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    Building, troubleshooting and testing of these amplifiers should only be
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Output tube for 800 Ohm speaker?

dovla

Member
2010-03-15 7:32 am
Can anyone suggest a (very) simple circuit to drive 800Ohm low-wattage speaker (3-5 W max).
I saw in other threads a link to Philips parallel EL86 OTL design, which also looks rather neat - any suggestions for a single tube that would do 2-3 W into 800 Ohm OTL?
Would 12B4A do? Is 6AS7 out of the game because its Rp is too low?
 

Arnulf

Member
2009-02-02 9:41 am
Can anyone suggest a (very) simple circuit to drive 800Ohm low-wattage speaker (3-5 W max).
I saw in other threads a link to Philips parallel EL86 OTL design, which also looks rather neat - any suggestions for a single tube that would do 2-3 W into 800 Ohm OTL?
Would 12B4A do? Is 6AS7 out of the game because its Rp is too low?

Look for high-current relatively low-voltage tubes such as ECL85 (PCL85/ECL805/PCL805 etc.). According to my calculations it will do a bit more than 2.5W out SE into 800R load and the triode section (mu = 60 or so) will provide enough swing to drive the amplifier with line level input and get some NFB going, plus it will work at relatively modest B+, just a single tube as you requested.

I'm not familair with this kind of speakers though, I imagine they require zero DC across the voice coil so you're going to have to parafeed your amplifier (or is there some sort of DC balancing mechanism incorporated into those speakers ?).

Oh and there is no such thing as rp being "too low" ;)
 
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As an alternative to a lump of iron (choke loading), CCS over a sweep tube, cap coupled to the speaker. Say, 50ma current, 300-ish V at the top of the CCS, 180V at the anode at idle, gives around 150V of swing to play with. Drive it with a common cathode voltage amplifier triode. As Arnulf says you can get tubes where voltage amp and output are in one bottle.

Figures are all first guesses and ymmv
 
You could try something like that:

Sch7.gif

Have fun ;)

Yves
 

dovla

Member
2010-03-15 7:32 am
Thank you all for suggestions - ECL86 or 6EM7 seem like the best (low part count) candidates but what Yves just posted is also interesting.
Speakers are Phillips from old radio and were originally transformed-coupled to EL84 and definitely do not have any DC protection components soldered on.
 
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Arnulf

Member
2009-02-02 9:41 am
I would take ECL85 (or any other incarnations of it as listed above) over ECL86 as it will allow larger current swing into such a steep load without getting anywhere near Vg1 = 0V with it (unlike ECL86).

ECL86 is in no way related to EL86 (= the tube which was designed to run those speakers), the pentode section of ECL85 is much more like EL86 than pentode of ECL86.

Use CCS (have a look at MJE350 + MPSA92 or similar combination for very easy implementation) and suitable coupling capacitor (10 uF for Fc = 20 Hz, use larger value if you can obtain it with suitable rating) - try to avoid electrolytics.
 

dovla

Member
2010-03-15 7:32 am
I would take ECL85 (or any other incarnations of it as listed above) over ECL86 as it will allow larger current swing into such a steep load without getting anywhere near Vg1 = 0V with it (unlike ECL86).

ECL86 is in no way related to EL86 (= the tube which was designed to run those speakers), the pentode section of ECL85 is much more like EL86 than pentode of ECL86.

Use CCS (have a look at MJE350 + MPSA92 or similar combination for very easy implementation) and suitable coupling capacitor (10 uF for Fc = 20 Hz, use larger value if you can obtain it with suitable rating) - try to avoid electrolytics.
I did mean ECL eighty-five (not six) - was just a typo