Voltage Supply Regulator for Audio Tube Amplifier

[Breezio]

Dear all,

I am rebuilding an older modification of my audio tube amp (AMC CVT3030) which is broken.
It's a source follower buffered Zener working as voltage regulator, mainly to supply the tube final stages of the amp.

I am attaching the schematic with the DC values marked and pics from oscilloscope measurements.
As you can see, the output is not stable when I connect the circuit to the tube modules (B+, G2, Plate on the schematic also attached).

I noticed that when I turn on the amp the regulator works fine and once the tubes start drawing more current, the output voltage (E) starts to wander
This is also in line with another test I did, with a couple resistors as a load (8k and 15k) where the output voltage stays very flat and everything works just fine.

I tried also to bypass R3 and get a higher Vds so that the mosfet has more room to regulate.
However the result is the same.

Does anybody have an idea of what is going on?

Thanks a lot,

Fabrizio

 

[NsrMagazin]

If the problem is removed by changing the load, that means you have issues with synchronization. I am not certain a voltage and current regulator is what you need. Unfortunately this is not my strong field.

 

[cowboybob]

Just a guess, but he source follower mosfet may be failing under load.

Pretty complex circuit(s) for an amp... .

 

[rjenkinsgb]

Your zener voltage is not stable - the ripple on that appears to be passed through to the output.

The zeners are labelled as 100V, so they will not be providing any regulation; it's totally dependent on having a stable voltage on C4/C5.

Try a rather lower value resistor for R9 and add a diode in series, so the caps are not discharged by the ripple on the main supply.
That may help the overall stability.

 

[Breezio]

Your zener voltage is not stable - the ripple on that appears to be passed through to the output.

The zeners are labelled as 100V, so they will not be providing any regulation; it's totally dependent on having a stable voltage on C4/C5.

Try a rather lower value resistor for R9 and add a diode in series, so the caps are not discharged by the ripple on the main supply.
That may help the overall stability.

Thanks,

yes the zeners appear to be the problem because the Vin (A - 401V) is too low and R9 is too high (very little current flwoing into them).
So the zeners cannot regulate to the expected 400V.

I am going to subtitute one 100V zener with an 80V one. Vout (E) can in fact be around 376V =380V (zeners regulated voltage) - 4V (Vgs).
Also I am going to bring R9 down at around 50k to increase the current.

Might need a diode later, but let's fix the zeners first.

Thanks again,

Fabrizio

 

[dr pepper]

I agree and also add that bias current could be so low that some of the fet gate drive is being lost in C5's leakage (is it an electro), this will increase ripple too.