Fender Twin Amp 6G8

[uki]

I forgot to mention, 2 things, 1- the electrolytic cap in the V5 part of the circuit, I couldn't find the 4µF-25v the one there now is 4.7µF-25v  ;  2 - The pot for Speed isn't the 4m as it says in the schematic, but 3m the only one I could find, would that interfere with the vibrato?

Remove that cable and replace it with five individual wires between the board and V5 socket. And don't bundle the 5 wires together. Leave them long enough to keep them separated from each other.

Aw man that was some work to put it there(it is bolted to ground), but let me guess it doesn't help anything with the hum, is that correct? Bundling them can also cause problems? 
What about just remove the outer cover of that cable, it will leave just the 5 wires, that will do yes?

Thanks a lot for all the help !! Already got the caps disconnected.

What next ?

I did recheck all components and connections in that area, they are in a correct position, so it looks like to be a bad component.
With those two caps disconnected there is even more hum, so I take the problem really lies in the V5 part of the circuit.

Wires replaced.

[uki]

Waiting for instructions !   

[sluckey]

I think you have narrowed the hum issue down to the V5 circuit. Use the attached pic to check every component in that circuit. Be sure the values are correct and the solder connections are good and in the right place. Check the ground connection. Check the B+ connection and be sure it connects to the right place on the cap board. Use a chopstick or other insulated utensil to move wires around and tap on components. Any effect on the hum?

I want to see some hi-rez pics of the V5 circuit, V5 socket, and interconnecting wires. I'm talking about the resolution like on your 'missing link' photo, but I want to see it all in one pic if possible. And take several pics from different angles.

I don't understand why the hum increased when you lifted the two caps from the trem oscillator. That may be a clue.

[uki]

I think you have narrowed the hum issue down to the V5 circuit.

Well not without lots of very skilled helpers !! 

After replacing the wires, the hum is the same, louder as before with the caps disconnected, there is also a whistle like sound when turning on/off the standby switch, and then if turning off right the after, the power switch does the same whistle.

... check every component in that circuit. Be sure the values are correct

I was reading resistors yesterday on that section, all looks fine but the two 470k (yellow-violet-yellow-silver) , they are reading 288k each, problem there or this happen because they are connected to other components?
I Don't have how to measure caps, no cap meter.

Use a chopstick or other insulated utensil to move wires around and tap on components. Any effect on the hum?

One wire coming from V5 from P7 does increase hum when touched. Besides that, while chopping B+ turret that is connect to filter cap does amplify a bit the tap.

Check the B+ connection and be sure it connects to the right place on the cap board.

It does go to the right point in the cap board, between those two 27k/1w resistors. It does read about 389-390dcv.... its off right? Or is it because of those detached caps ?

I did read again V5 pins:
P1 331dcv
P2 342dcv
P7 0dcv
P8 5dcv
P2 0-1dcv

[jojokeo]

What a brave build by uki for his experience level. You guys are all saints and should get a 12-pack each at least... (I just breezed over this about 15 mins ago & am surprised it took so long to recognize the filter cap string right at the input jacks - you guys are slippin'  ) With so many lo & hi Z and B+ wires all over the place next to each other it's amazing it's quiet at all? Well done just to get to this point everyone. This is like one big class project?

[uki]

What a brave build by uki for his experience level. You guys are all saints and should get a 12-pack each at least... (I just breezed over this about 15 mins ago & am surprised it took so long to recognize the filter cap string right at the input jacks - you guys are slippin'  ) With so many lo & hi Z and B+ wires all over the place next to each other it's amazing it's quiet at all? Well done just to get to this point everyone.

Crazy stuff isn't it ?!  Yup I did got lots of help, hints and tricks from the good folks around here. I'm too surprised with the result, for a first build and my lack of knowledge about tube amps and electronics.     

 Some shielded wires here and there, some rerouting of those Z(whatever it is) and B+ did the quiet state come about, it is indeed quiet, really quiet(without V5 tube)!! Just the V5 circuit hum to fix and then looks like it is good to gig !!!

