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
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
DC90 is a single triode. DCC90 is a double triode, as is 3A5, so I take you've got DCC90.
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
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
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
A 'CD Enhancer' design from the defunct UltrAnalog site is preserved here:
http://www.diyaudio.com/forums/tubes-valves/58007-using-plate-choke.html
'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:
ªü¤¯ªºµÅT¤u§@«Ç
http://www.diyaudio.com/forums/tubes-valves/58007-using-plate-choke.html
'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:
ªü¤¯ªºµÅT¤u§@«Ç
You can read more about this on Broskie's website, he calls it "cathode-coupled amplifier".
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...
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...
(You can find datasheets at duncanamps.com)
U mean 2.8volts on series connected filament? Or do u mean 2.8volts on the cathode resistor?If one end of filament is connected to ground, and the other to 2.8volts, you have bias from 0 to 2.8volts. Pretty nifty eh? I'd rather parallel the filaments and use 1.4volts, and use a cathode resistance that gives > filament voltage as bias.
90volts seems to be a decent starting point for this tube.
I believe this is a tube designed for AB2 or A2 with grid driven positive.
U mean 2.8volts on series connected filament? Or do u mean 2.8volts on the cathode resistor?If one end of filament is connected to ground, and the other to 2.8volts, you have bias from 0 to 2.8volts. Pretty nifty eh? I'd rather parallel the filaments and use 1.4volts, and use a cathode resistance that gives > filament voltage as bias.
90volts seems to be a decent starting point for this tube.
I believe this is a tube designed for AB2 or A2 with grid driven positive.
The regular data sheets aren't too helpful if you want to stick to negative grid bias. BTW, very nice curves in A1. See here: Phono Preamplifier with 3a5 Directly Heated Triode
Funny, I was just thinking about you a couple of hours ago, wondering whether you were listening to the phono stage or not. I remember you talking technical issues but not whether you liked the results. How did turn out soundwise? Are you happy with it?
It sounds very nice. No complaints at all. Care is required with these tubes, to minimize microphonics. But that doesn't require anything exotic - just some dampening between the tube section and vibration from the equipment rack. I did order extra tubes to match gain between channels, and it comes out well under 0.2 dB. I tested tubes for noise, and used the quietest ones for the input position. Variation is actually not too bad for these.
I think these are a nice tube for small signal amplifiers. You will need a quiet DC filament supply, and provide physical dampening. But that's the case for DHT's in general.
Sheldon
I think these are a nice tube for small signal amplifiers. You will need a quiet DC filament supply, and provide physical dampening. But that's the case for DHT's in general.
Sheldon
I am a bit confused because the regular data sheet writes:
90V plate voltage for -2.5V grid voltage
and the Phono Preamplifier with 3a5 Directly Heated Triode page shows:
ca.65V plate voltage for -2.5V grid vlotage
Tyimo
Well, you can pretty much bias and load your tube as you wish. These tubes work fine with relatively low voltage and in a phono pre the signal on the plates will be low, so even with 60-70volts on the plate the tube is happy.
Your bias voltage sets the operating point at a given voltage, and looking at the curves with -2.5volts bias, 90V on plates gives approx 4mA Ip, 60V on plates gives approx 1mA Ip. Both are acceptable but presents different working conditions for the tube.
My take on a preamp with these tubes is CCS for a high gain (phono), and either transformer or just a resistor load for a line stage. Filaments would have CCS, 220mA for 1.4volts.
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Think you should forget this as one triode will be biased between 0-1,4V and the other from 1,4-2,8V.
So this is a totally dead end. Do as the others recommend run 1,4V/220mA and CCS. My favourite is ca -2V/5mA/90V.
There is a nice solution where you parallell the triode-halves and run the heater current from the CCS, through the cathode resistor, to ground. You will then end up with a resistor in the ballpark of 10ohm. No need for bypass cap.
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