Help! I think I f**ked something up...

Trem Abuser · Post by **Trem Abuser** » Sun Apr 10, 2005 2:00 am

OK, with the 3 amp fuse, the fuse is surviving. However, if I turn the amp on and then throw the standby switch, the EL34s heat up super quick (way too hot to touch). In other words, that "glowing red hot" look. So I throw the standby to cool them off.

When I tried to measure the bias, I got a measurement of .37VDC with my Weber bias rite, but I get no change in the bias voltage no matter how I change the bias pot. I did the wiring on my bias rite, so that could be f**ked up as well and I do not trust it.

I pulled the power tubes and measured voltages with just the preamp tubes in. I get a bit different then before:

V1: pin1 = 191V (an increase), pin3 = 0.66 (decrease), pin6 = 191 (same), pin8 = 1.8 (same).
V2: pin1 = 150, pin3 = 1.06, pin6 = 280, pin7 = 150, pin8 = 152 (similar to before)
V3: pin1 = 325, pin3 = 28, pin6 = 175, pin8 = 23 (similar to before)

So I tried putting the power tubes back in and plugging in the guitar for a little bit just to see. It basically makes a loud hum through the speaker, but if I touch the standby switch with my finger, the hum is significantly reduced and you can hear the guitar, but super low volume.

I must have screwed somethin' up. Please help with some suggestions!

Thanks,
Scott

Flames1950 · Post by **Flames1950** » Sun Apr 10, 2005 9:16 am

As far as the bias pot, be sure to check the bottom of the board and make sure the turrets aren't touching. That problem has plagued a number of people lately. If they're touching you'll have to separate them with a Dremel or hacksaw blade.
Incorrect bias could cause a lot of hum so we probably need to deal with tht issue first before moving on to other possibilities.
If you aren't sure you can trust the bias rite, your easiest solution would be to put the one ohm resistors between pins 8 and 1 on the power tube sockets, so you know you can have some faith in what you're reading. Pin 1 will still continue on to ground, the one ohm resistor is just taking the place of the jumper between 8 and 1.

Trem Abuser · Post by **Trem Abuser** » Sun Apr 10, 2005 6:38 pm

Flames1950 wrote:As far as the bias pot, be sure to check the bottom of the board and make sure the turrets aren't touching. That problem has plagued a number of people lately. If they're touching you'll have to separate them with a Dremel or hacksaw blade.
Incorrect bias could cause a lot of hum so we probably need to deal with tht issue first before moving on to other possibilities.
If you aren't sure you can trust the bias rite, your easiest solution would be to put the one ohm resistors between pins 8 and 1 on the power tube sockets, so you know you can have some faith in what you're reading. Pin 1 will still continue on to ground, the one ohm resistor is just taking the place of the jumper between 8 and 1.

Flames, I pulled up the board and looked underneath. The turrets were not touching, and I could easily fit a hacksaw blade between them. (Correct me if I'm wrong, but the only one that isn't jumped is the one most "northerly" on the board, right?)

I don't have any 1 ohm resistors. I'll see if the local electronics store has them tomorrow, otherwise its back to Radio Shack for the crappy ones.

Flames1950 · Post by **Flames1950** » Sun Apr 10, 2005 7:23 pm

(Correct me if I'm wrong, but the only one that isn't jumped is the one most "northerly" on the board, right?)

You got it right.

Trem Abuser · Post by **Trem Abuser** » Sun Apr 10, 2005 8:37 pm

OK, so I don't think its the turrets touching. What should I try next?

Flames1950 · Post by **Flames1950** » Sun Apr 10, 2005 11:39 pm

Let's back up a second so you can clarify something for me. You said that with the bias rite you got .37VDC. But you didn't get a change in bias voltage turning the bias pot. Is it just the reading of the bias rite that did not change, or were you also measuring the voltage on pin 5 and getting no change?
I'd be sure to get some one ohm resistors so we can be sure of the bias readings. Did the bias rite have any one ohmers inside a tube socket when you assembled it that could be removed and used for your output tube sockets in the meantime?
With the power tubes out what voltage reads on pin 5 of each power tube socket (black probe to a mounting hole for ground so you're only putting one hand inside please

) and will it vary at all when turning the bias pot? If you can get it to vary the voltage at pin 5 set the pot to the most negative voltage you can (i.e. -44 is better than -30) before installing the power tubes again. The largest negative voltage is the least current you can set the pot to allow, and the coolest the tubes will run.

Trem Abuser · Post by **Trem Abuser** » Mon Apr 11, 2005 1:51 am

I was reading the bias rite only, I was not reading the voltage on pin 5.

(The bias rite has a one ohm resistor in there, but I'm just going to buy two new ones to put on permanently.)

(I actually have the banana clips, so I can attach the black lead of the meter to the chassis and just use one hand to hold the red lead. And looking into the amp and seeing the one 50/50 filter cap sitting there innocently just to my right, mounted to the side of the chassis, with that nice piece of buss wire between the two positive leads, ready to clean my clock with over 450VDC should I get careless....)

Trying to be as careful as possible, with the power tubes out, I get no voltage whatsoever on pin 5 of either power tube. This sounds like a culprit, eh?