This is like one big class project?

Just something I've always dreamed about, but those amps down here where I live are way too expensive, when I've found the schematics online I saw the way to make it happen !! It is been a big learning curve and I'm enjoying it a lot!! Thanks for step by !!

[sluckey]

I want you to reconnect the two yellow caps that have one lead dangling back to the turrets they were originally on. Continue to have V3 and V4 unplugged.

Next I want you to ground V5 pin 2 and pin 7 to chassis ground. Look at the attached pic to see how to do this. You will see three turrets circled in red. Solder two short pieces of wire to connect all three of those turrets together. Does the hum go away?

[uki]

Yes the hum is gone with this connection, standby switch does some pop when turning off, very low volume at VIB channel, I think that is the expected behavior since V3-4 are out.

We are getting very close I take !!! 

[Ed_Chambley]

What a brave build by uki for his experience level. You guys are all saints and should get a 12-pack each at least... (I just breezed over this about 15 mins ago & am surprised it took so long to recognize the filter cap string right at the input jacks - you guys are slippin'  ) With so many lo & hi Z and B+ wires all over the place next to each other it's amazing it's quiet at all? Well done just to get to this point everyone. This is like one big class project?

I did not notice either because I did not consider someone putting one there.  It is quite a project at any skill level.

I cant believe the normal channel is quiet, but if it works it must be ok.  I am surprised, but it looks like he may just get this thing quiet.

UKI, keep on going man.

[sluckey]

Ok, remove those two short jumper wires. Now look at the attached pic and remove the two 1M resistors that are circled in red. How's the hum?

[jojokeo]

What a brave build by uki for his experience level. You guys are all saints and should get a 12-pack each at least... (I just breezed over this about 15 mins ago & am surprised it took so long to recognize the filter cap string right at the input jacks - you guys are slippin'  ) With so many lo & hi Z and B+ wires all over the place next to each other it's amazing it's quiet at all? Well done just to get to this point everyone. This is like one big class project?

I did not notice either because I did not consider someone putting one there.  It is quite a project at any skill level.

I cant believe the normal channel is quiet, but if it works it must be ok.  I am surprised, but it looks like he may just get this thing quiet.

UKI, keep on going man.

This is the same issue that caused hum in Mike's Tweed build recently in that the B+ was right next to his input grid resistors and wiring. However I think uki changed his to using shielded cable on his inputs right?
 

[sluckey]

This is the same issue that caused hum in Mike's Tweed build recently in that the B+ was right next to his input grid resistors and wiring.

We thought that about a hundred posts ago, but it's not the case in uki's amp.

[uki]

Without that two resistors there is a little hum like if a guitar was plugged, not the big bad hum like before, a louder pop noise now when turning off standby switch.

[sluckey]

How does the guitar sound through the vibrato channel?

[uki]

I had one of the wires to V5 a bit loose its soldered properly now. There is some hum not as much as with the components, the VIB channel works, the guitar sound is coming through now, not as before in really low volume.

[sluckey]

Trying to get some kind of benchmark here. Would you say the level of hum is acceptable? Would you say the guitar sounds good through the vibrato channel? Would you say the only issue with the vibrato channel is the fact that the vibrato effect does not work?

[uki]

The hum is loud enough, not as bad as with the components attached, but not acceptable, sounds bad, the guitar sounds good in the VIB channel like it should be, besides the hum.

 I did remove V5 again to compare, very quiet that way, very good.

[sluckey]

OK. Leave those two resistors out for now. Pulling those resistors should have had the same result as disconnecting those two caps. I'm running out of ideas. I suspect a grounding problem in the V5 circuit. Or maybe a poor solder joint. Also, there are jumpers under the board in this circuit. Do you know they are securely in place? Describe how you did the under-board jumpers. Connect a gator jumper lead to the turret that serves as a ground point for V5 and connect the other end to chassis. Any better?

