Debugging a new biuld

[Slunderfungus]

I'm having a hell of a time debugging my amp on this, the 7th day after finishing wiring it up. I biult a bulb limiter, and repaired my variac( it needed a new receptacle) but absolutely nothing that I did would satisfy the bulb limiter and then I FINALLY found the short. When I originally wired the heater circiut I did so according to the layout, with all the grounds and the like.

When I got my PT it has a center tap so no need to ground at every tube, so I rewired the filaments when I did I accidentally left one of the cross grounds on one of the power tubes at pins1 & 8 that was hidden in thewires. The thing is though right after I removed it my voltage went from around 99v to 6-7 volts. Actually it reads 20 volts when off,9 volts when turned on, and 8to7 volts when swithched off standby to On I mean what now? Could the PT have blown just powering it up through the bulb limiter, a brand new  PT? At this moment I have had the amp on for a couple of hours and the the OT is still cold, PT on the warm side as are the tubes, but far from too hot to touch.
When the power is off it reads around 19-20 volts. These readings are from the fuse, then with the power OFF, I get 102 off the back of the switch, then when I turn the power ON the volts go back to 9 volts. Seriously, what the heck is that about????

It is a Fender 5E4-A Super with 2x6SL7 and1x 6SC7 preamp tubes and I wired in a post PI master volume:

[Geezer]

Have you actually played thru the amp yet? Is there a problem with the sound or tone of the amp (nasty distortion, buzzing or humming, etc?)

As for the voltages, turn the amp on (as if you were going to actually play a song on your instrument, off of "standby" & with all the tubes in the amp, of course), let it warm up few minutes, then take some voltage readings & post them here.

Here is a link to the "voltage results form" >>> http://www.el34world.com/Forum/index.php?topic=4194.0

Geezer

[RicharD]

Are you still hooked up to the lamp limiter?  Voltages will be lower and sounds nastier through a lamp limiter.

[Slunderfungus]

The readings I posted are what I am getting. No voltage anywhere else.

[sluckey]

The readings I posted are what I am getting. No voltage anywhere else.

So you have one meter lead connected to the fuse. Where is the other lead connected? Is your meter set to measure AC voltage? Did you install a 3 wire power cord with green connected to chassis?

[Slunderfungus]

The common lead on the DMM is alligator clipped to the mounting bolt of the PT, and yes I did use a 3 wired cord wit the green(ground) wire soldered to a ring and grounded to the chassis. The DMM is set to AC.

[Slunderfungus]

If anyone knows what is going on here I would greatly appreciate thehelp. I have a lot of time and money tied up in this project. I am hoping that the power transformer didn't fry. Its been on the bulb limiter since its been fired up so I can't see it being fried but staranger things have happened.

[sluckey]

Take it off the light-bulb limiter and plug straight into the wall so you can get some meaningful voltage readings. But first...

Pull all tubes. ALWAYS START THE FOLLOWING STEPS WITH THE POWER CORD UNPLUGGED FROM THE WALL.

1. Connect your voltmeter (set to measure 120VAC) ACROSS the PT primary (don't put one lead on chassis!). Use gator clips for no hands operation. Make sure you have a good fuse then turn the power switch ON. Now, while watching the meter, plug the cord into the wall just long enough to read 120VAC, but no longer than 5 seconds... Hopefully you got 120VAC (or thereabouts). If not, recheck your primary wiring.

2. Next, connect the voltmeter (set to measure 600-800VAC) to pins 4 and 6 of the 5U4 tube socket. Now, while watching the meter, plug the cord into the wall just long enough to read 600-800VAC, but no longer than 5 seconds... Hopefully you got 600-800VAC (or thereabouts). If not, the PT is likely bad or you're doing something wrong.

3. Next, connect the voltmeter (set to read 5VAC) to pins 2 and 8 of the 5U4 tube socket. Now, while watching the meter, plug the cord into the wall just long enough to read 5VAC, but no longer than 5 seconds... Hopefully you got 5VAC (or thereabouts). If not, the PT is likely bad or you're doing something wrong.

4. Next, connect the voltmeter (set to read 6.3VAC) to pins 2 and 7 of a 6V6 tube socket. Now, while watching the meter, plug the cord into the wall just long enough to read 6.3VAC, but no longer than 5 seconds... Hopefully you got 6.3VAC (or thereabouts). If not, the PT is likely bad or you're doing something wrong.

If you got good readings up to this point, plug in all tubes EXCEPT the 5U4. Plug the amp into the wall and verify that the tubes start glowing. Leave the cord plugged in unless the tubes don't glow, or you smell/hear something strange.

This is all the AC voltages checks you need to make. Report your results before moving on.

[Slunderfungus]

Thankyou Sluckey, you got me thinking again. I should have pulled it apart again and started testing in the manner you suggested. I am kind of in a state of lag, having spent the hours with my ass in the chair swiveling from laptop to amp to soldering gun. Add to that the fact that I am disabled from three seperate back injuries, I get fatigued from spasms and the like and some of the most obvious fixes may get lost in the fog. Add to that the fact this is my first ground up custom biuld from scratch I have yet to set up routines in this area of expertise. This is also a biuld into a very small chassis that used o be an integrated amp from the 50s I literally have to remove some parts in order to access others...ie. the pilot light is situated directly above the rectifier, and its a nice heavy duty military surplus pilot light it kind of covers the majority of the tube socket. My readings are as follows:

pins 4&6= 645V

pins 8&2= 5.73V

Heater= 7.2V

Back of the switch= 8 when turned on 90V when switched off I am tempted to say that the switch must have given up the ghost while I was hunting down the shorts that had the bulb limiter lighting up and subsequently took its crap right aboutthe time I found the short in theheater circiut.

