Simple high current VR tube regulator

VR and transistor regulator

If you’ve looked through many of the designs on this website, you’ll see I have a love of glowing things. A current project of mine requires a ~150V supply and my mind immediately went to the beautiful purple glow and sultry curves of the 0D3 VR tube.

close up 0d3

The problem was that I wanted around 40mA from the supply. In the usual VR tube shunt regulator configuration, we’d size the ballast resistor based on the load current and the current we want through the VR:

vr resistor calc.png

With a large load current, the ballast resistor (Rb) will be small. But at start up with a tube amp/preamp, the load current will be zero until the heaters are warm. This will force the VR to pass the entire load current (in addition to its own quiescent current) until the rest of the circuit is warmed up. VR tubes are generally specified for only 5-40mA. Too much current at start up will stress the VR, leading to a shorter lifespan and potentially arcing.

Transistors to the rescue. The simple schematic above uses a VR tube as a voltage reference on the base/gate of a BJT/MOSFET. The emitter/source provides a very low output impedance to the load.  The output voltage is the VR tube reference voltage (150V for 0D3) less the Vbe of the transistor (approximately 0.7V for BJTs and 4-5V for MOSFETs). The current limit in this configuration is limited by the pass transistor and heatsinking, rather than the VR tube.

I’ll be building and testing this supply in the near future with an 0D3, but I don’t see any reason it shouldn’t work with 0A3, 0B3, 0C3, and/or series combinations of these voltage references. Just be wary of the transistor max voltage.

Advertisements

Listening on someone else’s system

Like many audio enthusiasts, I have a general philosophy for audio that guides me when designing (or shopping for) new gear. In a nutshell, I value an objective and empirical approach to design, but this is tempered by the notion that music is art. At its core, art appreciation is a subjective, and often situational, experience. Objective design for subjective ends reads like a paradox; designers have egos too and so maybe conflict between engineering and ‘the feels’ is inescapable. If you’ve been on audio forums or blogs long enough, you know that objectivity and subjectivity do not usually mix in the audiophile hobby. I’ll steer clear of that morass, save to add one recently encountered perspective.

Last weekend I delivered a preamp (design write-up on the way) to its new owner, J. We spent a couple of hours listening to his system with and without the new piece. J’s system is different than mine and the music it makes sounds different, too (including recordings I know). We both had fun going through albums and cranking up the tunes. In a way, it was a little like seeing a favorite group perform live. You know the music but can appreciate fresh nuance all the same. That we got to do so together, on a social level, only added to the enjoyment.

Now what if we were all uncompromising in our objectivity? What if all systems and designers pursued the same goal and weighed compromises equally? Worse yet, what if compromises did not have to be made and all playback was “perfect?” While I know this is ostensibly what many of us seek in the audiophile hobby, where would it leave the hobby on a social and experiential level? I would visit J and hear the same songs in the same way that I always do.

The art in music is not a one-way street. The lenses and filters we use to experience and share art enrich both the art itself and culture as a whole. The process of internalization, expression, and rebirth keeps music relevant and vital. I’m off into abstraction, but there is a kernel of truth for audio here, too: how terrible the tyranny of ‘exactly as the artist intended’ could be if we took it too literally.

You are the artist of your listening, the world is your mixing console, seek out new stereos, and all that jazz.

 

Guest Post at Audio Primate: JDS Labs CMOYBB Review

023

One part market research and two parts DIY hobby service: click here for another review of a small solid state headphone kit/board at Audio Primate. JDS Labs has done an excellent job with this kit. Everything is clearly labeled, the board is good quality, and the documentation is excellent.

If you want a place to start with DIY amps and line-level gear, look no further than the classic CMOY.

DIY amplifier top plate easel

I wanted a better way to wire my amps and had been lusting after Decware’s amazing assembly room for a while.  While I can’t afford the custom extrusions and equipment that Decware has, I can get creative with common materials.  I decided to try a simple easel based on a couple of rails of t track and some standard angle and rod extrusion.

The basis of my easel is two 2ft sections of t track. Finding a 4ft section with bolts and knobs on sale at Rockler was what pushed me over the edge to build this daydream. The t track is 3/4″ wide by 3/8″ deep. Rather than trying to route a channel in a thick board, I sandwiched the t track in scrap with 3/4″ thick scrap under the track and 3/8″ to form the outside. These were glued and clamped face down so that I could be sure the t track would be flush with the top edge. 

My bench is cantilevered from basement joists with 2×6 vertical supports. I ran a 1/2 aluminum rod between the supports to provide lateral movement and adjustment to the t track rails. The rails simply have a 1/2″ hole through which the rod passes. By mounting the rod 12″ from the surface, the 24″ rails give a 30 degree angle. This leaves plenty of clearance for transformers and is comfortable to work on while standing or sitting). I purchased some 1/2″ washers and collars, but they may not really be necessary (I’ll find a use eventually). 

My horizontal ledges are 1/2″ by 3/4″ aluminum angle (3/4″ side is flat against the t track rails). I used self adhesive cork sheet to protect plates in the easel from scratching on the support. Depending on your knobs or wing nuts, you may have to trim some of the horizontal rails so that they can be completely tightened. I cut my rails to fit a 18″ wide top plate, but aluminum extrusion is cheap if I ever want to build something wider. 

All in all, this is a handy and relatively simple addition to my tube amp building bench. And it is a lot cheaper than custom extrusions or lab fixtures!

