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Old 7th April 2010, 02:19 PM   #1
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Default HELP!! Howling K12G!!

Hi Guys,
I had purchased a 12G kit and put it together. Used a couple of the mods that were recommended including bridging the filament wires and bypassing the input caps and the thing sounded

BEAUTIFUL. Unbelievable sound/dollar value. Built some Open baffle speakers and all was good with the world.
Well, after a week or so it was time to built a box to make it look as good as it sounds and make things work a little more practically.
I extended the leads from the board to the OTLs and the power Trans, put bullet connectors on them so I could run them though holds in the case easily. I moved the volume pot off the board so I could mount it on the front panel. I used approx 3" wires and twisted each pair. Finally I added a 3 channel switch and wired it to 3 sets of inputs and to the board, all the wires were made of twisted pairs from some cat 5 and braided together.

Now the bad part. The amp now howls. It got worse when you touched the volume control, or even when you put your fingers near it.
I removed the volume pot so I could try it with the pot mounted back on the board but found out that even with the pot and wires DISCONNECTED from the board, it still howls.
It gets worse when you put your fingers near the board in front of tubes 2 and 3.
I've checked all the ground wires and all my new solder joints..everything looks good

Anybody have any thoughts?
While my soldering skills are good my general electronics knowledge is deplorably weak.
Thanks in advance for any advice!!
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Old 7th April 2010, 02:56 PM   #2
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Try shortening the output transform leads, and twisting each set of leads (primary, secondary for each) - and could you have SWAPPED any of them when you extended them?
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Old 7th April 2010, 02:58 PM   #3
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Did you get the phase of the OPT's correct? If the amp uses feedback from the output transformer secondaries,and the phase is reversed,it will oscillate.
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Old 7th April 2010, 03:09 PM   #4
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Make SURE that the metal case of the volume pot is connected to the circuit ground! This can cause horrible noise if not. Might also want to add some carbon comp grid stopper resistors of 1k or so as close to the input tube grids as possible, as in 1-2mm. On circuit boards with no grid stoppers fitted i have in the past left the grid pin leg of the socket unsoldered and bent it outward then soldered a 1/4 watt CC 1.5k resistor from the pad to the tab.

Last edited by mctavish; 7th April 2010 at 03:15 PM. Reason: forgetful
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Old 7th April 2010, 03:11 PM   #5
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Also are the output transformers grounded

Also during your original mods that worked fine did you change or remove the ceramic cap in the feedback loop

Last edited by Soonerorlater; 7th April 2010 at 03:29 PM. Reason: more thoughts
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Old 7th April 2010, 03:41 PM   #6
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Default Aaaahhh

Volume Pots and OTs are grounded and nothing was changed in the electronics when I switched to the case...The OT leads are twisted and I will check and see if I switched any wires when I get home.....way to obvious....and jsut the things that I'd miss...lol
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Old 7th April 2010, 03:54 PM   #7
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I once reversed my output transformer leads and accidentally made myself a Theramin!

Best of luck, Bill
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Old 7th April 2010, 04:19 PM   #8
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Thats EXACTLY what mine is doing....would have been very entertained if I hadnt been so frickin' annoyed....lol.
Will let you know if it worked a bit later.
THANKS GUYS!!
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Old 8th April 2010, 12:10 AM   #9
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Okay, I didn't make THAT obvious mistake.
I didn't cross over any OT wires.
What I'm going to do is take out my bullet connectors and just have wire. While I'm doing this I can resolder all my OT connections at the board.
Anybody have any other ideas while the iron is hot?

Jeff
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Old 8th April 2010, 03:13 AM   #10
infobot is offline infobot  United States
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I got nervous being an amateur when I got my K-12G - could've built it myself easily - but got jumpy and instead had S5electronics build and test it before sending it.

Anyhow mine howled too but only with some speakers. Usually hard to drive ones. Amp seems fine. Swapped to easily driven dome / woofer (89-90 dB sensitivity) 2 and 3 ways and all noise totally disappears.

Howl on mine (with some speakers) disappears if volume raised past a certain point - at about 2 to 4 out of 10 on the volume control. Had to turn it up further (4 at most) with harder to drive speakers and all howl goes away.

I'd consider what others say first but just a thought. Amp runs fine, have had it in use for a couple of years, no problems whatsoever.
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