6CD6GA in Enhanced Triode Mode for SE Amp

[wrenchone]

Here is the schematic I have so far. The pot will be used to adjust the 6CD6GA screen to about 10V to get the desired idle current. The cathode follower current source loading allows me to adjust the grid voltage on the follower for proper 6CD6 screen voltage without the cathode follower bias current varying all over the place. Power supply schematic will follow.

[wrenchone]

Here is the first revision for the power supply. I'm using oddball transformers that I have on hand, elsewise I would cite part numbers. I'm using a lot of sand in the power supplies because space is tight in the chassis I've chosen.

[wrenchone]

This effort is not dead, just slow... I've simplified the schematic on both the amplifier and the power supply for my first effort. I'll post schematics and pics when the thing is up and running. It looks pretty utilitarian (gentle euphemism for butt-ugly), as I'm using a recycled chassis.

To stir the pot a bit, go to http://homepages.ihug.co.nz/~rwillis...ten_white.html and you'll find a simple schematic for an SE amp using a 6CD6 in ultralinear mode (perfect for one of the Edcor transformers with a tapped primary). The author claims about 10W RMS capability, but I don't know whether the amp in question was actually built. It certainly looks simple enough to try.

[tubelab.com]

I see the schematic, and I see the claim of 10 watts RMS, but I don't see where they come from.

Lets say that the 6CD6 posses this magical abillity to fully conduct (saturation voltage of zero). This means that when the grid is driven positive, the plate voltage equals the cathode voltage ( this is really impossible). This puts the plate voltage at 80 volts. Now as the grid is driven to cutoff the inductance in the primary will take the plate voltage an equal amount above the supply voltage (assuming no distortion) This puts the plate voltage at 420 volts. The peak to peak plate voltage swing is 340 volts. The RMS plate voltage swing is 120.6 volts. The given primary impedance is 8K ohms for an impedance ratio of 1000:1 with an 8 ohm load. This gives a turns ratio of 31.6. Dividing the RMS plate voltage by the turns ratio gives 3.81 volts RMS delivered to the load. This works out to 1.8 watts, not quite 10. Remember this was with a "perfect" tube and transformer.

Now I don't mean to bash this design. In fact I might even build one, just because it is so simple. I just know from my brief torture testing that I had to operate the poor tube well into the red glow region to get anywhere near 10 watts. I think that the load impedance needs to be far lower to get more power.

[wrenchone]

It may be closer to realization with 400-500V B+ and a 5k transformer. I personally would not like to push the plate voltage down past ~100V on the low side. Still, a simple design, and those Edcor transformers are cheap enough to make the experiment relatively painless. I'd probably use something other than the 6K5 for the front end, as I don't happen to have any on hand. A triode-strapped 6AH6 or 6AU6 comes to mind....

[wrenchone]

I've been looking at that simple ultralinear design some more and I better understand tubelab's reservations about it. .
Unless you get your hands on a better 6CD6 than the one tubelab played with, the screen grid might just become a second filament at 275V. Also, the direct coupling between stages forces one to have an huge amount of bias at the cathode, eating up voltage margin you didn't have anyway. This design would be more appropriate for something like an 807 or 1625, both of which have 300V rated screens. I would also hang a cap between stages and use fixed bias only or fixed bias along with a more modest amount of bypassed cathode bias, with a larger bypass cap to boot.
As tubelab pointed out, a lower transformer impedance is necessary in order to better use the limited amount of voltage swing available from the relatively low B+ value. 2.5k or 5k primary impedance is better suited with this design. 5k looks like it will get you 5-6W with about 300V B+. I have some 807s and 1625s (same thing, really) sitting around looking for something to do. If I can find an appropriate piece of power iron in my junk pile, I'll buy a couple of the cheaper Edcor transformers and try to get this to work after the 6CD6 design. Of course, tubelab will probably beat me to it....

[tubelab.com]

Unless you get your hands on a better 6CD6 than the one tubelab played with

I tried about 5 different kinds, with similar results. I have about 25 NOS Dumont (Sylvania) 6CD6GA's and about 50 25CD6's so I will eventually build something with them. I have been experimenting off and on with a screen driven P-P amp that currently runs 6AV5's. It runs on the edge of class B, so plate and screen dissipation is not an issue. I think the tubes with the hypersensitive screens will work good here, so I may try the 6CD6's in this design. Even with the 6AV5's I can get 80 watts, so I need to find a bigger OPT.

Of course, tubelab will probably beat me to it....

I have lots of 807's and an even bigger pile of 1625's, and I just built this SE UL amplifier that uses 6L6's and Edcor transformers. If you really want me to I could wire in one of those tubes, but I am itching to try a 6LW6. They are the king of sweep tubes.

Actually I have already tried 6L6GB tubes. It has been argued back and forth that they are (or are not) the same as an 807. It is my belief that they are the same (based on dissecting some broken ones). My operating point is a little to hot for these tubes, so I didn't play them long, but they do sound great. With 410 volts across the tube, 60 mA current, and a 5K load, I get about 10 watts in UL and 5 in triode, at 5% dist. That is a little too much power dissipation for an 807, so you would need a little less voltage or a bigger cathode resistor (currently 560 ohms).

The schematic for the PC board (transformer and switch connections are not shown) can be seen here:

http://www.tubelab.com/SimpleSE_schematic.htm

[wrenchone]

Tubelab,
The simple amp at your web site looks pretty much as I envisioned it would be, except for the current source loading on the input stage. I might try this on my version using a LV JFET current source cascoded with a PNP bipolar.

I looked up the specs on the 5842, and it is an interesting little beast indeed. It has a nice combination of low Rp, high gm, and reasonable gain. What I'll probably end up using in my setup is a triode strapped 6AH6. The mu is around 40, the plate resistance is not too bad (~3k), and the gm is respectable. I also have a bunch, which is more than I can say for either the 5842 or 12AT7. I'm also interested in the possiblility of triode strapping a 6AK5 (I have a bunch of these too, and they're cute little devils), but I don't think the specs will be as good as the 6AH6.

A local surplus outlet has some small cheap 115/230V stepdown transformers. If these have isolated windings, I'll press one into service for my plate supply. With a full wave bridge output, one of these would give me about 330V B+, which is just what the doctor ordered for both input and output tubes. If I put the 6AH6 filaments in series, I could run them and a pair of 1625s off a separate 12V transformer.

[wrenchone]

I just finished doing some curve plotting for the ultralinear amp mentioned above, using the 807 curves for G2 = 300V. I used 330V B+, 60mA bias current, which is right at the tube dissipation limit. With a 5000 ohm primary impedance, it looks like one could eke out about 5-6W RMS, assuming there isn't a lot of loss in the transformer primary. The amp would really like more B+ (say 400V), but this exceeds both the screen and dissipation limits. I don't know how that Kiwi designer ever thought he could get 10W at a much lower B+. Right now I'm thinking of using a Magnetek isolated 115/230V transformer that I picked up for 8 dollars. In step-up mode, rectified and, filtered, this gives me 320-340V, depending on my line voltage. It has way more VA capacity than I need (175 VA), but the price was right....

[Al3020]

Hi,
I am using these nice valves as substitute of EL34 on a PP-UL amplifier. B+ is 410 Volts and current 40mA, they are glowing blue (mine are 4 Philips) but sound much better than EL34. Pout i think is 45 W pch.
Ciao
Alberto