6CD6GA in Enhanced Triode Mode for SE Amp

[wrenchone]

Tubelab - be careful - the 6CD6 you want is the GA version, which has a 20W plate dissipation. The plain jane versions have lower plate dissipation. If you put the two versions next to one another, you notice immediately that the GA has a much larger plate.

[tubelab.com]

I have 6 6CD6GA's each one of them has a different internal construction. I have a Tung Sol that has a monstrous plate assembly that looks like they borrowed it from the 6550. I connected it up to the TubelabSE in place of the 6AV5. The whole thing is connected up to a variable power supply so that I could test at any voltage up to 400 volts. I used the same Edcor transformer so I could use the UL tap.

The short story is that the screen grid IS more sensitive than the one in the 6AV5. It does begin to glow at at voltages over 250 to 300 volts depending on the individual tube. The Tung Sol was the worst. Glow was visible at 250 volts. I tested triode, and UL up to 300 volts and 80 mA. The power at 5% distortion was similar to the 6AV5. I put the 6AV5 test results in the thread

The Edcor meets the 6AV5

Using a 4 ohm load on the 8 ohm transformer (2500 ohm load) caused increased distortion at any power level.

I got this idea that if I could run UL at a reduced screen voltage that this tube would rock, so I wired an 0A2 in series with the screen grid. Well this doesn't work too well in triode strapped mode because the plate voltage drops low enough to cause the 0A2 to go out, causing a whole screen full of distortion products (FFT analyzer). In UL mode this trick allowed 12.4 watts at 5% distortion with 400 volts on the plate (power supply maxed). I tried currents from 50 to 100 mA. 100mA is 40 watts, and the Tung Sol had no visible glow with the room lights off.

Due to the same effect described above (0A2 going out) the distortion comes up quickly when you hit this point. I had 12 watts at 2.5%, 12.4 watts at 5% but at 12.6 watts there is 12% distortion, really ugly distortion. A resistor from the screen to a negative supply could fix this.

I will try these tubes in my screen driven P-P test amp in the near future, but I need to find two that are the same. I have a bunch of Dumonts (Sylvanias) in the warehouse, I won't be over there until next weekend.

[wrenchone]

I have 6CD6GAs from GE, Sylvania, Cornell, Rad-Tel, and Raytheon. The plate structures are all about the same size. The Raytheon and Rad-Tel samples are interesting in that the plates have holes, so I could possibly see some interesting electron beam effects on the glass if I fired them up. I'm getting 10 NOS RCA units.

[tubelab.com]

I am at work now, so I can't see the tubes, but:

There were two Tung Sol's one had the 6550 looking plate with round holes. The other had a smaller plate with square holes.

Two RCA's, older logo. These had smallish plates with rounded corners. Unlike any of the other tubes. The "guts" looked the same, but the getters were in different places. I did not try these since there are no holes in the plates.

One Dumont, made by Sylvania. I have about 20 more of these in the warehouse. NOS. The guts look like the Sylvania 6AV5 only about 3/8 inch taller. This one had no problem at 300 volts. Large square holes in the plates.

There are a few others that I didn't test.

If there are holes in the plates, watch the screen grid wires carefully. If you have things set up so that voltage and (or) current are adjustable, you can turn up the screen voltage and see the grid wires begin to glow. Try this on a crummy tube so you know what to look for. Usually there will be one or two wires near the top or bottom of the grid structure that begin to glow first. Sometimes they will be quite bright while the other wires are not even glowing. Operation with even one wire glowing will lead to short unhappy tube life. If you are thinking of operating a tube above the screen grid ratings, find one with holes and check it out.

[wrenchone]

I just received a few more NOS 6CD6GAs - some from RCA, and some from Sylvania. Both brands are in a taller than normal bulb. The Sylvania plate stricture looks heftier, mainly because the heat radiating fins are larger. The Sylvania plates have square corners, making them appear larger than the rounded RCA plates. The Sylvanias also have a couple of pieces of blackened metal welded to the ends of the control and screen grid support bars. The piece welded to the screen grid supports is fairly substantial. I would assume these are serving a double purpose as stiffeners and heat radiators. When all is said and done, the Sylvanias look as if they can dissipate a bit more power, so I'll probably try then first.

I also received a pair of Hammond 125ESE transformers yesterday. These look and weigh a lot more substantial than I imagined they'd be. Are the Edcor transformers similar in size to these? Anyway, the amp output power will most likely be limited by the 80ma current rating of these output transformers, and not by the 6CD6.

