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6J1 preamp
6J1 preamp
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Old 10th February 2018, 11:28 PM   #1
adason is offline adason  United States
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Default 6J1 preamp

Bought this little 6J1 based preamp. It runs on 12 vac, and has small mains hum. Any suggestion how to get rid of the hum?
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Old 11th February 2018, 12:32 AM   #2
rayma is online now rayma  United States
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Could be how the RCA grounds or volume control grounds are connected
to the power supply common. Maybe the filament rectifier circuit is adding
noise to the grounding. Try feeding the filaments DC from an external 12V
power supply, disconnecting the pcb filament supply by removing D1,
and see if the hum is reduced.

Last edited by rayma; 11th February 2018 at 12:45 AM.
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Old 11th February 2018, 01:20 AM   #3
adason is offline adason  United States
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Ok, will do, thanks for the suggestion rayma.
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Old 11th February 2018, 01:24 AM   #4
rayma is online now rayma  United States
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It's only a half wave rectifier in the filament supply, so lots of ripple will be there.
If this helps, maybe you can rework it as a full wave rectifier with more capacitance.
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Old 11th February 2018, 07:15 AM   #5
auriga2001in is offline auriga2001in  India
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Default Similar 6j1 preamp thread

6J1 China preamp thoughts
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Old 11th February 2018, 12:55 PM   #6
adason is offline adason  United States
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Thanks. I searched the forum for '6J1' repeatedly and it always ended with 'sorry no results'...not sure why.

I have some success with hum. I separated normal power supply, now 14 volts AC, from heater, now 12 volts DC. I use separate trafo. Hum went down from 3-4 mV to about 0.3 mV.

Since i separated these two supplies, i can raise the voltage, but i need to replace power supply caps, since they are rated to 35 volts.

Last edited by adason; 11th February 2018 at 01:15 PM.
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Old 11th February 2018, 02:00 PM   #7
Ketje is offline Ketje  Belgium
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You'r not the first with problems with this circuit.
R16 to ground not -Vb !
Some sugestions to improve.
Mona
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Old 11th February 2018, 08:01 PM   #8
adason is offline adason  United States
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Great, thanks Ketje, will test all.

I thought that R16 was only drawn incorrectly in the schematics, but in reality was ok. Will check that out.

Most important suggestion in your picture is the change from preamp to buffer. That of course will require some copper trace cutting. I will consider doing so.

Well, i am already using two separate trafos, so i got that already on my own.
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Old 11th February 2018, 08:03 PM   #9
adason is offline adason  United States
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I replaced some caps in the power amp and i am running it +/- 48 volts, which is significantly better than original.
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Old 11th February 2018, 08:16 PM   #10
adason is offline adason  United States
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Just checked, R16 is fine, there is no DC on output.
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