Dyna ST70 bias help needed

I have run into a problem that I have not seen before. I have a vintage Dynaco ST70 in very nice condition that is the property of the Antique Wireless Association. I am restoring it to as close to original is I reasonable can. Also trying not to spend any money that is not essential to do so. I donated one of my four section CE twistlocks for the project. I did replace the rest of the caps but have only spent $20 or so. I had to change some resistors but I had all the right values so we are all set there. One problem is the original circuit board has some bad traces. I have worked around this issue before by gluing the component down on the board. Then lay the new component lead along the trace and solder. I would not do that to my equipment but buying a new board is not really what they want to do. A good set of old stock tubes and the amp should be all set. We are not.

The bias will adjust just fine with no signal. For now I have the outputs at about 40 mA each. At low volumes all seems well. Once the volume is raised the bias reading starts to go up and down and will not stabilize. The harder you push it the worse it gets. It is worse in one channel but both are affected. I changed out the bias supply caps thinking they were bad (although new and tested fine). Not the problem. Tried a new set of output tubes. Not the problem. Checked voltages throughout the amp and everything seems fine. Looked for bad solder, bad grounds, wrong wiring. Nothing shows up.

So please if you have seen this issue before please respond with some help. No thank you to a new board. Or further modifications or additions. I want to fix this in original design state. I have gone so far as to check the voltage drop across the output tube grid resistors to see if something is off. Seems fine.

Any constructive help is appreciated. Thanks WLT

Hi WLT. Sounds normal to me. You don't bias with signal. I bias my amp with the inputs shorted.

WLT wrote:I have run into a problem that I have not seen before. I have a vintage Dynaco ST70 in very nice condition that is the property of the Antique Wireless Association. I am restoring it to as close to original is I reasonable can. Also trying not to spend any money that is not essential to do so. I donated one of my four section CE twistlocks for the project. I did replace the rest of the caps but have only spent $20 or so. I had to change some resistors but I had all the right values so we are all set there. One problem is the original circuit board has some bad traces. I have worked around this issue before by gluing the component down on the board. Then lay the new component lead along the trace and solder. I would not do that to my equipment but buying a new board is not really what they want to do. A good set of old stock tubes and the amp should be all set. We are not.

The bias will adjust just fine with no signal. For now I have the outputs at about 40 mA each. At low volumes all seems well. Once the volume is raised the bias reading starts to go up and down and will not stabilize. The harder you push it the worse it gets. It is worse in one channel but both are affected. I changed out the bias supply caps thinking they were bad (although new and tested fine). Not the problem. Tried a new set of output tubes. Not the problem. Checked voltages throughout the amp and everything seems fine. Looked for bad solder, bad grounds, wrong wiring. Nothing shows up.

So please if you have seen this issue before please respond with some help. No thank you to a new board. Or further modifications or additions. I want to fix this in original design state. I have gone so far as to check the voltage drop across the output tube grid resistors to see if something is off. Seems fine.

Any constructive help is appreciated. Thanks WLT

That current through the power tubes fluktuates when signal is applied is normal.
The important thing is that the moment the signal stops the current is back at normal.
btw 40mA is a little low, if it's a dynaco st70 then the bias is supposed to be 1.56 Volt
across a 15.6 ohm resistor, 100mA for 2 tubes.

WLT wrote:The bias will adjust just fine with no signal. For now I have the outputs at about 40 mA each. At low volumes all seems well. Once the volume is raised the bias reading starts to go up and down and will not stabilize. The harder you push it the worse it gets. It is worse in one channel but both are affected. I changed out the bias supply caps thinking they were bad (although new and tested fine). Not the problem. Tried a new set of output tubes. Not the problem. Checked voltages throughout the amp and everything seems fine. Looked for bad solder, bad grounds, wrong wiring. Nothing shows up.

Do you mean that the bias voltage "pulses" up and down as music plays? It should do that, and it should do it in time with the music actually, and the voltage swings will get bigger with increased volume. This is normal.

Or do you mean that if you play music and then check the bias after, the bias voltage has changed from its original voltages and won't stabilize? This is not normal.

Does the bias settle back to where you had it set once the music has stopped? It should.

Play music through your ST 70 for 15 minutes. Turn off ST 70.
Unplug the inputs on the ST 70.
Turn on ST 70 and check and set your bias. Turn off ST 70.
Plug in the inputs, turn on the ST 70 and enjoy.
(Don't turn the amp on/off quickly.)

ps I don't even unplug the inputs, I just turn off my pre-amp.

I tried to post a response this morning but I guess it did not go thru. Trying again.

Thanks for the responses. I do need to give you more info so you can see what I think is the problem. More to the story.

All my Dynaco amps are set for reduced output tube current. A suggestion from this forum and it makes sense. EL34s around 40 mA and the KT88 around 50mA each. I have my pair of MK IVs connected to my Altec speakers. The Altec are 97dB/1 watt so they are efficient. I set the bias to 1.10 VDC across the 13.5 ohm resistor. This yields about 40mA per tube at idle. I then turn up the volume some till it is a moderate but nice output. Not blasting. My Fluke meter will move some but does not exceed 1.14VDC. Maybe a one or two mA change. I think that is reasonable.

