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PostPosted: 20 Mar 2018, 14:56 
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Joined: 20 Mar 2018, 14:32
Posts: 4
Hello everyone,

I've been lurking on this site for a while and just got finished building the K-502 HiFi Tube Amp from Amplified Parts. Kit was pretty easy to build and I enjoyed building it. However, once I fired it up it has a high pitched squeal at very LOW and Very HIGH volumes, but with the volume turned a tad past 9:00 to about 3:00, it sounds very good.So here are my questions........

1. What would cause the squealing/interference and how do I fix it?

2. There are a couple of extra soldering points for capacitors and resisters, should I find out their values and I install them, and how do they effect the amplifier. The list below are the parts not mentioned or included in the kit.

Items not in kit
C5, C6, C11, C19
R9 & R19

I've always building things and once I get the bugs ironed out and a good case made for this amp, I'll probably build a "MOD 102+" for my guitar.

Any help is GREATLY appreciated.


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PostPosted: 21 Mar 2018, 17:30 
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Joined: 04 Jun 2008, 20:59
Posts: 4014
Location: Arizona, USA
Hi, If it squeels like you indicate there are a number of possible causes. The phase on the output transformers may be reversed and causing the negative feed back to be positive. If you followed the color codes when building it this should not be an issue. If not go back and rewire them. Another issue with these amps is that the case of the volume control often needs to be grounded. It can cause this. What you do is fasten the bare end of a wire around the shaft of the control, you can solder one to a washer or large lug and put it around the shaft as well, and attach the other end of the wire to a circuit ground. I recall there are a number of locations right near the physical location of the control. If it still squeels then there is likely a wiring error. Possibly a bad solder connection.


Good listening
Bruce

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PostPosted: 24 Mar 2018, 19:39 
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Joined: 20 Mar 2018, 14:32
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gofar99, thanks for your input. Looks like the problem was the device I had it connected to as my tiny MP3 player worked GREAT, however my laptop, of which is not on a grounded outlet and my compact CD player on a wall wart seem to make it squeal. I've moved the amplifier and have connected it to my Recording PC and I'm powering a couple of EV Studio Monitors. Awesome sound.

Any idea of what the additional resistors and caps do if incorporated into the amp? There's no mention of what they would be for or use, I can only assume that the Caps my be for more filtering, but not sure.

Thanks again, I love this little amp.


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PostPosted: 24 Mar 2018, 21:42 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, You should read the projects on the K-12 amps. The amps benefit greatly with replacing the coupling capacitors with a quality brand. Other changes include better output transformers, plus a number of things like better rectifiers. The problem is ...you can spend a lot of money doing all the upgrades, but still have an amplifier that is entry level. My first tube amp was one and I went that route. In the end it was not bad, but for the amount of money invested a different ones would perform better. It was my initial incentive to start designing equipment. The K series of amps are decent starter amps and for the initial price will give you a taste of tubes. If you get serious about sound there are lots of ways to go.

Good listening
Bruce

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PostPosted: 30 Mar 2018, 14:31 
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Joined: 20 Mar 2018, 14:32
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Thanks for the information! Yea I have a couple of Tube Amp Gurus as friends so I'm lucky enough to pick their brains and get suggestions. To me, so far this is a really great sounding amp, but yeah I have been kicking around changing some of the Caps and stuff, but wondered if it's worth while if I don't change the Transformers?

I've got the bug now because I plugged a Digitech Valve FX (80s vintage, with 12AX7 Tube) Guitar Preamp/Processor into the 502 and ran it into my EV Sentry 100 monitors..........WOW. So now I'm contemplating building another 502 or a K-16LM Mono amp to marry to the Valve FX. The main concern is running the amp at 4Ohm as I've build a custom cabinet for the project and it has two 8 Ohm speakers in it so I'd be running the "mono" amp at 4 Ohms.

Thanks a bunch for your input.


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