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Old 28th April 2003, 05:33 AM   #1
karma is offline karma  Canada
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Default the case of the arcing tube

ok im new to tubes and i just finished building a preamp using
6414 and 5687 works great no hum at all my problem is when i
first turn it on tube 6414 arcs in the filament for a few seconds then stops. it fine fully powered up.when i turn it off it does the same thing.is this tube on the way out? or is the field to high?
or do i need a better bleeder resister? im lost here

300v is the max for 5687 so i read tested around 270 is that to high
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Old 28th April 2003, 06:18 AM   #2
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Sure it's arcing? Might be like the Mullard 12AX7s with the heater flash, I suppose...
If it is indeed arcing, you should measure all the voltages and see if anything goes over somehow. If it's false, replace the tube, it's probably messed to all heck if it's doing that within ratings.

Tim
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Old 28th April 2003, 08:08 AM   #3
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Hi karma;

Sorry to hear about the arcing situation. If it really is arcing, it could be a case of a healthy valve that's being presented with too high a B+ during the time when the plates are cold. Have you tried measuring the B+ before the valves are hot? It can get quite a bit higher than when the amp is drawing current, trust me.

Yes, a lower resistance bleeder resistor (with a suitably high power rating!!) could help load down the amp's power supply and keep that from happening - at the cost of more heat dissipation inside the chassis during normal operation and more strain on the PS trannie. If you add a second very high current bleeder, you should see if you can put a switch or a relay on it so that you can break the connection with it (ALWAYS LEAVE ONE BLEEDER IN THE CIRCUIT FOR SAFETY'S SAKE!!) after 30 seconds or so.

Are you using any kind of time delay on the "B+" so that you can preheat the filaments on your valves? This has been an issue over here on a couple of my DIY amps - in one case I added a "standby switch" and in another I'm going to add a relay with a 30 second delay (for now I'm just using one brand of valve that can take the high B+ turnon spike). You might try a rectifier tube with 'slow start' as a quick fix.

If there's room under the hood, you could see about adding a second filament trafo instead and switching it so that you can preheat all your valves.

Don't know about the spike on power down - are you sure it's inside the valve and not an artifact caused by 'arcing' inside the switch? The cure there is a 3kV rated .01uF ceramic disc cap placed across the "throws" of the switch so that there is an alternate pathway for the "arc" (okay, it's really "back EMF" for the EE's out there) inside the switch when it opens. Switches last longer with bounce caps anyway, so I always use 'em in power applications.

Good luck with it and all the best!!
Morse
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Old 28th April 2003, 12:29 PM   #4
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Default ouch

ok i tested it on startup it jumps from 360 down to 270 i would say thats a problem ill change a few parts to bring it down
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Old 28th April 2003, 06:25 PM   #5
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Default This sounds odd.

Agreed, off-load HT can be higher than on-load, which is a very definite problem, but I've never actually seen an arc, and you said "filament". Are we talking about AC heaters, or DC heaters fed from a regulator that might be misbehaving?
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Old 28th April 2003, 06:36 PM   #6
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OBTW -
Tube ratings are as though it's pushing a transformer. If you run a tube at 300VDC plate, and it pulls fully down for the negative peak (plate = 0V), then the positive peak will want a similar excursion - up to 600V, since the transformer's voltage adds to the DC supply.
This means, for a 150VDC rated tube, it is actually rated to 300VDC max., so you can use it in a resistance-coupled amplifier, since the voltage can never go higher.

And besides, tubes are hearty...they can take overvoltage...

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Old 28th April 2003, 10:09 PM   #7
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well that good to here that thay can take some over voltage. as for the arcing i would say it was from the plates im working on it now to see if i can lower the voltage see if it helps
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Old 28th April 2003, 11:19 PM   #8
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Default FLAMING COMPUTOR TUBES AND MORE...

Hi,

Quote:
ok i tested it on startup it jumps from 360 down to 270 i would say thats a problem ill change a few parts to bring it down
Well, yes... and you're damn lucky your caps still survided too.

Did you put in bleeder resistor in your PSU, etc?

If you did than I really doubt the tubes are arcing in the first place, computor tubes are designed for on/off service and that lighting up you see may be just their fast turn-on.

Still, going from 360V to 270V at switch-on is not your normal turn on cycle.

Anyway, I don't think using computor tubes for audio is a good idea unless you really understand what they're made off.

Cheers,
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Old 29th April 2003, 12:14 AM   #9
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computer tubes? im working on a preamp for my zen anyway it working now.i just down graded the b+ tubes are fine. now that its finished ill make a case for it and move on to bigger and better

the caps wear not cheap

multicaps 200v-2
wonder caps 630v-2
infinit caps 630v-2
rubycon 360v-2

thay made it and the tubes

i over did it with the rubycon caps i only needed one opps
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Old 29th April 2003, 12:21 AM   #10
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Default SURPRISED?

Hi,

Quote:
computer tubes?
Just the 6414.

You seem surprised?...The first computors relied on electron tubes to operate, you know.

I'd rather use shark teeth then those in my gear but that's just me I reckon.

Cheers,
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