Apex 260 PSU zeners

[sedrec1]

Hi,

I thought about replacing the zeners in the Apex 260 PSU with 62v zeners. However, I have a couple of questions...

- Is it possible, instead of swapping the zeners, to add a trimpot after the the two zeners, and adjust the B+ more precisely? If so what ohm value trimpot would work for this?

- Or, can the two zeners be replaced entirely with just resistors? To lower the voltage more like in an unregulated supply like an original U47 or C12. And are there any sonic advantages to this.

Thanks

 

[RuudNL]

I suppose you want 120 V? In that case it is better to use a 62 V and a 56 V zener, that will bring you closer to 120 V than two 62 V.zeners. I would use 1.3 Watt zeners.
Zeners produce noise, for this reason it is better to place the zeners not at the output, but over the before last capacitor. A trimpot to replace the resistor in the last C-R-C stage is possible, but you have to take care of the dissipation (voltage drop x current). Value depends on the current and the adjustment range you want.

 

[mad.ax]

Since there will be a voltage drop in the last RC stage, might as well keep two 62V Zeners and drop 4V in the last resistor (or trimpot).

 

[sedrec1]

Thanks, the capsule is a K47 type, I am aiming to make the mic a little closer to U47. I realise that a U47 sound isn't really acheivable without everything else. However, I suppose I could lower the voltage further if this is more like a U47, but I am not sure what that voltage should be?

I will consider the trimpot, I could use a large wattage one, but without knowing the formulas to calculate it, or current draw, I suppose the resistance value will be trial and error, and I am still considering whether there any advantages to being able to fine tune it.

 

[sedrec1]

It seems in this PSU (schematic - https://www.deeringamps.com/workbench/images/apex460_ps_lg.jpg), the two zeners are after the final RC stage - without altering the board or other modifications (although perhaps the trimpot might not fit anyway) I guess the best I could do is change the zeners where they currently are in the circuit, and add a trimpot in the RC stage prior to the zeners? But I guess it doesn't work that way round as the zeners have not already dropped the voltage.

I could keep the existing zeners and add trimpot after (with some sort of modification to board/holes etc.), but there will be no final RC stage after the zeners as it stands. I suppose you are suggesting this as zeners can introduce noise?

Does anyone know how much voltage is being dropped at these zeners? Can I just replace one with a high wattage resistor and the second with a trimpot? Or is it just a crazy amount of dissipation due to the the U47 for example using a choke... does the C12 PSU use a choke too?

 

[RuudNL]

add a trimpot in the RC stage prior to the zeners?

That doesn't make any sense.
The zener will stabilize the voltage, but with a trimpot before the zeners, the voltage may become lower than the zener voltage and there will be no stabilization.

 

[sedrec1]

Thanks, so I think there are two options - 1) simply swap the zeners for lower voltage, or, 2) remove the zeners, and replace with a bleeder resistor (the value for which I guess is trial and error, somewhere around 200K?), and possibly swap the final RC stage resistor with a trimpot.

The question is, will removing the zeners and going with just resistors have a positive effect on the sound, or is it better to go with the zeners? I guess zeners introduce noise, but I think I am more concerned with whether using zeners affects tone/dynamics, as I understand they are not present in classic tube mics.

 

[RuudNL]

Keep in mind that the polarisation voltage for the microphone capsule is derived from the same HT voltage from the power supply. Imagine that you would accidentally switch the power supply on without the microphone connected. (Something you should never do.) In this case the voltage without zener diodes could rise to an unacceptable high value.
An example: an unstabilized U47 power supply can produce 385 V. without a load! (Desired output: 105 V...)
The capacitors will hold this voltage for a long time, you connect the microphone and the polarisation voltage will for a moment be more than 3 times the nominal voltage... The capsule won't like this!
Most tube microphones don't load the HT with more than 1 mA. If the last capacitor in the power supply doesn't have an extremely low value, there is enough 'buffer' for a very good dynamic range.

A 200 k.ohm pot is way too high! At 1 mA you will theoretically get a voltage drop of 200 V, a voltage that isn't even available. I would start with 22 (25) K.ohm. At 1 mA this will give a range of 22 V.
For 'fine tuning' 10 K.ohm might even be better.

 

[sedrec1]

Thanks again. I think I will stick with the zeners. Do you think that including the trim pot to slightly alter the voltage after the zeners would typically produce an audible difference at different voltages? (assuming I have already changed the zeners to 118V, so the capsule is already seeing a more suitable voltage)

 

[RuudNL]

The effects of a minimal variation in the HT voltage are highly overrated.
118 V. will sound exactly the same as 120 V.
Even with the voltage lowered 10% you won't notice much difference, only a drop in sensitivity of 1 dB...