Hello! I have recently purchased an ST-70 from someone who had it for 4 years. He bought it on ebay from a store that got it from the estate of a deceased musician. The late musician either modified it him/herself or had it done by someone else. There is no schematic or record of what was done. All I know is that it comes from around the Woodstock, NY area. I have never seen mods like these before. Bias pots under the chassis (in addition to the regular pots on top!) LED light in the front left octal socket?? Why??? Not sure why they upgraded the quad cap to 80-40-30-20 .... maybe it has to do with all of the extra wiring under the hood. I'm trying to retrace their steps and understand what was done! Hopefully, someone here can help me figure it out. If you need more detailed photos, I'm happy to provide them.
Any thoughts or advice or input you have is welcome!!! I'm a n00b so no advice is too small.
Thx!!!
Any thoughts or advice or input you have is welcome!!! I'm a n00b so no advice is too small.
Thx!!!
Mostly still stock. The extra pots are probably DC balance controls. Hard to tell about the LEDs.
The new main capacitor is ok. Hopefully the 80uF section is not connected just after the rectifier tube, or the tube will be unhappy.
Be careful not to push any of the wiring around, there are some untethered solder joints floating that must not touch anything.
The new main capacitor is ok. Hopefully the 80uF section is not connected just after the rectifier tube, or the tube will be unhappy.
Be careful not to push any of the wiring around, there are some untethered solder joints floating that must not touch anything.
@rayma I finally plugged it in, warmed it up and checked the bias today. It hasn't been checked in over 4 years, it was reading about .48! I was very concerned about the inner pots so instead of raising the bias, I just turned it off until I got some second opinions. It doesn't look they did the "yellow sheet" mod. Not sure what resistors might have been added that could lower the bias. That said, the music that was playing as it warmed up did sound good, so I'm happy about that.
Odds are that the bias voltage readings no longer use the 15.6 ohm white resistors of the original (I don't see them),
and should not be adjusted for a reading of 1.56V. In fact, do not change any settings until you are certain of the proper way.
In order to know what and where to measure the DC output tube bias, and how to adjust it properly, you'll have to trace out the circuit.
Start by tracing the connections to the two front panel octal tube sockets.
and should not be adjusted for a reading of 1.56V. In fact, do not change any settings until you are certain of the proper way.
In order to know what and where to measure the DC output tube bias, and how to adjust it properly, you'll have to trace out the circuit.
Start by tracing the connections to the two front panel octal tube sockets.
I think I see a 10 ohm cathode resistor on one of the EL34 sockets, but I am not 100% sure on that. (If that is the case you would want about 0.45 - 0.50V on the bias test point.)
This appears to be a later ST-70 based on the PCB, I assume this unit uses 7199, the board does appear to be original Dynaco. The cover is not original, in fact this would be the first time I have seen one. (As long as components are not overheating it's not a problem.) No cage?
While I would definitely quibble over the quality of the workmanship, the features added are a good idea. (I used to make mod board kits for ST-70s from the late 1980s until about 2001)
This appears to be a later ST-70 based on the PCB, I assume this unit uses 7199, the board does appear to be original Dynaco. The cover is not original, in fact this would be the first time I have seen one. (As long as components are not overheating it's not a problem.) No cage?
While I would definitely quibble over the quality of the workmanship, the features added are a good idea. (I used to make mod board kits for ST-70s from the late 1980s until about 2001)
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@kevinkr That's great news! I'm excited to know about the (possible) 10 ohm cathode resistor. It would explain why the readings I got was so low.
That PCB shield is definitely an aftermarket piece. I have seen 1 or 2 in the endless searching of Dyanco-related info. I believe the unit dates from 1976 from the numbers on the transformers. It did not come with a cage, it may have been discarded by the original owner.
Pretty certain this was built as a kit. Pretty sure the input tubes are 7199, the markings are gone except for the Hytron brand. Not sure what kind of rectifier tube is in the unit, there are no markings on the glass. I am wondering if it would be better to swap it with a solid state rectifier. (I like the idea of a little warm up before the current flows through.)
That PCB shield is definitely an aftermarket piece. I have seen 1 or 2 in the endless searching of Dyanco-related info. I believe the unit dates from 1976 from the numbers on the transformers. It did not come with a cage, it may have been discarded by the original owner.
Pretty certain this was built as a kit. Pretty sure the input tubes are 7199, the markings are gone except for the Hytron brand. Not sure what kind of rectifier tube is in the unit, there are no markings on the glass. I am wondering if it would be better to swap it with a solid state rectifier. (I like the idea of a little warm up before the current flows through.)
That's 76-51, late 1976, probably the next-to-last production batch of the dyna transformers.
NO solid state HV rectifier instead of the tube, that is not a valid substitution.
NO solid state HV rectifier instead of the tube, that is not a valid substitution.
If there are separate readings per output tube, using 10 ohm current sensing resistors, the readings should be 0.50VDC each for all four.
But you'll need to trace out the circuit to determine how the bias pots are supposed to work, since there are four of them now.
There could be four bias adjustments, or else there could be two bias adjustments and two balance adjustments.
But you'll need to trace out the circuit to determine how the bias pots are supposed to work, since there are four of them now.
There could be four bias adjustments, or else there could be two bias adjustments and two balance adjustments.
It shouldn't be.
With that quad capacitor, the 80uF section should be connected just after the choke.
With that quad capacitor, the 80uF section should be connected just after the choke.
I would add a single 1N4007 rectifier diode in series with each plate of the 5AR4/GZ34 rectifier to prevent arcing during hot restarts, etc. The most reliable 5AR4 I have encountered in recent decades was Chinese made.
Sorry to be negative but you best option is to remove all "fluff" and rebuild it according to the dynaco manual. Then
you will have a documented amp with good performance. The alternative is to reverse engineer the amp creating
documentation for yourself and future owners.
The task is most likely easy as all building blocks are there, even the PC-3 board seems fairly standard.
you will have a documented amp with good performance. The alternative is to reverse engineer the amp creating
documentation for yourself and future owners.
The task is most likely easy as all building blocks are there, even the PC-3 board seems fairly standard.
Those flying leads are scary as hell! All they have to do is bend a little bit and you have a short somewhere. Super dangerous, it looks like pin number four has a resistor coming off of it just hanging in the air…. There is B+ on that connection……
Please keep in mind these amps have high volts and can kill in an instant. If you’re not sure what you’re doing, at least make sure to never work on it while it’s on, and always keep your left hand in your back pocket. It doesn’t take much voltage across your heart to stop it.
Please keep in mind these amps have high volts and can kill in an instant. If you’re not sure what you’re doing, at least make sure to never work on it while it’s on, and always keep your left hand in your back pocket. It doesn’t take much voltage across your heart to stop it.
Also, I can’t tell if there is a bleeder resistor, but if not, keep in mind that the capacitor can hold high voltage for quite some time….
The 7199 circuit includes a bleeder resistor, which is intended to add some fixed bias to the 7199 pentode stage.
If those two extra pots are for output tube DC balance, they are certainly well worthwhile.
And still worthwhile even if they are only for AC balance instead.
If those two extra pots are for output tube DC balance, they are certainly well worthwhile.
And still worthwhile even if they are only for AC balance instead.
I did a very similar mod for someone decades ago, but my workmanship was substantially better. All of the changes IMHO are worthwhile, but might be worth a good tidy.
There is no need for AC balance when a concertina is used, it's inherently balanced. DC balanced is not needed if properly matched power tubes are used. At least this is my personal opinion.