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Old 12th March 2006, 03:27 AM   #1
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Default 6CD6GA in Enhanced Triode Mode for SE Amp

On a whim, I've been picking up 6CD6GA sweep tubes (they were soooo cheap). Following a further whim, I picked up a couple of Hammond 125ESE single ended output transformers. Not the best iron in the world, but they'll make enough noise to get me started.

Does anyone have some suggestion for rough operating conditions for the 6CD6GA in enhanced triode mode? I was thinking of running 400-600V plate voltage, depending on what I find in the transformer elephant's graveyard in my basement, but I have no idea as to what would be appropriate for quiescent current. The 6CD6GA can dissipate up to 20W, so I can afford a few mils of bias current.
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Old 12th March 2006, 05:53 AM   #2
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Those octals look darn cool too!

I did some testing on an Edcor equivalent and you'll find the 6CD6G need big iron for the final product.

Usual question lineup here: How are you doing enhanced triode? Driving g1 and g2 equally? How are you driving it and with what?

If you park g2 at 150 to 200V, that HV will be OK. If not, you may have some trouble with g2 nonlinearities messing things up on the peaks.

For the heck of it, give a listen in pure triode at 300 to 325V before deciding
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Old 12th March 2006, 06:56 AM   #3
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In pure triode, you're limited to 175V because of Vg2(max).
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Old 12th March 2006, 10:34 AM   #4
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'Tis true, the 6CD6GA is limited to 175V screen voltage. This doesn't give you much swing at all, even using the 2.5k tap on the output transformer. Higher plate voltage and enhanced triode mode it is... I'll probably have to characterize the tubes myself for this operating mode unless someone has already blazed the trail.
The 6CD6GA is capable of fairly hefty peak current, so I might be reasonably comfortable operating off the 2.5k tap on the transformer. The 125ESE transformers are rated at 80ma - I wonder if that is a saturation limit or a power dissipation limit. The saturation current should vary depending on which tap is used. Fewer turns wil require more current to saturate the core.
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Old 12th March 2006, 10:43 AM   #5
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I just looked at the Hammond web site and answered my own question. The transformer ratio is varied using taps on the secondary side, so the primary turns are fixed, and the current rating of 80ma is a hard and fast limit. For an 8 ohm load and 2.5k effective impedance (turns ratio of 17.6), this gives you about an 8W RMS maximum output capability, which is not too bad for a small SE amp, especially using $2 tubes and cheap iron...
To get that 8W RMS, you need about 400Vp-p swing at the plate of the tube, so a plate supply of probably at least 500V is needed to keep things linear.
I did a few numbers, and the powers mentioned above are pushing the tube to its max limits using the 2.5k tap. If you are banging the tube from cutoff to the 80ma peak rating, you'll need to bias the tube at 40ma for a symmetrical swing. That's a quiescent power dissipation of 20W, right at the max rating of the tube. Obviously the bias current and peak swing will need to be smaller. Problems, Problems... Perhaps a higher plate voltage and higher impedance tap would be more appropriate. I'll ponder this later, as it's 4 AM here for me.
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Old 12th March 2006, 12:05 PM   #6
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Helpfully, the GE data sheet http://frank.pocnet.net/sheets/093/6/6CD6GA.pdf shows curves for Vg1 = 0 and Vg2 being the variable.
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Old 12th March 2006, 06:42 PM   #7
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This is true, SY, but they don't show any data for negative screen voltage, and all the currents are above the 80mA rating for the 125ESE - how inconsiderate of them.... I did some back ofthe envelope stuff last night before turning in, and 700V plate voltage and 20mA bias current looks plausible using the 10K tap (35:1 turns ratio). I'll probably use my good friend Mr. 6GF7 as preamp and driver, as I have a lot of them kicking around.
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Old 12th March 2006, 06:52 PM   #8
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Some more back of the envelope fiddling, and 500V and 16 mA bias may be the ticket. 10W may be possible if the tube lets me - I need to sit down and figure out the dissipation.
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Old 12th March 2006, 09:36 PM   #9
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6CD6, I thought that number sounded familiar. I got a box of loose tubes cheap at the Orlando Hamfest. There are 5 or 6 of them in there. They look like 6AV5's on steroids with a plate cap. If I don't get interrupted, I will wire a pair in place of the 6AV5's that are still rigged up to the TubelabSE amp. Can't do screen drive with this amp, but I can try UL and triode strapping. I know that there is a "maximum screen voltage" of 175, but that didn't stop me before, and it won't stop me now.

5 Bucks for the box, about 10 6SN7's 3 6L6GB's 1 6L6GA, 12 6V6's and a bunch of other good numbers, I can afford to abuse the 6CD6's.
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Old 12th March 2006, 11:38 PM   #10
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Quote:
Originally posted by SY
In pure triode, you're limited to 175V because of Vg2(max).
I guess I'm just plain lucky I haven't killed a couple on the bench

I guess my signature is true
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