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PostPosted: 14 Mar 2012, 23:20 
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Location: South East US - Tennessee
jetorama wrote:
Hi all.
I'm just about finished building this phono stage. This is my first diy electronics project since helping my brother construct frankenstein stereo systems when I was 10. It is amazing we didn't electrocute ourselves back then. Anyway, I have a very simple question. Do the 4 LEDs require directional placing? If so, how do I know which side takes (+) and which (-)?
Thanks for any info! Bill


For future reference:
The PCB silkscreen (component side - component outlines) has the orientation for the LEDs indicated. If you look closely, you'll see the the outline for the LEDs has a flat side. This corresponds to the flat side of the actual LED body. This is the most accurate identification for polarity as sometimes leads are not correctly cut. I do not rely on lead length. Flat side is minus side (cathode). As Mark correctly stated, they serve no functional use in the circuit and they may be left out. They are just for show - the idea being you would bend the LED to shine up the body of the microtube and give the ambiance of "filament glow". If you choose to leave them in and you're concerned with the HF noise they produce, they can be snubbed with a 1nF capacitor to quiet them. If you choose not to use them, then you can also leave out R201 and R202 as they won't be needed - they provide current limiting for the LEDs.

:beerchug: Cheers!

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 17 Mar 2012, 17:39 
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I have a question for you guys. I'm somewhat of a newbie but tried it anyways. I tried replacing the power supply by 5 6v lantern batteries hooked in serie. So what I did what hook the batteries directly to the amp in place of the power supply. I switched it on and everything other than some buzzing noise seemed to be okay. Then, there was a puff of smoke that came out of the amp's enclosure. I switched it off and, when I looked inside, c6 and c106 had exploded (the metal cap literally poped off the capacitor). So, I was wondering if any of you guys knows what could have turned wrong or what is it that I did wrong.

There's really only c6 and c106 that blew up, everything else seems ok. So' I'm gonna replace these 2 capacitor hoping that it is the only damage.


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PostPosted: 17 Mar 2012, 18:36 
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In your case I would say either you had the batteries in the wrong way (positive of batteries to negative of amp) or the new batteries, fully charged were over-volts for the caps. Not hard to do if each new battery is close to 7V that would make 35V or more and maybe the caps are rated 30V??

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 17 Mar 2012, 18:48 
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Joined: 22 Sep 2011, 11:22
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Maybe I did wire it in the wrong way... I really don't remember. As for the cap, I doubt the blew up because they had too much voltage. If I'm not mistaken, I think they are rated for 63v...


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PostPosted: 17 Mar 2012, 18:54 
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Then I would say the batteries may have been in the wrong way. If that is the case you may have done more damage than a couple of $1 caps.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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PostPosted: 17 Mar 2012, 21:52 
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Interestingly, the parts list doesn't seem to list the 2 capacitors indicated in the schematic, which are power supply filters (actually there a four). The 4.7uF electrolytics aren't listed. Which are C6, C106, C8, & C108. I'll bet they were all the same voltage rating. I could pull mine out and look, but I'm not really keen to disassembling my amp to the extent where I could put an eyeball on those caps. They should have been at least 35V rated. Either they were installed backwards, or your battery was reverse connected. As Mark indicated, if the later was the case, you probably want to also check all 4 of the FETs (Q1, Q2, Q101, & Q102). I don't believe the tubes would've been damaged. Look at C8 and C108 and inspect R11 and R111. These are in the filament supply. R11 and 111 are the dropping resistors the are feeding C8 & 108. They're probably OK, since the voltage there should be somewhat less than 30v, say about 14vdc. Everything else should be just fine as they aren't polarity sensitive.

Watch the polarity of those electrolytic capacitors when installing them... :up:

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 18 Mar 2012, 09:31 
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Joined: 22 Sep 2011, 11:22
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Thanks for the info. I'll try to check it all. It really must have been the battery that was reversed connected beacause I've been using the amp for a couple of month with the power supply that came with the kit.


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PostPosted: 20 Mar 2012, 14:56 
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Joined: 13 Mar 2012, 16:53
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Thanks for the previous info Les. Sure, it seems obvious now.

All going strong and it sounds pretty good.

I didn't separate the power supply from the signal paths. There is a little hum, but not much. However, it is enough that I've decided to change chassis and use a Hammond 1444-14, allowing a little separation.

I'm thinking about mounting a mains fuse, either a panel mount or internal chassis mount. Does anyone have any thoughts about amp rating for the fuse?

Cheers.


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PostPosted: 20 Mar 2012, 15:57 
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If you are talking a mains fuse always use a slow blow. A tiny tranni like this will not have much of a surge at turn-on. I would think 250mA slow blow would work. Though these trannis are small you will always get hum is a high gain cct. like this if the tranni is close by. Either use one big case to separate them or make the PS remote. I have done both and both work well.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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PostPosted: 27 Dec 2012, 19:54 
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Joined: 23 Dec 2012, 05:18
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Season's greetings all.

I just built this kit last night and wonder if anyone has had an issue with distinctly noticeable channel imbalance (definitely the preamp). I was fortunate in that I had two very closely matched pairs in the five tubes supplied so CH1 got two tubes with 134 & 88.2 uA respectively and CH2 got 133 & 92.4 uA. I'm certain I did not misplace them on the board having marked them up beforehand (the obvious thing to suspect in the first instance). I don't know if it is valid to perform the matching test again on a populated circuit board, but for what it's worth even though the values were different (much closer) repeating the test on the mounted tubes yielded CH1: 108.5 & 107 uA, and CH2: 108.7 & 107.8 uA.

It's a pretty straightforward build and I have double checked all the other components are placed correctly. Does anyone have any ideas please? should I test the FETs? Only change I made was to use a 12VAC wall wart instead of the supplied PCB transformer since I had one and moving the transformer further away seemed like a good thing. Hum isn't an issue.

Thx in advance.


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