Hi everyone,
This is my first post. I have found the more I learn about the ST 70 the more questions I have. My ST 70 is mostly built from old stock I purchased from Stereo Cost Cutters in the mid 80's. They didn't have any power transformers so I bought one from a highly regarded name in tube electronics (he is still in business). This replacement transformer doesn't have a 5 volt tap for a 5AR4 which he claimed was better because this tube was getting rare and expensive.
So for the next few years I tried a couple of different driver boards and even splurged on a SDS Labs cap board but the amp never sounded quite right so I put it in storage. A couple of weeks ago I pulled it out of storage and installed a Diytube driver board (the one that uses two 12au7's and one 12ax7.) The manual can be found at tubezone. (I have the link but I am too new to post links.) The documentation that came with this board has a voltage chart and that's when I discovered the real problem. The power transformer only puts out 630vct instead of the 720vct for a
PA060. So now I have many questions. On the driver board J19 should be 300v and J20 should be 350v. Both were 50v lower so I changed the 6.8K power supply resistor to 3K and that brought the voltage back up to where it should be. There is only 390v on pin 3 of the EL34's and I'm using the 15.6 ohm resistors. My EL34's are original Dynaco and I would like to keep the bias current on the safe side. How do I figure this out? What value would a safe current be? 35ma or should it be lower? The two pots on the driver board (R29 and R30) are supposed to be adjusted for zero volts but the best I can get is minus .9 volts. How important is this setting? Can I change values on the driver board to get it closer to zero? I would be grateful for any advice. Maybe I need to buy a new power transformer? I wouldn't mind lower wattage if it can be done properly.
Thanks,
Gary
This is my first post. I have found the more I learn about the ST 70 the more questions I have. My ST 70 is mostly built from old stock I purchased from Stereo Cost Cutters in the mid 80's. They didn't have any power transformers so I bought one from a highly regarded name in tube electronics (he is still in business). This replacement transformer doesn't have a 5 volt tap for a 5AR4 which he claimed was better because this tube was getting rare and expensive.
So for the next few years I tried a couple of different driver boards and even splurged on a SDS Labs cap board but the amp never sounded quite right so I put it in storage. A couple of weeks ago I pulled it out of storage and installed a Diytube driver board (the one that uses two 12au7's and one 12ax7.) The manual can be found at tubezone. (I have the link but I am too new to post links.) The documentation that came with this board has a voltage chart and that's when I discovered the real problem. The power transformer only puts out 630vct instead of the 720vct for a
PA060. So now I have many questions. On the driver board J19 should be 300v and J20 should be 350v. Both were 50v lower so I changed the 6.8K power supply resistor to 3K and that brought the voltage back up to where it should be. There is only 390v on pin 3 of the EL34's and I'm using the 15.6 ohm resistors. My EL34's are original Dynaco and I would like to keep the bias current on the safe side. How do I figure this out? What value would a safe current be? 35ma or should it be lower? The two pots on the driver board (R29 and R30) are supposed to be adjusted for zero volts but the best I can get is minus .9 volts. How important is this setting? Can I change values on the driver board to get it closer to zero? I would be grateful for any advice. Maybe I need to buy a new power transformer? I wouldn't mind lower wattage if it can be done properly.
Thanks,
Gary
Not knowing any more about your transformer than you have said, I think your best bet is to buy a new clone power transformer from a more mainstream source. Here are two popular ones; you probably can't go wrong with either.
DynakitParts
PA060 POWER XFMR - Dynakit Parts
Triode Electronics
PA060 S Dynaco ST70 Upgrade Power Transformer Replacement MADE IN USA
Both of these have the 5-volt winding needed for the 5AR4 rectifier tube. Since your transformer doesn't have this winding I suspect that it really wasn't intended as a drop-in replacement like these two are.
After installing the correct transformer, I would return all of the resistors you changed to their stock values before powering up. This should get you the proper DC voltages as the Dynaco engineers intended.
Good luck.
DynakitParts
PA060 POWER XFMR - Dynakit Parts
Triode Electronics
PA060 S Dynaco ST70 Upgrade Power Transformer Replacement MADE IN USA
Both of these have the 5-volt winding needed for the 5AR4 rectifier tube. Since your transformer doesn't have this winding I suspect that it really wasn't intended as a drop-in replacement like these two are.
After installing the correct transformer, I would return all of the resistors you changed to their stock values before powering up. This should get you the proper DC voltages as the Dynaco engineers intended.
Good luck.
Yes, time for a new clone transformer, dynakitparts.com is best.
While you are waiting for it, return the circuit to original voltage spec parts.
Also check the 15.6R bias resistors for the proper values.
While you are waiting for it, return the circuit to original voltage spec parts.
Also check the 15.6R bias resistors for the proper values.
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Either of those two transformers will work well. I have had very good experiences with Triode Electronics.
#1. The way you done it with lowering the 6.8K to 3K is fine.
#2 390V B+ on a PP EL34 is fine. You get less power but definitely with-in the good sounding region. IMHO, The lower B+ also allow for some 6L6Gx tube rolling fun. Dynaco recommend 50mA. I would bias the El34 to 50mA instead of 35mA. At 50ma, it is still well within the 25W EL34 limit. Too low bias lower the Class A region and might effect sound.
#3 This two pots looks to me are for DC balance for each channel. It allow for some adjustment of current split between the two PP EL34 by changing the negative supply load.
Thanks all of you for your kind advice. The power transformer is a drop in replacement I purchased from Frank Van Alstine in the early 90's. It turns out all I had to do was change the value of two resistors and up the bias to 1.45v which then allowed the balance pots on the driver board to be set. The amp is now rock solid and sounds very good.
Cheers,
Gary
Cheers,
Gary