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Old 4th January 2013, 12:31 AM   #1
Crypto is offline Crypto  United Kingdom
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Default EL84/UL84 single valve amp

Has anyone got a schematic for this pentode in use as a single valve audio amplifier, i.e. as a sole valve in the circuit? I couldn't find one on Google images. Thanks
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Old 4th January 2013, 01:03 AM   #2
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This thread has a stereo amp about as simple as I've seen. magnavox 8301 revamp, need's a second look
If you want only one channel you could use a 6C4 instead of a dual triode tube.
Your tube EL84 takes high voltage swings (7 vac?) to amplify things, and I don't know how you would operate it with any reasonable source without a voltage gain stage. Cd players, dvd players etc put out 1.5-2 vAC signals. If you have a speaker level signal as input that can put out about 7 vAC, you could ground reference it, build a bias resistor to put the input plate at a linear operating point, and capacitatively couple the signal into the EL84 grid.
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Old 4th January 2013, 01:08 AM   #3
Crypto is offline Crypto  United Kingdom
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Quote:
Originally Posted by indianajo View Post
This thread has a stereo amp about as simple as I've seen. magnavox 8301 revamp, need's a second look
If you want only one channel you could use a 6C4 instead of a dual triode tube.
Your tube EL84 takes high voltage swings (7 vac?) to amplify things, and I don't know how you would operate it with any reasonable source without a voltage gain stage. Cd players, dvd players etc put out 1.5-2 vAC signals. If you have a speaker level signal as input that can put out about 7 vAC, you could ground reference it and capacitatively couple it into the EL84 grid.
Good point, on second thoughts maybe it can't be done.

Last edited by Crypto; 4th January 2013 at 01:11 AM.
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Old 4th January 2013, 01:28 AM   #4
cotdt is offline cotdt  United States
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You can use EL84 without a driver stage if you parallel several of them, like 4 per channel.
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Old 4th January 2013, 01:31 AM   #5
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"indianajo" has given correct info. You need a voltage amplifier between the signal source(s) and the power O/P tube.

It's very easy to make a high performance, low cost, B+ PSU that will power a SE 'L84 stereoblock. An inexpensive Triad N-68X isolation trafo will feed a Greinacher, AKA "full wave", voltage doubler. The uploaded "El Cheapo" schematic shows how it's done. Triad's N-77U and N-68X are valuable to DIYers world wide, thanks to dual primaries.

The signal topology of the RH84 amp is a reasonable point of departure, for what's desired. Regulating g2 B+ is an obvious improvement. Approx. 5 WPC is the expected power yield.
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Old 4th January 2013, 02:12 AM   #6
nhuwar is offline nhuwar  United States
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you could use a step up transformer on the input.
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Old 4th January 2013, 02:23 AM   #7
dgta is offline dgta  United States
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Not sure what the big deal is about a single tube, but yes easily done.

Just use one of the many preamps available that put out 10V or more.
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Old 4th January 2013, 02:26 AM   #8
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Quote:
Originally Posted by dgta View Post
Not sure what the big deal is about a single tube, but yes easily done.

Just use one of the many preamps available that put out 10V or more.
The preamp should be considered an additional stage, so it would be 2 stages with the preamp.
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Old 4th January 2013, 03:26 AM   #9
dgta is offline dgta  United States
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Originally Posted by cotdt View Post
The preamp should be considered an additional stage, so it would be 2 stages with the preamp.
He said "audio amplifier", which you take to mean "integrated amp" and I take to mean "power amp". But even if we were to accept that there is no preamp, you can still do it. With a 2V standard signal source it would work just fine, just not a lot of power output. He didn't say how much power he wants

See where this is going? No specs, so we can only speculate. These threads are always unhelpful because (no offense to the OP) like they say in computer science, garbage in, garbage out. You need to define to a reasonable degree what it is you're trying to do.
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Old 4th January 2013, 03:49 AM   #10
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Best way is to use an input transformer like this one http://www.lundahl.se/pdf/1922.pdf set up for 1:4 or 1:8 depending on the available drive voltage.
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