raising filament voltages above cathode 6v6 please check. - Page 2 - diyAudio
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Old 6th May 2012, 12:02 PM   #11
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Cool!!!,
I shall get into action It is likely that i will try both methods.
thanks for the advice, much appreciated.

cheers
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Old 6th May 2012, 04:45 PM   #12
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Quote:
Originally Posted by magnetman View Post
with regards to the hum issue there is enough to make a fuss. I have honed the grounding a little more. I have a star ground at the transformer this takes the mains the CT for HT and filament, also the power caps. At the other end of the chassis i have all preamp parts connected with ground buss to the input jack.
If I understand your description correctly, you have the input jack grounded to the chassis. You can reduce hum further by isolating the input jack from the chassis, then run your preamp ground buss to the star point.

The AC ground should not be connected to the signal star point. For safety, this is bonded to the chassis by itself near the AC entry point.

Scott
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Old 6th May 2012, 05:32 PM   #13
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Check out this article:

http://www.geofex.com/article_folder...nd/stargnd.htm
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Old 6th May 2012, 09:21 PM   #14
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Great article! I have shifted the mains earth connection, I will now lift the jacks from the chassis, however i cant seem to get my head around how i will do this, I have Neutric Jacks metal open type they also have a switch. I have wired it exactly like this:

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Old 6th May 2012, 10:38 PM   #15
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The rule for avoiding ground loops is that there must be one and only one current path between any two points.

All good fortune,
Chris
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Old 6th May 2012, 10:57 PM   #16
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Quote:
Originally Posted by mctavish View Post
By connecting the ct to pin 8 of the 6V6 you are raising the heaters potential to that of the output tubes cathode. This reduces hum by eliminating the diode-effect between the heater and the cathode in indirectly heated tubes. This technique was commonly used in Magnavox amps back in the day and is a way to accomplish raising the heaters without adding to the parts count ( and cost) of the amp. The ac from the heaters goes thru C6 to ground and the dc potential is at whatever the cathode of the 6V6 is, somewhere around 12v give or take. I say go for it, worst it can do is reduce hummmmmmmmmm.
Quite effective with 12AX7A, has no effect on the 6V6 hum wise but will trivially ease the electrical stress on the filament to cathode insulation, but given the signal levels at this point shouldn't be a problem anyway.
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Old 6th May 2012, 10:59 PM   #17
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Quote:
Originally Posted by magnetman View Post
Great article! I have shifted the mains earth connection, I will now lift the jacks from the chassis, however i cant seem to get my head around how i will do this, I have Neutric Jacks metal open type they also have a switch. I have wired it exactly like this:
You'll need jacks along the lines of these:

Neutrik - Audio - M-Line - NMJ4HC-S

I think you'll find that the correct grounding scheme will do more for hum reduction than floating the heater voltage.

Best regards,

Scott

Last edited by scott17; 6th May 2012 at 11:08 PM. Reason: additional info
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Old 7th May 2012, 12:10 AM   #18
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Since this is a guitar amplifier thread it occurs to me that it really belongs in Instruments & Amplifiers.. I'm going to move it there.
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Old 7th May 2012, 12:42 AM   #19
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The mum was significantly decreased in a AX-84 P1 project when I connected the heater wiring through 2 100 Ohm resistors (transformer did not have center tap), as per the schematic (120K and 56K if i remember). Heater voltage was around 45V above ground. This is higher than you can achieve by connecting heaters to 6V6 cathode.
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Old 7th May 2012, 09:57 AM   #20
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Hi all,
thank-you for your help. I have just turned the amp on again its very quiet. now i think there is a dodgy solder somewhere it is cutting in and out a bit however it is soft loud soft, no crackles at this point. it seems to respond to a strong strum then it goes quieter again.
Anyways

thanks again!
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