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Boy, do I have my wires crossed or what! Help???


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3/2/2004 4:13 AM
Brian Stear
Boy, do I have my wires crossed or what! Help???
Just got it all put together. I'll swear it looks like the diagram, but I know it's not right ( maybe the fact I get only low level hum and that's all..)I knew when I turned the volumes up and got nothing i was on the right/wrong track!  
 
Here's some of my voltage readings. I KNOW some aren't right!  
 
16Vac on pin 2 and pin 8 on rectifier socket. Also 371Vdc on same pins!  
Same voltage from standby to first 20uf 500V cap.  
10Vdc on pin 8 of power tubes.  
263Vdc between the 5K1 2W resistor and the 22K 1W resistor.  
 
363V to 371V on OT brown and blue wires ( at least THAT seems normal! )  
On 1st preamp tube, 112V on each side.  
On 2nd preamp tube, 168V on pin 1, 204V on pin 2  
 
The only thing I did different was in the picture of Bruce's board, the Hi-V center tap is connected to the first 20uf 500V. In the picture, this has a ground wire going to it.  
I connected it there, but i ran a wire over to the 22uf 63V cap and ran my ground wire there, like the layout sent with the kit.  
Thanks for any and all help. my poor amp could use it! :-) Brian
 
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3/2/2004 4:47 PM
Bruce /Mission Amps

Are you using shielded cable from the junction of the two volume pots to the grid of the 12AX7?  
If so, make sure it's not grounding out the signal by having the shield touch the center conductor anywhere.  

 
You'd still hear audio with this but, 10v on the cathodes is too low so I suspect your power tubes are not turned on all the way at idle. Check your screen voltage on both tube sockets. It should be about 15-30v less then the plate volatge.  
Bruce
 
3/2/2004 5:27 PM
Brian Stear
Hi Bruce, funny you should mention that. I double checked, and I'm only getting 6.4 filament voltage to pin 7, not 2!  
 
So I disconnected the wire from pin 2 and behold, there's my voltage! I must be losing it somewhere.  
 
As far as the shielded wire, I ran it like the pics. I used some heatshrink tubing to cover where the sheild cable connects to the pot so it wouldn't touch the pot, and I put some more on where it connects on pin 2 on the 12AX7.
 
3/2/2004 6:46 PM
Brian Stear
Well, as soon as I reconnected the filament wire, the voltage dissapeared. So I must assume it's going to ground from there?  
 
Bruce, another question. on the layout, there's the wire that goes from the two 220K resistors. Doesn't it connect to the 56K resistor and the 15uf cap, then connect to the right 20uf 500V cap and go to ground? The only thing I did different was I ran my ground wire from the 56K res. and the 15uf cap, and ran a wire from the 20 uf cap over to tie it in.  
 
 
On my output tubes, I get 364DC on pin3, 264DC on pin 4, nothing on 5 and 6 , and of course 10 on pin 8.  
Plus as soon as I connect pin 2, the filament voltage dissapears.
 
3/2/2004 9:59 PM
Bruce /Mission Amps

The junction of the two 220K resistors is grounded at the negative end of the 15-16uF filter cap... that's it.  
They aren't connected to anything else, well, other then the two wires that feed the grids of the power tubes and the two .1uF coupling caps.  
You can't have 264vdc on lugs 4 of the two 6V6s unless something is wired wrong Brian!!  
zero filament vac at lug 2.... are you sure you have the two 100 ohm resistors wired right?  
 
Bruce
 
3/3/2004 12:26 AM
Phil Loarie
"You can't have 264vdc on lugs 4 of the two 6V6s unless something is wired wrong Brian!!"  
 
Hi Bruce,  
Isn't pin 4 of the 6V6 tubes connected to the B+  
between the 5K and 22K resistors? If so, then  
wouldn't 260 ish VDC be normal?  
 
FYI: i have about 21 VDC at the cathodes of my  
power tubes, (pin 8).  
 
Thanks, sorry for the noise,  
-Phil
 
3/3/2004 4:28 PM
Bruce /Mission Amps

[QUOTE]Hi Bruce,  
Isn't pin 4 of the 6V6 tubes connected to the B+  
between the 5K and 22K resistors?[/QUOTE]Yes! That's the big clue that something is wrong.  
[QUOTE]If so, then  
wouldn't 260 ish VDC be normal?[/QUOTE]  
No because because of simple Ohm's law.  
The voltage and subsequent current fed to the 22K resistor (after the 5100 ohm resistor) is the result of the sum of the two preamp tube's idle current and the sum of the two screen's current.  
Nothing else if it's wired right and no tube is bad.  
If each 6V6 screen pulled say, 2ma and each preamp tube pulled say, 2ma that's a total of 8ma.  
8ma across the 5100 resistor would create only a 41v drop.  
If his B+ plate voltage is +360v (supplied in front of the 5100 ohm resistor) then it could only be 360v - 41v or 319vdc at the screens.  
If it's lower then that then one of the screens is pulling way too much current or one of the preamp tubes is biased on full bore drawing way too much current, or there us a small short to ground somewhere after the 5100 resistor that is pulling much more current then the estimated 8ma.  
Plus or minus about 5% to 10% of those base figures is normal.  
 
Bruce
 

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