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Main :: DT50/DT25

dt 50 2x12 motorboating!
by bluepete222 on 2011-09-07 05:53:51.3980

i have a dt50 2x12 which is fantastic. i changed to buffer and phase inverter 12ax7 valves to make it a bit less hissy.

i have noticed a tendency for slight oscillation on voice 4 if the preamp is on full. this results in a very low but noticeable oscillating hum due i think

to the fact that voice 4 cuts negative feedback in low frequencies. also connected to my hd500 with the divide by 13 patches the same thing is there

due i think to no negative feedback in voice 3. amp techs will tell u that having no negative feedback can cause amps to 'run away' with themselves resulting in

a 'motorboat' sound in the background. this is not a mega issue as loud its not a prob but is an issue? is it a design limitation?

note if i use the jcm800 patch into voice 2 up load with plenty of gain there is no noise which fits the parameters of voice 2 which has more negative feedback

achieving that tight marshall sound!

Re: dt 50 2x12 motorboating!
by Nick_Mattocks on 2011-09-07 14:28:35.3920

What brand tubes did you use?  Could one of these tubes be faulty?

Re: dt 50 2x12 motorboating!
by bluepete222 on 2011-09-08 02:38:46.0710

Hi Nick

no its not a tube issue. i know a bit about amps having restored a vox ac10 and sound city l100. tapping the valves does not induce any noise. however i have found the sensitive spot!. tapping behind the carry handle to the left induces the pulsing noise. also if u tap underneath directly on the metal case same thing. i suspect that there is a capacitor at this location with either dry joints or something and in the more loose voicings 3 & 4 is oscillating. dont think it is a major smoothing capacitor as when they go it tends to make loud wolf bark noises. this is just an annoying low level oscillation which is noticeable on the low power setting and as background hum when loud. i think it will need to be checked out by a service centre.  Is this a known problem line6? 

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