using flash camera (and maybe laptop screen or fluorescent) inverters to power tubes

[ionomolo]

Some years ago i had a dramatic experience: I was repairing a bad joint in a digital camera and suddenly i found myself into the floor. It took me about five minutes to realise that something inside the camera shocked me (I did the repair with no safety practices since i thought that the highest voltage inside the camera was 3Vdc).

Later i discovered that the flash uses an inverter that can give up to 200V at 1 mA.

Now i'm doing a super-compact chipamp and i wanted to add a tube buffer to it, so i picked up the digital camera inverter and used it to power a low quiescent current PCC88 cathode follower. The results are very nice, the buffer has a distortion under 0.5% (the residual at the scope is under 0.5% but it looks like distorted fundamental, so if i used anything finer than substracting a scaled input to the output maybe i would find the distortion to be under 0.1%).

Digital cameras have evolved a lot in the past few years and many of us have one or many broken ones which aren't worth repairing, so using the inverter to power small tube buffers seems a very cheap and interesting option (adding the tube buffer to my compact gainclone left it's weight and size unchanged!).

While the flash camera has a 100 uF cap that filters most smps noise (i had to add a RLC filter to get rid of it completely), i have no idea of whether the laptop screen inverter output is DC or AC, despite it seems to be a digitally controlled CCS (that would be great to power tubes if it was easy to interface).

I would like to ask if someone has used a lcd inverter to power tubes, which tubes work well at low quiescent currents (PCC88 is wonderful in that sense) and if somebody has used a fluorescent bulb inverter to power tubes, as transformers are the most expensive parts in any tube amplifier.

[jackinnj]

I recall that Audio Amateur had an article using one of these chips -- there was also an article in Nuts N Volts one or two issues ago using one for running nixies.

These chips are pretty noisy. A better route would be to use one of the low-noise switching chips from Linear Tech. These are slew controlled so the switching chips go through the linear portion -- albeit generating heat in the process.

[pilli]

...maybe off-topic, but one way of having an easy power supply for tubes could be to put two "small" transformers "back-to-back", like
220 || 12 into 12 || 220

In countries where the network works at 220V or 240V, this gives above 300V when rectified and smoothed with caps.

Then of course you also need a supply for the heaters, almost inevitably DC.

But transformers under 100VA can be found relatively cheap, including from wrecked equipment (VCRs of the pre-SMPS age, the older the better...)



Edit: wrecked SMPSes can give high voltage caps, though the tedious part is to make sure that *they* (the caps) are not damaged.

_

[ionomolo]

Originally posted by Jackinnj
These chips are pretty noisy. A better route would be to use one of the low-noise switching chips from Linear Tech. These are slew controlled so the switching chips go through the linear portion -- albeit generating heat in the process.

These chips look nice, i don't see noise as a problem since the low current drawn by the tube stage allows for the use of a rc network of, say 2KOhm - 10 uF would provide a rejection of 62 dB at 50 Khz, a choke would increase it even more and the large reservoir cap would almost null it, while this is feeding a cathode follower which itself has a good PSRR. A grounded cathode amplifier might give more problems.

Originally posted by pilli
Edit: wrecked SMPSes can give high voltage caps, though the tedious part is to make sure that *they* (the caps) are not damaged.

Yes, i use these caps, and also some from crt tv's. Using a couple of small transformers instead of a big one may save money but not weight and size. My compact tubed gainclone looks wonderful!

[merajsalek]

Hey Pilli,

You don't need another transformer for the heater supply. Just wire it like this:


220 || 12 into 12 || 220
|
Diode Bridge
|
RC Filter
|
Regulator
|
Heater


Cheap and easy :-)

[merajsalek]

forum formatting

220 || 12 into 12 || 220

Diode Bridge^ (from the 12V secondaries)
|
RC Filter
|
Regulator
|
Heater

[jackinnj]

Since we're on the topic -- Jim Williams wrote a nice article "High-voltage, low-noise dc/dc converters" in the August 7, 2008 issue of EDN. It can be found here:
http://www.edn.com/article/CA6582859.html