Second channel of AB763 behaving oddly

[evancorylevine]

Good evening!

I just finished an AB763 and the second channel is really quiet unless I bang on my guitar and then it makes really loud fuzzy loudness.

Any ideas?

[shooter]

follow this thread, slowly, n see where it gets you;

http://el34world.com/Forum/index.php?topic=17701.0

[evancorylevine]

follow this thread, slowly, n see where it gets you;

http://el34world.com/Forum/index.php?topic=17701.0

Hi shooter!

I went over everything just like that said, and I came up with nothing. What I'm seeing is that my second channel makes almost no sound unless my EQ knobs are turned up, which amplifies the signal. At that point I can turn up the volume with the volume knob, but it's very quiet. Also, there's no tremolo no matter the settings, but I can see the lamp flashing.

My soldering all looks shiny and happy and there's connectivity everywhere where there should be.

Is it time for me to take voltage measurements? Where should I grab them from? Lend me your wisdom! <3

[shooter]

Is it time for me to take voltage measurements?

maybe, I typically do plate, and cathode on all small tubes + every tap on the B+ rail
which AB763?  Your schematic would help?  I'm assuming dougs twin version
did you put the foot switch in to make trem work, did you verify it switches.
does the reverb channel work normal?

[bnwitt]

Filling out and sharing a voltage chart might be helpful.  I know this is a Weber kit which is the original LDR vibrato circuit but the attached blank chart will work just fine.

[evancorylevine]

Is it time for me to take voltage measurements?

maybe, I typically do plate, and cathode on all small tubes + every tap on the B+ rail
which AB763?  Your schematic would help?  I'm assuming dougs twin version
did you put the foot switch in to make trem work, did you verify it switches.
does the reverb channel work normal?

Hey shooter.

I'm actually building a weber kit. Linking layout and schem.

I tested the foot switch in the amp, although, I didn't check it on its own. The reverb itself isn't hooked up, so I didn't check that. Right now I'm just trying to get a decent signal on the channel.

Where exactly should I measure the voltage? I've looked around and am struggling to figure out what exactly you want me to measure.

Thank you so much!
e

[evancorylevine]

Filling out and sharing a voltage chart might be helpful.  I know this is a Weber kit which is the original LDR vibrato circuit but the attached blank chart will work just fine.

omg. how did I miss this post. On it!

[evancorylevine]

oh gosh. are all these measurements to ground? should I just clip to the case?

[shooter]

all these measurements to ground?

yep, chassis is fine for the black lead.

[bnwitt]

and make sure you have your meter set to the proper function i.e. DC or AC for each reading

[evancorylevine]

alright. hit me with the good news.

[shooter]

good news.

They appear good to me, so it's on to signal path, do you have a listening tool, a scope, a sig-gen?

[evancorylevine]

good news.

They appear good to me, so it's on to signal path, do you have a listening tool, a scope, a sig-gen?

I think I've got most of that. A listening tool could be broken out audio cable with a probe and clip, right? Could a sig-gen be my synth? What's the protocol here?

[bnwitt]

All of your voltages are too low.  Heaters, plates etc.  I seem to remember you were deciding which primary wire to use on that Weber transformer.  I'd say you've got the wrong one hooked up.  Attached is a Twin Reverb voltage chart with voltages taken from the schematic and layout.  Your voltages will probably be a little higher throughout the amp due to higher voltages we have now compared to when the schematic was produced once your primary is wired correctly.  I've put in a line for you to fill in the actuals from your amp once you get the primary sorted out.  your A, B, C and D B+ power rail voltages should be closer to the 460, 458, 450 and 410 of the original design when the amp is on and off the standby switch.

[bnwitt]

What does your outlet house voltage measure with your meter?  After looking at the Weber transformer diagram it looks like your primary only has two choices.  120V or 125V.  Is that right?  Your voltages are about 80% of what they should be. 

[mresistor]

have you tried both of the input jacks on the Vibrato channel ? Both have weak sound? The Normal channel is fine?

[bnwitt]

I don't know how either channel could sound right with 5 volts A.C. on the heaters and only 350 volts on the highest B+ voltage for a twin

[shooter]

5 volts A.C. on the heaters

I think you're right, the other volts should make sound, maybe less than twin, but I've got great roar from 6v6 at 350, and the pre#'s look pretty close to "normal", but 5VAC might take the life out

[bnwitt]

The preamp voltages are way off too.  All voltages are at approximately 80% of the original when you'd expect them to be at 100 to 105% depending on which primary was used.  It's a shame Weber doesn't put voltages on their schematic or layout but their amp is pretty much identical to the original Fender model and Fender's drawings have that information.  That data is what I used to create the voltage chart.

