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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: 6973-any substitute? posted by Gary Kaufman on April 14, 2001 at 12:18:03:
I would like to elaborate on Gary's comments about the 6973, since I find them rather timely. I recently acquired a GE amplifier, called a Classic 7700. It's a stereo integrated, using one pair per channel. The previous owner probably didn't want to spring for 6973's, so he plunked a quartet of 6CZ5's in instead.
I had a problem with one of them going into a marginal runaway, you could see some nasty orange plate glow visible even in full room light. I also wanted to substitute the 6CZ5 for the 6973, since whilst rummaging around I found I had only about a dozen or so 6973's in varying condition (mostly rather knackered, maybe I can scrounge up one good set). In contrast, I stumbled across an old purchase of yummy NOS Canadian GE 6CZ5's, a box of 100! Another tube with identical pinouts to the 6CZ5 is the 6EM5, and I have 300 NOS's of those!
But what Gary says is true, you have to verify that pin #8 is not used in order to substitute to either tube (pin #8 performs the same function as pin #2, which is the screen grid).
But there are other differences, and they would account for the problems I had with the GE amp. I expected to find a leaky coupling cap causing the bias problem, but alas, twas not to be! The amp is in very minty condition, and all the candy-stripe caps are fine.
Rather, the difference between the 6CZ5 and the 6973 is down to a very narrow & specialized one: tolerated grid impedance for fixed bias operation. Whew!
There are also differences in specified maximum anode & G2 voltages, plate & screen voltages to you Yanks. But I doubt that this is the problem, as voltages in my GE amp aren't THAT high, and the difference in practice between a 6BQ5 and a 7189 is largely fiction, even though those tubes are rated quite differently in that one area, maximum permissible voltages.
In cathode bias mode, both tubes can tolerate a grid resistance of up to 1 Megohm. But when used with a fixed bias supply, the 6CZ5 grid impedance is rated to be no higher than 100 Kilohms, whilst the 6973 can tolerate five times that, or 500 Kilohms.
If it turns out that your Magnavox is cathode bias, you're home free if the grid resistance is 500 K or lower. To verify this, you'd need to see where pin #7 goes (Cathode & suppressor G3). If it goes directly to ground, or you can see bias pots, well...
If there are power resistors of several hundred ohms there, then you can get away with this substitution, just double check the value of resistors connected between pin #3 and/or pin #6 (both pins are G1 grid) and ground. There would probably be a lowish voltage (25-40V) electrolytic capacitor in parallel with the power resistor as well, or at least a wire going to a low-voltage section of one of those aluminum multi-section can capacitors in the power supply.
The GE turns out to be a fixed bias amp, and the designer took the grid resistance to the maximum 500 Kilohms! No wonder the 6CZ5 was problematic in this amp...
Now, the 6EM5 beats both the other tubes in this regard, with a rating of a whopping 2.2 Megohms of permissible grid resistance.
Of course, there is a rub. In all probability, your pin #8 connection will have been left unused. If not, just move the wires over to pin #2 instead, unless there is already something connected there, in which case you can just omit the pin #8 components if you wanted to (just be careful that you actually know what you are doing!). So the pin #8 thing is unlikely to be a problem.
To the rub. The 6EM5 is a filament pig. Everything is pretty similar to the 6CZ5 (bias requirements may differ slightly, but probably not much), but filament current jumps from 450mA (same as 6973) to almost double that, or 800mA. That means that the filament transformer needs to be able to safely put out that extra third of an ampere per tube.
Since you describe your amps as monos, that is only 700mA extra on the filament circuit, probably doable.
The way to verify that is to measure the AC filament voltage on either 6973 (pin #4 to pin #5) and write it down. Then insert the 6EM5's (if there is a rectifier tube, yank it while doing this test, otherwise check pin #8 first!) and note the filament voltages. If they are more than 5% lower, the transformer may be overstressed trying to run it. This is more likely to be a problem in a stereo amp than a mono one.
And there is a second rub, as if that one wasn't bad enough. Whilst the 6EM5 is rated for 2.2 Megohms of grid resistance in fixed bias, this is only if the tube is used as something called a "Vertical Deflection Amplifier". Even the 6CZ5 is rated for 500 Kilohms in this service. Problem is, it is a pulse-type application, and circuit techniques for these TV applications may also have some protective cathode resistance thrown in as a matter of course. I'm not familiar with VDA's, so I can't comment authoritatively. But what IS interesting is that the tube manuals specify the two applications, VDA or Class A1 amplifier, with dramatically lower grid resistance tolerance in the latter mode (which unfortunately is the one that concerns us here). There is no Class A1 amplifier grid resistance ratings for the 6EM5, probably since it was never really intended to be used in audio amps. Still, it's grid ratings are even higher in VDA mode than the 6CZ5 by a factor of 4.4, so it would seem likely to be able to handle the 500 Kilohm grid rating as well as the 6973.
IT WOULD SEEM.
What I suspect is going on here is that the 6973 may be a 6CZ5 with one critical difference, perhaps a specially-treated or gold-plated grid that the 6CZ5 DOES NOT have!
Again, three of the four 6CZ5's I have in the amp work fine. I haven't tried any others yet, but I do know the problem follows the tube, not the socket and is not related to a stability problem (ultrasonic oscillation in the tube, etc.).
So you may get the results you want with the 6CZ5 or 6EM5, but you may need to order some spares to plug'n'play. Again, not really a good recommendation to someone lacking electronics experience, but if you trust your eyes you can watch for excess bias current draw. Just look for the tell-tale anode/plate orange or red glow along either seam in the centre (height-wise) of the stucture.
I haven't listened to see how similar the 6973 sounds to the 6CZ5, but they are VISUALLY identical tubes, and otherwise electrically identical except for the pin #8 connection (internal capacitances are even identical, but maximum plate and screen voltage ratings are lower for the 6CZ5, plate voltage is only 350V vs. 440V, screen is 285V vs. 330V, or 285V vs. 410V if the amp is Ultra-Linear! Whew! Talk about complicated and caveat-filled ratings! All of which are specified using the "Design Maximum" criteria as opposed to the "Design Center" system sometimes employed...).
This goofy little tube never really took off in popularity the way the 6BQ5/EL84 did, never mind the 6GW8/ECL86 or 6BM8/ECL82. It was only used in some American designs, and was never used by the major players (at least that I've ever seen) such as Dynaco (6BQ5), Fisher (6BQ5), H.H. Scott (6BQ5,6GW8), Griefkit (6BQ5/6GW8) or Eico (6BQ5).
As a result, some use by GE, Magnavox & Seeburg resulted in just enough demand to create a pricey NOS market! As far as I can tell, the 6973 was ONLY produced by RCA in the USA, and they probably stopped making them around 1970 or so, since they started heavily outsourcing to Japanese factories around that time. On the other hand, the 6CZ5 was made by both RCA & GE, and in Canada as well as the USA. It may have been made by Westinghouse & Sylvania, too, on that I'd have to check a little more thoroughly.
To the extent that I can judge sound quality in the GE amp in its present condition, the sound of the 6CZ5 is quite distinct and engaging. Very powerful, clean & clear, much less coloured and shut-in than a 6BQ5 at first blush. Certainly a tube worthy of closer examination, but rarity does make it a pain in the butt, and an expensive little one at that!
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Follow Ups
- Re: 6973-any substitute? - Joe Rosen 05:38:20 04/16/01 (0)
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