EML mesh plate 2a3 in DRD amp - diyAudio
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Old 24th April 2011, 03:00 AM   #1
debk is offline debk  United States
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Default EML mesh plate 2a3 in DRD amp

I am looking at building a 2a3 DRD amp. I have heard that some amp have problems with the EML mesh plate 2a3's. Would this be a problem in a DRD design using SS rectification? Would I need some sort of softstart circuit? In my research I have come across some people saying that the ultapath capacitor in this topology lessens the effect of the inrush current?

Also in looking at schematics electraprint is using an electrolytic instead of an oil capacitor. Any pluses or minuses to using the electrolytic here.

thanks

Debra
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Old 24th April 2011, 09:05 PM   #2
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Why not use a TV damper diode in series with the +B supply...?

This will give a nice smooth ramp up of the supply to the amp. The high current capacity of the TV Damper-diode will prevent too much sag at high current demands too...
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Old 26th June 2011, 05:46 PM   #3
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Quote:
Originally Posted by debk View Post
I am looking at building a 2a3 DRD amp. I have heard that some amp have problems with the EML mesh plate 2a3's. Would this be a problem in a DRD design using SS rectification? Would I need some sort of softstart circuit? In my research I have come across some people saying that the ultapath capacitor in this topology lessens the effect of the inrush current?

Also in looking at schematics electraprint is using an electrolytic instead of an oil capacitor. Any pluses or minuses to using the electrolytic here.

thanks

Debra
I've built a 2A3 and 300B using Sophia meshplates with no problem. You will need some type of delay if you're using solid state rectification. I'm doing it manually with two toggle switches, one for the filaments, and a second for the B+. The tv damper tubes are an option, but require a lot of current. You may need a seperate filament transformer. Solid state is perfectly acceptable.

As for the ultrapath cap, an electrolytic is cheap and has better bass coupling, but that's about it. I've tried a lot of caps in this position and prefer by far the older PIO caps bypassed by a smaller cap. GE, Cornell Dubilier (CDE), and Sangamo's all give excellent sound, but are getting expensive and hard to find, plus they contain PCB's and can be dangerous in the very rare instance they leak. I've also had good results using half Mundorf and half PIO's bypassed by a smaller cap. The ASC motorstart caps used in the original DRD circuit should be avoided at all cost. Just my opinion. Try it all and see what you like.
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Old 27th June 2011, 08:10 PM   #4
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It does show an electrolytic cap on the schematic, but in practice when Welborne made a kit of this circuit using EletraPrint iron he used a 50uF ASC motor run metallized polypropylene oil cap Mfr. Part#: X386S-30-10-330.

I have used a AVVT 2A3 mesh plate tube in my DRD amps for almost a year without any issues. I use a 5AR4 for rectification. I would not use SS rectification without separate transformers for the filament and B+ and two switches. I have a Welborne DRD amp that uses the ASC cap and a scratch built amp that uses a Solen 50uF poly cap. I don't think there is much difference, but it is an easy swap to see what you prefer. I would definitely not use an electrolytic.
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Old 29th June 2011, 03:52 PM   #5
debk is offline debk  United States
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What about using an Obbilgato 70uf 630vdc Film oil for the ultrapath capacitor?
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Old 1st July 2011, 01:26 PM   #6
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What about using an Obbilgato 70uf 630vdc Film oil for the ultrapath capacitor?
You can use any cap as long as it's rated above 450V. I didn't care for the sound of the Obbligato, but try it anyway as your taste may be different than mine. Aside from the output transformer, i find the cathode bypass cap on the 6AN4 has the biggest impact on sound.
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Old 1st July 2011, 06:15 PM   #7
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i find the cathode bypass cap on the 6AN4 has the biggest impact on sound.
There is an easy, cheap and painless way to do away with that cap: use an LED to bias the input tube.

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Old 1st July 2011, 10:37 PM   #8
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There is an easy, cheap and painless way to do away with that cap: use an LED to bias the input tube.
Thx for the advice. Does this reduce gain?
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Old 2nd July 2011, 02:11 PM   #9
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No, with the plate choke you get almost the mu of the tube. Notice I get a gain of 32 with this setup.
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