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Old 28th August 2008, 09:01 PM   #11
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by sunrise
i figure that E180F would be enough - in triode connection it would probably go with 50Vrms output for 1V (or so) input and that on 10-11mA bias current....right or not???
You might want to be able to swing a little more, but you might get away with that. At 10mA, and a 300V B+ your driver plate load is limited to about 10k, for a plate voltage of about 150 on the e180f. Fairly high 2nd harmonic.

http://www.diyaudio.com/forums/attac...amp=1063412366

You can swing a bit more with a 27-30k load line and lower 2nd, if you drop to about 6mA at 125V. But that may be on the margin for driving the output tube capacitance.

Or, if you don't have sand allergies, you can push 6-10mA at 125-150V with a CCS and have plenty of current drive and voltage swing and low 2nd in the driver. And, if I understand correctly, by using a CCS and connecting it as a u follower (low output impedance), you can eliminate the driver tube bypass cap.

Which sounds best? Up to you, but they will all sound a bit different.

Why not try several different operating conditions for the driver? Resistors are cheap and easy to change. A DN2450 or IXCP 10M45 (or either cascoded) is not expensive either.

Sheldon

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Old 28th August 2008, 10:00 PM   #12
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Sheldon -
thanks a lot....
that's what i was hoping to hear.....
some tech info....
it seems to me that CCS version will be the best or to use E280F tube connected as triode.....????
what do You think???
i thought that E180F might be a bit not enough....well, well, well..... decisions deciosions....
but the guys are telling on the net that E280F is a bit dry.....

well, maybe to use E180F with CCS after all....
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Old 28th August 2008, 10:43 PM   #13
Sheldon is offline Sheldon  United States
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I don't have any direct experience with the 2A3. I take it that you don't either. So leave yourself room to experiment. Start with a very good power supply, with plenty of extra current capacity, and low DCR chokes. Check your designs with PSUD. If you design it for choke input to start, you can increase the voltage by tuning with input caps. Pick some good OPT's.

Then try a couple of different driver/tube configurations. As I mentioned, they are easy to change and not expensive to experiment with. Just give yourself plenty of room to work with. You might even consider making a modular driver section, to make this easier. Look here for inspiration on how to play: http://www.tubelab.com/ You can do the same thing on perf board.

Then you don't have to worry about conflicting opinions on sound. Pick what sounds good to you on your system, in your room.

sheldon
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Old 29th August 2008, 06:25 AM   #14
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that is the idea i was considering to use.... few options on PSU, output tubes are set....only driver...
build everything on the mdf board or something and then put into the box........
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Old 29th August 2008, 07:18 AM   #15
mach1 is offline mach1  Australia
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The audioroundtable group build forum contains a wealth of discussion on using trioded D3a, E180F, E280F and C3g to drive a 300B.

try

http://www.audioroundtable.com/Group...ages/1909.html

for starters.
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Old 29th August 2008, 07:26 AM   #16
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heheheh - yes - i have seen this post.... and this post the one to start me thinking about using penthodes for my parallel 2A3... only i wanted to get away with a resistor, not CCS... but it seems that i will have to use a CCS afterall....

can somebody tell me if i will have to disconnect the driver output until the situation with the ccs stabilizes (when powering on and off).....
i am not sure how CCS will behave of power ON... maybe to use a C4S ccs - easier to implement....the components are not so problematic to find for C4S....???


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Old 29th August 2008, 02:46 PM   #17
Tweeker is offline Tweeker  United States
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JJ offers a 40watt plate dissipation "2A3".

Said tube could go 350V B+, 50V cathode bias, 100ma, 3K load. Gives more B+ for resistive plate load in driver. Easier load to drive than pair of other 2A3.

Since budget isnt an issue, sand could also be avoided by using a choke load in the driver, either DRD or otherwise. Not that sand is a bad idea with these demands.
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Old 29th August 2008, 03:20 PM   #18
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by sunrise
can somebody tell me if i will have to disconnect the driver output until the situation with the ccs stabilizes (when powering on and off).....
i am not sure how CCS will behave of power ON...
It's always good to analyze your circuit for various power up and failure scenarios. If B+ is live and the driver tube is not conducting, then the full B+ will be seen at the plate of the driver tube. That's also true with a resistor or choke plate load. It won't be a problem with the tubes and voltages here, but the coupling cap should be rated for the full B+, better yet for the full B+ with no current draw.

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Old 29th August 2008, 03:36 PM   #19
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o.k. - so - first be sure that your tubes are heated well.....
than turn on the plate voltage.....but still - i think that i might get some weird noises with ccs..... i have some when using ccs in the plate of the 12B4 that i have as a preamplifier.....
sure the noises here will be smaller because of the transformer on the output - but how much smaller i am not sure...

oh well..... i will have to try that.....
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Old 29th August 2008, 05:34 PM   #20
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by sunrise
o.k. - so - first be sure that your tubes are heated well.....
than turn on the plate voltage.....but still - i think that i might get some weird noises with ccs..... i have some when using ccs in the plate of the 12B4 that i have as a preamplifier.....
sure the noises here will be smaller because of the transformer on the output - but how much smaller i am not sure...

oh well..... i will have to try that.....
I think that sequencing the plate voltage is not necessary at these voltages, and complicates things for little practical benefit. Damage to tubes due to cathode stripping is negligible unless you are dealing with something like high voltage transmitter tubes. After all, rectifier tubes have the full B+ before the filament heats up. Ask SY about the issue. People with much more experience than I have not had problems. I haven't seen any actual reports here either. I'd be more worried about a sharp transient to the CCS.

If you want a switch, a switch on the B+ side will almost certainly give some transient noise. I would put the switch on the primary connection to the supply, so that the charging time of the caps will smooth out the rise time some. Or, of course, use an indirect heated rectifier, either to rectify, as a hybrid bridge, or as an in-line slow switch after a SS bridge or rectifier. In the second case you could use a damper diode.

Sheldon
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