Line level connected direct to split phase on EL34 PPP? - diyAudio
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Old 24th October 2014, 02:25 PM   #1
apnneto is offline apnneto  Brazil
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Default Line level connected direct to split phase on EL34 PPP?

I'm currently working on a vintage Philips PPP EL34 amp, 70 watts. The output tubes are driven by an ECC82, whose sensitivity input is 2V. My question is: Do I need a pre-amplifier stage or can I connect the line level signal from passive preamp directly at the input of the ECC82? I'm a little confused by adding or not a tube preamp considering that the split phase needs only 2V, exactly what can be got from line level.
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Old 24th October 2014, 03:53 PM   #2
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It depend from output level of different sources. Some input devices have output signal about 2 and more V, some - about 0.5-1 v. If second - You will need add active preamp with gain about 20-50 /6SN7, 5687, 6S45P, ECC88/, passive prestage will decrease signal level.
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Old 24th October 2014, 04:35 PM   #3
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If that vintage Philips produces 70 W when input level is 2 V, then:

- with 1 V input, the output power is 17,5 W
- with 0,5 V input, the output power is 4,4 W

Some signal sources have less than 1 V output level, so is some 10...20 W from 70 W amplifier enough ?
That is what you have to deside.
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Old 24th October 2014, 04:57 PM   #4
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IMO, implicit in the other posters' remarks is a need for a LOW gain line stage. I suggest you scan the archives for 12B4 based designs.

Please post a schematic of the Philips amp. A schematic gives the membership important clues, which lead to better advice.
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Old 24th October 2014, 06:27 PM   #5
apnneto is offline apnneto  Brazil
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Thank you guys for the answers. I appreciated very much.

The scheme is attached. I changed the output stage from penthode to UL and is working very well.

My problem is now in the input stage. The amplifier (monoblock) has inputs for microphone and pickup (ECC83) with a sensitivity of ~ 300mv. The stage for EF86 predicts the connection of microphones ~ 5uv sensitivity. So much gain...

I'm making small changes in the stage of ECC83 to slightly decrease the gain. But for stage lines the gain is still very high.

So, I will replace the EF86 stage by one with the lowest possible gain and directly connect to ECC82. There a switch already installed by Philips. Do you recommend any preamplifier that can meet this amp?

The original thread for this power amp is here. I will be back to it as soon as I choose a good preamp.
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File Type: jpg Philips Cinema Amp.jpg (271.4 KB, 109 views)

Last edited by apnneto; 24th October 2014 at 06:29 PM.
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Old 24th October 2014, 07:02 PM   #6
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For ease, you can use just half of the 12AX7 acting as a line stage and bypass the EL86 and first half of 12AX7. The amp section alone with 12AU7 as driver might not have enough gain. With half 12AX7 ahead of it you should have plenty gain but not over driving the amp. If it's still too much gain, try eliminating 12AX7's cathode bypass cap. Of course you can always install a volume control in front. If you have a single tube gain stage preamp section, the amp will invert absolute phase so just swap speaker cables.

Click the image to open in full size.

Last edited by directdriver; 24th October 2014 at 07:22 PM.
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Old 24th October 2014, 08:31 PM   #7
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Both triodes ECC82 must be together, first is prestage, second - phase splitter.
You can try disconnect ECC82 from ECC83, put in front to ECC82 potentiometer about 50 kohm.
470 kohm on first grid ECC82 must be connected.
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Last edited by azazello; 24th October 2014 at 08:33 PM.
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Old 24th October 2014, 08:43 PM   #8
apnneto is offline apnneto  Brazil
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Quote:
Originally Posted by directdriver View Post
For ease, you can use just half of the 12AX7 acting as a line stage and bypass the EL86 and first half of 12AX7. ...
Hello directdriver

I could not visualize the image. The solution seems pretty simple and I would try it. If it is not asking too much, would it be possible to post the picture again?
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Old 24th October 2014, 08:46 PM   #9
apnneto is offline apnneto  Brazil
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Quote:
Originally Posted by azazello View Post
Both triodes ECC82 must be together, first is prestage, second - phase splitter.
You can try disconnect ECC82 from ECC83, put in front to ECC82 potentiometer about 50 kohm.
470 kohm on first grid ECC82 must be connected.
Hello Azazello,

Thanks for the response. I tried this solution and it worked well with a common sony dvd player. Perhaps this is the solution I will use, but I will still try the solution of the above post.
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Old 24th October 2014, 09:12 PM   #10
apnneto is offline apnneto  Brazil
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I connected the output of the tone control directly on the ECC82, using only the first half of the ECC83. The result was very good. Do you see any problem in this configuration?

This way I can use the volume control.
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