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Old 8th March 2009, 07:53 PM   #1
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Default "Junkbox" 2A5 SE

I have never designed a pentode amp before.

I tried my hand at it with VR screen regulation. Most if not all the components are "junkbox" stuff. I happened to have (4) 2A5's.

Any comments? Suggestions?
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Old 8th March 2009, 08:47 PM   #2
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My gut says you will want some feedback. It also says that the regulator can be built off of the B+ supply saving you a transformer. Also, I think the supply is cleaner than necessary, and the last RC filter can likely be dropped (and the draw from the regulator may be enough to drop any extra volts you may have). Mind you, this is all without looking at the datasheets, so I may be totally wrong.
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Old 8th March 2009, 09:12 PM   #3
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I guess you don't need the high amplification of the SL7. Exchange it for a SN7 which has lower Ri so the bandwith will be higher.
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Old 8th March 2009, 09:20 PM   #4
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I would think the one transformer would do - the 2A5s can run from the 6.3V winding with a 1 Ohm (or so) dropping resistor. I agree you may want feedback - the 6SL7 has enough gain for that. If you go without feedback,the 6SN7 has enough gain.

The cap across the VR tubes shouldn't be more than .047 uF, according to the data sheets. You can use something larger IF you add a resistor in series (forget the value...).
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Old 8th March 2009, 10:51 PM   #5
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Proper pentode operation requires that g2 be at the same AC potential as the cathode. A cap. is needed.

I agree with other posters about some NFB being in order. You really need to do something about an inadequate damping factor. The so called plate to plate connection might work out very well. Of course, the "classic" global connection from O/P trafo secondary to voltage gain cathode is also available.
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Old 8th March 2009, 11:21 PM   #6
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Quote:
Originally posted by dsavitsk
My gut says you will want some feedback. It also says that the regulator can be built off of the B+ supply saving you a transformer. Also, I think the supply is cleaner than necessary, and the last RC filter can likely be dropped (and the draw from the regulator may be enough to drop any extra volts you may have). Mind you, this is all without looking at the datasheets, so I may be totally wrong.
I might add FB from OPT to the Cathode of the SL7 once I get up and running.

(how can a PS ever be "CLEANER THAN NECESSARY"?)

The 2A5's filaments draw 1.75A so I NEED the extra tranny no matter what because the 6 volt winding on my tranny is barely 2 amps and I would rather not "push" it.
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Old 8th March 2009, 11:26 PM   #7
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Quote:
Originally posted by disco
I guess you don't need the high amplification of the SL7. Exchange it for a SN7 which has lower Ri so the bandwith will be higher.
Unfortunately my experience with the source I have (A PC sound card) requires a bit more gain than the SN7 (I actually prefer to use 6FQ7/6CG7) can give. In addition I might want to up the B+ in the future and I will need the gain to drive the tubes with more bias.
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Old 8th March 2009, 11:30 PM   #8
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Quote:
Originally posted by Eli Duttman
Proper pentode operation requires that g2 be at the same AC potential as the cathode. A cap. is needed.

Eli,

Can you explain this Cap?

Thanks,
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Old 9th March 2009, 06:20 PM   #9
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Lightbulb REVISIONS NEED HELP! NFB

Well after careful consideration of everyones suggestions I am encorporating them into a revision.

By switching out to a 6V heated Rectifier I "freed up" the 5V winding for the 2A6's (DUH why didn't I think of that the first time?)

I am pretty sure the schematic should function as it stands now just need a little help calculating a possible starting point for the NFB. I will probably place a switch and some "breadboard" off chassis to allow me to switch in and out some different amounts and with different capacitors. Can anyone help me out with a starting point. (First timer at NFB)
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Old 9th March 2009, 06:26 PM   #10
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There is a small error in the PS section, a 1K 5W resistor S/B in series with the supply to the VR tubes.
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