D3a Experimenters Breadboard Project

[dsavitsk]

This is a project I have been working on for a while and thought others might think it interesting. This is a small board for testing various circuits, all of the grounded cathode variety. The idea is that by changing a few jumpers, there are a few hundred different circuits that can be breadboarded. I am planning to use this to test drivers for a choke phase splitter push-pull amp, but it would be fine as a single ended driver, preamp, part of a phono stage, headphone amp, etc.

I used the D3a as it is reputed to be about the lowest distortion small signal tube there is. It has high mu and high Gm making it a great driver. Also, being a pentode, it will work as both a pentode and a triode allowing for trying various circuits. But, the board should also accommodate other tubes like the 6688, 7788, 7721, 7722, etc.

Below are the board design, and the circuit schematics that can be produced.

[tubelab.com]

If you have some of these boards made, I would like to get 2 or 3. I have been told by the "gurus" that the D3A is a magic tube. Of course I didn't have any, and they aren't too common here in the US. I bought a pair of D3A's from Germany for too much money, but haven't had the time to play with them yet. Your board takes care of the hard stuff.

I have several other tubes with the same pinout and would like to compare them all. The common 6EJ7 and 6EH7 has the same pinout except that pins 8 and 9 are swapped, but your board will handle this with a flying wire jumper. Can you post (or email me) the Eagle schematic for your board? I am having a hard time visualizing it all from the PCB artwork.

The little 7 and 9 pin test boards you did do come in handy for "testing" tubes. I haven't had too much time to use them lately, but I have "seen the light" come out of some 6AQ5's. They make it easy to wire a 7 or 9 pin tube into my octal based Simple SE board.

[dsavitsk]

Schematic is at http://www.ecp.cc/images/D3a_experiment_board_sch.gif
The things that look like LM317's are 10M45's. "out" is the anode, "in" is the cathode, and "adj" is the gate.

I'll probably get a few made w/o soldermask and screen to keep costs down. That way, if we split them it will cost about $35 for a pair -- this drops a bunch if anyone else wants a few. It also drops having them made overseas, but it takes a bit longer.

I haven't been able to find jumpers yet. I assume they exist as they come on the back of harddrives, but Mouser has not been forthcoming so far. Spacing on jumpers is 0.1"

[athos56]

I got some D3a's for a project that since fell on the list of priorities, if you have a bunch made I'll take 4.

[Tweeker]

Its not shown in circuit options, is there plenty of room for ferrite and/or R||L stoppers near the leads(not just grid)? D3A is an serious hotrod.

[Tubes4e4]

Hi dsavitsk,

granted, the D3a had been considered a "secret tip" over here in Europe some years ago, but since then rumours and hysteria settled quite soon. Actually, due to its extreme gm it is quite hard to tame, and also for the same reason, its factory tolerances are rather wide. If you don´t believe, have a look at the TFK D3a spec sheet .

Due to its extreme gm, to get stable operation, one is well adviced to use the somehwat self-stabilizing cathode resistor auto bias method, using *exactly* the suggested Rk values and operation points as given in the TFK sheet. Sorry to say so, but there really is not much room for any experimentation with this tube.

I used the D3a as it is reputed to be about the lowest distortion small signal tube there is.

I don´t know where this "reputation" may have originated from, but neither spec sheet plots nor my own tracings of NOS/NIB TFK and Siemens samples suggest that. Neither in native pentode mode, nor when being triode-strapped. Granted, when considering its extreme gm its linearity is rather good, but otherwise almost any medium-to-low gm (or mu) tube can beat D3a in linearity any day, in any operation mode.

In my humble opinion, in audio the D3a just should be left to tasks that come in "naturally" by its data: Constant current sources, cathode followers and "upper" position in mu-followers (see Morgan Jones book), or even better, shunt-loaded mu-followers like Alan Kimmel stages (aka "mu-stage"). The later application is quite popular and proven for D3a over here in Europe, for those, who know how to handle and tame it for stable operation and suppression of parasitic oscillations.

Be aware that w/ D3a, parasistics (especially in CF duty) can occur at several hundred MHz - chances are you wouldn´t even detect them with a 100MHz scope. A friend of mine was disappointed by a somewhat "harsh" sound of a D3a circuit, but, according to his "scoping" of the circuit, was sure about not having parasitics going on. But there _were_ parasitics; rather easily detected by my old and trusty HP 410C VTVM, which is spec´d up to about 1GHz, IIRC. Eventually, an additional cathode (!) stopper cured the problem.

