P2P Redd 47 - a few questions

[abbey road d enfer]

Maybe something like this...?
http://www.jlmaudio.com/VU%20DRIVER.pdf

You may not need the diodes if you use a meter with built-in rectifier.

This buffer needs 9V to 30V DC. Would it be best to derive this from my 48V phantom supply,

Yes, you have enough power to spare on it

 

[earthsled]

What's your opinion about the 0A2 neon regulators? I'm finding that there's no current production of these tubes, but there seems to be a decent NOS supply. Is there an advantage to keeping true to the original power supply circuit?

Thanks!

 

[abbey road d enfer]

You could replace the OA2's with a string of power Zeners. Total voltage is 310 V and dissipation is about 6W.
You could use 6 of those:
http://uk.mouser.com/ProductDetail/ON-Semiconductor/1N5942BRLG/?qs=sGAEpiMZZMsnCtsnHQqDlfeX5dtOLFRiEzavffudQtE%3d
You may also want to use a high-voltage ballast transistor and a zener reference, but I don't think it's worth it.
Most of the times, the use of a voltage regulator was deemed necessary to tame unwanted VLF oscilations (so-called "motor-boating"), not for a particular sonic preference.

 

[earthsled]

You could replace the OA2's with a string of power Zeners.

I wan't sure what to do with R17 in this case...

 

[abbey road d enfer]

R17; you don't need it any more.

 

[fazer]

Avoid OA2's,  there are many left but most of them are noisy.  +1 for zeners.

 

[earthsled]

Thanks for the advice fazer! I was able to find some 0A2WA tubes that are supposedly better designed than the original 0A2s. I may decide to scrap the tube regs altogether, but for now, I'm planing to have the option for zeners in case they cause issues.

Speaking of noise issues, what are your opinions about DC heater supplies? Is it possible to change 6.3V AC into DC using a voltage doubler, rectification, smoothing, and regulation?

Thanks!

 

[abbey road d enfer]

Speaking of noise issues, what are your opinions about DC heater supplies? Is it possible to change 6.3V AC into DC using a voltage doubler, rectification, smoothing, and regulation?

Although theoretically possible, it may not be. Regulating a voltage implies wasting energy, and your transformer may not have enough power to start with.
Voltage doubler is a nasty thing because it is single-wave rectification, meaning you need huge caps to maintain a low enough ripple, which means the peak current will be very high, and may create too much stress to the transformer, and the voltage loss created by the DCR of the windings may be too high.
DC heaters voltage is a nice thing if it's built-in from the start or you take it as a redesign, with properly dimensioned transformer, full-wave rectification, reasonably large caps and quiet regulators (some are less than others).
I've seen too many improperly DC heaters that were noisier than a clean AC. Clean AC is 50/60Hz with very small harmonic content. Dirty DC is full of upper harmonics that cunningly go direct from the filament to the grid.

 

[earthsled]

I've seen too many improperly DC heaters that were noisier than a clean AC. Clean AC is 50/60Hz with very small harmonic content. Dirty DC is full of upper harmonics that cunningly go direct from the filament to the grid.

Okay, I'll plan on using the 6.3V AC from the power transformer. What about the center tap? Should it be connected to the same ground as the audio circuit and DC? Are there any tricks to reduce noise in the heater supply?


Also, the VU meters I ordered came with 2 LEDs wired in series for illumination. The label says "3.6~5.2V 20mA". I'm wondering if I should barrow more current from the 48V DC supply, or if it would be better to rectify and smooth the 5V AC filament supply for these LEDs. I'll have 2 meters in the same chassis, so a total of 40mA.

 

[abbey road d enfer]

Okay, I'll plan on using the 6.3V AC from the power transformer. What about the center tap? Should it be connected to the same ground as the audio circuit and DC? Are there any tricks to reduce noise in the heater supply?

You can connect the center tap too the audio ground.
Another possibility is to use a "hum-balancer"; a potentiometer of low value (100r to 1k) across the filament voltage, the wiper connected to ground.

  Also, the VU meters I ordered came with 2 LEDs wired in series for illumination. The label says "3.6~5.2V 20mA". I'm wondering if I should barrow more current from the 48V DC supply, or if it would be better to rectify and smooth the 5V AC filament supply for these LEDs. I'll have 2 meters in the same chassis, so a total of 40mA.

I would use the rectified heater voltage. Less heat.

 

[Jean Clochet]

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[earthsled]

Another possibility is to use a "hum-balancer"; a potentiometer of low value (100r to 1k) across the filament voltage, the wiper connected to ground.

Would this need to be a high-watt potentiometer, or could I use a trim-pot?

I would use the rectified heater voltage. Less heat.

Attached is my best guess for the needed values. Please let me know what needs to change.

-----------------------

I've succesfully heated 6.3V valves with as low as 5V so you could do the maths using a low drop regulator and see where you are.

Alternatively, have you thought of adding the 5V heater to the 6.3V heater before rectifying?  Might work if they aren't too dissimilar in proportions although it would be wasteful of power since you'd then have to throw quite a bit away to get it down.

Thanks for your suggestions, these are both good ideas. Of course, noise floor is a big concern with any mic preamp. This being my first build of a tube-based mic pre, I'm not sure how much of an improvement the DC heater supply would make. From your experience, would you say there is a significant reduction in noise with DC?

Thanks!

 

[abbey road d enfer]

Another possibility is to use a "hum-balancer"; a potentiometer of low value (100r to 1k) across the filament voltage, the wiper connected to ground.

Would this need to be a high-watt potentiometer, or could I use a trim-pot?

6.3V into 100r makes 0.4W so a trimpot would not handle the power, but if you use a 220r or 470r it will work.

Attached is my best guess for the needed values. Please let me know what needs to change.

I would use one 100uF cap and increase the resistors to 100r.

 

[Jean Clochet]

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[abbey road d enfer]

...From your experience, would you say there is a significant reduction in noise with DC?

Providing the D.C. heater supply is a good one as Abbey said, I don't think it's unreasonable at all to get a 12dB reduction in low frequency noise.

Can you substantiate that? The reduction of LF noise in the case of perfect DC would be infinite. But the adjustable virtual tap trick can improve AC heaters hum so much it is buried in the tube's intrinsic noise (particularly in the case of a pentode).
With proper implementation, the hum voltage can be significatly lower than 1uV - probably 10-15 dB below because the source impedance seen by the grid is much lower than the recommended 500k. The intrinic noise is likely to be of the same order, even higher because of partition noise.
I'm not questioning the fact that DC makes less noise, just drawing attention to the fact that it may be totally unsignificant.
Me, I couldn't put a figure.

 

[rock soderstrom]

A lots of good information in this thread, thanks for that!

I will go the humdinger route in my build.

 

[Jean Clochet]

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[Jean Clochet]

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[bernbrue]

E80F would be a good substitute for the EF86.
regards
Bernd

 

[abbey road d enfer]

The original CV4085 pentode was a "long-life" version of the EF86, which doesn't say much about the actual performance.