5U4 revisited

8/9/2004 11:04:42 PMBubba

Well I unhooked the electrolytic so nothing was touching the rect tube at all but it's still arching. I tried a 5AR4 as well and it arched. Measure 720 Plate to plate with no tube in there. and 5.6V on the filaments. Somthing isn't right here. Here is the schamatic I'm working from. If it's completly unhooked from the circuit then what could be causing it to draw high current?? I give.

http://www3.telus.net/demos/6V6cathode.jpg

8/10/2004 12:49:22 AMBradF

The only way I can see arcing happening is if the 5 volt filament winding is grounding.Could not get to your schematic page...referred to a Fender schematic...high tension leads to plates and center tap grounded,filaments and their windings float at B+.I would get the caps and their bleeder resistors hooked up before testing the power supply as I suspect no load voltage might be quite high.Ol'Leo used two 350v caps in series with bleeder/voltage dividers across them to get 700v rating!(70mfd with 220k resistors) With the standby switch open these caps stand at 500+ volts! Check and be sure the filament winding and its circuit is not grounding,and make sure the tube isn't damaged (the plates glowing followed by arcing indicates damage),then use caps with high enough voltage rating.Be careful and good luck!

8/10/2004 12:50:06 AMNorm Leal

Could the tube socket have arced over and carbonized? Is the 5 volt filament winding shorted to chassis?

Norm

:Well I unhooked the electrolytic so nothing was touching the rect tube at all but it's still arching. I tried a 5AR4 as well and it arched. Measure 720 Plate to plate with no tube in there. and 5.6V on the filaments. Somthing isn't right here. Here is the schamatic I'm working from. If it's completly unhooked from the circuit then what could be causing it to draw high current?? I give.
:
: http://www3.telus.net/demos/6V6cathode.jpg

8/10/2004 1:00:27 AMBbba

Does the fact that I grounded the center tap on the rectifier (5V) make a difference?

8/10/2004 1:06:56 AMBubba

:Does the fact that I grounded the center tap on the rectifier (5V) make a difference?
:
Is it because I grounded the center tap? Why the heck does hammond give you a center tap for the 5V??? argg

::Well I unhooked the electrolytic so nothing was touching the rect tube at all but it's still arching. I tried a 5AR4 as well and it arched. Measure 720 Plate to plate with no tube in there. and 5.6V on the filaments. Somthing isn't right here. Here is the schamatic I'm working from. If it's completly unhooked from the circuit then what could be causing it to draw high current?? I give.
::
:: http://www3.telus.net/demos/6V6cathode.jpg

8/10/2004 10:06:14 AMNorm Leal

Do not ground center tap of the 5 volt winding. B+ should be taken from the center tap. This is the reason for red plates on your 5U4.

Norm

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::Does the fact that I grounded the center tap on the rectifier (5V) make a difference?
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:Is it because I grounded the center tap? Why the heck does hammond give you a center tap for the 5V??? argg
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:
:
:
:::Well I unhooked the electrolytic so nothing was touching the rect tube at all but it's still arching. I tried a 5AR4 as well and it arched. Measure 720 Plate to plate with no tube in there. and 5.6V on the filaments. Somthing isn't right here. Here is the schamatic I'm working from. If it's completly unhooked from the circuit then what could be causing it to draw high current?? I give.
:::
::: http://www3.telus.net/demos/6V6cathode.jpg

8/10/2004 12:19:05 PMBubba

Thanx Norm I had a brain fart on that one..

:Hi
:
: Do not ground center tap of the 5 volt winding. B+ should be taken from the center tap. This is the reason for red plates on your 5U4.
:
:Norm
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::
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:::Does the fact that I grounded the center tap on the rectifier (5V) make a difference?
:::
::Is it because I grounded the center tap? Why the heck does hammond give you a center tap for the 5V??? argg
::
::
::
::
::::Well I unhooked the electrolytic so nothing was touching the rect tube at all but it's still arching. I tried a 5AR4 as well and it arched. Measure 720 Plate to plate with no tube in there. and 5.6V on the filaments. Somthing isn't right here. Here is the schamatic I'm working from. If it's completly unhooked from the circuit then what could be causing it to draw high current?? I give.
::::
:::: http://www3.telus.net/demos/6V6cathode.jpg

8/15/2004 12:37:47 AMThomas Dermody

The reason for that center tap is to neutralize hum induced from the AC filament supply and to get equal draw from both sides of the high voltage winding. That's a fine set if it has a center tapped filament winding. If they wired the filaments of the 5U4 tube in parallel, you could just pick a pin, and get balanced B+. Wiring them in parallel, though, would require reducing the current carrying capabilities of each filament, because each filament would have to draw half as much current so that they both equal the proper filament current draw. Since the filament is also the cathode, though, they must make the filament heavy for this extra current. To do this and not draw excessive filament current, the two halves are wired in series. It is interesting to note, for those of you who have never seen this before, that when you test a filamentary full wave rectifier on your tube tester, you will see one diode test higher than the other. You may think that this is because one cathode is just worn out. You don't know why one wore out sooner than the other, but it just did. If you reverse the filament connections, though (on my Eico, if you're testing an 80, you'd reverse lever 1 and lever 4--if 1 was in the middle, and 4 was down, 1 would be put down, and 4 to the middle), you'll find that the low reading on one of the plates reverses! Kind of interesting. I didn't know this for quite some time. Well, if you had a sophistocated tube tester that could center tap the filament, you'd get nearly equal readings for the two diodes. Since you have a sophistocated set with a center tapped filament, you can at least enjoy less noise in your audio. Try tapping directly off of one of the filament pins on the tube, instead of at the center tap. You should notice an increase in hum. Amazingly most radios are wired in this inferior, yet less expensive way.

An interesting note to be made is that the 5V4 tube is electrically quite similar to the 5Y3 tube in its pin connections and current carrying capabilities. It differs in that its cathodes are indirectly heated. In many cases not exceeding 400 volts per plate, a 5Y3 tube may be substituted with this tube. This tube alleviates the hum problem and the filament problem by wiring the filaments in parallel, yet keeping their current draw low, as they are not used as cathodes. It then has cathodes wired to one pin. The cathodes are connected to the filament supply, but since both are tied to one side, they will both react in the same way.

Thomas
:Thanx Norm I had a brain fart on that one..
:
:
::Hi
::
:: Do not ground center tap of the 5 volt winding. B+ should be taken from the center tap. This is the reason for red plates on your 5U4.
::
::Norm
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:::
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:::
:::
::::Does the fact that I grounded the center tap on the rectifier (5V) make a difference?
::::
:::Is it because I grounded the center tap? Why the heck does hammond give you a center tap for the 5V??? argg
:::
:::
:::
:::
:::::Well I unhooked the electrolytic so nothing was touching the rect tube at all but it's still arching. I tried a 5AR4 as well and it arched. Measure 720 Plate to plate with no tube in there. and 5.6V on the filaments. Somthing isn't right here. Here is the schamatic I'm working from. If it's completly unhooked from the circuit then what could be causing it to draw high current?? I give.
:::::
::::: http://www3.telus.net/demos/6V6cathode.jpg