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Old 16th April 2006, 02:58 PM   #1
jane is offline jane  Norway
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Default Just another PP...

This design is a result of a too high B+. In my case I had to deal with ~380VDC together with a pair of EL84s and a ECC83 for driver. The question was where to burn away the not needed voltage. Here, the cathode resistors (R6/R7) did the trick. The voltages over these are ca 100V, which makes it possible to do the driver stage direct coupled. The feedback is partial, ~10dB from plate back to grid at the output pentodes and ~10dB loop feedback.
Not that much more to tell -- it's just another small amp.

Mini PP schematics:
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On the work bench:
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Old 16th April 2006, 03:56 PM   #2
Giaime is offline Giaime  Italy
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Good! There also could be a possible adaption for cheap E/PLC86-82 etc etc...

What about the diodes between the power tubes grids and cathodes?
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Old 16th April 2006, 04:56 PM   #3
jane is offline jane  Norway
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Quote:
Originally posted by Giaime
What about the diodes between the power tubes grids and cathodes?
They are there to limit the Ugk voltage at start up. If not present this voltage will be equal to B+ before the circuit is stabilized.
I'm not sure how bad this is for the tubes, but 1N4148s are quite cheap (and Iíve got a box full of them)
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Old 16th April 2006, 07:09 PM   #4
Giaime is offline Giaime  Italy
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Oh yes, in DC coupled designs it's better to use it. Althought I've never seen a faliure due to the omission of that...

Probably it's related to the warmup times of the drivers versus the one of the power tubes.
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Old 16th April 2006, 07:26 PM   #5
Yvesm is online now Yvesm  France
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So simple !

Congrats.

Yves.
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Old 17th April 2006, 12:33 PM   #6
ttan98 is offline ttan98  Australia
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JAne,

where do you get the pCB with pins mounted on it?

thanks
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Old 17th April 2006, 01:09 PM   #7
jane is offline jane  Norway
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Quote:
Originally posted by ttan98
where do you get the pCB with pins mounted on it?
Partsconnexion: http://www.partsconnexion.com/catalo...nalStrips.html

Click the image to open in full size.
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Old 17th April 2006, 01:57 PM   #8
ttan98 is offline ttan98  Australia
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Thanks Jane,

please report back the results you get, eg Distortion figure as the feedback is quite low.
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Old 17th April 2006, 04:41 PM   #9
ttan98 is offline ttan98  Australia
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Jane,

BTW what input tube would you use and what current will you run it at, just wondering..

Also you may need a balancing pot at the cathode of the input tube to balance the anode so that the EL84 can be correctly bias. I like the Jfet Yves uses for his amp. at the cathode of the input tube to determine the cathode current... Just a thought...
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Old 17th April 2006, 05:36 PM   #10
jane is offline jane  Norway
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Quote:
Originally posted by ttan98
BTW what input tube would you use and what current will you run it at, just wondering..

Also you may need a balancing pot at the cathode of the input tube to balance the anode so that the EL84 can be correctly bias. I like the Jfet Yves uses for his amp. at the cathode of the input tube to determine the cathode current... Just a thought...
I have some ECC83s that I might use...
Since Vak is approx 100V, I suppose I have to "starve" the input tubes a little bit.
Ia about ~0.5mA seems to fit in this case.

Click the image to open in full size.

The EL84s will have quite large cathode resistors (3-4k), so a few volts out of balance will not alter the anode current too much. That is the reason why I donít use a shared cathode resistor.
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