5f4 tweed super build?

[chuckb]

Hey guys, I just finished wiring up a 5f4 tweed super and I have no sound. I checked all my solder joints and grounds and also removed the board and everything seems to be ok.I checked and replaced the tubes as well I did have some very low distorted sounds on the normal channels but that's it.I'm using Webers layout, so no bias pot. Any help in which direction to take to track down the problem would be greatly appreciated.

Chuck.

[loudthud]

Do you have a DVM? In cases like this I first check for a short at the speaker jack. This is a little tricky because the speaker winding on the output transformer is almost a dead short. Measure the resistance across the speaker terminals at the speaker. You should get a very low reading, lower than 1 ohm. Now unplug the speaker from the amp and measure again at the speaker. You should see something like 6 ohms for an 8 ohm speaker. Now unsolder one of the wires from the output transformer to the speaker jack. Plug the speaker back in and you should still see the 6 ohms at the speaker terminals.

If that check is ok, (re-solder the wire from the OT) there could be a wiring mistake at the input jacks. Look at my profile for a link to a thread were I outlined a resistance check for the input jacks.

[Chuck H]

There almost has to be a fault somewhere that has been missed. If checking the input and output jacks doesn't reveal anything it would be time to take some voltage readings at the tube sockets.

[chuckb]

Hey loudthud, I checked the reading across the speaker terminals with the speaker plug into the amp and I got a reading of 1.5 to 1.7 ohms. I then unplugged the speaker from the amp and got a reading across the speaker terminals of 7ohm. I then unsoldered one of the wires to the speaker jack on the amp and plugged in the speaker with a reading across the speaker terminals of 7 ohms. I have a very low stactic sound when I strum the guitar plugged into the normal inputs 1 & 2. I get no sound when plugged into the bright channels 1or 2. Where else can I check/ I checked the wiring of the input jacks and I do have them wired correctly.

Chuck.

[Chuck H]

OK... I guess this is the part where the invisible man is supposed to go screw himself?

Voltage readings.

[chuckb]

Hey Chuck, thanks for your reply, I'm in the process of taking the voltage readings. i'll post them as soon as I have them.

[loudthud]

SwitchCraft jacks and the ones made in China have the terminals in different order. No telling what was used on the layout you followed, or what type you used. So you could have followed the layout exactly but the amp won't work. That's why you need to do the resistance check. I see you haven't looked at my profile. Click on my name above my picture at the left and a little menu pops down. Click on View Profile. Scroll down the messages until you find the link to the resistance checks.

[chuckb]

Hey Loudthud, I did all the resistance checks on the inputs and everyone checked out the same with the readings you stated in your profile. I have all switchcraft 12a input jacks . I also did some voltage checks as well here are the results.

Rec tube- p2=355vdc
p8=355vdc
p4=380vdc
p6=381.5vdc

6l6's were the same p3= 440vdc
p4=440vdc
p6=44.5vdc
p2&7=0vdc
p8=0vdc

12ax7 #1 p1=425vdc
p2=0vdc
p3=63vdc
p6=343vdc
p7=30vdc
p8=75vdc

12ax7 #2 p1=0vdc
p2=ovdc
p3=0vdc
p6=380vdc
p7=243vdc
p8=243vdc

12ax7 #3 p1=348vdc
p2=0vdc
p3=3vdc
p6=377vdc
p7=0vdc
p8=1v
I used a 12ax7 in place of the 12ay7 untill I get one.


Chuck.

[Chuck H]

There are several problems I'm seeing.

First, is that 44.5V on pin 6 of the 6L6's actually -44.5V? And is that same voltage present on pin 5?

On the driver/PI tube you have plate and cathode voltage that is much too high on the driver side. There is almost no current going through that triode. Maybe a ground fault or an incorrect resistor value. Looks like cathode and grid voltages are off by a lot on the PI side too.

Obviously NO voltage on one triode of the second preamp tube is wrong. The other triode for that tube has VERY high voltage and no bias differential, indicating no current. I think the bottom of the 100k cathode resistor may not be grounded.

