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Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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Old 20th January 2010, 03:04 AM   #11
trigger is offline trigger  United States
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ok one question,
when i plug in the amp, it has a ac buzz/humm, its seems to be coming from the PT
see any thing missing from the power supply? i must have sniped out its liver or something?
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Old 20th January 2010, 06:07 AM   #12
rotaspec is offline rotaspec  New Zealand
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Was the 6CA4 new or the original? Are there sparks inside the 6CA4 when this happens?
Does it buzz without the 6CA4 plugged in? If so, the power transformer may be history Or there is a short on the heater lines (unlikely unless you altered something)

Assuming it's still buzzing with all tubes unplugged, check the voltages coming out of the transformer. Careful, the ac on the HT winding could be well up over 300v. I suggest disconnecting any heater wiring you added/altered first to see if that helps.
If it stopped buzzing without the 6CA4 (rectifier), then the problem is along the high voltage DC (hvdc) line. Again this is where a schematic makes it easier for those of us who can't be present to see what is going on.
This is also where it could get risky. I could suggest disconnecting the hvdc that leads from the 6CA4 to the rest of the tubes and checking the hvdc, but doing that will drive the DC voltage quite high and the risk of exploding the big old filter capacitor (top left of pic, the can shaped thing that sticks thru the chassis) becomes greater if it's not in good condition. It could be what is causing the buzzing if it begins to get warm after a short time. If so, it's probably toast and will go bang at some inconvenient time and must be replaced.

Hope that helps, and BE SAFE while doing these measurements. Read the sticky thread on this forum about safety, as your life depends on it.
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Old 20th January 2010, 08:00 AM   #13
trigger is offline trigger  United States
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why thank you for the advice, i take safety first, i use a shield over the caps when testing and all other necessary measures.
the amp was buzzing with out tubes,,,i discovered the chassis was running 12vdc hot, a lead for the radio's luminescent was bent touching the chassis... eaggg should have looked better before i turned on, the short was on the top tubed side witch i overlooked.
after 3 more long stare downs the amp was fired up and works great, there is a real scratchy short coming from the 7025 tube socket, a very faint quiet crackle-fizz, besides that clean loud and i didn't listen to long, i will get the schematic and some post surgery pics up.
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