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Old 20th April 2010, 07:25 AM   #1
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Default What can you do with a 3a5?

I'm playing around with the 3a5. Also called DC90. This is a tiny and very cheap double DHT. Mu 15 and Rp - 8.5. filaments can be run in series or parallel since there's a centre tap. Sound is pretty good - has a lot of those special DHT qualities. It's used in chinese amps like the Music Angel 3a5>2a3>300b where it sounds really nice - that's a good amp.

It's a bit microphonic, but I've found it useful to put two or three rubber O rings round the bottle, which is quite easy. It doesn't get hot.

So I'm wondering how to use it and looking for any experiences and thoughts from past or potential users. Possibilities:

1) Phono stage - there's already one design on the forum.
2) Line stage
3) Differential pair input to amp

Has anybody tried balanced cascode, SRPP or combining the two sections as a SE stage?

Also any preferences for resistor load, active load, transformer out etc?

Andy
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Old 20th April 2010, 08:57 AM   #2
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by andyjevans View Post
I'm playing around with the 3a5. Also called DC90. This is a tiny and very cheap double DHT.
DC90 is a single triode. DCC90 is a double triode, as is 3A5, so I take you've got DCC90.

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Has anybody tried balanced cascode, SRPP or combining the two sections as a SE stage?
SRPP or cascode with twin tube that has its cathodes connected together ? Even if you used two separate tubes, you'd either have to parallel both sections of the upper tube or leave one section unused rather than using one half per channel
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Old 20th April 2010, 10:28 AM   #3
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Hi Arnulf -

Memory slip, yes DCC90. Actually you set me off on looking at some of the DC tubes like DC70 and DC93...... probably quite rare but possibly interesting on a first look. I did try some of the sub-miniature DHTs like 5676 - sounded OK but burned out pretty fast - I think they were used as fuses or something.

Andy
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Old 20th April 2010, 12:47 PM   #4
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A 'CD Enhancer' design from the defunct UltrAnalog site is preserved here:

using a plate choke?

'Raj1' also writes a little about it here:

Audiophile Journeys with a PC - AnandTech :: Your Source for Hardware Analysis and News

He might be still using it...Raj1 if you are out there?



And here is a version from Taiwan:

*Tu@
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Old 20th April 2010, 01:31 PM   #5
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Such tubes make very nice regenerative AM radios - built a number of them when I was 12-13 years old.

Regards, Allen
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Old 20th April 2010, 03:08 PM   #6
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You can try making Rozenblit's grounded grid design with it; the cathodes are commoned in that design. I would use SS CCS's on the tail and plate.
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Old 20th April 2010, 04:34 PM   #7
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Mmmmm - Rozenblit. I'd forgotten him. I have one of his books Audio Reality.

Andy
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Old 20th April 2010, 04:47 PM   #8
Arnulf is offline Arnulf  Europe
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Originally Posted by andyjevans View Post
Mmmmm - Rozenblit. I'd forgotten him. I have one of his books Audio Reality.

Andy

You can read more about this on Broskie's website, he calls it "cathode-coupled amplifier".
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Old 10th December 2010, 09:30 AM   #9
Tyimo is offline Tyimo  Hungary
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Hi!
Could somebody tell me what is the optimal plate voltage of the 3a5 tube with 2.8V bias?

Greets:
Tyimo
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Old 10th December 2010, 10:42 AM   #10
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NOOOOO....dont tempt me, please! I've got a bunch of those tubes, but never got around doing much other thatn a few breadboard experiments. I just dont have time for another project. Lets see, in the works as we speak: circlotron OTL, totempole OTL, tube microphone, new speakers, new phonopreamp, small shop amp, guitar amp (two of them), new signal generator for the shop, ... oh somebody stop my compulsive project habit.
And now I want to dust off those 3A5s I've got...will it ever get better? ...and I've got FOUR kids, one wife and two dogs, oh and a big old house that needs work. oh and a daytime job...help...
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