B+ 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

 

[Breezio]

Vin AC

 

[Breezio]

Vout - AC

 

[Breezio]

VB - AC

 

[Breezio]

PCB

 

[Breezio]

Tube Module

 

[abbey road d enfer]

Does anybody have an idea of what is going on?

The regulator is out of breath; the voltage at the input is only a couple of volts above the desired output, due to the voltage drop in the 47r resistor. The output MOSFET needs about 10 volts more than the desired regulated voltage  to operate correctly, particularly at the lower points of the rectified voltage.
You must increase the rectified voltage, which implies a change of power transformer, or decrease the regulated voltage by decreasing the voltage of one of the zener diodes by about 10V.
Do you use this regulator to power also teh output stage ( center-tap of output xfmr)?
You should use it to power only the module and connect the output xfmr to the unregulated (rectified) voltage.

 

[ruffrecords]

Couple of things come to mind after looking at the schematic.

First there is no dc path from the regulator output to ground. This explains why the behaviour changes once the tubes warm up and start to provide one. You really should have something like a 270K 2W resistor across the output just to make sure the caps get discharged safely

Secondly, the bias current through the zener chain looks a little light. Looks like there is 15V across the 100K resistor which is only 150 micro amps. Most likely before the tubes start to conduct, the HT voltage is higher and there is more current through the zener so they work OK but once the tubes start pulling current the raw HT drops and the zeners are struggling.

Cheers

Ian

 

[abbey road d enfer]

According to the schemo, and voltage chart, there is 15V across the 47r resistor; that accounts for 300mA!
There is definitely something wrong; either the voltage measurements are wrong or the resistor is actually much higher than 47r.

 

[Breezio]

Thanks Ian and Abbey Road for the valuable comments and suggestions.

The current flowing through the regulator is usually aorund 260mA.
The particular measure I indicated might be flawed by fluctuations.

Of the 260mA, less than 10mA powers the integrated preamp, the rest goes to two push-pull EL34G power modules: anodes (4x50mA), grids (7.5x4) and driving circuit (2x11mA).

As mentioned, when I bypass the 47ohm resistor and raise Vdrain by more than 10V nothing changes.
I cannot change the power transformer, so all I am trying to do is to improve the regulator which is fed with the 480V rectified AC voltage.

Let me know if you have any other comments.
I'll try the changes suggested and let you know how that goes.

Thanks again!

Fabrizio

 

[abbey road d enfer]

Thanks Ian and Abbey Road for the valuable comments and suggestions.

The current flowing through the regulator is usually aorund 260mA.
The particular measure I indicated might be flawed by fluctuations.

Of the 260mA, less than 10mA powers the integrated preamp, the rest goes to two push-pull EL34G power modules: anodes (4x50mA), grids (7.5x4) and driving circuit (2x11mA).

As mentioned, when I bypass the 47ohm resistor and raise Vdrain by more than 10V nothing changes.
I cannot change the power transformer, so all I am trying to do is to improve the regulator which is fed with the 480V rectified AC voltage.

Let me know if you have any other comments.
I'll try the changes suggested and let you know how that goes.

Thanks again!

Fabrizio

Hey, the zeners are 100V, for a total of 400? You feed them with 401V via a 100k resistor. Assuming the gate current is near-zero, you have only 10uA available. The zeners are not in their regulation area.

 

[Breezio]

Yes, that seems to be the problem!

In fact, after

- changing R9 from 100k to 62k
- adding a 200k resistor towards ground at the output
- bypassing R3

The problem persists.

I will substitute a 100V zener with one close to 70V since the nominal B+ value is supposed to be 375V.

Just to give you some background, I'm trying to remake/review and old broken mod.
While the acoustic result was very good, the execution, as you can see in the picture, was outrageously sloppy if not dangerous.

 

[Breezio]

Correction, the substitute zener will have to be close to 80V

 

[CJ]

Fairchild 670 reg is the bomb, big ol EL34, and a freaky cool OD2 gas thing that performs it's own light show on startup,  adjustable pot, rock solid to .001% regulation what more could you ask for, lower parts count? you get out of life what you put into my friend, 

 

[Breezio]

Ha! I wish I had enough space in my amp to fit one of those!

 

[Breezio]

Most likely before the tubes start to conduct, the HT voltage is higher and there is more current through the zener so they work OK but once the tubes start pulling current the raw HT drops and the zeners are struggling.

Yes, in fact when I turn on the amp, Vin (A) is 464V and Vout (E) is 388V.
Once the tubes are warm enough, they drop to ~401V and ~381V respectively

 

[Monte McGuire]

One suggestion, since you're cutting the zener voltage margin closely, is to measure specific zeners and select a set of diodes that have the right drop at the current you will actually use them with. This will be a lot more precise than buying low tolerance parts, since all of the errors will be taken into account. I find that a typical pile of zeners will have a fairly wide spread, and there's no reason to guess if you're making a one-off like this.

 

[abbey road d enfer]

The current flowing through the regulator is usually aorund 260mA.AC voltage.

As I wrote earlier, you should take the plate current out of the regulator, power the center-tap of the output xfmr with the unregulated voltage. That would solve many problems.

 

[Breezio]

One suggestion, since you're cutting the zener voltage margin closely, is to measure specific zeners and select a set of diodes that have the right drop at the current you will actually use them with. This will be a lot more precise than buying low tolerance parts, since all of the errors will be taken into account. I find that a typical pile of zeners will have a fairly wide spread, and there's no reason to guess if you're making a one-off like this.

Thanks for the advice!

 

[Breezio]

As I wrote earlier, you should take the plate current out of the regulator, power the center-tap of the output xfmr with the unregulated voltage. That would solve many problems.

Ok, so you are suggesting to provide a regulated supply only to the integrated preamp and the tubes driver circuit?
This would in fact leave out the grid and plate supplies (see output trafo pic attached) for a remaining supply current of 32mA, whcih is definitely more manageable.

Question> how would the acoustic performance be affected in this case?

Thanks again for the help,

Fabrizio