I'd heat up every turret in the V5 circuit and reflow some fresh solder. Make sure the solder connections at the socket are secure also. Make sure the B+ connection to the filter cap board is good. And make sure the negative side of that filter cap is securely connected to chassis.

Tell me what you find.

[uki]

Also, there are jumpers under the board in this circuit. Do you know they are securely in place? Describe how you did the under-board jumpers.

There are 3 jumpers in that section.
I did check those jumpers several times and just now as I read your last post, did the checking with meter for continuity, they are secure, unless this isn't a good way to check. I did plugged the tip of the wire in the bottom end of the turrets and soldered them, I've used the same type of solid wires as all else.

Connect a gator jumper lead to the turret that serves as a ground point for V5 and connect the other end to chassis. Any better?

I tried that and no difference.

I'd heat up every turret in the V5 circuit and reflow some fresh solder. Make sure the solder connections at the socket are secure also.

Did that as recommended but no change.

Make sure the B+ connection to the filter cap board is good. And make sure the negative side of that filter cap is securely connected to chassis.

Checked the connections for those points as well they read good with the meter.
With the digital meter at 200 the connection from the board to ground read 1.2 ohms, that is the lowest my meter can go.
When reading B+ point in the board to ground in ohms, it does close circuit and then start dropping slowly like the a cap been discharged.

Tell me what you find.

Did the chopping again, and there are 3 noise points that does get amplified, all else doesn't do it.
The noise points are the 2 ceramic caps(blue ones) when tapped at the top, the noise does get amplified, and the P1 connection to the 470k and 100k turret.

I did read the electrolitic cap there under ohms, it does give some reading at the max of the meter, did it with the analog one.

Mentioning again that cap is a bit out of specification, schematic says 4µF/25v  the installed one is 4,7µF-25v, also the 470k resistors are reading 288k.

[sluckey]

Mentioning again that cap is a bit out of specification, schematic says 4µF/25v  the installed one is 4,7µF-25v, also the 470k resistors are reading 288k.

The cap is fine. The resistance reading is fine too. When you put your probes across one of those 470Ks if you look you will see that the probes are also across a 100K + another 100K + the other 470K. That adds up to 670K. The 670K is parallel to the 470K so the total resistance calculates to be 276K.

The problem has to be right there around the V5 circuit. I don't think I can do anymore without actually having the amp in my hands. I seem to remember that at one time you were using double sided carpet tape as an insulator under the board? (Or was that not you?) That has been fixed, right? And there is no carpet tape residue on the bottom side of the board?

[uki]

I don't think I can do anymore without actually having the amp in my hands.

Man you did a lot and I'm forever thankful for all your help !! 

I seem to remember that at one time you were using double sided carpet tape as an insulator under the board? (Or was that not you?) That has been fixed, right? And there is no carpet tape residue on the bottom side of the board?

Nop, nothing like that in the amp, nothing bellow the board but some wires and the chassis.

The problem has to be right there around the V5 circuit.

It is probably right in front of us but we can't see it, someone with fresh eyes and mind would help.

A bad component maybe? Maybe the other guys come up with some ideas! 

[tubenit]

I would be inclined to replace all the V5 vibrato components ......  OR  change vibrato to reverb (dwell & reverb pot).


Sluckey's the best there is in trouble shooting help on this forum.  If you haven't goten it fixed with his help, I'd look at plan B.


With respect, Tubenit

[uki]

I would be inclined to replace all the V5 vibrato components ......  OR  change vibrato to reverb (dwell & reverb pot).


Sluckey's the best there is in trouble shooting help on this forum.  If you haven't goten it fixed with his help, I'd look at plan B.


With respect, Tubenit

My plan B is to completely remove the vibrato. I'll look for replacing components for now. Which ones in V5 you think are the more critical to problems ?

[sluckey]

I don't really suspect a failed component, but you never know for sure. There are only 14 resistor/capacitors in the V5 circuit. I would replace all of them. And there's always a chance that the problem will inadvertently be fixed during the process of replacing them.

