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Old 28th September 2007, 05:39 AM   #71
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carbon film is fine, it's the value that'll need tweaking. At your 180mA, it's only running slightly over rated dissipation @ 0.32W (if it's 1/4 watt). Bumping that up to 22 or 27 ohms @ 1/4 watt will probly be good enough and will burn up quickly should you lose bias. Don't use 1/2 watt in this case...smaller is better.

good luck
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Old 28th September 2007, 08:20 AM   #72
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I did hear that carbon film resistors can burst into flames if overloaded. You might want to try something less flammable.
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Old 28th September 2007, 01:31 PM   #73
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Quote:
Originally posted by ray_moth
I did hear that carbon film resistors can burst into flames if overloaded. You might want to try something less flammable.
Yeah, unfortunately I can confirm this from experience, and boy they really burn baby.. In particular the ones RS sells are extremely flammable.. The aftermath is that they leave quite a mess behind them as a result of their violent demise.

There are metal oxide fusible resistors available, however in my experiments they proved to be somewhat non-linear (measured thd) and produced a gritty sound quality when used in the cathode circuit of one of my amplifiers..

Frankly a good old fashioned fuse may sound better.. You can fuse it well beyond the normal operating current to avoid nuisance blowing, but still protect the amplifier when an output tube fails. The safest place for a fuse is usually in the cathode circuit because of the normally low voltages encountered.
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Old 28th September 2007, 02:03 PM   #74
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would a 125mA fuse be sufficient?
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Old 28th September 2007, 02:19 PM   #75
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Quote:
Originally posted by sorenj07
would a 125mA fuse be sufficient?
I would think that would be fine if on a per tube basis. I have found it helpful sonically to put a small film cap across the fuse (well it seemed to help, nothing scientific about this suggestion therefore YMMV - try with/without if curious) - should have a voltage rating higher than the raw supply even though it's in the cathode circuit, and anything around 0.22uF - 0.47uF should be fine.
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Old 28th September 2007, 04:52 PM   #76
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cool. I think I still have a few standard .22uF 1KV C-D dipped radial caps. I want to get my amps working with the bias pots and then, it's modding time All of this will be made simpler with an oscilloscope that I might possibly be getting.
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Old 29th September 2007, 06:42 AM   #77
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Until I get these pots and a drill to mount them on my chassis, I cut the 100K bias connection resistors and brought them to ground. I cut out the little biasing boards and brought each 6L6's cathode to ground with a 470 ohm 5W resistor bypassed by a 470uF 63V electrolytic. Took out the 5U4 tubes and put two 1N4007's in series for each leg of the HV winding. Not my intended "final" fix but for now it's working OK. There's some hum but I think it's from ground loops in my crappy dorm wiring. For example, connecting my powered USB hub to my macbook makes hum increase a TON.
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Old 4th October 2007, 01:05 AM   #78
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First thing - reconnect the chassis/bus ground to earth. I'd cut it but measured 10-15VAC between the two, and touching the amps increased hum a bit. This is my left monoblock by the way, the one that had buzz on top of the hum and noise. I've since installed RCA brown-base 5R4GYB's.

Then I measured plate voltage on all the tubes and found a stupid wiring mistake, where the 47K plate resistors in the driver stage weren't in parallel - I had 47K going to one plate, then 47K off that PLATE to the plate of the other tube - in effect, one tube had 94K. I'm surprised the amp worked at all the way it was set up. I fixed this but didn't notice much difference in buzz.

Then, another "doh!" moment. It turns out, that I stupidly had left the negative bias tap off the B+ connected, thinking that with the tubes disconnected, no current would be drawn. Of course, there's a bleeder resistor after the rectifier to ground. One of the capacitors was only rated for 160V and was seeing 300V on it, and was bulging. I disconnected the coupling cap from B+ and let that wreckage sit on the ground bus - can't do any harm the way it is. I had hoped that this would have fixed my problem, but I think it only decreased buzz a bit.

I'd been using a portable CD player to test the amp but when I connected it back to my laptop, I had the most horrible hashy noise on top of the buzz. I tried cutting earth ground, which made my laptop have only as much buzz as with the CD player, but had 15VAC on the chassis again!

Damn, this ain't very easy. The only nice news is that with a B+ of 420V, and a 30V drop on my 470 ohm bias resistors for the KT77's, I'm dissipating a comfortable 25W (64mA per). I'm considering just keeping cathode bias because this lower plate voltage/higher current seems a bit closer to class A - am I right?

Anyway I have work to do but I'll keep plugging away at this amp (the left one) until I get it shipshape. I wish I had my workshop - I'm soldering on the floor in my dorm hallway!
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