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Old 14th February 2003, 06:02 PM   #1
Joel is offline Joel  United States
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Default Gas-coupled 6L6/6BG6-G SE

This is my next in line to be built I think. I have a few beautful ST shaped 6BG6-G's to use up, although you could substitute any 6L6 variant.

Q: if I added a 2nd output tube and changed the whole thing to parallel feed, does the DC current rating of the output transformer matter at all anymore? Or do you still need to obey the ratings? And... do you choose the same primary impedence as though you were using the tranfrmr in the standard manner? So, in this case, 2500 ohms? (2x 6L6)
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Old 14th February 2003, 06:57 PM   #2
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Hi Joel,

My answers to your questions are:

1) Parallel feed has a capacitor at one end of the transformer so no dc can flow through the OPT.

However I find a small gap i.e. 10mA worth of dc current gap helps OPT performance so I always spec. my PP transformers for 10mA dc current...

2) The value of OPT impedance should indeed be as per two parallel tubes so 2500 is fine.

I would go for slightly higher impedance and sacrifice a little power for more linear performance...

Hope this helps

ciao

James
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Old 14th February 2003, 07:04 PM   #3
Joel is offline Joel  United States
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James, I think you misunderstood my question?
I know that in parafeed you have a cap - so of course there is no DC in the transformer. But, my question is what is the limitation on the AC signal voltage + current (wattage) that can be handled in parallel feed mode?
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Old 14th February 2003, 07:26 PM   #4
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Default ...Sorry...

Joel,

Sorry, I completely misunderstood your question I thought it was strange that you didn't realise the answer

To answer your question... to a first approximation, you would rate the OPT as you would a PP transformer with the two valves ...so in CLASS A1 triode connected about 4-5 watts and Class A pp ultralinear about 15 watts

hope thats a better answer...

James
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Old 14th February 2003, 10:22 PM   #5
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Assuming the same transformer is used, (125ESE), the AC voltage across it will be exactly the same. AC current will double, but you weren't going to listen to sine waves, were you? You'll use a different secondary tap, of course. The low frequency saturation will occur at the same point (twice the power, though, with two tubes), the low level bass response will extend a litle lower, and the high frequency end may suffer a little compared to a single tube. Maybe your dog can hear the difference
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Old 15th February 2003, 02:52 AM   #6
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My first thought is with no DC in the core using up magnetic headroom you would be able to push the tranny harder with AC signal.

You could either push more power through the tranny or go lower in frequency before saturation would raise it's ugly head.

Gary, (who has never built a non parafeed SE amp...)
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Old 15th February 2003, 01:42 PM   #7
Joel is offline Joel  United States
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Quote:
Originally posted by Gary P
My first thought is with no DC in the core using up magnetic headroom you would be able to push the tranny harder with AC signal.
You could either push more power through the tranny or go lower in frequency before saturation would raise it's ugly head.
Ah ha! That's exactly what I was getting at, Gary. Let's take the 125ESE for example: it's rated at 80mA DC, and the frequency response at 15W is 150-15kHz +-1dB... not so hot. But I and others have observed that with 30mA or so of plate current, and low Rp in the output tube, you can easily get it flat from 20-20kHz. So... eliminating the DC altogether should show further improvement.
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Old 15th February 2003, 06:27 PM   #8
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Default EXTENDING THE BANDWIDTH.

Hi Joel,

Quote:
But I and others have observed that with 30mA or so of plate current, and low Rp in the output tube, you can easily get it flat from 20-20kHz. So... eliminating the DC altogether should show further improvement.
Isn't that rather obvious?

I wouldn't expect miracles though, but parafeding should help here if you're happy having the cap there.

Cheers,
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Old 16th February 2003, 02:49 AM   #9
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Frank says

[but parafeding should help here if you're happy having the cap there.]

Keep in mind that if you are doing a standard series feed output arrangement you also have that cap in the singal path. It's the last cap in the power supply.

Parafeed does not add an additional cap to the signal path. It mearly moves one that is already there.

Gary
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Old 16th February 2003, 05:38 PM   #10
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Are you saying that the power supply no longer has a capacitor on it?

Indeed, it does add another cap there. If its value is significant, of course, it won't acceft (damn my spelling sucks today.. I mean affect ) the performance because of its low reactance.

Tim
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