I think I killed my amp. No output [Resolved]

[casssax]

I've been working on converting a mono block amp into a guitar amp.

It's a 12Watt 6V6 push/pull with a 5Y3 rectifier.

I have another post about the voltage measurements here:
http://el34world.com/Forum/index.php?topic=18972.msg194758#msg194758

My amp was working, but today I have no output at all. If I turn the volume up almost all the way I hear the typical hum of an amp with the volume turned up but if I plug a guitar cord into it and touch the end I hear nothing.

All the tube are lighting up like normal.

There are two possible causes that I know of.

The first:
Last night after I had replaced some resistors (I played the amp and everything was sounding good) I decided to check the voltages again to see if they had changed with the new/correct resistors (they did, the voltages looked much better than they had before). While checking the voltage on the AC side of the rectifier tube my current limiter bulb lit up!     I had the probes on the multimeter hooked up  the wrong way ( the red probe was hooked up to Amps rather than the Volts side  ) I put the red probe in the correct spot and checked the voltage and everything seemed fine until I tried to play the amp this morning and nothing.

The second:
To make matters worse, I put the amp back in it's box and set it up on top of my other amp that I've been using as a speaker and plugged the power cord in. I walked a way for a while (maybe 15 or 20 min) and when I came back and was going to play I realized that when putting the amp back in its box the power switch had been flipped on. So the amp sat with the power on for 15 to 20min with no speaker hooked up to it 

When I did hook the speaker up and turned it back on I found I had no sound coming out.

I don't know which one of these was the cause of my problems. What should I do to figure this out.
I assume it has to be either the tubes or the output transformer (I would be happy if it was just the tubes).

I've been taking a look at the GEO tube amp troubleshooting page on output transformers.
http://www.geofex.com/ampdbug/outtrans.htm

It says to "measure the resistance from the B+ lead to each plate of the output tube(s)". On a 6V6 tube would this be the resistance between pins 3 and 4?
If so, I'm getting readings of 3.26Kohms on both tubes.

Any advice would be greatly appreciated,

Thanks. 

[Willabe]

Last night after I had replaced some resistors (I played the amp and everything was sounding good) I decided to check the voltages again to see if they had changed with the new/correct resistors (they did, the voltages looked much better than they had before). While checking the voltage on the AC side of the rectifier tube my current limiter bulb lit up!     I had the probes on the multimeter hooked up  the wrong way ( the red probe was hooked up to Amps rather than the Volts side  ) I put the red probe in the correct spot and checked the voltage and everything seemed fine until I tried to play the amp this morning and nothing.

With a amp known to be working you don't check voltages with the limiter hooked up. With the limiter hooked up it will give you false readings. You use the limiter to test an old amp or new build for shorts. Once the amp under test passes with the limiter you unplug it and then you can test for ac/dc voltages.

But since you did have the limiter hooked up and as you where trying to get an acv from pin 4 or 6 to ground on the rectifier tube socket, and the bulb lite up that would indicate a short. My guess is because you had the meter probes hooked up for amps, the meter acted like a shunt to ground?

Take the rectifier tube out and check for AC voltage, 1 probe to pin 4 the other to ground, no limiter. Then do the same for pin 6.

Then test for DC voltage on pins 2 and 8, same way, no rectifier tube, no limiter, 1 probe to a pin (1st pin 8, then measure pin 2) the other probe to ground, but set meter for DC.

Post what you get.

The OT is probable fine. As long as you don't run a signal through the amp with the speaker unplugged it shouldn't blow up the OT. 

[sluckey]

Then test for DC voltage on pins 2 and 8, same way, no rectifier tube, no limiter, 1 probe to a pin (1st pin 8, then measure pin 2) the other probe to ground, but set meter for DC.

You must have the rectifier tube plugged in to measure any DC voltages.

It says to "measure the resistance from the B+ lead to each plate of the output tube(s)". On a 6V6 tube would this be the resistance between pins 3 and 4?
If so, I'm getting readings of 3.26Kohms on both tubes.

That reading of 3.26K is correct, but that's not what that procedure means. The B+ lead that procedure refers to is the center tap (brown wire) of the OT primary. Readings would be 150Ω and 140Ω. You have a resistance chart on your schematic. Use it.

Recheck voltages on all tube pins. Also recheck the AC voltages at V5 pin 4 and 6.