Pin 3 and 8 of the first preamp tube should be electrically connected via a jumper. They should have the same voltage.

I see more that I think is wrong but can't be sure. But you definitely have some miswiring, ground faults or incorrect component values in there. And not just one problem, but several.

[chuckb]

Hey Chuck H, I re-checked the voltages and this is what i have. The first preamp tube (#1) being next to the 6l6 and 2,& 3 following it. This is how I listed the voltages.

6l6,s p6 & 5 =-44.7 v

1st 12ax7- p1=386v
p2=0v
p3=6.1v
p6= 313v
p7=27v
p8= 74v

2nd 12ax7- p1= 222v
p2= 0v
p3= 1.728v
p6= 339v
p7= 222v
p8 = 222v

3rd 12ax7 p1= 306v
p2= 0v
p3= 3.676v
p6= 315v
p7= 0v
p8= 3.637
Tube # 3 has a jumper on pins 3 & 8.

Chuck.

[Chuck H]

Thats a bit better.

1st 12ax7- p1=386v
p2=0v
p3=6.1v
p6= 313v
p7=27v
p8= 74v

Pin 3 is too high. More than twice what it should be. Check the cathode resistor value. The relationship between pins 7 and 8 should be a couple of volts. They're too far off from each other. Not sure why.

2nd 12ax7- p1= 222v
p2= 0v
p3= 1.728v
p6= 339v
p7= 222v
p8 = 222v

Pins 6, 7 and 8 are too high and there is no bias differential between pins 7 and 8. I don't think this triode is conducting. Check the cathode resistor ground.

3rd 12ax7 p1= 306v
p2= 0v
p3= 3.676v
p6= 315v
p7= 0v
p8= 3.637

This looks fine.

[chuckb]

Hey chuck, I checked the cathode resistor value for p3 and it is as it should be. Although I accidentally grounded pin3 of tube #1 and I could hear my guitar loud and clear? I checked the cathode resistor ground for tube#2 because you said that pins 6,7&8 voltages were to high and the ground seems to be fine as well. I re-checked the voltages on these pins (6,7,&8)and the voltages are still the same.Anything else i can try?

Chuck.

[Chuck H]

Make sure the resistor on the cathode follower (pin 8 second preamp tube) is 100k, not 1M. If all is right, test resistance from grounded end of that resistor to the chassis. It should be virtually zero.

Test from the driver/PI tube pin 3 to the chassis. Do you get 1.5k? (not 15k)

Obviously the cathode for the input tube shouldn't be grounded. Why that would "fix" the amp I can't understand since that is the only tube that has correct voltages on it. Do you have grounds daisy chained? Test ALL grounds from the component lead to the chassis. Is it possible that you have flux in your tube sockets? Were any of the tubes especially hard to insert? Are all the tubes known to be good. Not just new out of box, but good?

[chuckb]

Hey chuck, the resistor on the cathode follower is 100k,when testing the resistance from the ground end of that resistor to the chassis the reading is zero. P3 of the PI to the chassis is 1.5k. The 1st tube next to the 6l6 is the tube which when pin3 is grounded that the sound improves. tube #3 was the tube that had the correct voltages. I do have the grounds daisy chained and when tested at the component end I get 0v reading at all points where components are grounded. none of the tubes were hard to insert and I had all the tubes tested and all tested good. I don't believe any flux went into the tube sockets,nothing that i can actually see. I really appreciate you taking the time to help me out here.

Chuck.

[chuckb]

Hey ChuckH,
I put a ground wire on the 1st lug of the presence pot and grounded it to the chassis and it lowered the voltage on p3 of the #1 tube from 6.1v to 4.62v. I now have c guitar sounds from the normal channel 1&2 inputs. P3 of the #1 tube goes to the presence pot and that's what I was talking about earlier when I said that I accidentally grounded p3 of that tube and I got guitar sound. I did have both channels working but now only the normal channel is working. Could it be a problem with insufficient grounding of the pots? Maybe thier not grounding to the chassis well. I'm at a loss to what else to try here.

Chuck.