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Old 8th January 2005, 05:45 PM   #1
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Exclamation arcing in rectifier tube

after replacing my psu caps, i am now getting some arcing on power up in the 5u4gb.

i need to increase series resistance?
how to stop the arcing?
it goes 5u4gb to a 40uF, 100 ohm filter resistor, 40uF,10k,20uF,10k,20uF...
the output tubes are 6l6gcs, in push pull, cathode biased with a 500uF and a 500ohm resistor across that.
my plate current is btwn 0.044-0.051mA on the 6l6gcs.


thanks,
scott
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Old 8th January 2005, 07:28 PM   #2
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What was the size of your original input cap? I think 40uF is the recommended maximum for the 5U4G.
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Old 8th January 2005, 07:41 PM   #3
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Default Re: arcing in rectifier tube

my plate current is btwn 0.044-0.051mA on the 6l6gcs.

This seems adrift......are you able to re-measure ??

rich
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Old 9th January 2005, 04:27 AM   #4
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40uF is indeed the maximum.
40uF is what the original schem calls for.

the original cap was a 40/10/10/10, I replaced it with a 40/20/20/20.
so i have effectively increased the capacitance.

there was a second can cap at 10/10/10, i replaced this with a 20/20/20.

these values were more easily obtained by me, and truth be told, the amp did not arc until i replaced the cathode bypass capacitor for the 6l6s, this was also electrolytic, i replaced it with a 550uF/ 500v

as for the bias current, one pair (channel) is reading 0.044
the other channel is reading 0.051mA.
all of the tubes are GE 6l6gcs, and once the amp is up to temp, there is no drift. i should have specified that.
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Old 9th January 2005, 04:31 AM   #5
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also,
my line voltage averages 125v. perhaps i should think about getting a variac to run it on 117v?

or just replace the power tranny with one rated for 120v?

the sams photofact i have shows all of the voltages with a line of 117v primary.
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Old 9th January 2005, 04:46 AM   #6
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Default Re: arcing in rectifier tube

Quote:
Originally posted by el capitan83
as for the bias current, one pair (channel) is reading 0.044
the other channel is reading 0.051mA.
I really think 44mA and 51mA is not much for a pair of 6L6s...
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Old 9th January 2005, 08:31 AM   #7
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Default Re: arcing in rectifier tube

[QUOTE the output tubes are 6l6gcs, in push pull, cathode biased with a 500uF and a 500ohm resistor across that......my line voltage averages 125v. perhaps i should think about getting a variac to run it on 117v?
[/QUOTE]


I'm curious about that ouput stage...it sounds like a 500uF/500V across each cathode resistor.....complete overkill when cathode voltage is less than 50V.......the ESR of a 500V Cap is way higher than a lower voltage unit. Choose low voltage electrolytics with low ESR i.e those used in switchmode power supplies are excellent and will sound better

You don't need a variac........a tube amp should work throught large variations of line voltage....my line voltage varies from 210-238V AC and my amps work despite slight power droppage.

rich
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Old 9th January 2005, 10:50 AM   #8
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Hi,

The datasheets for the 5U4G state 32F as Cmax.
This value is relative to B+ voltage and current and is almost invariably stated within the context of a C-L-C PS.

What I think happened is that the PS you were using kept its PS caps charged even after power off.

The caps are now replaced with new and bigger one and the sudden charge up surge is what's causing the retifier to arc over.

Using a variac will overcome the problem but can't be a permanent solution so I'd reduce Ctot to the original value.
Keep in mind that the U4G may now be damaged because of the arcing over as well.


Cheers,
__________________
Frank
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Old 9th January 2005, 07:50 PM   #9
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frank, i don't want to challenge you, but-
i have rc-27 of rca receiving manual. for 5u4gb, input capacitor max is 40uF. then they state:

"higher values of capacitance than indicated may be used, but the effective plate-supply impedance may have to be increased to prevent exceeding the maximum rating for peak plate current."
so maybe 32uF is the mean, with 40uF being the MAX in certain situations. i was always taught to keep the input capacitor between 20-50uF, and that the remaining capacitance (within reason) may be increased slightly without putting undue stress on the rectifier.

so,
i have the 40uF/100ohm resistor/40uF for the pi section of the filter. the filtering for the pi and the preamp plates is all 20uF.

I have around 530V on the plates of the 6l6gc(without tubes installed, measurement taken at socket with fluke 73)


the 550uF/500V bypass cap is overkill, however, there is not enough room in the amp to add separate resistors and caps for each tube, so all 4 6l6s share a 550uF/500v+ 500ohm resistor.
I suppose I could lower the voltage rating to a 250v or so, even, as the schem only shows about 30v for each cathode. this will lower the ESR, no?

lastly, this is just my "daily driver" amp, i just need something sweet and reliable until i can get something decent made, hence, this is being done as time and budget allows. however, i need to stop that arcing!

i was thinking it would be something easy, like running it on regulated voltage, or adding a resistor or an inrush current limiter?

apologies if i may seem a bit "rude and crude", i cut my tube teeth on guitar amps, and all of my gear reflects this attitude...

also, how to attach large files? or is it easier to just set up a photosite and link to that? my schems are way too big to fit on here.

thanks for all the continuing help...
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