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Old 4th January 2013, 10:18 PM   #1
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Default fu-29 Audioromy

Hi,

Im looking for some tips on using the Audioromy FU-29 amplifier. I just got one in the mail the other day and powered it up for the first time. Awesome! Sounds great and runs quite silently. However... the day old problem of one of the plates inside the output tubes started to 'red-plate' after 40 minuets of running. I wasn't even running them loudly at all. I turned off the machine and let them cool down for a bit. Turned it on and within another 40 minuets, the one anode started to gently glow red again. All the other plates were happy looking.

So in an effort to double check all the calibrations, I flipped it over to check if the output was balanced (by metering each plate to each channel together). Pretty darn close, about 0.3v off on both channels. But the real question is where do I meter to check the cathodes of each channel? The manufacture requests that the shared cathodes are biased to 0.5v off of ground. So where do I meter? (check out my photos because the new circuit layout is not the typical one seen on previous posts) I tried checking Pin 4 to ground, and its not showing me results I am looking for.

Also, I have 2 convenient GU-29 tubes ready to go if red plating means absolute failure, but if I do install a GU-29, I want to make sure I bias them correctly so that damage wont happen in the future. Does anyone have any tips?
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Old 4th January 2013, 10:41 PM   #2
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If I remember correctly, these dual tetrode tubes are notorious for for imbalance in bias specs between tube halves. They were meant for class B/C operation where matching is not important. Your best bet would be to swap out the tube that is red-plating - hopefully the replacement won't be just as bad.

I was fairly gung-ho about building an amp with 829/832 outputs, but the mismatch issue has set me back.
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Old 5th January 2013, 03:35 PM   #3
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Interesting, I figured that would be the case. If you had to double check the bias of the shared cathodes, how would you go about doing it?
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Old 5th January 2013, 05:21 PM   #4
wicked1 is offline wicked1  United States
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I would measure the voltage across that 10ohm 5watt resistor. If it says for example, .500v, you are pulling 50ma. I'm not sure what good it will do, though. That only tells you what they are both pulling combined.
You could add a 10ohm at the plate of each section, and then you could tell what each section is pulling. But again, not much use unless you have a way to make adjustments.
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Old 5th January 2013, 08:45 PM   #5
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The problem is that I cannot seem to locate the cathode resistor to meter across. There are two 1 ohm resistors connecting to the top of each plate but I don't think that is the proper area to gauge the bias. Metering both of those gets me the balance between them.
I think the cathode resistor may be on the underside of the board. Does this seem right? Why would they try to hide that?
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Old 5th January 2013, 08:58 PM   #6
wicked1 is offline wicked1  United States
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Oh, the 1 ohms on the plates aren't on the schematic.. But, yes, you can measure the current each section of the tube is pulling w/ that.
Here's a thread about it.
measuring current using 1ohm resistor. How?

The 5 watt cathode resistors maybe large, so they put them on top to have physical space for them... Who knows....
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Old 5th January 2013, 11:09 PM   #7
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Interesting idea of using a resistor to measure the Amps running through it. So when I read across each plate resistor, I get about 0.025v. So being that it runs through a 1 ohm resistor, each plate is using 25ma? Combined at the cathode it would be 50ma? So if trust that there is a 10ohm resistor down there, 50ma would make for .5v bias before that resistor? Is this the correct way of doing this?
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Old 5th January 2013, 11:30 PM   #8
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Quote:
Originally Posted by woodrough View Post
Interesting idea of using a resistor to measure the Amps running through it. So when I read across each plate resistor, I get about 0.025v. So being that it runs through a 1 ohm resistor, each plate is using 25ma? Combined at the cathode it would be 50ma? So if trust that there is a 10ohm resistor down there, 50ma would make for .5v bias before that resistor? Is this the correct way of doing this?
You already have the 10R resistors on each of the plates (highlighted in pink circles). So you could take voltage measurements across each of them. But please be extra careful, and make sure your DVM can handle the HV, use banana clips/clip leads on the resistors, do not attempt to hold the test leads with your hands while taking the measurements, one little slip, you can kill yourself and/or damage the tubes.

After taking the measurements, you can then adjust the 33K pot on each channel to balance the bias voltages to each half of the tube, unless the two halves are very mis-matched, you should not have to turn the dial much at all to get the bias voltages in line. If it still red-plates after the adjustment, something else is wrong...

Good luck and please be very careful when you do this!

Click the image to open in full size.

Jaz

Last edited by jazbo8; 5th January 2013 at 11:33 PM.
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Old 6th January 2013, 12:11 AM   #9
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Those resistors you circled are 1ohm resistors right? I had some confusion about them being they are not on the schematic. The colors are brown, black, gold and gold. The first gold being a 0.1 multiplier making it a 1 ohm resistor. If I wanted a bias voltage of 0.5v, I would want to adjust the trim to produce 0.025v across each plate resistor correct?
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Old 6th January 2013, 01:04 AM   #10
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Quote:
Originally Posted by woodrough View Post
Those resistors you circled are 1ohm resistors right? I had some confusion about them being they are not on the schematic. The colors are brown, black, gold and gold. The first gold being a 0.1 multiplier making it a 1 ohm resistor. If I wanted a bias voltage of 0.5v, I would want to adjust the trim to produce 0.025v across each plate resistor correct?
I think they are 10R, since there are 5 band not 4, either way, so set the DVM to read 25 or 2.5 on either the 100mV or 10mV scale, then 0.050*10=0.5V is the bias current for both halves.

Jaz
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