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Old 6th July 2006, 04:48 AM   #1
arnoldc is offline arnoldc  Philippines
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Default My 2A3 is being left behind

That is a JE Labs 2A3 design with the SRPP 6SL7 drivers...

Now that I've progressed my 45 to 5842 choke-loaded + LED bias, and my 10/50/300B amp got some new soul with 5842 CCS-load + LED bias, I feel that my 2A3 with the expensive MagneQuest iron it got is being left behind sonically.

I think it's going to be boring to use my last pair of 5842 for this amp My headphone amp is also using 5842, did I say I like 5842?

Maybe I should graduate to another driver

I'm thinking of-

a) Pentode driver, I have a Western Electric 404A but it's just a pair!
b) DHT driver, I have 01A but I want to make a pre-amp from it. I have another 2A3 2A3 drives 2A3?
c) My friend bought about 80 pieces of C3m and I can get them really cheap. But I've read the C3m mostly for 300B applications...

Can the bright minds here suggest to this bored DIYer what to do with his 2A3?
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Old 6th July 2006, 05:26 AM   #2
2wo is offline 2wo  United States
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So you can get the C3M really cheep.
Don’t make me come all the way to the Philippines just to smack you
If it can drive a 300b it can drive 2A3.

Your mission, should you chose to accept it. Is to give it a go. LED bias sounds like a good idea. how about choke or CCS anode load?
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Old 6th July 2006, 05:38 AM   #3
arnoldc is offline arnoldc  Philippines
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Where did I read this? TubeDIY at AA perhaps... That CCS is not good for Pentodes.

What's your take on this?

ps.

The japs came here in the Philippines in the 80's to hoard hundreds of Western Electric 300Bs... They're all NOS.
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Old 6th July 2006, 05:44 AM   #4
lndm is offline lndm  Australia
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I'd be curious as to comments on a pentode driver. Think I'll make some popcorn and sit back for a bit
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Old 6th July 2006, 06:37 AM   #5
arnoldc is offline arnoldc  Philippines
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An exercise on Visio
Attached Images
File Type: jpg c3m-2a3.jpg (14.8 KB, 1183 views)
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Old 6th July 2006, 08:02 AM   #6
mach1 is offline mach1  Australia
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Arnold,

It's very interesting that you have shown a grid choke and a pentode driver in the schematic.

I haven't seen that done before, although I can't see any reason why it shouldn't work well if the pentode plate resistor is kept fairly low.

paul
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Old 6th July 2006, 08:32 AM   #7
arnoldc is offline arnoldc  Philippines
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Hi Paul, I misplaced the notes I took for the C3m. But I remember the plate resistor is 12K.
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Old 6th July 2006, 01:42 PM   #8
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Hello Arnoldc,

Your circuit is very close to the one I and my firends use with the 2A3. Differences are that the VR tube should return to the cathode not the ground connection. THe cap across the VR tube should be replaced with a zobel network after MArk Kelly's posting on AA on the subject.

We found the grid choke didn't add anything and size the grid leak resistor at 4-5 times the load resistor for the pentode. 3Cm is good in this application but we found C3g better - more life in the sound and better tone.

We also use the WE connection on the power tube. Other than these small tweaks it is spot on for a design we have proved works extremely well.

Good Luck

James
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Old 6th July 2006, 02:15 PM   #9
Ryssen is offline Ryssen  Sweden
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Quote:
That CCS is not good for Pentodes.
Not even if it is connected as a triod?
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Old 6th July 2006, 03:18 PM   #10
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Triodes theoretically work the best when loaded with an infinite impedance load. Pentodes are supposed to work best into a constant resistance load.

In the real world these "ideal loads" do not exist. One of the best compromises for the triode case is the CCS load. They work best when the following stage has a high input impedance. In practice some triodes really like a CCS load (the 5842) and some work OK but not that great. In practice some pentodes work OK with a CCS load and some distort heavilly ( I have limited experience here). Triode strapped pentodes behave like triodes, and therefore should benefit from a CCS load. Every case is different and depends on the individual tube and the load conditions presented by the following stage.

If you present the tube with a perfect load for the DC condition (a CCS, or resistor) but the input impedance of the next stage is low, or non - constant (worse), the tube will still not be happy because the AC load conditions are not satisfied. Every circuit behaves differently in this regard and is highly dependent on the operating point.
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