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Old 24th March 2011, 01:01 PM   #11
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by kmtang View Post
I bought a pair of 71A few months ago and I don't know what to do with it.

Possibly it would be good to use them for the line amp.

Johnny
Also you could build a 71A SE amp, in case you have a high sensitive SP system.
It was a fantastic sound, though the output power was only 0.9W.
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Old 24th March 2011, 01:11 PM   #12
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I'm going to use my 71As for a electrostatic headphone direct drive amp! (I have 9 pieces of them now total.)

If I weren't going ESL, I'd build a regular headphone amp. Single ended, direct coupled, with mu follower 27 driving!

H.Michi, great work! What is the purpose of the metal 'tube' around the 71As?

Now that I have 9 71As (RCA, Tung-Sol, Westinghouse, Raytheon), I can see that the ST types all seem to have practically identical construction, with production dates over 10 years apart. Perplexing. Perhaps they were in reality made by only a few number of factories, then labeled for different brands. The globe types have a bit different construction.

I'm a bit worried about microphonics, many of them ring like a bell when I put them next to my ear and flick with a fingernail.
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Old 25th March 2011, 06:31 AM   #13
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by MrCurwen View Post
I'm going to use my 71As for a electrostatic headphone direct drive amp! (I have 9 pieces of them now total.)

If I weren't going ESL, I'd build a regular headphone amp. Single ended, direct coupled, with mu follower 27 driving!

H.Michi, great work! What is the purpose of the metal 'tube' around the 71As?

Now that I have 9 71As (RCA, Tung-Sol, Westinghouse, Raytheon), I can see that the ST types all seem to have practically identical construction, with production dates over 10 years apart. Perplexing. Perhaps they were in reality made by only a few number of factories, then labeled for different brands. The globe types have a bit different construction.

I'm a bit worried about microphonics, many of them ring like a bell when I put them next to my ear and flick with a fingernail.
The purpose is to seal electrostatic noise as well as shielding direct coupling between two tubes. Additionally suppressing air-borne vibration is achievable.
Mechanical osillation (around 2kHZ) like a tuning fork occured on my naked experimental chassis.

Yes it is sensitive to the microphonics, so I built the amplifier in a stiff enclosure for shutting off air-born vibration and mounted 71A`s sub-chassis on to the main chassis via the rubber suspensions.
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Old 25th March 2011, 04:28 PM   #14
Carter is offline Carter  United States
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Quote:
Originally Posted by H.Michi View Post
Also you could build a 71A SE amp, in case you have a high sensitive SP system.
It was a fantastic sound, though the output power was only 0.9W.
The best 71A is the predecessor tube, the 71. The 71 has a .5 amp
thoriated filament. Of course the bright glow is fun. But the stronger
cathode produces richer tone, maybe more important in a 750 milliwatt
amplifier. I have several 71's, including one "WLS" brand tube [71, but
with WLS number]. I'll try to take a photo of one of them, "lit up."

--Carter
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Old 26th March 2011, 05:43 AM   #15
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by Carter View Post
The best 71A is the predecessor tube, the 71. The 71 has a .5 amp
thoriated filament. Of course the bright glow is fun. But the stronger
cathode produces richer tone, maybe more important in a 750 milliwatt
amplifier. I have several 71's, including one "WLS" brand tube [71, but
with WLS number]. I'll try to take a photo of one of them, "lit up."

--Carter
Interesting the 71 !
Attached is a schmatic drawing of 71A/6DJ8 sound processor.
Attached Images
File Type: jpg 71A-6DJ8 sound processor.jpg (114.1 KB, 832 views)
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Old 26th March 2011, 06:11 AM   #16
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by H.Michi View Post
Interesting the 71 !
Attached is a schmatic drawing of 71A/6DJ8 sound processor.
PSU for 71A/6DJ8 sound processor.
Attached Images
File Type: jpg PSU for 71A-6DJ8 sound processor.jpg (126.5 KB, 700 views)
File Type: jpg PSU for 71A-6DJ8-2.jpg (141.5 KB, 688 views)
File Type: jpg Botom view of PSU.jpg (302.0 KB, 669 views)
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Old 27th March 2011, 02:33 AM   #17
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H.Michi:

Where did you pick up the white terminal strips? ie the ones in your photo above of the bottom of you power supply? Do they screw mount to the chassis?
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Old 27th March 2011, 04:01 AM   #18
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Those terminal strips look a lot like the ones I have so diligently removed from many a Tek scope for my projects. Worth the effort
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Old 27th March 2011, 07:27 AM   #19
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by boywonder View Post
H.Michi:

Where did you pick up the white terminal strips? ie the ones in your photo above of the bottom of you power supply? Do they screw mount to the chassis?
I bought thease terminal strips at a shop in Akihabara, Tokyo and some of them via internet. Yes, they are fixed to the chassis by screws. They are three kinds; 2mm,2.5mm and 3mm. Thease terminal strips were originally used in professional comunication or measurement equipments. some of recent products are not so good in quality and pron to separate metal part from ceramic body.
Attached Images
File Type: jpg Terminal strips.jpg (243.1 KB, 627 views)
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Old 3rd April 2011, 03:44 PM   #20
H.Michi is offline H.Michi  Japan
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Updated status of the 71A/6DJ8 sound processor

By increasing plate voltage and its current of 71A. I could see wider in dynamic range and more in clarity. For further touch-up, this time I put emphasis on my ear through listening rather than via measuring instruments. For references, I tried routin measurements and found that the output voltage was lower than the input one! I would say it is an active attenuator. However, it is not an issue for me as my main amps can generate maximum output power by 0.4 to 0.6V input and the setting position of main volume is around 11:00 to 12:00 O'clock. The residual noise level was 0.004mV which was far lower to what I assumed. It is attributed to the massive incorporation of passive LC filters and provided independent power transformers to those: HT, filament and bias circuit respectively.

Results measured at 1KHz, 600 ohm load

Output voltage, distortion , input voltage,
1.0V , 0.4% , 1.3V,
0.5V , 0.18% , 0.64V,
0.1V , 0.018% , 0.12V,

R-channel of 71A
Plate voltage: 134V
Plate current: 13.8mA
Grid bias: -25.8V
Attached Images
File Type: jpg 71A-6DJ8-2.jpg (114.3 KB, 379 views)
File Type: jpg PSU for 71A-6DJ8.jpg (152.4 KB, 267 views)
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