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Some Dynaco ST-70/VTA Low Gain voltage questions

Jeffery

Jeffery

High Powered Mutant
Subscriber
In my First tube conflagration thread, shelly_d mentions that the grid voltage on the output tubes should be in the -32v ~ -35v range and no more positive than that.

On the socket where the tube erupted, I get -47v. All the other tubes measure -35.5v. So not more positive, but quite a bit more negative.

The bias is fine, or I should say I have no trouble setting the bias on any of the other tubes I substitute into this socket.

This more negative voltage comes right off the VTA board, but I don't have a schematic and haven't seen one. Maybe an incorrect resistor just prior to the lead out, not sure, and there's a cap right over the whole shebang.

Any other ideas as to this incorrect voltage? Can somebody confirm the correct resistor value before I unsolder the caps and look?
 
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bktheking said:
Measure your resistors at pin 5, should be 1K resistor connected to pin 6.


http://www.tubes4hifi.com/VTA-Rebuild.PDF
Click to expand...

That's where I measured, but aside from the resistor, the only other connection is the single wire going to the edge of the VTA board. Pin 5 is where I get -35.5v on 3 out of 4 tubes and -47v on the tube (socket - the tube is no longer in the amp) that erupted.

OK, I read the .pdf. I was taking the voltage measurement off where the wire connected to the tube socket, which would be pin 6, then the resistor provides power to pin 5 -the grid. I'll double check to make sure I'm getting the same voltage on all pin 6s and then pin 5s and see it the 1000 ohm resistors are all actually 1K ohm. But my gut feeling is that I'm still -11.5v off at the pin 6 connection of this socket. It'll be a little over an hour before I can confirm this.
 
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OK, no matter how you cut it, the voltage on the grid of this tube is -12v different from the other 3.


The resistor leading up to the wire connected to pin 6, and hence to pin 5 via the 1K resistor, are Dales and situated so I can't make out the value. The 1K resistors between pin 5&6 are all fine.

I guess my next move is to pull the driver board and see if I can figure out where the problem is, I sure wish I had a schematic.
 
Aren't the resistors a mirror image on the board? Just write down the values and compare one site to the other.
 
Jeffery said:
OK, no matter how you cut it, the voltage on the grid of this tube is -12v different from the other 3.


The resistor leading up to the wire connected to pin 6, and hence to pin 5 via the 1K resistor, are Dales and situated so I can't make out the value. The 1K resistors between pin 5&6 are all fine.

I guess my next move is to pull the driver board and see if I can figure out where the problem is, I sure wish I had a schematic.
Click to expand...

I wonder if you have to lift that resistor to make sure you get an accurate reading.
 
bktheking said:
Aren't the resistors a mirror image on the board? Just write down the values and compare one site to the other.
Click to expand...

They appear identical, but so do most all of the resistors on the board. The Dale resistors are just tan with some printing, no banded codes. I can't see the side of the Dale resistors with the values in these four particular resistors.
 
cc43fan said:
I wonder if you have to lift that resistor to make sure you get an accurate reading.
Click to expand...

I don't know, I can't tell if there's anything other than the wire going to the socket connected to the outboard end. But whatever is connected, it should be the same for all 4 resistors. I guess I can measure all of them and see if this one is different. Even if I don't get an absolutely accurate value, maybe it'll be enough to tell if one is that much different.

ETA

All resistors are the same.
 
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Jeffery said:
The bias is fine, or I should say I have no trouble setting the bias on any of the other tubes I substitute into this socket.
Click to expand...

If I'm understanding, your coupling caps are connected to pin 6. I'm wondering if the tube was bad. Have you tried another tube in this socket? I also wonder if somehow the bias adjust pot got moved. I built this driver board, but it's been a couple of months.
 
Jeffery shoot me a PM with your number an a good time to call.
I should be able to walk you through all of your questions easily.

If you want..

--

P.S. Unless something is wrong with your bias pot you should be
able to adjust that voltage to be the same on all 4 output "grids".
(can pull All your tubes, then try adjusting the bias to see there)

Also your bias and output section on the VTA should look like this.
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attachment.php
 

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  • bias_output.GIF
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cc43fan said:
If I'm understanding, your coupling caps are connected to pin 6.
Click to expand...

No, there's a wire from one side of R31, a 221K resistor, going to pin 6. Then a 1K resistor between pin 5 and 6. The 221K, 1K, and the 10 ohm resistor that the bias reading is taken across all measure fine.


I'm wondering if the tube was bad. Have you tried another tube in this socket?
Click to expand...

The tube was definitely bad. I can put any of the other 3 tubes into this socket and adjust the bias via the trim pot very easily.


I also wonder if somehow the bias adjust pot got moved. I built this driver board, but it's been a couple of months.
Click to expand...

I'm not having any trouble getting 40mv across the 10 ohm resistor, just having a hard time wrapping my head around why the grid voltage is -47v on this socket as opposed to -35.5v on the other 3.

OK, no tubes I can set it to the same as the others via the bias trim pot. Then, when I put a tube in the socket that biased at 40mv with -35.5v in a different socket, the meter goes off the chart on this socket at -35.5v. I can get it back to 40mv across the 10 ohm, but not at the same -35.5v it took in a different socket.
 
