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k-12G turret board version
k-12G turret board version
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Old 5th April 2011, 03:37 AM   #1
ferronier is offline ferronier  United States
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Join Date: Mar 2010
Default k-12G turret board version

I have built a turret board version of the K-12G. A separate transformer providing DC on the filaments eliminated most of the usual needed mods, made a quieter filament supply, and separated it from possible interference from the HV power supply. Using a larger enclosure allowed me to route wires away from interference; the power supply is in a separate bay inside the metal enclosure. Normal upgrades on caps and OTs make this a capable amp.

I can't figure one thing--it works great without feedback---but makes a terrible squeal if I try to connect it. Any ideas on that?

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Old 5th April 2011, 05:19 AM   #2
payitforwardeddie is offline payitforwardeddie  United Kingdom
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Join Date: Feb 2011
Location: South Wales, UK
Could be that the output transformer secondary connections are phased wrong. One connection is grounded, the other goes back to the input stage via the feedback components. Try swapping these two connections on one channel to see if it cures the squeal. If it works do the other channel too, if that is squealing. Alternately, you could could swap the anode connections to the transformer, if that is easier.
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Old 5th April 2011, 10:02 AM   #3
Soonerorlater is offline Soonerorlater  United Kingdom
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k-12G turret board version
Made that mistake when fitting Hammond 1609's to the similar K502 kit.

If Payitforwardeddie's advice doesn't work perhaps you could check to see if your negative feedback resistor has been bypassed with a small cap, my K502 didn't like it being left out. In this case the oscillation was more high pitched, unlike the terrible wale I got from the positive feedback caused by getting the phase wrong on the output transformers.

HTH Bill

Last edited by Soonerorlater; 5th April 2011 at 10:04 AM.
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