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It is currently 15 Dec 2018, 16:51

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PostPosted: 06 Jan 2018, 20:24 
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Joined: 13 Jul 2017, 21:24
Posts: 13
with tubes in I see about 20mW through the resistors in the power supply (dropping 20V across them) and in the region of 160V on the tube. Do I need to swap the resistors out for something of a little higher value to bring the tube voltage down?
Tube specs suggest 130V max...

cheers


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PostPosted: 07 Jan 2018, 00:09 
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Joined: 17 Mar 2016, 07:26
Posts: 29
Location: Prêles, Switzerland
Hi Beardy,

I had the same issue with B+ at 170V. By setting both R2 at 1.8K (5W), I now have about 145-147 V on B+. I will write a distinct post about my construction soon.

I reached the same effect by using 1K for the "second" R1 (instead of 500), but I thought better do the further voltage drop at R2. Not sure which option is the best in regards to the B+ stability. Intuitively I would think rising R1 is better, but the values I had at hand made me rise R2's.

Regards,
Charles


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PostPosted: 07 Jan 2018, 21:16 
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Joined: 13 Jul 2017, 21:24
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many thanks Charles, I will get some ordered and swapped in.
cheers


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PostPosted: 12 Jan 2018, 14:09 
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Joined: 30 Nov 2014, 06:38
Posts: 4
I'm just gonna leave this here.

Image

Come see more at: https://riverty.net/bludit/6dj8-vacuum-tube-headphone-amplifier

Special thanks to Bruce Heran and Matt over at Cascade Tubes.

Enjoy...

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Ty Unes - sales@sevenhifi.com
Seven HiFi Amp Co.
https://sevenhifi.com


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PostPosted: 12 Jan 2018, 22:05 
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Joined: 28 May 2008, 21:53
Posts: 4575
Location: Winnipeg, CANADA
Hi riverty, welcome to the forum.

Fantastic work on the head amp. The wood shell really pops.

If the transformer is putting vibration into the chassis you could consider using rubber and grommets to try mechanically isolate the transformer from the chassis.

Enjoy the headamp.

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 14 Jan 2018, 20:02 
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Joined: 13 Jul 2017, 21:24
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Hi Bruce and other knowledgeable folk,

Apologies for being a time sink. As you may know I am a newbie and having issues with lots of noise. I have read various articles including Bruce’s own guidance on noise and ground connections, loops etc. However, there still exists some considerable confusion at my end. Please can you give guidance on where the various grounds should be connected:

There are multiple wires coming from PSU board and presumably each of these needs a return back to the PSU board..?
B+ left
B+ right
Filament heater 6V supply

There are multiple ground/common connections related to the signal:
Input jack common should be tied to…
1. One end of 100k ohm resistor that runs from input signal
2. Pin A of voltage regulator/pin 9 (internal shield) of tube
3. One end of 47k ohm resistor that hangs of pin 2 of the tube

All of these come together and go down to PSU where they are tied to via type x2 cap and parallel resistor to chassis earth...
this should be part of a twisted pair with 6V filament supply?

One end of 1uF capacitor hanging off pin 2 of output transformer – ground/return to PSU? twisted pair along with B+ supply?

Pin 8 output transformer – headphone jack common (both channels) and only connected here?

Thanks


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PostPosted: 21 Jan 2018, 12:47 
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Joined: 11 Oct 2014, 03:56
Posts: 14
So I've almost got my build completed. I've wired everything up with the exception of the inputs to my pot and amp and the outputs from the trafos to a dpdt switch and subsequent headphone jack.

My 6v voltage reads out at 5.99v and my B+ at power up started at around 165-166V and has dropped to about 157V on one side and 158V on the other. This is after about 30 minutes of being powered on. Do I need to make adjustments on R2 or should I be good to go?

Tonight I'll probably finish up the amp and throw in a pair of crappy headphones for good measure.

One more thing to note: When I put my ear close to the amp I can hear a very slight buzz. It sounds like it's coming from the power supply area of the amp. Is this normal?


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PostPosted: 21 Jan 2018, 15:16 
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Joined: 17 Mar 2016, 07:26
Posts: 29
Location: Prêles, Switzerland
Hello Mullet,

I had the same issue, my B+ was at 170-180V. Bruce told me here to adjust the resistors. By setting both R2 at 1.8K (5W), I now have about 145-147 V on B+.

Regards,
Charles


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PostPosted: 21 Jan 2018, 19:53 
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Joined: 11 Oct 2014, 03:56
Posts: 14
Yeah mine is a little lower so perhaps I'd only need 1.4K instead of 1.8K.

Either way I'm noticing that after powering off I still had 18V with a multi-meter, even hours later. When I put the probes on B+ it will come down about .01V a second. I'm still not clear on how to properly drain the voltage. I suppose once the amp is fully in use I won't really care, but while setting things up I'd rather not have 18V hanging around.


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PostPosted: 22 Jan 2018, 02:35 
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Joined: 17 Mar 2016, 07:26
Posts: 29
Location: Prêles, Switzerland
opinions diverge about the need of a bleeding resistor. I prefer to have a deterministic approach and know that the tension will be low enough after 2 minutes when I want to work on the amp. Consequently, I have added a 150K / 2W resistor (or maybe 100K, don't remember) across one of the B+ (the other one will drop as well through both R2). I can make a picture and check the exact value today evening if you want.


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