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I've just done a few things on the other amp (same output design and very similar preamp) when I noticed that the circuitboard material is different. I think...
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I think so as well. I'll do a bit of work on the grounding and see if I can separate the preamp stages into a couple ground connections. I'm also considering...
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The preamp is also on a board. I've grounded the board to the star ground but I'm not looking to rebuild the preamp as well (though it would be a suitable...
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Eliminated the hum there by rebuilding it onto tagboards instead of old nasty circuitboard (not all circuitboard is nasty, but this stuff is) .
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no, it refers to the resistors between the grids of the kt120's and the bias.
so for your bias you would have a negative voltage supply into...
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Thanks steve.
I dont think its heater hum, its very buzzy and not a sin wave by any means. I've tried different tubes including some with spiral...
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Didnt fix it Tubeswell.
I know that the issue is within the LTP PI, most likely the input. I've isolated the PI/power amp so I know it is not...
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didnt completely short it, just took it out of circuit. I'll do that when I get back from work.
Thanks for helping me on this invisible problem...
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wasnt the issue tubeswell, had me excited though. This amp has a resistor bypassing the presence pot which had blown and I've replaced previously, but wasnt...Last edited by black_labb; 05-20-2011, 11:48 AM.
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I've just measured voltages on the PI which seem correct. When probing pin 2 or 7 (the grids) of the ltp 12a*7 the hum goes down alot, so the issue is at...
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Thanks tubeswell. I've done that thinking that that was the issue but it made no noticeable difference.
I'm pretty stumped at the moment. I...
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The buzz seems to be coming from the PI area. I'll try replacing the CC resistors.
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The voltage on the plates doesnt change as I adjust the volume control to where the buzz goes away. I get the same bang noise from when I probe with the multimeter...
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Thanks again Jmaf
I've just checked the grounding and the positive rail in the preamp and everything seems fine. I'll have to try another cap....
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seems that valve 2 is breaking up. check the voltages on each of the pins and get back to us.
edit: sorry, had it in thread mode and didnt see...
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Thanks Jmaf, I typed a reply a while ago but must of not clicked send.
the hum in the power amp is gone. was partly the power tubes and partly...
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Thanks jmaf
I'll check the tubes. I dont have a well matched set but I have 12 used 6550's, so I'll be able to see if it makes a difference....
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quick update
retouched the ground connections, same hum.
The caps draining i'd say is from the tubes still being warm enough...
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Quieting an old beast
I have a couple old amps that run 4 6550's. one of them has a problematic hum issue since I got it. What is curious is that the hum is coming from the power...
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fixed the PI issue. realising that that wasnt the major problem. only putting out a couple watts from the 4 el84's. will make a thread in the proper troubleshooting...
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