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5842 45 SE DC
5842 45 SE DC
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Old 13th June 2018, 12:03 AM   #1
Gungnir is offline Gungnir  United States
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Default 5842 45 SE DC

5842-45-edit (10).pdfI was thinking about building this amp, but I do not think that the B+ shown is high enough? Also, the schematic shows 3k for the outputs, shouldnt they be around 5K?
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Old 13th June 2018, 01:50 AM   #2
AllenB is offline AllenB  Australia
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The 5842 doesn't need a lot of V to be at a good bias point.
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Old 13th June 2018, 02:07 AM   #3
Gungnir is offline Gungnir  United States
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I need to do some more research on direct coupling. I thought that I would need more B+ on the plate of the 45? I just seems 330v on the plate with 4.33k on the cathode would pretty much starve the tube?
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Old 13th June 2018, 04:48 AM   #4
PRR is online now PRR  United States
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Quote:
Originally Posted by Gungnir View Post
Attachment 686499...I do not think that the B+ shown is high enough? Also, the schematic shows 3k for the outputs, shouldnt they be around 5K?... I just seems 330v on the plate with 4.33k on the cathode would pretty much starve the tube?
Work-out the actual plate-cathode voltage and current.

Back before 1938, Tung-Sol worked to $ell tubes and gave suggested Operating Conditions. Study them. The plan you found is essentially what Tung-Sol suggested. I don't think 2K7/3K or 31mA/35mA is any real difference.

Yes, there IS another condition making >twice the Power Output. Also needs near twice the grid drive. Which may be especially awkward in a direct-coupled scheme. And while 2 Watts may have been "High Output" in the 1930s, it seems small today, and isn't much louder than 0.8 Watts.

The 4K-5K load conditions go with higher B+ at similar current (Ohm's Law).
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Last edited by PRR; 13th June 2018 at 04:51 AM.
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Old 13th June 2018, 05:04 AM   #5
Tjj226 is offline Tjj226
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Also keep in mind that the 5842 tube is kind of a noisy tube. And I am not even sure its necessary. A lot of people use a 5842 in conjunction with an interstage 1:1 transformer to drive a 300b. Its slightly overkill for a 45. Not to mention that the tube that you really want to use is a 417a and finding a good matched set of those is difficult and expensive to say the least.

If you want a single triode to drive a 45, I would look elsewhere. A 6j5 might be interesting.
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Old 13th June 2018, 02:00 PM   #6
Tubelab_com is offline Tubelab_com  United States
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If you want a single triode to drive a 45, I would look elsewhere. A 6j5 might be interesting.
Over 200 TSE's have been built in the past 12 years. They use a 5842 or 417A to drive a 45 or 300B. The TSE is a highly regarded amp, and dead quiet. The gain is about right for the 45 and the 5842 is CCS loaded for maximum gain.

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Its slightly overkill for a 45. Not to mention that the tube that you really want to use is a 417a
The 5842 is the industrial equivalent of Western Electric's 417A. They are supposed to be identical. My testing has revealed that the WE tubes are a bit more consistent that the 5842 regarding the tube to tube variations. The WE tubes seem happiest at around 125 to 140 volts of plate voltage while the 5842's that I have (mostly Raytheon) seem happiest around 175 volts.

I run my 45's at 300 to 320 volts of B+ and about 30 mA of current. This provides a bit over 2 watts and does not seem to stress the tubes. My personal amp uses NX-483's (a 5 volt 45) pulled from an old Sparton radio chassis with 1929 dates on the store stickers. They have been in my amp for about 10 years......still work great.

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A 6j5 might be interesting.
A 6J5 in a real circuit is going have a voltage gain between 10 and 15. The 45 will need 60 to 100 volts of P-P drive. There won't be enough gain unless a preamp or line stage is used. That's why I'm using a CCS loaded 5842.
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Old 13th June 2018, 05:27 PM   #7
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Huh, interesting.

I have a few things that use 417as ( I have a couple 5842s for backup) and all they want to do is make random noises and oscillate like mad.
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Old 14th June 2018, 01:23 AM   #8
Tubelab_com is offline Tubelab_com  United States
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all they want to do is make random noises and oscillate like mad.
They are high Gm RF tubes. They will oscillate in the RF frequency range while trying to process audio leading to all sorts of unusual sounds. Careful lead dress, grid stoppers and a clean PC board layout (if used) is needed with these tubes.

I was an RF engineer for 41 years in my day job, and it took me a bit of doing and a few tries to get my PCB right.
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