Got another quad you could try? Just try two tubes in the one side that's causing an issue, sound like a bias pot is bad. Any discoloration near the pots?
 
I can bias every tube in any socket to read 40mv across the 10 ohm resistor, it's just that any tube in this particular socket ends up about 12 volts more negative than it does in any other socket. Put the same tube in any other socket and I can get the 40mv at -35.5v, put it in this socket and it's at -47v to get 40mv. Only this socket.

But I'm not going to spend anything else on it, it's got more issues than I'm prepared to deal with. I'll set it aside and maybe give it another shot when I feel more in the rebuild mood.
 
Does that one socket adjust at all or is it stuck at -47?

Measure the 50 k resistors in the bias circuit on the board, something sounds smoked, look for a burned resistor, as I'm sure you have.

sounds like Vr1-vr4 or a bias pot is pooched.
 
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bktheking said:
Does that one socket adjust at all or is it stuck at -47?
Click to expand...


It adjusts fine. Like I mentioned above, I can pull all 4 tubes, set all 4 sockets to -35.5v, and stick a tube into any socket. On 3 of the sockets, the bias is now right on 40mv across the 10ohm resistor. On the trouble socket, it gets to over 100mv and I shut it off. But I can then adjust the bias pot to read 40mv no problem, it's just that the grid is now -47v. Put a different tube in the trouble socket and it reads 40mv so long as the grid is -47v.

There didn't appear to be anything burnt, but the cover is back on it and it's boxed up heading to the storage shed.
 
Jeffery what your describing is/should be more then likely an bad/leaky coupling cap.
(if so will need to be replaced, even new ones can fail an often do early if they are)

----------

Just for those reading that may not understand how an "adjustable bias" amp works.
(also known as fixed bias quite often, like an ST-70 for example)

There is an negative voltage applied to grids of the output tubes, the more Negative
you make this voltage the Less current the tube draws and you check this current in
the Cathode of the output tube by measuring the voltage dropped across the 10ohm
"current sense" resistors generally found there, using ohms law to figure that current.
(say .5v dropped across an 10ohm resistor will give you 50ma of current .5/10 = .05)

Now the coupling caps(in this case C5 and C7) couple the driver stage to the output
tube grids to drive them, they block the DC voltage at the drivers stage from getting
to the output tubes grids, if those caps fail or "leak" some of that DC voltage will get
through and mess with the Negative voltage you are using to bias the tube, so you'll
have to turn up the Bias Voltage(more negative) to try and compensate for it.

---------------

Only other thing I can think of for this scenario is a bad output transformer where the
plate winding has failed an or got hurt to where the resistance is very very high which
would cause the screen to do all the current to where any tube would try an runaway.
(but I'd venture to say it's an bad/leaky coupling cap as my first culprit though here)
 
Kegger,
Thanks for giving us rookies a really good explaination of "adjustable bias." As one who finds himself "addicted" to tubes, any bits of knowledge I can absorb is a huge help.

Chuck

Kegger said:
Jeffery what your describing is/should be more then likely an bad/leaky coupling cap.
(if so will need to be replaced, even new ones can fail an often do early if they are)

----------

Just for those reading that may not understand how an "adjustable bias" amp works.
(also known as fixed bias quite often, like an ST-70 for example)

There is an negative voltage applied to grids of the output tubes, the more Negative
you make this voltage the Less current the tube draws and you check this current in
the Cathode of the output tube by measuring the voltage dropped across the 10ohm
"current sense" resistors generally found there, using ohms law to figure that current.
(say .5v dropped across an 10ohm resistor will give you 50ma of current .5/10 = .05)

Now the coupling caps(in this case C5 and C7) couple the driver stage to the output
tube grids to drive them, they block the DC voltage at the drivers stage from getting
to the output tubes grids, if those caps fail or "leak" some of that DC voltage will get
through and mess with the Negative voltage you are using to bias the tube, so you'll
have to turn up the Bias Voltage(more negative) to try and compensate for it.

---------------

Only other thing I can think of for this scenario is a bad output transformer where the
plate winding has failed an or got hurt to where the resistance is very very high which
would cause the screen to do all the current to where any tube would try an runaway.
(but I'd venture to say it's an bad/leaky coupling cap as my first culprit though here)
Click to expand...
 
Thanks, Kegger. I'll keep that in mind if I decide to dig this thing back out.

Any recommendation on some decent, reliable examples?
 
Jeffery said:
Thanks, Kegger. I'll keep that in mind if I decide to dig this thing back out.

Any recommendation on some decent, reliable examples?
Click to expand...

Almost any new coupling cap is as reliable as any other here.

Just every now and then you get 1 bad one, I've gone through
probably a couple hundred of those Russian oil caps on ebay and
I had 1 fail on me in the same manor you seem to be experiencing.

But I still do like those Russian caps quite a bit an trust them, from
my experience you get a bad part here an there, I also actually like
the Yellow IC film caps for vintage equipment
 
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