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Discussion in 'Amp Tech Center' started by rojo412, Aug 8, 2019.
black to ground
red to power tube pin 5
get that # and the rest
Now that you have B+ to the filer capacitors you need to be checking and draining the caps before you are going inside the chassis. Any work should be done with the amp unplugged and caps drained.
Taking voltage reading with amp on. I assume you have read the safety concerns and will apply them to all the steps that are suggested. Be careful.
I'm definitely being as cautious and careful as I can be. And I'm asking what may sound like dumb questions to people who know what they're doing, but I'm still new at this, so I appreciate people bearing with me.
TO VERIFY: Testing the points requires the power to be on and careful probing, but before I solder is when I need power off, caps drained, amp unplugged. Correct?
Black lead secured to chassis
One hand behind your back
Turn on amp
probe pins with red lead
With the bias dialed down to lowest negative, I get it at -63.5 VDC.
Inserted tubes, flipped back on, no limiter light, tested again. Both sides (tube sockets) are the same readings:
3&4: 13ish vdc, then drops steadily
5: -62.8 vdc
Does it matter if standby is on or off at this point? It's been in standby for everything.
And when I turn the amp off, I unplug it and then test the caps, which are steadily dropping to 0 the longer I wait.
put the standby in play position
you have no plate voltage P3 recheck
P2 and p7 will be vac reading
2 & 7: 3.2 vac.
Getting spotty readings on p3, blips but then nothing.
You did reinstall the O/T right?
Where is the O/T center tap connected and what is the voltage there.
Edit: should be connected at the standby switch.
What is the voltage DC on either side of the standby
Here's my earlier findings on the OT:
Red (tap, I'm assuming) connects to the standby. I was trying to get a reading there, but it's wonky. It will show voltage, then drop or just cut out.
That said, there's a grey wire connected there as well, which goes to the other capacitors.
With standby in play position does either side of the switch show a good voltage reading?
No, not a consistent one. It will get some kind of reading then either continue to drop or just say nothing.
Keeping the standby in play
Do you still have B+ at the orange wire on the board?
We need to be able to determine at what point in the schematic we lose B+
Starting at the board follow the orange wire and take readings at every connection up to the standby
a few good pics of this area would be good. Not real close
What kind of meter do you have.
Try putting the tubes in and see if the bulb limiter is still good. Do you have sound with a guitar plugged in. Some meters have difficulty reading higher voltages. Not saying you need a new one just a thought.
Okay, my bad, I had the meter set to the wrong DCV number.
- Checked B+ at orange, 518 DCV
- Checked the #3 pins, 518 DCV with standby in PLAY mode.
- The tubes have been in since I turned the bias down. The limiter hasn't lit since that diode was removed.
- I've gotten a few "POPS" from the speaker when checking points, but nothing consistent there.
- Haven't plugged a guitar into it, no. Was afraid to try that yet.
It probably won't sound good on the limiter but we will know if it is passing signal.
It is making noise through the speaker. I started with the cable and just tapping it, which made noise, then plugged in the guitar and I got some signal as well.
measure resistance in ohms from power tube pin 8 to chassis. Do this on each tube. Looks like you have resistors there but look to be incorrect value for measuring bias easily. They will work tho.
Then measure vdc with the amp on. Set the meter to read millivolts.
we need to bring the bias a little hotter which in turn will bring down your B+ some.
Off: 1.5 ohms on both.
On, play mode (standby off), 6 mV.
Adjust that pot until you are around 20mv, Then recheck plate voltage and cathode mv.
read through the how to bias link above. Use the bias calculator to keep you in a safe place. I may recommend getting to about 60%. You can go a little higher. Those resistors should be 1ohm 1% resistors. The mv reading translates equally to mA. Do some reading and play with the bias. I have a football game to get to! have fun. Good job getting this going.
@rojo412 I have nothing to offer as you have the best amp minds working with you on this project.
But I wanted to say one thing, way to go! You are making progress and I promise some of this stuff will start making sense. Keep it up and you will be like @robrob and @D'tar before you know it!
@D'tar You rock! Thank you so much for helping me here. I hope you have a great time at the game.
- Plate voltage is now 500 VDC
- Cathode is pretty close to 20 mv.
I'll definitely be reading and re-reading the info on that site.
@Preacher I'm certainly hoping that some day, I can be a fraction as good as these guys.