Hello
I just purchased my first tube amp, a Dynaco ST-70. I opened it up and with the help of the original assembly manual and compared what I have to it . What I found
were a number of things that have been changed.
1) The selenium rectifier has been removed and from what I have read is a good thing to do. There is a modification for this that replaces the selenium rectifier with a 1N4007 diode but this has not been done.
2) There are two 1N4004 diodes that have been added to the base of the rectifier tube. This was based on another recommended modification and the diodes are in the correct locations but 1N4004 diodes were used instead of the recommended 1N4007's.
3) The 2 capacitors soldered to the 7 lug terminal were changed from 50uf (not sure of their original volts) to 200uf 100volt capacitors. In my research I did not find this to be a recommended modification so I'm not sure if this was a good idea.
4) RCA style inputs and outputs replaced the original screw set ups. For the outputs two transformer wires from each side that were used in the screw type terminals were capped and not used. I could not find information online of how to do this change correctly so I can't verify if this was done properly.
5) The main PC board was replaced with the Vintage Electron Inc 6GH8A stuffed board which appears to be installed correctly.
Other than these changes it appears stock. My main concern is changes number 1 and 2. Did the addition of the two Diodes in modification #2 to the base of the rectifier tube make the selenium rectifier modification unnecessary? and should the 1N4004 diodes be replaced with the recommended 1N4007 diodes.
Thanks for any help you can offer
Bob
I just purchased my first tube amp, a Dynaco ST-70. I opened it up and with the help of the original assembly manual and compared what I have to it . What I found
were a number of things that have been changed.
1) The selenium rectifier has been removed and from what I have read is a good thing to do. There is a modification for this that replaces the selenium rectifier with a 1N4007 diode but this has not been done.
2) There are two 1N4004 diodes that have been added to the base of the rectifier tube. This was based on another recommended modification and the diodes are in the correct locations but 1N4004 diodes were used instead of the recommended 1N4007's.
3) The 2 capacitors soldered to the 7 lug terminal were changed from 50uf (not sure of their original volts) to 200uf 100volt capacitors. In my research I did not find this to be a recommended modification so I'm not sure if this was a good idea.
4) RCA style inputs and outputs replaced the original screw set ups. For the outputs two transformer wires from each side that were used in the screw type terminals were capped and not used. I could not find information online of how to do this change correctly so I can't verify if this was done properly.
5) The main PC board was replaced with the Vintage Electron Inc 6GH8A stuffed board which appears to be installed correctly.
Thanks for any help you can offer
Bob
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The selenium bias rectifier and caps have been replaced with a diode and the two blue 220uF caps. That's perfectly fine.
The 1N4004s are underspec'd and should be replaced with 1N or UF4007s. The previous ownder apparently thought they were sufficient but they're really not. Otherwise it looks like a nice unit.
The 1N4004s are underspec'd and should be replaced with 1N or UF4007s. The previous ownder apparently thought they were sufficient but they're really not. Otherwise it looks like a nice unit.
It appears that the diode replacement in change 1 is actually there. It's attached to the red wire from the transformer with a piece of black electrical tape over part of it. This is the negative bias supply and quite necessary for the output tubes to work properly. The two 220 ufd capacitors are the bias filter capacitors. They are larger then the originals but will function OK since there is a new solid state diode.
The two 1N4004 diodes on the rectifier tube socket are there to protect the newer manufactured 5AR4 tubes which are not as robust as the original older tubes were. They are rated at 400 volts reverse voltage and are working right at the edge or their ratings. Replacing them with 1000 volt unites should be done. They're cheap so why gamble. And even if one should short you may not notice it because the tube will probably take it and be OK. But again why gamble.
The two 1N4004 diodes on the rectifier tube socket are there to protect the newer manufactured 5AR4 tubes which are not as robust as the original older tubes were. They are rated at 400 volts reverse voltage and are working right at the edge or their ratings. Replacing them with 1000 volt unites should be done. They're cheap so why gamble. And even if one should short you may not notice it because the tube will probably take it and be OK. But again why gamble.
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THANK YOU!
You are correct, I found the diode. It's a 1N4004 also. I will change all 3 to 1N4007's. I'm going to replace all the wiring and the last 2 remaining
capacitors while I'm at it.
sincerely
Bob
You are correct, I found the diode. It's a 1N4004 also. I will change all 3 to 1N4007's. I'm going to replace all the wiring and the last 2 remaining
capacitors while I'm at it.
sincerely
Bob
Just so you know, the single bias diode (1N4004) is in a lower voltage part of the circuit and is OK. If you do replace it anyway make sure to keep the cathode (band end) toward the transformer. It supplies a negative voltage to the blue filter caps which are probably OK.
The HV rectifier 1N4007 diodes have nothing to do with the bias rectifier diode.
The bias circuit capacitors can be a larger value with no problems, just less hum.
That's a half-wave bias rectifier, and it needs all the help it can get.
No need to change the caps again, or the 1N4004 diode, it's very well rated for that use.
The bias circuit capacitors can be a larger value with no problems, just less hum.
That's a half-wave bias rectifier, and it needs all the help it can get.
No need to change the caps again, or the 1N4004 diode, it's very well rated for that use.