I located a 700VCT, 90ma transformer in my "elephant's graveyard", which will do OK for supplying B+ for the output tubes and filament current for the drivers, but I'll need to have a supplemental transformer to power the 6CD6 filaments (5A total!). I may also want to have a separate supply for the drivers, as there will not be much of the 90ma left over for drivers after supplying the output tubes.

Next up will be to characterize the 6CD6GAs for screen grid drive at 500V plate voltage, with the intention of using them with the 5k tap on the output transformers.

[tubelab.com]

The Edcor transformers are the same height and width as the Hammond 125CSE. They look like they have one or two more laminations in the core which makes them a little thicker. The Hammonds have a multi tapped secondary where the Edcors have only one secondary winding. The Edcors have a UL tap, and the Hammonds do not. I do not have a 125ESE to compare it to.

In testing with SE triodes (45, 300B, triode wired 6AV5) the two transformers sound very similar. It takes me about 5 minutes to change transformers, so direct A-B comparison is impossible. I tried using one of each in the two different channels. I think the Edcor might sound better, but it is close. I am now testing UL with cathode feedback using the 6AV5's so the Edcors are the only transformers that I can use.

The Edcors are only $18, maybe that is why I think they sound better.

[wrenchone]

The 125CSE transformers are a couple of sizes down from the ESE. The current rating on the Edcors is probably not as high as the 80ma listed for the 125ESE. I did some back of the envelope stuff today, and it looks like optimum usage for the ESE woulld be to run using the 5k tap. I'm using a 700VCT power transformer, which means a B+ of about 500V. If I choose to live dangerously and run each tube at 30ma bias (15W dissipation), I get 8W maximum if I drive hard enough to push the tube into cutoff, less if I'm more conservative. This is substantially less than the filament power needed to run the tubes. Obviously I'm not going to be doing this experiment to build an efficient amp...

Hammond has just added a couple of larger transformers in their single ended "replacement transformer" series, with 90 and 100 ma bias capability. With these it may be possible to use a lower B+ with the 2.5k tap and take advantage of the peak current capability of the 6CD6. With the ESE, I'm limited by the current capability of the transformer at low B+ and lower turns ratio, and by tube dissipation at higher plate voltage.

[wrenchone]

Strange to relate - I just visited the Edcor site, and they don't list any current ratings for their single ended transformers - a serious omission. Do they supply the data with the transformer? I don't see how one could properly design an SE amp without it.

[Geek]

From an email from Edcor:

Here is the maximum DC bias 1250 - 3000 ohm 150ma, 3500 - 5000 ohm 120ma, 5200 - 7600 ohm 100ma.

[tubelab.com]

Geek:
You got more from Edcor than I did. I got " I don't know anything about transformers, I just take the orders. All of the data is on the web site."

No data came with the transformers. If you make amps like I do, you don't pay too much attention to manufacturer data. Use it for a starting point, then measure everything yourself. It would be nice to know the inductance of the primary, since my inductance bridge is dead. Measuring LF response of (-3db) 22 Hz at 5 watts says that it is good enough though.

I tested distortion VS current with several different triodes (45, 300B, 2A3, 6AV5, 6CD6, 6L6). I find a definite null in the third harmonic at 80 mA, regardless of the tube or supply voltage being used. This null is NOT the point of lowest total distortion. The null DOES correspond to the bias point which gives the best sound. I have not tested the Hammonds to see if they do the same.

There are people that claim that the Hammond 125CSE is the best sounding one of the 125 series. I have also heard the same about the 125ESE.

The maximum current rating has to do with the flux density in the core. The transformer should be able to handle more current (within reason). The results are highly dependent on the impedance that your speakers present to your amp in the bass region. If you have a scope watch the output as you play LF test tones at volume levels near clipping while you adjust the tube current. You will learn the difference in sound between clipping (bad) and transformer saturation (gross) real quick.

I ran the Edcor transformers at 80 mA with 300B tubes for 2 weeks of daily use. No ill effects were noted, and the transformers did not get warm. They sounded good and no saturetion effects were heard or seen at maximum volume (some slight clipping on bass notes).

I started to experiment with 6AV5's in UL mode with CFB. This sounds extremely good, but the sharp null at 80 mA is no longer present. I have had no more time to experiment, and won't until mid April.