FPIII Transformer

[Lynxo]

Hi,

if I want to use a 230V toroid transformer in place of the 150V, would I just need to increase the value of the 2.7K 5W resistor?
what value should I use?

The 230V toroid is rated at 110mA.

Thanks

[Paul Joppa]

You would really have to re-design the power supply. All the capacitors would need higher voltage ratings, to allow for start-up transient voltages and the possibility that the gas regulator tube might fail or have a bad connection.

[Lynxo]

I could add a series resistor before the 2.7K 5W.
What is the target voltage before and after the 2.7K 5W with the 150V trafo?

Thank you

[Grainger49]

Lynxo,

When you turn on the power transformer everything up to the tube plates "floats" to the highest voltage available.  When the circuit starts to draw current then the series resistors in the power supply start to drop the voltage and the filter smooths it.  But for a few seconds everything gets a higher voltage than is the designed supply voltage.

That is what PJ is saying.

[Lynxo]

Hi Grainger49,

thanks for dumming it down further so I can understand :-)

if higher rated caps are used, it should be okay?

[Paul Joppa]

It's a significantly different design. An engineering review would take some time and effort, and until that is done nobody can say "it should be OK". It's very much like replacing 15" tires with 23" tires on a car - you might need a new suspension, as well as new fenders; the car might become prone to tipping on sharp turns, you'd probably need a different drive train and gearing, the odometer would need a different calibration, ...  :^)

[Lynxo]

understood.

Do you know of any online sources that explains theory/operation of the OD3/VR150?

I've googled but see limited information on these tube regs.

Your Extended FPIII still uses the stock 150V trafo? If I read correctly.

Thank you Paul.

[porcupunctis]

Try googling "tube voltage regulator" or "tube shunt regulator".  The 0D3 will be one of several that are discussed in general. 

Wikipedia has a fairly good general explanation of how these things operate at: http://en.wikipedia.org/wiki/Voltage-regulator_tube

They are quite a departure from most tubes.  No vacuum, filled with a mix of gases.  No heater filaments, they just start working when sufficient voltage is applied.

The one in my Foreplay is a JAN 150 and it has the open top on the well so you can see all the gaseous glow.  Others have a cap on the top that limits the view.  I think they are just fascinating.

[Paul Joppa]

Yes, limited information is readily available on gas tubes. But I also have found it nearly impossible to find detailed information, such as impedance and noise as a function of frequency for example. Or whether there are better or worse choices for the emissive materials - things that might make a difference in the sonic quality or lifetime.

The stock Foreplay uses a voltage doubler to generate a bit over 400vDC. This allows a large resistor to drive the gas tube, without the current being unduly sensitive to power line fluctuations. The eXtended Foreplay uses two independent fullwave bridge power supplies to generate about half that voltage, along with current sourced to drive the hybrid regulators - this gives less wasted heat and produces better independence form the power line. Even though the preamp circuits themselves operate at identical voltages and currents (it's just the current sources that replace load resistors) the power supplies are very different designs.

[howardnair]

lynxo-i have a disc and paperback  basic electronics book that gives a good  11 page tutorial on gas voltage regulator tubes-i cannot seem to save those pages from the disc  to pdf, disc or otherwise -i can print them and use ye ole snail mail --or print them and then scan and send -just ask

[Lynxo]

Hi Paul,

would 140VAC or 160VAC work as well? If used the 160VAC trafo and changed the 2.7K
to 3K, that should be okay overall?

I might have misread, the OD3 is working as a shunting circuit and not a regulator, so the
+B is not 150V or it is?

thanks

[Doc B.]

What is the reason for wanting to use a toroid? Are you wanting to use it in place of the stock transformer in a foreplay? Or is this a scratch built preamp?

[Lynxo]

toroid has lower profile, scratch build in smaller height chassis.

Thx