[uki]

What is the chance of the wires from OT primary(they are going right bellow that area) and that red wire B+  from the diode board and/or even the diode board position be the cause of hum? All that is very close to V5 circuit.

[sluckey]

Sure it could be something like that. It all goes back to the layout issues that have been discussed. I don't know how to fix that. Well, I do, but I know you don't want to do that.

[uki]

A - What would you do?  What do you have in mind?

B - Lets say plan B, I've modified the schematic/layout without vibrato would that work? (Did it as some study few days ago)


(Images fixed)

[sluckey]

B - Lets say plan B, I've modified the schematic/layout without vibrato would that work? (Did it as some study few days ago)

That's not quite what we did. That will leave your vib channel much much weaker than the nor channel. This is what you need to do for plan B...

[uki]

Cool thanks !!
So I did somewhat good redraw, I wouldn't know about that little detail.

It is good to have a plan B, but I'm not at plan B yet, nono, I'll try harder, gonna get some spare components and replace them, one at a time to find out where the problem lies, gonna take a few days until I have those components thou, some break will also be a good thing.

[uki]

I did tried plan B, but it didn't was what I was thinking, so I went back to plan A and thanks to plan B I found a missing link, from a resistor to ground, that didn't make the vibrato to work but it did reduced the hum about 40%, but I'm not fiddling with it for now, gonna just play and enjoy w/o V5 which makes all the hum disappear.



Here a sample of the amp sound !!
! No longer available

[uki]

... it does seem to be finicky about what tube is used in the vibrato circuit. I made a list of 7 brands and only two were usable the other 4 had a loud hum increasing and decreasing with the vibrato. With the right tube it doesn't do it at all....

http://el34world.com/Forum/index.php?topic=20798.msg222410#msg222410

The JAN, and Sovtek were both quiet. I have the Jan in now. The JJ's were the absolute worst they made it into a hum vibrato. China made were next to the worst...

So vibratos are picky about tubes...

After reading this I got a new perspective about the problem in the 6G8 amp, since my amp had the same problem, a bad hum in the second channel "related to the vibrato"... well it was the JJ tubes.... I have replaced all of them(JJs) for old tubes and the hum is completely gone!!

 

Well but the vibrato still not working, the only thing I get is some noise when turning the intensity pot between 9 and 10.

What can I do about it ?!

[EKDENTON]

It was working but with the hum reduced in post 179,  and you haven't changed anything since it was working except for swapping tubes?

[uki]

It was working but with the hum reduced in post 179,  and you haven't changed anything since it was working except for swapping tubes?

Vibrato never worked. 

Haven't changed anything besides the addition of the wire that connect the board to the jack for the foot switch which were missing.
I have swapped all the preamp tubes, they were all JJ(cheap stuff man) now I got only old tubes into it, the tone is way better now, the tube in V5 have less gain than all others, it has no print, it is maybe a 12at7 or 12au7 or something like that for sure not a 12ax7.

The only thing remaining to complete the amp is the vibrato effect !! 

[uki]

 

Hey guys I have managed to get the amp working nicely, no more hum or anything weird going on, like when I've started. 

 Now the last thing to get working is the vibrato,         

The only thing happening is a small noise when turning the intensity pot to max or zero, nothing on speed pot, what to do ? 

Thanks in advance !   

[Willabe]

Hey guys I have managed to get the amp working nicely, no more hum or anything weird going on, like when I've started. 

That's great, what did you do to fix it? 

[sluckey]

Check the voltages on V3, oscillator tube. If pin 1 is not jumping all over the place the oscillator is not working. Check voltages on V4 also.

[uki]

Hi Willabe ! Here how !!

That's great, what did you do to fix it? 

I have swapped all the preamp tubes, they were all JJ(cheap stuff man) now I got only old tubes into it, the tone is way better now, the tube in V5 have less gain than all others, it has no print, it is maybe a 12at7 or 12au7 or something like that for sure not a 12ax7.