[sluckey]

I have a couple questions...

First, did you take those voltage measurements with the line cord plugged directly into a wall outlet? Not using a current limiter light bulb?

The schematic you posted is for the 6A80 (a 4 x 6L6 amp) but you listed 6V6s in your tube voltage chart. And you listed 12AX7 for V3 and V6, but they should be 12AT7s. So, what tubes do you really have in this amp?

You show -36V on pin 5 of the output tubes. This is typical for 6V6s but 6L6s should be about -50v. So, if you really have 6L6s in this amp they are running extremely hot, maybe even redplating (a bad thing). If the metal inside the tubes is glowing red the tubes are probably shot by now. Hot running 6L6s could be pulling the B+ voltage down, explaining your low voltage readings.

As Barry says, your B+ is low, about 100V too low for a 6A80. If you're not using a current limiter light bulb, are you using a PT that's different from the one Weber includes with the 6A80 kit?

Show us some pics of your amp chassis, inside and outside.

[bnwitt]

Steve, he used the chart I gave him which was created for a BFDR.  That was my oversight.  I'm pretty sure he has 6L6s.  I thought about the limiter too.  Good question

[bnwitt]

Here is a voltage chart with Fender values and his values together

[evancorylevine]

I was measuring through a current limiting bulb. Lesson learned!

I'm out of town. Can't wait to get back home and get this all straightened out!!!

[evancorylevine]

and gosh bnwitt! Thanks so much for the help, with this awesome chart!

For the record, I use male pronouns. But I'm flattered you mistook me for the fairer gender. <3

[sluckey]

I was measuring through a current limiting bulb. Lesson learned!

I'm out of town. Can't wait to get back home and get this all straightened out!!!

OK, that explains the low voltages. The bulb limiter has served it's purpose. Now it's time to plug straight into the wall.

The first thing you should do when you when you plug the amp straight into the wall is pull the output tubes, then adjust the bias pot to give you approx. -50vdc on pin 5 of each output tube socket. Then insert the output tubes and re-evaluate the amp.

Also, refer to the two channels as "Normal" or "Vibrato" rather than "first channel" or "second channel". Doing so will make it perfectly clear which channel you are talking about.

[sluckey]

What I'm seeing is that my second channel makes almost no sound unless my EQ knobs are turned up, which amplifies the signal.

FYI, the tone stack in this amp (or any other Fender TMB tone stack) will give zero sound anytime all three tone controls are set to zero. That's just how it's designed.

[bnwitt]

Attached is a cleaned up layout of the Fender AB763 Twin Reverb Amp with voltages from the original layout and schematic.  FYI there is a discrepancy between the original layout and schematic for V5 pin 8.  The layout calls out +17.0 volts but the schematic says +5.6 volts  The Weber Twin is almost identical to the Fender amp so this should be a good guide.

[bnwitt]

and gosh bnwitt! Thanks so much for the help, with this awesome chart!

For the record, I use male pronouns. But I'm flattered you mistook me for the fairer gender. <3

My apologies, I fixed the posts.  It was the Corylevine in your handle that threw me.  I was thinking Coraline for some reason

[evancorylevine]

and gosh bnwitt! Thanks so much for the help, with this awesome chart!

For the record, I use male pronouns. But I'm flattered you mistook me for the fairer gender. <3

My apologies, I fixed the posts.  It was the Corylevine in your handle that threw me.  I was thinking Coraline for some reason

Hah! No worries! I was hoping it was my feminine energy that brought you to that conclusion. ;)

Alright! Thank you so much for the help, y'all!

I've attached a new spreadsheet with updated values, after biasing to -50v. What mess have I done?!

[bnwitt]

Well I'm assuming your voltages are taken without the tremolo on?  Heater voltages are just a little low.  V5 and V6 voltages aren't exactly right.  How does the amp sound on both channels now?

[evancorylevine]

Well I'm assuming your voltages are taken without the tremolo on?  Heater voltages are just a little low.  V5 and V6 voltages aren't exactly right.  How does the amp sound on both channels now?

