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Old 13th June 2003, 11:45 AM   #21
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Quote:
Been there , done that but the CCS will only force AC to be symetrical . In DC terms a pot between the cathodes would be required to optimise DC balance (and cancel any DC in the core) otherwise variable fixed grid bias will be required . I don't think this is possible with the IT transformer shown , unless it has separate secondary windings .
All the necessary info is hidden subliminally in my post. Look closely.

Quote:
...with a CCS the output stage will only run in class A
Yep, push-pull class A in this case.

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Not a bad thing but power will be far less than the original design's 10w
A pair of 6AQ5's will easily produce 10 watts in class A - at least the one I built in 1975 did. I'll go so far as to suggest Joel's cct is already class A so there would be no problem.
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Old 13th June 2003, 11:48 AM   #22
316a is offline 316a  England
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Default Class A

Quote:
Originally posted by Circlotron

A pair of 6AQ5's will easily produce 10 watts in class A - at least the one I built in 1975 did.
with only 258V of HT on tap ? If my memory serves correct a factory specified operating point for 6AQ5 in **pentode** with 10k a-a optx will only provide 10w of class AB1 power , it would be less for UL , surely ?

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Old 13th June 2003, 12:00 PM   #23
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UL. Oh yeah. Whoops! I was in fact thinking pentode mode.
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Old 13th June 2003, 12:07 PM   #24
316a is offline 316a  England
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Default Still...

...it's an interesting idea ! I find those DN2540 depletion mosfets are a 'lazy man's' way of implementing CCS cathode bias for PP triodes , which works rather well .

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Old 13th June 2003, 01:32 PM   #25
Joel is offline Joel  United States
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Hi guys,

Whew, lot of comments....
Let me try to address a few points.

EC8010's cold startup: this could be an issue, and is an excellent question. The 0A2/0B2 combo can sink 75mA averaged over a 10 second startup period. A reasonable guess for the voltage across the 100uF cap in the first few seconds is 300V or so... (anyone have another guess??) so, the gas tubes will want to pull down 40V across the 320 ohm resistor.... hmm, I think increasing the resistor will solve it. The real question is does the choke act like a choke immediately, or only after the load comes on line?

Why the transformer phase inverter? Excellent balance (AC, and output impedance), no adjustments needed, and excellent frequency response granted you have a low Rp driver - hence the Nuvistor. Yes, it did cost twice what the output transformer did...

Cathode bias? Well, if you want exquisite control over the output stage bias, this is not going to do it for you. But, when you really start to think about how it is nearly impossible to truly balance an output stage anyway - is there tangible benefit? Is the AC balanced? Ok, is the current balanced? Is the gain of each output tube identical? Are the windings on the output tranny identical? Is your speaker really producing 2% THD???
James - I like your battery idea though - and Energizer does make a 15V battery... I had thought of that before, but there will be a bit of heat in the chassis from the PSU crap.

Another note on the schematic - I wrote "+250V" for the plate voltage on the 6AQ5's. Of course that is merely the target voltage for the plate-cathode measurement, and of course you have to subtract the bias voltage of +15V. I have no idea where the gas tubes will finally decide to settle out, so until I build it, it's only a rough guesstimate. I think somewhere around 258-265 is reasonable from the datasheets. That would mean around +240 from plate to cathode on each 6AQ5, which is ok. Only a small loss in power.
Also, 15V bias puts these into class AB, but just barely. And 10W may be very optimistic! We'll see.
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Old 13th June 2003, 01:42 PM   #26
Joel is offline Joel  United States
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Default Re: Question

Quote:
Originally posted by Positron
Wonder how the regulators handle the current and line regulation since the max current is 30 to 40ma, depending on the regulator...
Pos,
The max is 30mA, but they are only going to be running at 15mA due to the size of the resistors chosen and the accompanying load.
An 0A2 has a drop of 150V, so if we present a voltage of 160V, we need to regulate 10V. If the load from the system is 10mA, and we assume the reg will be at 15mA, then we have a total of 25mA. So, placing a 400 ohm resistor in front of the regulator will set that condition, and drop the required voltage. 10/.024=400

Make sense?
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Old 13th June 2003, 05:46 PM   #27
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Default So then....

To someone who posted above, the regulator current is enough for the choke to "keep" the pleak DC, no load voltage down.

Since the DC voltage doesn't peak out, the regualtor current is somewhat contolled?

Someone mentioned 75 ma for 10 seconds. I take it, some testing has been done to verify this is ok?

Any noise problems?
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Old 13th June 2003, 06:58 PM   #28
Joel is offline Joel  United States
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Steve,

That "someone" was me... in the post right above yours... haha

Anyway, the max startup current rating is from the tube manual. I have not personally verified that it is accurate.
As for noise - as Crowhurst says, a device that can pass the slow changes in load current has very little reactance at audible frequencies.

Yes, the whole start situation is more complex to model than it seems at first - the gas tubes will draw a big load (over 150mA?), which will be impeded greatly by the choke no doubt. Then you will have a true choke-input-filter, and the voltage should crash, bringing the VR tubes back into regulation, and normal currents.
So, to sum up - I'm not worried at all.

Anyone have experience with these?
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Old 13th June 2003, 07:49 PM   #29
Colt45 is offline Colt45  Serbia
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are you sure B+ is high enough to strike the gas tubes?

doesnt seem like it would be, but maybe..

And you should probably put a resistor from B+ to the middle of the gas tubes, and then another from the middle to ground, so that one of them doesnt hog the current and blow itself up.. er.. yeah. i think.
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Old 13th June 2003, 08:12 PM   #30
Joel is offline Joel  United States
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Quote:
Originally posted by Colt45
...And you should probably put a resistor from B+ to the middle of the gas tubes[/B]
Interesting. To make sure the bottom one lights? Something big like 1 meg?
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