According to your schematic you have still not corrected the grid leak bias for V1A and V1B. Need two caps. See my previous post.
 

[Willabe]

You must have the rectifier tube plugged in to measure any DC voltages.

My mistake, I meant for him to check for the 5acv rectifier heater voltage.

[PRR]

> volume up almost all the way I hear the typical hum of an amp with the volume turned up but if I plug a guitar cord into it and touch the end I hear nothing.

The amp *works* from output to a stage before the volume control.

Typically a bad or miswired input jack.

In this case, Sluckey is bugging you about grid-leak bias details which *could* be the problem.
_______________________

Do NOT plug into the AMPS hole on your meter!!! Or rather, when you "must", try to find a way using Volts (drop across a resistor). If you truly must use the AMPS hole, treat the meter as a LOADED GUN. Do what you have to do and then MAKE-SAFE (take the probe out of the Amps hole) before you do ANYthing else.

The AMPS hole is not just a dead-short, it is an expensive short (if the meter is any good it has eXpensive fuses; if the meter is cheap it may die). Aside from any damage it may do to your gear.

[casssax]

Good news! The amp still works and there is nothing wrong with the tubes or output transformer.

I think when I was checking voltages I must have moved some wires around and somewhere (I'm not sure where yet but it looks like it's on the V1 tube, probably where the shielding from the input wire is grounded) the signal was going to ground. I fiddled around with it and now there is signal going to output. I'm waiting on the parts to convert from grid leak to cathode bypass so when I get those I'll clean up some of the bad soldering jobs I did early on in my project and make sure 

I also found that I have a problem with a ground wire (the ground coming from the volume and tone stack), if I touch it with a wooden pointer while the amp is on it makes noise. I think I have a bad solder joint. I wonder if this is why I'm getting some scratchiness on the pre-vol pot?

So, a couple of things to clean up and hopefully everything will be working then. And I learned an important lesson about not using the Amp jack on my multi meter!

With a amp known to be working you don't check voltages with the limiter hooked up. With the limiter hooked up it will give you false readings. You use the limiter to test an old amp or new build for shorts. Once the amp under test passes with the limiter you unplug it and then you can test for ac/dc voltages.

Just curious, the limiter I'm using has a 300W bulb. It's exactly the same as the 'Uncle Doug' version on his YouTube channel. Would a bulb that size throw the reading off by much? In his video he says that the 250W bulb he uses creates about 1.6Ohms of resistance. I posted the readings I got the other night on the link in my first post. Once I get everything fixed up I'll take the readings again without the limiter and see if there is a difference.

Thanks for all the replies.

[HotBluePlates]

Just curious, the limiter I'm using has a 300W bulb. ... Would a bulb that size throw the reading off by much?

Maybe not, but it's a poor choice. Use a 100w bulb.


By design, the lightbulb limiter should reduce current going into the amp, which reduces voltage readings as a byproduct. But if you use a bulb rated much, much higher than the amp's power, you reduce the current-limiting feature, which is precisely what protects the amp from faulty wiring.


So using a too-big bulb actually defeats the purpose of using the limiter in the first place...

[Jack_Hester]

I use four bulbs.  A 25, 40, 60, and finally 100 watt.  Mostly (when work allows) I'm bringing some old amp back to life.  Most of the time, problems show up right off.  If the 25 watt burns bright enough to burn your hand immediately, you don't want to go any further.  If not, by the time I'm up to 60 watts, I'm hearing a working amp that I can try with the guitar.  I'm also hearing any serious hum that the volume control will not reduce.  Caps in the power supply are my first fix, before starting all over again with 25 watts. 

I typically have something else going on, to work on.  So, I will leave the amp on each bulb for 45 minutes to an hour.  By the time I reach 100 watts, I know that I have a working amp.  All wattage bulbs are still available, though your local Home Depot or Lowes may not have them.  They are really pushing LED bulbs, and not restocking others. 

Look for Newcandescent ( http://www.newcandescent.com/ ) bulbs.  I bought some to have on hand when I need new ones.  If I remember correctly, they have a lower power consumption than indicated, and a much longer life than conventional.  Competition for LED's.  I haven't tried them on my limiter yet.  So, I don't know the difference in current draw between one of them, and that of a conventional one.  I can't pull this website up right now, as I'm doing too much on the computer (this site, and loading a couple others), and downloading Windows 10 upgrade.  All on slow DSL. 

Jack