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Old 20th August 2003, 03:50 PM   #1
zxx123 is offline zxx123  United States
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Default High Frequency Clipping in EL84 SE

Especially, in soprano's high pitched climax
or flute solo, high frequency range is clipped.
I have tried to fix it, however, I fail.

I appreciate if you find some abnormal resitor or
capacitor.

The speakers is 89db/MAX 200W bass reflex type.
It has 1 high tweeter, 1 mid-high, 2 8-inch woofer.

The output transformer is from old PA monitor system.
It is UL tapped. The tubes used are JJ.

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Old 20th August 2003, 04:12 PM   #2
Fuling is offline Fuling  Sweden
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The schematic looks OK, I would suspect the output transformers.
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Old 20th August 2003, 06:01 PM   #3
G is offline G  United States
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You don't have enough power for those speakers. I had the same problem with mine until I finished my Fostex FR speakers. Also your speakers are probably not very linear in the load they present to the amp. Try some more sensitive speakers.
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Old 20th August 2003, 07:05 PM   #4
316a is offline 316a  England
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Default Re: High Frequency Clipping in EL84 SE

Quote:
Originally posted by zxx123

The speakers is 89db/MAX 200W bass reflex type.
It has 1 high tweeter, 1 mid-high, 2 8-inch woofer.

The output transformer is from old PA monitor system.
It is UL tapped. The tubes used are JJ.

Hello ,
The schematic does not indicate any feedback is used . This amp will have a very high output impedence / low damping factor . Not a good idea with bass-reflex speakers with their wild impedence fluctuations . You could always connect a resistor between 6BQ5 and 12AX7 plates (say 150k) , this would implement a little local feedback . Otherwise I'd tie g2 to plate (via 100r resistor) on the 6Bq5 output stage and run as a triode but you'd end up in flea power territory (about a watt or so) . Was this amp intended for hifi usage ?

316a
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Old 20th August 2003, 07:11 PM   #5
316a is offline 316a  England
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Default One more question

What value is the anode load for the 12AX7 ?

316a
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Old 20th August 2003, 08:08 PM   #6
dhaen is offline dhaen  Europe
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Looks like 27K, and it's a 12AU7.

Cheers,
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Old 21st August 2003, 12:11 PM   #7
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If the output tranny is from an old PA system it probably isn't designed for good HF response, so if it is not to be replaced any NFB circuit should include the tranny to get the best of what's available. So rather than having anode to anode feedback, running a little feedback from the secondary of the tranny via a resistor to the cathode of the 12AU7 (losing the cathode bypass cap in the process) might be preferable.

If NFB is not used I suggest a small cap, say 0.001uf connected across the primary of the output tranny to help reduce high frequency transients. A cap of about 1000v DC working should be used.
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Old 21st August 2003, 01:07 PM   #8
wkopacz is offline wkopacz  Poland
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Transformer is for PP amp? SE trannys should have air gap to prevent saturation ( DC - bias current).
???
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Old 21st August 2003, 01:46 PM   #9
zxx123 is offline zxx123  United States
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Default Thanks!

Thanks, Folks!
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