It's quite the same sounding as it was. The tremolo was off. The LDR lamp used to flash, but it hasn't been, no matter the settings. Also, clearly, some voltages have dropped drastically since the last test which leads me to fear that I fried something. :'(

[bnwitt]

Well unplug it, drain it and then check the values of the resistors and the proper route and continuity of the wiring going to and from V5 and V6

[evancorylevine]

Well unplug it, drain it and then check the values of the resistors and the proper route and continuity of the wiring going to and from V5 and V6

All the continuity and routing is good. It seems there may be a couple discrepancies in my readouts.

R64 on this schem is reading as 89Ω. R31 and R32 are both reading out as 150kΩ.

[sluckey]

R64 on this schem is reading as 89Ω. R31 and R32 are both reading out as 150kΩ.

That's exactly what you should measure in circuit.

I still don't know which channel you are referring to when you say "second channel".   

[evancorylevine]

R64 on this schem is reading as 89Ω. R31 and R32 are both reading out as 150kΩ.

That's exactly what you should measure in circuit.

I still don't know which channel you are referring to when you say "second channel".   

Thank you for the reminder. My reverb/tremolo channel isn't working properly. The normal channel sounds great.

[evancorylevine]

have you tried both of the input jacks on the Vibrato channel ? Both have weak sound? The Normal channel is fine?

yup

[evancorylevine]

Maybe this photo album could help pinpoint my grave error?

[sluckey]

Are you ready to do some troubleshooting? Gonna use the divide and conquer method. Follow my instructions to the letter and don't read between the lines or assume I meant to say something I didn't say.

First check...

V2 pin 6 connects to a .022µF cap and 100K resistor on the board. Unsolder that wire at the board. Swing the wire around and temporarily solder it to V1 pin 6. When done you will have V2 pin 6 connected directly to V1 pin 6.

Now turn on the amp and plug guitar into VIB channel. Does the VIB channel work now and sound just like the NOR channel? Never mind that there will be no reverb or tremolo.

Report back. Your answer determines what we do next. We can significantly speed up this process if you and I can be online at the same time and you can do this and report back quickly.

[sluckey]

I see you've been on the forum three times since I posted. Are you not interested?

[evancorylevine]

I see you've been on the forum three times since I posted. Are you not interested?

Hey! I'm deeply interested, and very grateful for your generosity! I just had a very busy weekend of gigging, and I'm recovering and doing office labor today.

I'm around today or tomorrow. Thursday, too.

[sluckey]

Let me know what happens when you move the wire.

[evancorylevine]

When I moved that wire, the VIB channel works just like the NOR channel.

[sluckey]

OK. That means everything from the input jacks to the plate of V2B is working. That only leaves V4B and a very few components. Connect the wire back to the .022µF cap and 100K resistor on the board.

Now look at this pic of your intensity pot and switch. Do I see a bare wire connected from your ground buss to the switch? If so, remove that wire. Does the VIB channel work now?

[evancorylevine]

yup. that made it work. without vibrato, it seems, though.

[sluckey]

OK, making progress. Let's move on to the trem circuit.

First, verify that the intensity pot is wired correctly. The left lug should be connected to the ground buss. The center lug should connect to the opto-isolator (let's call it a roach) on the board. And the right lug should connect to the switch on the back of the pot. I could not see the INT pot good enough to verify in your pics.

You said at one time you could see the roach flashing, but later on you said that it had quit. This circuit requires the footswitch to be plugged in to make the roach flash. Or you can put a short jumper across the Vibrato Pedal jack. So, do one of these. Does the roach flash now? Do you have a tremolo effect?

[evancorylevine]

should I reconnect the switch to ground?

[evancorylevine]

the intensity pot is wired correctly.

I was mistaken about the tremolo. It was working with the footswitch connected before I disconnected that ground.

[sluckey]

should I reconnect the switch to ground?

No. There should be no ground connected to that pot switch. That ground wiring error is what was killing your VIB channel. Look at the layout diagram.

[evancorylevine]

This layout has a little arrow coming off that terminal.

[sluckey]

I was mistaken about the tremolo. It was working with the footswitch connected before I disconnected that ground.

Disconnecting that ground wire from the switch has nothing to do with making the roach flash unless you have another wiring error.

Let's stay in sync with each other. Answer these three questions...

Does the VIB channel work now that you removed the ground wire from the switch?
Does the roach flash now?
Do you have a tremolo effect?