
Click to expand...
The TAD 5AR4 has been rock solid since replacing the JJ that shorted out.
 
gadget73 said:
The supply of Russian-made ones is a bit thin but you should be able to get JJ or Shugang versions. I have both in an EL34 and think they sound fine. Of my small collection the JJ actually perform the most like my old production Mullard tubes do in my Pilot SA-260 at least.

Are the originals bad or missing? If not maybe give them a run and see what you get.
Click to expand...
i do have three of the originals the 4th is clearly burned out and they look like mullards , but unit was shipped to me across the usa with the tubes in amp and the box and amp took a beating , one small tube even broke up , i wont be using ant of those tubes
 
You must log in or register to reply here.
Back
Top Bottom