See:

I got my cabinet in today. It turned out great. This is a very good tube effects project. it does seem to be finicky about what tube is used in the vibrato circuit. I made a list of 7 brands and only two were usable the other 4 had a loud hum increasing and decreasing with the vibrato. With the right tube it doesn't do it at all. I would recommend it because you can use it with any amp. The cost was pretty low to build because of the small trannies and one output tube. Thankfully Sluckey didn't give up on me getting it fixed and helped throughout the project. Thanks to Sluckey and everyone else that helped out!

The JAN, and Sovtek were both quiet. I have the Jan in now. The JJ's were the absolute worst they made it into a hum vibrato. China made were next to the worst...

@Sluckey here are the readings.

The voltages on V3 and V4:

V3
P1:   111-160vdc (jumping)
P2:   0
P3:   0
P6:   280vdc
P7:   112-160vdc
P8:   61vdc

V4
P6:   248vdc
P7:   0
P8:   61vdc

Ok what next ?

[sluckey]

V3 pin 8 should be more than pin 7. Probably about 165V. Why is it so low?

V4 pin 7 should be some relatively big positive voltage, but less than pin 8. Why is it zero?

[PRR]

> V3 pin 8 should be more than pin 7. Probably about 165V. Why is it so low?
> V4 pin 7 should be some relatively big positive voltage, but less than pin 8. Why is it zero?

Meter loading in high-resistance grid circuits?

May not be a classic 11Meg VTVM meter.

[uki]

V3 pin 8 should be more than pin 7. Probably about 165V. Why is it so low?

V4 pin 7 should be some relatively big positive voltage, but less than pin 8. Why is it zero?

I have no idea   

> V3 pin 8 should be more than pin 7. Probably about 165V. Why is it so low?
> V4 pin 7 should be some relatively big positive voltage, but less than pin 8. Why is it zero?

Meter loading in high-resistance grid circuits?

May not be a classic 11Meg VTVM meter.

What is that   

[uki]

Oh man i got it wrong ... 
V3 P8 have indeed more voltage than P7

I did the readings again:

V3
P1:   105-158 vdc (jumping)
P2:   0
P3:   1.1-1.4 vdc
P6:   277 vdc
P7:   105-158 vdc
P8:   122-193 vdc

V4
P6:   239 vdc
P7:   3.46 vdc
P8:   53 vdc

While reading pins 6 there is a loud noise.

[sluckey]

Go back to reply #159. Did you put those two 1M resistors back?

Read forward from reply #159 to present. Several things were tried to bypass the vibrato circuit. Has all that stuff been returned to original?

[uki]

Yes all that stuff is been put back as original at that time, the circuit is as in the schematic/layout. The small alterations for testing were undone short after the tests. Does the voltages read good ? What should I look after next ?

[sluckey]

Those voltages indicate that the oscillator (V3 and V4) are working. That just leaves V5. And we have been all over V5 chasing the hum issue. I'm out of gas and I'm not gonna go back over that old stuff again. I can't fix it without being able to put my hands on the amp.

[uki]

All good Sluckey !!

Thank you so much for the help !!

[Willabe]

Uki, you have come a long way from when you 1st got here. 

I think you can fix it now. The oscillator/LFO is working and Sluckey said "That just leaves V5."

So focus on V5.

[uki]

Uki, you have come a long way from when you 1st got here. 

I think you can fix it now. The oscillator/LFO is working and Sluckey said "That just leaves V5."

So focus on V5.

Thanks Willabe !! I will keep looking, it is probably something trick and almost invisible that I'm missing...

[uki]

Hey guys !

You know I wont give up on this, I have been playing with the amp, and now that the second channel works, after fiddling with the controls I notice a 2 things:

1- When volume is at zero on both channels there is a low guitar sound coming through the speakers.
2- The second channel when treble is at max and volume at about 8-9 the amp squeals, and the noise change as the pot goes up until 10.

Could this info help solve the problem ?