All in all, I wish you "good luck" with your D3a experimentation board - you probably will all luck, since using a PCB definitely is not the easiest way / method of choice when experimenting with extreme gm vacuum tubes, except you have designed it right from the start to prevent parasitics. Usually, it is quite more feasible to do a well-thought-out RF circuit style air- or hybrid- wiring.

Regards,

Tom Schlangen

[SY]

Tom, in my measurements of the D3a, the linearity was outstanding, easily surpassing any other tube I've had on the bench. At the same signal output level that Morgan used for his 417 tests, and similar setup (CCS plate load, LED cathode bias), in triode mode, the second harmonic was at -70dB, third at -90dB, and nothing higher order was measurable. I had to double-check my measurement setup since I thought something must broken! At a higher signal level (25VRMS), I measured -58dB second, and everything else was below my measurement threshold (-100dB).

Now, I agree that it's not an easy tube to use (tendency to oscillate, lots of unit-to-unit variation, tendency to draw grid current), but the effort is certainly worth it. In my phono stage, it's dead quiet and does not significantly contribute to the overall distortion. What else could one ask of a tube?

On the other hand, please keep telling everyone that it's nothing special. That will keep the prices down.

[Tubes4e4]

Hi Stuart,

first of all, I don´t doubt your measurements, since you certainly know how to do such stuff.

On the other hand, you agree on the comparatively large unit-to-unit variations, so I suppose you made a lucky pick with your samples at hand and the operation point you use, too. I (and many others) just didn´t.

I mean, considering that for the spec sheet given (pentode) op point an Ia range of +/- 14% ~ 30% explicitely is allowed for brand new out of the box tubes, should ring some bells regarding also to be expected linearity variation, when just plugging in another D3a from a different batch (in the same, unaltered circuit, of course). The OP wil move considerably, as also will the distortion spectrum distribution and hence absolute THD numbers.

Further on, looking at the "end of life" data in the mentioned TFK sheet, things get even more extreme. So, real world circuits (not talking about measurement setups) using D3a as they come out of the box and and age during use must be designed to be able to deal with such extrema in a most forgiving manner, or else one will get a "D3a rolling required - pick 1 out of 20 lottery" circuit, so to say.

Essentially, all tubes giving comparatively data and performance to D3a are beasts to deal with. And in audio, such extrema rarely are needed. Backing off somewhat from such extrema saves a lot of trouble - and money. I mean, who cares, say, if a CF gives an A of 0,97 using a bronco D3a, or A=0,95 using a tame workhorse EF184/6EJ7, or A=0,94 using a completely unsexy but made by the millions EF80/6BX6 (numbers just made up, you know what I mean).

On the other hand, please keep telling everyone that it's nothing special. That will keep the prices down.

In Europe, over the last two years or something, street prices for D3a dropped considerably from their peak. Just as some other fashionably "high gm´ers" like, say E280F, did. Just a sign that many gave up designing "real world" circuits with them, I suppose. I don´t expect falling prices for TFK EC8020, though

Uhmmm, but, well, D3a and the like _are_ special, at least for their extreme gm. But that is no hard criterium for good audio duty and sound, and often not worth the hassle. I mean, in audio we still are real men when _not_ showing off that we can tame D3a and the like, but instead design real world usable circuits that sound good using generic/average replacement tubes and keep on sounding good even after thousands of hours of parts wearing and aging.

Just my opinion, but I admit to be biased towards using unsexy and cheap and plentyful (think "TV" <gasp>) tubes and trying to get the most out of such anyway.

Regards,

Tom

[SY]

so I suppose you made a lucky pick with your samples at hand and the operation point you use, too

Tom, for the record, six samples, all Siemens, with two out of Morgan's stash (he had baked them to try to reduce grid current), four from Frag Jan Zuerst. 1.7V red LED for cathode bias, 10mA current. No more than 1 dB variation in 2nd, and 3rd plus higher were so far down in the noise that I would hesitate to specify variation.

I can't disagree about using cheap tubes when possible; after all, I use sweep tubes and bargain-bin 6BG6 for my power amps! I've saved my D3a for phono stage use, where getting the noise performance I want from cheaper stuff is not a trivial exercise. Fortunately, D3a were relatively inexpensive when I bought them ($15 or so each, including shipping and duty when I bought a dozen).

[tinitus]

Triode Dick has used that tube fore some time

http://www.triodedick.com/ceasar_2/c...versterker.GIF