I tried to follow the voltage back through the filter caps. I do have voltage on the 100/82 resistor pair, but I find no voltage whatsoever on the resistor pair in a V-shape directly below them.

Flames1950 · Post by **Flames1950** » Mon Apr 11, 2005 7:56 am

Now we're getting somewhere. It's the resistor pair below the 82K/100K that we're concerned about (two 220K's in a "V") because that's where your bias voltage should go through to pin 5 of each power tube.
And sure enough, now that I go back and look at your pics from the other thread, you're missing the white wire that would connect to the 220K bias resistor to the left of the two 10uF bias diodes. On 50 watt amps there is no tranny tap for the bias; it's typically taken as a white wire that goes over to the standby switch and taps off one of the blue wires. Since those blue wires come right up to the rectifier diodes next to that 220K resistor anyway, you could just add a jumper from the 220K to the first diode to its right, where the blue standby wire connects.
At that point you should be getting bias voltage through the circuit, through the (2) 220K resistors in a "V", and through the (typically) orange and green wires to pin 5 of each output tube. With the power tubes out, you can check and adjust that voltage to the largest negative value as described above, and then be able to put the power tubes in safely and adjust the bias current up to a safe operating value.

Billy Batz · Post by **Billy Batz** » Mon Apr 11, 2005 9:53 am

Its crazy how many times this stuff turns out to be an error like a forgotten connection or mixed up connection and all it takes is a careful look. Im not dissin you Trem Ive done that sort of thing myself many times but I cant say enough how important it is to take a break for a few hours or the day when you get done building an amp so you can come back at it fresh without tunnel vision which makes you more likely to spot something like that. When you spend at least $1k on something if its gonna possibly be damaged you can live with many reasons but impatience is one that will really kill ya. To this day I still take Hoffmans suggestion and grab the schematic and trace every single connection highlighting the schem after its verified.

Flames1950 · Post by **Flames1950** » Mon Apr 11, 2005 10:40 am

No kidding. I still keep all the schematics and information I've got close at hand when working so I can refer to it, even when I'm sure of what I'm doing.
Even then, it's easy to miss a wire or connection, like the screen voltage wire on my McIntyre-mod Bassman. Talk about cold bias and crappy sound........

Trem Abuser · Post by **Trem Abuser** » Mon Apr 11, 2005 2:28 pm

DOH! Something of a Homer moment.

Guys, thanks for the posts. As I stated, I'm so green to this stuff that I wouldn't have known that I was missing the bias wire even though I had taken some time off from the amp before proceeding. While George's instructions are great for the 100W, since I didn't have a step by step for the 50W I was flying blind.

My big problem is that I'm not able to truly follow and understand schematics. I try to pull the schematic out and follow the things around, but when I do not understand how it relates to my amp in front of me, I lose focus.

Anyway, this is a super busy week at work and school for me, so I probably will not get a chance to fiddle with the amp until the end of the week.

But again, thanks, your help is much appreciated. I'll get back to it at the end of the week.

Guest · Post by **Guest** » Sun Apr 17, 2005 2:52 am

Flames1950 wrote:Now we're getting somewhere. It's the resistor pair below the 82K/100K that we're concerned about (two 220K's in a "V") because that's where your bias voltage should go through to pin 5 of each power tube.
And sure enough, now that I go back and look at your pics from the other thread, you're missing the white wire that would connect to the 220K bias resistor to the left of the two 10uF bias diodes. On 50 watt amps there is no tranny tap for the bias; it's typically taken as a white wire that goes over to the standby switch and taps off one of the blue wires. Since those blue wires come right up to the rectifier diodes next to that 220K resistor anyway, you could just add a jumper from the 220K to the first diode to its right, where the blue standby wire connects.
At that point you should be getting bias voltage through the circuit, through the (2) 220K resistors in a "V", and through the (typically) orange and green wires to pin 5 of each output tube. With the power tubes out, you can check and adjust that voltage to the largest negative value as described above, and then be able to put the power tubes in safely and adjust the bias current up to a safe operating value.

OK, I make the jumper from the turret where the closest blue wire from the standby switch attaches to the diode, to the adjacent turret at the bottom of the 220K resistor. I put the one-ohm resistors on pins 1 and 8 of the output sockets (grounding from pin 1).

I go through the voltages with just the preamp tubes in the amp. I get pretty much the same readings I got before (posted above). I can now measure the bias current. I seem to be able to get from about -0.26mV to -0.40mV. The tube on the right reads about 0.02mV greater than the other tube.

I plug the guitar in and get ... a loud hum. No guitar sound at all.

The agony of defeat sets in. I type a post on the Metro forum.

Trem Abuser · Post by **Trem Abuser** » Sun Apr 17, 2005 2:57 am

Last post was me of course.

Dax · Post by **Dax** » Sun Apr 17, 2005 3:15 am

We Need Pics!!! I don't think you need that jumper for a 50 Watter, i had to remove mine at george's request.

Dax

Trem Abuser · Post by **Trem Abuser** » Sun Apr 17, 2005 4:27 am

Dax wrote:We Need Pics!!! I don't think you need that jumper for a 50 Watter, i had to remove mine at george's request.

Dax

Close up of jumper.