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Old 1st August 2012, 04:05 PM   #1
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Default need help to fix the "hiss" on my tube preamp

Hi,

I have the hiss issue on my clone CAT SL preamp. The power supplier have been changed to 80 rectifier tube with EL84 and EF86 for stabilizer.

Everything going well with it but the very noticable "hiss" from the tweeter. I can hear the "hiss" from 5 feet distance.

The heater of the ECC82 and ECC83 have been grounded, and the 6DJ8 have been hanged at 150Vdc.

This kind of "hiss" noisy never happend when I replace the preamp with a passive preamp so I petty sure that it was came from the preamp.

Can someone help to advise where is that kind of "hiss" noisy came from and how to fix it...

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Old 1st August 2012, 04:11 PM   #2
DF96 is offline DF96  England
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Circuit diagram? Hiss can mean a fault, or just too much gain. If you were previously running with a passive preamp then too much gain is quite likely.
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Old 1st August 2012, 04:18 PM   #3
kevinkr is offline kevinkr  United States
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The additional gain always comes at a price, if things were OK gain wise with the passive it is likely the active stage has too much gain. Most of these clones are relatively suspect wrt design quality.
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Old 1st August 2012, 04:26 PM   #4
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Be sure that series regulator is not oscillating. This oscillation can be radiated and pick up by the first stage. An oscilloscope will be of great utility to this task. In such case. add grid stopper resistors in the EL84/6BQ5)
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Old 1st August 2012, 06:04 PM   #5
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I assumed the root cause is the too much gain on my system. I believed the problem is came from the high gain of the power amp.

See the schematic and what will be the best way to lower down the gain?
remark : there is a 100K resistor at the grid of the input stage (6SN7) connected to the ground

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Old 1st August 2012, 06:09 PM   #6
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If the hiss appears when reforming the power supply, then the cause is the power supply, not the amplifier stage. Unless u also modified the preamp. stage. Also may be the amp itself, self oscillating.
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Last edited by Osvaldo de Banfield; 1st August 2012 at 06:12 PM.
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Old 1st August 2012, 09:08 PM   #7
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Quote:
Originally Posted by Angelpat View Post
I assumed the root cause is the too much gain on my system. I believed the problem is came from the high gain of the power amp.
You can experiment by removing the first gain stage from the circuit. Remove the .22uF coupling cap between the stages and use a piece of wire to connect the RCA jack input to the hole where the cap connected to pin 4, the grid of the second stage triode. This will bypass the first stage and let you know if there is too much gain with the two stages.
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Old 2nd August 2012, 07:28 AM   #8
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Hi Osvaldo,

Would you please help to advise if there is the grid stopper resistor on my circuit already.

Noted that I changed the EZ81 to 80 rectifier tube.

Many thanks!

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Old 2nd August 2012, 12:14 PM   #9
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OK, then, the voltage at the rectifier's output, is the same?
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Old 2nd August 2012, 01:09 PM   #10
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I would install parasitic suppressors (grid stoppers) to the first two stages. If nothing more than precaution, but I would suspect an RF oscillation problem. I will not comment on the design or changes to lower gain.

Cheers.
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