 

Letters to WTF: What kind of rectification am I supposed to use with this power transformer?

I was helping someone with an amp build over Telegram (chat app) yesterday when this question came up. He had in fact been trying to use a diode bridge with a center tapped transformer with both the center tap and the bridge grounded. He released the magic smoke from his transformer, though there were a couple of other issues that may have contributed to this.

When I was starting out, I had some confusion with power transformers and rectifiers, too. Probably like many others, I started with small solid state circuits, where center tapped transformers are rare. Once I started building with tubes, the secondary ratings of center tap transformers were another source of confusion. So here’s a by no means complete rundown of transformer configurations.

1. My transformer doesn’t have a center tap and I want a full-wave rectified DC output.

You want to use a diode bridge (figure 4-8). This is four diodes arranged to rectify both positive and negative phases of the power transformer’s AC output. Your ground will be taken from the bridge, NOT THE TRANSFORMER. This ground at the junction of the diodes creates a return path for current that ‘switches’ with the changing phase of the secondary’s AC output.

2. My transformer has a center tap and I want a full-wave rectified DC output.

You want to use a “conventional” full-wave rectifier (figure 4-5A). This requires only two diodes (solid state or a rectifier tube). Your ground is taken from the center tap of the transformer (which is then the return path for current). Many center tapped transformers are rated as the full end-to-end secondary voltage. For example, a 300VAC center tapped secondary would actually provide 150VAC into a conventional full-wave rectifier. You’ll sometimes see the same transformer listed as 150V-0-150V.

Here’s a great clarification of what’s going on with full-wave bridges and conventional full-wave rectification.

How much voltage do I get?

With either of the above, the unloaded DC output into a capacitor-input filter is approximately the AC output from the secondary times the square root of two, minus the voltage drop across the diodes (minimal for solid-state, can be considerable for tube rectifiers). Into a choke-input filter (unloaded, ignoring diode drop), the output will be approximately two times the square root of two divided by pi (about 0.9) of the AC output of the transformer secondary.

3. My transformer secondary has a center tap, but I want a bipolar power supply.

Here you can combine the center-tapped transformer and the aforementioned bridge style rectifier. See figure 5.1c here. This creates two separate full-wave rectified voltages, one positive and the other negative with respect to the center tap. If you read a lot of TubeCAD, you see bipolar tube circuits pretty regularly.

4. My power transformer is 300VAC (150V-0-150V) center tapped, but I want 400VDC!

Another way to combine the center tapped transformer and bridge rectifier is to ignore the center tap altogether. Do not connect it to ground; just SAFELY tape it off and tuck it away. Now you have basically a non-center tapped transformer and you can treat it like number 1 above. Note that current capacity in this configuration is typically half of what the transformer was originally rated for.

5. My power transformer is 120VAC without a center tap and I want 300VDC!

To achieve this, you can use a voltage doubler (see figure 4 “Delon circuit). This requires two diodes and two capacitors. Because the capacitors will see large pulses from the diodes and will be supplying the rest of the circuit continuously, they need to be a fairly large value. But because each only sees half of the supply voltage, their voltage ratings are a little more relaxed in comparison to what is required in a filter. The unloaded DC output into a capacitor-input filter is approximately twice the AC voltage from the transformer secondary times the square root of two. Current capacity must be de-rated at the output voltage by a factor of at least two.

6. My power transformer has dual matching secondaries and no center tap. What do I do?

This is common with toroidal power transformers in particular. You can wire the two secondaries in parallel (making sure the polarities are matching) and use a bridge rectifier like number 1 above. The AC output of the transformer will be the same as either secondary by itself (and current capacity will be doubled). You can also wire the secondaries in series by connecting a positive and negative from each secondary (not the positive and negative from the same secondary!). This creates a center tap at the junction. The AC output end-to-end will be twice the AC output of a single secondary if the secondary is not grounded (see number 4 above). If you ground the secondary you created, you can use a rectifier like number 2 above.

The Joy of Local

RCA ST 2I’ve mentioned here and there the clubs and local organizations I’m involved in (Wisconsin Antique Radio Club, represent). I also try to get to local DIY meet-ups and sip and spins wherever possible. Getting to know and network with experienced designers and builders was and is one of the best parts of the hobby for me. Having someone you personally know and who personally knows you (and your experience and capabilities) is an important resource as you get started in DIY. A lot of the knowledge, especially in our hobby, doesn’t live online and the perspective of an ‘old timer’ is far more insightful than a website (my own included). So get out there and make friends as a newbie.  Most of us in the hobby do it for the passion and fun of it and are happy to share thoughts and suggestions with others.

RCA ST 1

In addition to getting to know other DIYers, being involved in local clubs puts you in touch with local vendors. Online stores are a great way to find that specific part or tube without a hunt, but part of the fun of DIY is scrounging and working with what you find. Unless you are from a big city, there probably aren’t any “DIY tube amp builder association” meetings in your area, so look for HAM or radio clubs or meets, too. Chances are good that there is at least one local organization that you can join and start attending events. Many small vendors are willing to make quite a trip to offer their parts and tubes and meeting these local businesses is an adventure in itself.

The NOS matched RCA carbonized ST envelope tubes pictured above came from the swap meet I attended this past weekend.  The price would make you jealous.  Since joining  my local radio club, I’ve started buying almost all my tubes at the monthly meets. Dave at Electric Guru Parts House is my local go-to.  Join your local club and find your own!