My ST70 on the bench is connected to some low efficiency drivers for test purposes. I set the bias to 1.30 VDC across the 15.6 ohm resistor. This yields about 41mA per tube at idle. I then turn up the volume some till it is the same moderate but nice output. My Fluke meter will move much more and will hit 1.75VDC. Maybe a 10-20 mA change. No red plating as these movements are up and down as spontaneous/momentary readings. I think that is unreasoanble.

I will test some other amps on different speakers to get more info. Back to you you soon with that info.

I hope this helps and you can see what my concern is.

WLT wrote:I tried to post a response this morning but I guess it did not go thru. Trying again.

Thanks for the responses. I do need to give you more info so you can see what I think is the problem. More to the story.

All my Dynaco amps are set for reduced output tube current. A suggestion from this forum and it makes sense. EL34s around 40 mA and the KT88 around 50mA each. I have my pair of MK IVs connected to my Altec speakers. The Altec are 97dB/1 watt so they are efficient. I set the bias to 1.10 VDC across the 13.5 ohm resistor. This yields about 40mA per tube at idle. I then turn up the volume some till it is a moderate but nice output. Not blasting. My Fluke meter will move some but does not exceed 1.14VDC. Maybe a one or two mA change. I think that is reasonable.

My ST70 on the bench is connected to some low efficiency drivers for test purposes. I set the bias to 1.30 VDC across the 15.6 ohm resistor. This yields about 41mA per tube at idle. I then turn up the volume some till it is the same moderate but nice output. My Fluke meter will move much more and will hit 1.75VDC. Maybe a 10-20 mA change. No red plating as these movements are up and down as spontaneous/momentary readings. I think that is unreasoanble.

I will test some other amps on different speakers to get more info. Back to you you soon with that info.

I hope this helps and you can see what my concern is.

The tubes will draw more current as volume goes up. Much more as you starve them closer
to class-B. Set bias to 1.56 Volt and retest !

I suppose the bias comes back to the value you set it to when signal is zero ?

WLT wrote:I tried to post a response this morning but I guess it did not go thru. Trying again.

Thanks for the responses. I do need to give you more info so you can see what I think is the problem. More to the story.

All my Dynaco amps are set for reduced output tube current. A suggestion from this forum and it makes sense. EL34s around 40 mA and the KT88 around 50mA each. I have my pair of MK IVs connected to my Altec speakers. The Altec are 97dB/1 watt so they are efficient. I set the bias to 1.10 VDC across the 13.5 ohm resistor. This yields about 40mA per tube at idle. I then turn up the volume some till it is a moderate but nice output. Not blasting. My Fluke meter will move some but does not exceed 1.14VDC. Maybe a one or two mA change. I think that is reasonable.

My ST70 on the bench is connected to some low efficiency drivers for test purposes. I set the bias to 1.30 VDC across the 15.6 ohm resistor. This yields about 41mA per tube at idle. I then turn up the volume some till it is the same moderate but nice output. My Fluke meter will move much more and will hit 1.75VDC. Maybe a 10-20 mA change. No red plating as these movements are up and down as spontaneous/momentary readings. I think that is unreasoanble.

I will test some other amps on different speakers to get more info. Back to you you soon with that info.

I hope this helps and you can see what my concern is.

Correct me if I'm wrong, but don't you change the tube bias current by changing the value of the bias resistor, and setting the bias trim pot to the same voltage (1.56v)?

You can the bias change as you stated. However there is no real good reason to change out the bias resistor. Just measure it so you know what the exact resistor value is. Then measure the voltage drop thru it to ground. Then adjust the trim pot to whatever voltage you want that corresponds to the output tube current you desire. V=IR. Lets set each output tube at 50 mA so that is .100 amp times the 12 ohm resistor or 1.2VDC. Change the resistor or the current and you get a different bias voltage. The ST70 uses 15.6 ohm resistors and the MK IV uses 13.5 ohm. All three amps use the EL34 so decide what value you want for output tube current and then do the math. The original Dynaco bias settings were set very high which causes shorter tube life. Several reasons. Higher current puts the amp further into class A operation. Output tubes were cheap in 1960. They also set them for 1.56 VDC so you could compare your volt meter to a fresh battery and use that as a reference. Today tubes are expensive so we turn down the idle current and even our cheap meters are very accurate when measuring in the 1-2 volt range. Reduced output tube current can sound just fine unless you go to low.

I have fixed my bias variation problem. The bias does not vary much under load unless you really push the amp pretty hard. It is now very stable. If your bias voltage does very considerably when playing music at normal listening levels than you do have a problem.

Changing the bias level can result in an increase in distortion and or Xover distortion, so whilst it's good to give valves an easy time it's worth checking the OP on a scope and also check THD.