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-   -   Help with an S5 K12-G Tube Amp (http://www.diyaudio.com/forums/tubes-valves/239439-help-s5-k12-g-tube-amp.html)

bananajamslam 18th July 2013 09:39 PM

Help with an S5 K12-G Tube Amp
 
Hello everyone,

I have been lurking around the forums for a while, and eventually got interested enough to buy a tube amp kit. (S5 electronics, K12-G) The kit arrived in the mail yesterday and I spent the evening putting it all together.

However, upon testing it, I have discovered a very strange issue. My problem is that, after connecting a speaker (an Audio Nirvana Super 8) and a source (an mp3 player) and turning the amp on, I am getting a loud buzzing noise coming out of the speaker. The buzzing noise does not occur immediately, but rather after the amp has been on for a couple of minutes. (Until then, the amp sounds perfect.) The noise is so loud I have to pull the plug.

I have also noticed that the main transformer is making a buzzing noise, although, it is fairly quiet.

Again, I'm very new to this, so I'd really appreciate any help you can give.
Thanks! :)

P.S. I'll try to post some pictures later tonight, as well as anything else that I try when I get home from work.

Soonerorlater 18th July 2013 10:48 PM

1 Attachment(s)
Hi Bananajamslam, welcome to the forum.

Attachment 360980


I believe this a schematic of your amp. I assume you have attached a pair of speakers and the increasing buzzing sound is equally loud across both channels.

JonSnell Electronic 18th July 2013 10:49 PM

Is the hum from both chanels? If so then there may be a smoothing issue. If not then a misbalance of output valves could be it. Swop one from each output stage to the other side.

JonSnell Electronic 18th July 2013 11:02 PM

Interesting selection of valves used here ... ECL85 with an odd heater voltage!

bananajamslam 18th July 2013 11:39 PM

Quote:

Originally Posted by Soonerorlater (Post 3566288)
Hi Bananajamslam, welcome to the forum.

Attachment 360980


I believe this a schematic of your amp. I assume you have attached a pair of speakers and the increasing buzzing sound is equally loud across both channels.

Yes, that's the same schematic that came with the amp.

I have pair of speakers attached to the amp. The buzzing seems to be coming from the left channel of the amp. There also seems to be some loud crackling noises coming from it now.

bananajamslam 18th July 2013 11:50 PM

Swapping the valves does not seem to do anything.

Also, upon closer inspection, it seems like the hum and crackling noises appear on both channels. It is just not as severe on the right channel.

bananajamslam 19th July 2013 12:33 AM

Quote:

Originally Posted by bananajamslam (Post 3566340)
Swapping the valves does not seem to do anything.

Also, upon closer inspection, it seems like the hum and crackling noises appear on both channels. It is just not as severe on the right channel.

Sorry, I mean the left channel. (I can't edit my post for some reason.)

Soonerorlater 19th July 2013 10:43 AM

Think you only get half an hour to edit. :o

Amp built as is, ie on the wooden board and not cased in a chassis ?

Tried a different source such as a cd/dvd player ?

Earthed the volume pot ? ( IIRC there was a tab from the pot to be connected to the pcb )

bananajamslam 19th July 2013 02:57 PM

So I was reading through the manual again and I noticed the words "Do not operate the amplifier without proper speakers connected." Unfortunately, that's exactly what I did, and I assume I broke something by doing so. :(

So now the question is: "What do I need to replace?"
Output amps?
Tubes?
Main transformer?

I guess I'm just trying to figure out how much it's going to cost. Because, realistically, if I need to replace everything, it's not worth trying to repair it. Is there any way I can test these things with basic tools? (I only have a multimeter.)

Thanks guys.

P.S.
Just to answer a few questions:
-The volume pot is indeed earthed.
-I have not tried a different source, but the amp creates this strange distortion even without a source connected. (Question: Is this a bad thing to do too?)
-The amp is attached to a wooden board. (Believe it or not, the kit came with a big slab of maple to attach it to.)
-I have also tried using the amp in a different location and using different speakers.
-I checked all solder connections and reflowed all of them.

Soonerorlater 19th July 2013 05:48 PM

Operating your amp without speakers may damage your output transformers (OT's). The channel that had the speaker attached during your initial test should be fine, the other OT may have been damaged ( or not, you may have been lucky :) )

Is the problem confined to the channel that had no speaker attached ?

Final proof of a faulty OT is to swap them over and see if the fault goes with it.

( The problem might still lie elsewhere, it could be oscillation of some sort )

Original OT's supplied with the kit are not well thought of by some. Replacement with Hammonds or Edcors usually are considered a worthwhile upgrade. I have the similar K502 from S5 which has been upgraded to Hammond 1609's and I wouldn't go back to the stock OT's.


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