[sluckey]

pins 4&6= 645V

pins 8&2= 5.73V

Heater= 7.2V

Looks good.

Back of the switch= 8 when turned on 90V when switched off I am tempted to say that the switch must have given up the ghost while I was hunting down the shorts that had the bulb limiter lighting up and subsequently took its crap right aboutthe time I found the short in theheater circiut.

I still don't know what this means, but the fact that your secondary voltages are good means the primary has to be good also. I really wanted you to do step 1. Is it possible that the switch readings to chassis are really millivolt readings? Do you have that ground switch and cap wired in? If so, remove the cap. It's not needed with a 3 wire power cord.

Does the amp make any joyful noise?

[Slunderfungus]

Step 1. 122V And the original readings of the switch were in voltage. I've never seen anything like that either. I may be green with amplifiers but I have worked on a lot of electrical equipment and never seen a reading like that before. My only conclusion is the switch had to have gone bad while debugging the the board and preamp tubes. I had one discepency on the preamp tubes which is pretty good considering that the schematic calls for 3x12**7 tubes and I used octals, 2x6SL7 and 1x6SC7.  I'll have to try getting a hold of the supplier of the switch. I could go to BiMart and get a good DPST like the standby I have but I went with the rotary 1M on/off/volume because of space constraint. The chassis is 10.5"X6.5" so everything has been carefully mapped in order to fit and try and cut down on heat as much as possible.

[Slunderfungus]

I now have power at the(255V)at pins 8&2 and 48V at the fuse and switch but nowhere else Man, I can't see where I've gone wrong here.

[sluckey]

I now have power at the(255V)at pins 8&2 and 48V at the fuse and switch but nowhere else

I assume the 5U4 is plugged in and that 255v is DC and the amp is plugged straight into the wall? That may be correct IF the STBY switch is open and/or no filter caps are connected to the rectifier. Quit worring with the fuse voltage.

[Slunderfungus]

  I frygot to switch back to DC last night It gots power everywheres now...still no sound, but that should be easy enough to track down...knock on wood.

[Slunderfungus]

I am wondering if I might be needing to change master volumes. I have it wired in right now in what I am told is a Post PI Master volume which seems wrong because it is actually before the PI using a dual ganged pot, and Pre PI is running off the middle post of the treble in a 1M single pot. Well the PostPIMV replaces the two 220ohm resistors and attaches to the two .1 caps that would go to pins 1&6 of the PI. The Fender 5E4-A has those two legs running to pins 8&6 instead of 6&1, then the pin 1 is going to the .02 cap just left of the those two legs. I believe that is what is known as a Cathodyne Inverter. So I am thinking that the PostPIMV is designed to work with a long tailed pair PI. Am I right, and if not where am I wrong? Also if I am right can I run the lead from the pot all the way over to the the .02 cap, or will that not be feasable. I can always go with the PrePIMV if I need to or for that matter just use myWeber MiniMass to get the overdrive at livingroom levels. I just thing the MV would be easier to deal with...less cords and stuff for my cats to play with while I'm setting up to jam out.

[Slunderfungus]

 I have good voltage in my Filter Caps, and my bias circiut. Heaters are all working, but still no sound. In fact the preamp up to the 8uf filter cap is giving good readings until I get to the .01 cap beside it. There the readings start to sky rocket, or are non existant. Below are the readings for the preamp including the PI. Mind you, this is a 12a*7 preamp amp, and I am using two 6SL7,and a 6SC7 tubes in this biuld.

First tube is the PI and is a 6SL7 in order left to right down the board:

pin 4= 73v grid 1
pin 6= 77v cathode 1
pin 5= 301v plate 1
pin 1= 0v grid 2
pin 2= 61v plate 2
pin 3= 0v cathode2
pin 1 goes to the center node of the treble pot, and pin3 goes to righthand node of presence pot

second preamp tube is also a 6SL7:

pin 5= 349v plate1
pin 6= 314v cathode1
pin 1= 0v grid2
pin 3= 3v cathode

pin 1 goes to center node of volume pots, and pins 4,5 and 2 are bridged.

third preamp tube is a 6SC7:

pin 5= 318v plate1
pin 2= 198v plate2
pin 4= 0v grid1
pin 3= 0v grid2
pin 6= 108v cathode

pins 3&4 go to the inputs. These are all the pins from left to right that go to a preamp tube.

[sluckey]

You have several things wrong. See comments in red, particularly that bridge between pins 4,5,2.

First tube is the PI and is a 6SL7 in order left to right down the board:

pin 4= 73v grid 1
pin 6= 77v cathode 1
pin 5= 301v plate 1
pin 1= 0v grid 2
pin 2= 61v plate 2 (too low)
pin 3= 0v cathode2 (too low)
pin 1 goes to the center node of the treble pot, and pin3 goes to righthand node of presence pot

second preamp tube is also a 6SL7:

pin 5= 349v plate1
pin 6= 314v cathode1 (too hi)
pin 1= 0v grid2
pin 3= 3v cathode

pin 1 goes to center node of volume pots, and pins 4,5 and 2 are bridged. (pin 2 and 4 are bridged. DO NOT BRIDGE TO PIN 5!)

third preamp tube is a 6SC7:

pin 5= 318v plate1 (too high)
pin 2= 198v plate2 (pin 2 and 5 should be about the same)
pin 4= 0v grid1
pin 3= 0v grid2
pin 6= 108v cathode (way too high! should only be 2v.)

pins 3&4 go to the inputs. These are all the pins from left to right that go to a preamp tube.

It's very difficult to troubleshoot your amp with the substituted preamp tubes. I had to print the 5E4 schematic and write in the correct tube pins. If you would post a corrected schematic you would get lots more help.