The phono preamp is the "gotcha". AFAIK, no DHT meets both the gain and noise factor requirements.
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I haven't built or heard such a system, but this article may be of interest:
Phono Preamplifier with 3a5 Directly Heated Triode
3A5 DHT | Bartola Valves
http://www.diyaudio.com/forums/tubes-valves/169428-unholy-alliance-phono-amp.html#post2372139
http://www.diyaudio.com/forums/tubes-valves/148225-all-dht-phono-stage.html
Phono Preamplifier with 3a5 Directly Heated Triode
3A5 DHT | Bartola Valves
http://www.diyaudio.com/forums/tubes-valves/169428-unholy-alliance-phono-amp.html#post2372139
http://www.diyaudio.com/forums/tubes-valves/148225-all-dht-phono-stage.html
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3A5's mu is about 15, versus good IDHT "phono" tube's (triode or trioded pentode) mu: 50-100.
The DHT tubes best amplification factor is about 30 (for example EML 30a: 32).
I have several 841 DHT tubes (mu=30). It is good VAS device with small distortion even at high swing, but requires enormous plate load (over 250k) and potentially microphonic and hum sensitive. At large (line) input signal it's not issue, but phono input has mV magnitude.
Vinylsavor (Thomas Mayer) like this tube as phono voltage amplifier device loaded by transformers.
I developed phono stage (in LTSpice) with two stage 841s, but amplification is over average (below 40dB). With 3 DHT stages the implementation would be very expensive.
The DHT tubes best amplification factor is about 30 (for example EML 30a: 32).
I have several 841 DHT tubes (mu=30). It is good VAS device with small distortion even at high swing, but requires enormous plate load (over 250k) and potentially microphonic and hum sensitive. At large (line) input signal it's not issue, but phono input has mV magnitude.
Vinylsavor (Thomas Mayer) like this tube as phono voltage amplifier device loaded by transformers.
I developed phono stage (in LTSpice) with two stage 841s, but amplification is over average (below 40dB). With 3 DHT stages the implementation would be very expensive.
You could think about using the 6S17K-V which is a metal planar dht.
http://www.shinjo.info/frank/sheets/113/6/6S17KV.pdf
It was used by Erci Barbour in a phono stage in Audio Express November 2001.
I've tried it in a line stage and as the front end to my GK71 amp. It is not microphonic and it is very clear and detailed.
Audio ratbag: The heart attack special - 6S17K-V powerdrive GK71 SE amplifier
I haven't used it in a phono stage yet but it could be a future project.
ray
http://www.shinjo.info/frank/sheets/113/6/6S17KV.pdf
It was used by Erci Barbour in a phono stage in Audio Express November 2001.
I've tried it in a line stage and as the front end to my GK71 amp. It is not microphonic and it is very clear and detailed.
Audio ratbag: The heart attack special - 6S17K-V powerdrive GK71 SE amplifier
I haven't used it in a phono stage yet but it could be a future project.
ray
6S17K-V looks like a disc seal triode . One side of the heater is the cathode connection . Indirectly heated . Or am I missing something ?
316a
Yes, one side of the cathode is the cathode. Eric Barbour used 6.3 vdc to heat it and connected one side to ground. The tube self biases and does not need a cathode resistor. The tube is directly heated.
Looking at the tube as in the pdf diagram -
- high voltage to the top
- input to the grid
- ground
- +6.3 VDC
They are very inexpensive at about $2 each.
6S17K V UHF Triode Tubes Lot of 10 New Boxed | eBay
ray
Looking at the tube as in the pdf diagram -
- high voltage to the top
- input to the grid
- ground
- +6.3 VDC
They are very inexpensive at about $2 each.
6S17K V UHF Triode Tubes Lot of 10 New Boxed | eBay
ray
You are mistaken . The datasheet clearly shows a heater not a filament . If it had a filament , it would not have a cathode connection .
316a
OK you are correct and on closer reading of the blurb it even says indirectly heated.
It really is a curious beast, quite unlike most indirectly heated tubes with the cathode separate from the heater. Looking at the topology of the tube itself, the heater and the cathode are almost one and the same thing.
Perhaps we could say semi directly heated. I wonder if there are other tubes with this topology.
ray
It really is a curious beast, quite unlike most indirectly heated tubes with the cathode separate from the heater. Looking at the topology of the tube itself, the heater and the cathode are almost one and the same thing.
Perhaps we could say semi directly heated. I wonder if there are other tubes with this topology.
ray
The family that these tubes come from are called disc seal triodes . These are indirectly heated types . The DET23 datasheet details how these would be used . Note the cathode is tied to one side of the heater
http://www.mif.pg.gda.pl/homepages/frank/sheets/139/d/DET23.pdf
316a
Thanks for the info.
By the way, the datasheet solves a problem for me in that it points out the need for cooling the anode. I have found that the sound would go away when there was not enough ventilation and now I see why. Luckily I was using an alligator clip to hold the anode but now I will add extra mass to the clip.
ray
By the way, the datasheet solves a problem for me in that it points out the need for cooling the anode. I have found that the sound would go away when there was not enough ventilation and now I see why. Luckily I was using an alligator clip to hold the anode but now I will add extra mass to the clip.
ray
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