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Old 3rd May 2004, 12:56 PM   #601
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is my case shielded ? its made out of aluminium sides and top and bottom out of thin steel bought from one of the electronic chain stores here in oz. during this testing phase the top is off.

the chassis ground is made by a steel screw coming through the bottom steel plate and the connections Joe has labelled G chassis are connected to this point . From there another wire links the screw and its connections to the earth wire of the mains supply.

there is no resistor or cap anywhere in the grounding part of the circuit. The components of the circuit are as Joe described in his latest posts in this thread and the power supplies are as he describes on his website.

I should emphasise that its very faint this radio broadcast ,only just audible when the ear is inches from the speaker. When the music is on the broadcast is well drowned out. Its only the obsessive part of me that says I would like to get rid of it ,I assume it has an influence on the sound of music however faint.

from memory Greggc in a photo of his "ultimate gainclone" has a cap and resistor connecting the signal and ground terminal on his input RCA plugs to stop RF hash . This is the thread
[URL=http://www.diyaudio.com/forums/showthread.php?s=&threadid=23353&perpage=40&pagenu mber=1]

Gregs is not a tube buffer though I assume that shouldnt make any difference. Thanks again
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Old 3rd May 2004, 03:02 PM   #602
Nuuk is offline Nuuk  United Kingdom
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Try connecting the case to the power ground star using a 100R resistor in parallel with a 0.22 uf cap. Put the lid back on and have a listen.
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Old 3rd May 2004, 11:15 PM   #603
Ropie is offline Ropie  United Kingdom
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Picture of my last VBIGC.
Superb amplifier; thanks for sharing the design, Joe

A bit more info here


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Old 7th May 2004, 06:13 PM   #604
Sheldon is offline Sheldon  United States
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Default 6gm8 buffer stage?

Anyone want to take a stab at component values if substituting a 6gm8 for the 6922, and running the buffer stage with batteries (12 or 24v)?

Sheldon
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Old 7th May 2004, 06:30 PM   #605
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*My only mission now is how to reduce further that hum. Would a capacitance multiplier or a LM317/337 for the tube psu help on this hum reduction?

Indead my experience is that a capacitance multiplier can help to reduce hum. Mine went from been very quiet to absolutely quiet with the addition of a cap multiplier !

Shoog
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Old 8th May 2004, 11:37 AM   #606
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Default Re: shoog

Quote:
Originally posted by Shoog
*My only mission now is how to reduce further that hum. Would a capacitance multiplier or a LM317/337 for the tube psu help on this hum reduction?

Indead my experience is that a capacitance multiplier can help to reduce hum. Mine went from been very quiet to absolutely quiet with the addition of a cap multiplier !

Shoog

This is actually what I did in my next VBIGC project. The last one I built sounded great, I hope I can make this one better.

PSU boards for tube, heater, GC amp.

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Old 11th May 2004, 07:01 AM   #607
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Quote:
Originally posted by JojoD818
Hi!,

I tested a valve buffered gc as per Joe's schematic. I connected a load, but without any signal. When I turn the pot clockwise, the speaker moves in and when I turn the pot counterclockwise, the speaker moves out.

When I connect an input (cd player), the amp works fine, but when I turn the pot, the same weird thing happens.

Any suggestions would be very helpful. Thanks.

JojoD

This weird thing happened again on my other VBIGC project. I measured my heater voltage at 6.4VDC near the filament. It plays but when I adjust the volume pot clockwise, the speaker moves out and vice-versa.

I have rechecked my wirings and no fault found.

I removed the pot, then I measured from the 3.9K resistor (other end is connected to the grid of the tube) to ground and got a reading of -14.5VDC. Is this correct? Must I have such a DC voltage at my input? When I reconnect my pot, the voltage disappears. I tried inserting a small 0.22 capacitor between the pot and the 3.9K resistor and my problems were solved.

What went wrong? Any ideas?

JojoD
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Old 11th May 2004, 07:10 AM   #608
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This is what I meant when I removed the pot.
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Old 11th May 2004, 07:13 AM   #609
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This was the only solution I can think of.... inserting a cap between the pot and 3.9K resistor.

What could probably caused this voltage to be present in the grid of my tube? I used only one tube for 2 channels, both have -14.5V on the grid.
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Old 11th May 2004, 07:23 AM   #610
moamps is offline moamps  Croatia
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Quote:
Originally posted by JojoD818
This weird thing happened again on my other VBIGC project. I measured my heater voltage at 6.4VDC near the filament.
Hi,
6,4V is OK, little bit too high for my taste.
Quote:
It plays but when I adjust the volume pot clockwise, the speaker moves out and vice-versa.
Check your source for DC offset.
Quote:
.....I removed the pot, then I measured from the 3.9K resistor (other end is connected to the grid of the tube) to ground and got a reading of -14.5VDC.....
Grid must be allways pulled down to ground with a resistor. (DC bias set, resistor can be high ohmic like 470k, 1M). Your measurements method is faulty.

Regards
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