Bias setting problem

[Leevi]

I create the negative bias voltage from high voltage secondary of the PT.
The secondary AC voltage drops remarkable when I change from the standby to the operation mode which
of course affect the negative bias voltage as well. I can go around the problem by
setting the bias based on the dropped voltage but there will a while (some seconds) when the tubes get too much current
because of the capacitors in the bias setting circuit. Any ideas how to fix this?


/Leevi

[sluckey]

Surely you adjust the bias while in operate mode for the desired idle current through the tubes. When you switch to standby mode the bias voltage increases to some greater negative value. Now when you switch back to operate mode this greater bias voltage has the tube current throttled back initially. As the bias voltage slowly decreases the tube current will slowly increase until it reaches the point you had set it to.

I don't see a problem.

[Leevi]

The problem occurs when I switch from the standby to operate mode.
I have adjusted the bias while in operate mode.



In the standby mode the bias voltage is on the grid -29V.
In the operate mode the voltage is in the same point -37V which is correct value for the power tubes.


After I have switched to the operate mode the negative voltage starts slightly getting closer to the -37V level
which takes in total some seconds. During this time the bias is set too high which causes that the power tubes
get too much current. I can not say how much this will shorten their life time but a current peak will
happen every time you switch from the standby to the operate mode.


/Leevi

[sluckey]

This is just opposite of what you said in the original post. Can you show a schematic? I have a couple ideas to make the bias voltage be rock solid but would like to see your circuit to best adapt my ideas.

[Leevi]

I'm building a Deluxe type amp with 2 x 6V6s and using a PT without CT.
The rectifier and the bias circuit is like in the VOX AC50 but I use only one trimmer (in the VOX schematic there are 2)

http://www.drtube.com/schematics/vox/ac501976.gif




/Leevi

[sluckey]

I don't see a standby switch???

[VMS]

Did you check Merlin's page:


http://www.valvewizard.co.uk/bias.html

[Leevi]

You are right, I don't see either.


In my circuit the standby is placed after the rectifier, pls see the pdf below.


/Leevi

[sluckey]

That Vox bias circuit is overly complicated. May I suggest you build the much simpler circuit as found in this Marshall JCM900 amp?

     http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm900_25xx_45xx_50w.pdf

[Leevi]

That Vox bias circuit is overly complicated. May I suggest you build the much simpler circuit as found in this Marshall JCM900 amp?

That's one option but how does the Marshall circuit be a better solution? The AC voltage will drop anyway when the operate mode is switched on.


/Leevi

[sluckey]

When the AC voltage drops the DC voltage should drop also. This is the part I don't understand.

[Leevi]

I have one idea. I have now quite big resistors like 220K around the trimmer that probably
slow down the voltage change. By decreasing the resistor values and allow more current the capacitors will discharge
faster.


/Leevi

[silverfox]

Late to the party. Doing an Internet Crawl... Anyway, if the 220k resistor you're talking about reducing is the one feeding the bias circuit, use caution in changing that value as the voltage for the bias circuit is going to rise closer to the HV level. Change it too much and you'll have very high bias voltage. In any event, if you reduce the value of the resistor the cap will charge faster. If it's the one feeding the grids, more current will flow into the tube.

silverfox.

[2deaf]

The standby arrangement is causing the phenomenon.  If you unload the B+ with the standby switch like that, the bias voltage will indeed drop (become more positive) and the 47uf cap. will charge up to a possibly alarming voltage.  If you move the standby to the position in the JCM900, the bias will increase (become more negative) on standby to a level that might exceed the voltage rating on your capacitors.

If you want to have a standby, then I second Sluckey's motion to use the JCM900 bias.   

[Leevi]

Thanks for the replies, I'll do some tests tomorrow. It's year 2017 here already.
Happy new year!
/Leevi

[2deaf]

I have one idea. I have now quite big resistors like 220K around the trimmer that probably
slow down the voltage change. By decreasing the resistor values and allow more current the capacitors will discharge
faster.

The 220K's going to the power tube grids have essentially no current flowing through them, so changing the value will not make any difference.  The 220K's across the 4uf capacitors coming off of the transformer winding are merely balancing resistors just like the ones across the 64uf filter capacitors.  The voltage across the 4uf's will be similar to the voltage across the 64uf's, so they stacked both of them to get the voltage rating they needed.  A single capacitor without any resistors will achieve the same goal if it has a voltage rating that is sufficient.

The whole thing looks sort of like a voltage doubler gone wrong, which is probably how it got invented.  If you alter it correctly, it just might turn into a voltage doubler gone right with a bias voltage of zero.

[sluckey]

There's some big money betting on that bias supply to work well every time you turn the amp on. I like simple.

So, you have an odd project here... Deluxe type amp, PT that requires a bridge, that weird AC-50 bias supply. How did you come to put these 3 ideas together?

[whoops]

You will not correct that with that standby switch arrangement.

If you want to make it work and go on with your life do like sluckey said already and use the JCM900 bias circuit.

[Leevi]

The 220K's going to the power tube grids have essentially no current flowing through them, so changing the value will not make any difference.

It's not a question about grid resistor. The 220K resistor is in the bias controlling circuit in order to to limit
the current flow through the bias trimmer. When you switch from standby to operate mode the capacitors (in bias circuit) will discharge
through this resistor which makes the voltage change slower.


/Leevi

[2deaf]

It's not a question about grid resistor. The 220K resistor is in the bias controlling circuit in order to to limit
the current flow through the bias trimmer.

I only see four 220K resistors.  If they aren't the resistors going to the grids, they must be the balancing resistors across the 4uf capacitors.  These resistors have nothing to do with the current flow through the bias trimmer.  The combined total capacitance of the two series capacitors makes a difference.  If the value increases, the bias will become more negative. If the value decreases, the bias will become less negative.  You can replace the two 220k resistors and the two 4uf capacitors with just one 2uf capacitor and get the same results.

[Leevi]

I have attached a picture below so that we are talking about same thing.


/Leevi

[VMS]

This traynor amp has got similar bias supply as the jcm900:


http://traynoramps.com/downloads/servman/sm_dh40h.pdf

[Leevi]

I changed to Marshall bias circuit that works perfectly.
I have never fully understood the VOX bias circuit what is the point there.


Thank you very much for your replies, this forum helped again.


/Leevi

[sluckey]

I changed to Marshall bias circuit that works perfectly.

I like happy endings.   

[whoops]

Happy that you were able to implement the Marshall circuit and that it worked.

Time for playing now!!!!