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Old 18th August 2005, 01:37 AM   #1
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Default PPP El-84 Amp

Am thinking about building something like this, perhaps triode strapped, should get approx 11 watts in class A. Anyone seen a schematic of that nature?

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Old 18th August 2005, 07:18 PM   #2
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Hi Peter,

There are various schematics available on the 'net, but one of the more trusted amplifiers is the old Mullard 5-10, which in pentode mode gives you about 11w.

Have a look at page 15 on the following url:



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Old 18th August 2005, 08:19 PM   #3
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There is a recent thread about PPP with EL84. For two weeks it was in the top10

But anyway...when you connect the EL84 in triode you don't have to worry that much with Miller capacitance...what means that you can use any available schematic and parallel output tubes. Note that current dragged from the power tranny doubles (for both HV and heaters) and the output tranny must have half the impedance (about 4 to 5k) for double power.

Try a search...that thread has lots of interesting information.
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Old 20th August 2005, 12:09 AM   #4
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Eric; thanks for the info, although I would really like to use the EL-84 in PPP I appreciate the good info on this site.

Erik; haven't been able to find the thread that you referred to. If you should recall it's name I would appreciate it. meanwhile I'll continue to look. Thanks for your help.

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Old 20th August 2005, 06:46 PM   #5
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Take a GOOD look at the PP 6V6 amp of Eric Kingsbury, AKA Poindexter. The 5965 is anything but a wimpy tube and should be able to drive a quad of EL84s, without "burping". The drive voltage is what you need. Use O/P trafos designed for EL34s. Individual bias adjustments for the "finals" are IMPORTANT.

Eric Kingsbury's Site
Eli D.
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Old 20th August 2005, 08:07 PM   #6
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Hi Peter

The thead I referred to wasn't just for EL84, but for conversion from PP to PPP. Probably this made your search more difficult. Here it is


And here is another (old) thread about power supply for PPP EL84. Maybe there is some interesting info in it.

Double EL84 Push-Pull supply

Good luck and all the best

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Old 21st August 2005, 04:19 PM   #7
amperex is offline amperex  United States
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Default Parallel Tube Tip

I have seen some old '40s & '50s amps that use a 1/4" diameter x 1" long inductor on each tube plate when paralleling tubes. I guess it makes for better sonics.
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Old 24th August 2005, 12:11 AM   #8
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Gentlemen; My apologies for this late response. A few days in the hospital for tests has kept me away from "the box".

Erik; thanks for the info on the threads, I will take a close look at them in the next few days.

Eli; Poindexter's site is very interesting and you are right, there is plenty of "juice" to drive a 4x EL 84 final.

I have read in more than one place that the quad output seems to suffer some loss of detail traded for increased Bass, in my case it is the detail that I am seeking, the lower end being covered actively.

I suspect I'll go with a single set of EL-84's in UL for the 9+ watts and be happy with that. Will send pictures when project is complete.

By the way Eli, how are things in Forest Hills these days? Haven't been there since the late fifties!


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Old 24th August 2005, 05:34 AM   #9
PRR is offline PRR  United States
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> inductor on each tube plate when paralleling tubes. I guess it makes for better sonics.

We didn't have "sonics" in those days.

Vacuum tubes have gain way up into the radio range. I proved that myself once: thought it was an audio amp but it wanted to be a ~6MHz radio transmitter and tube-melter. Paralleling plates through several inches of wire can bring out the radio oscillations. The sure cure for the parallel-plates oscillation (there are other modes) is 50-100 ohms carbon resistor in series with each plate. That can be a significant loss in spec-sheet power rating, so try inductors. Problem is, the wrong value will make things worse, and there is no way to compute the total distributed system unless you chew radar-waves for a living. In DIY, use a resistor.

> in the hospital for tests

I hope you failed all your tests, nothing needed doing, and the doctors threw you out in disgust. Happened to a friend: head-pain led to a head-scan which found a smudge which suggested a brain aneurism (blood-pipe about to burst in the brain). That would be very costly to fix, so of course it had to be double-checked before they could get paid to cut her. The second scan said the first scan was flawed, her head was fine, all plumbing in order. The real cause was an atypical sinus infection, slow to cure, but the head-doctors were not going to admit that when they have those lovely big-ticket scan-toys to play with. (OTOH, if it were an aneurism, modern scans would have saved her life. As mine was saved when atypical appendicitis could not be diagnosed the old ways, but was quite obvious on CT. "Big black baseball" inside me. They had me on the table in hours. If they hadn't, I probably would have been dead in a day.)
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