Ab763, suddenly amp plays when volume is off + Hiss. Worked fine before.

[cboysen]

Hi all! I'm really sorry if I've come across a bit too elaborate and needy from earlier posts. I do immensely appreciate your help. Unfortunately I don't have an amp tech anywhere near my town, and thus try to resort to the internet for solutions. 

I've build sluckeys TDR / Single channel ab763 to his specs. It worked perfectly and sounded amazing for a while. Then suddenly it started to hiss. I interpret this as a faulty component, soldering or tube. Even more bewildering, the signal goes through to the master volume knob, even when the regular volume knob is turned all the way down.

I've swapped tubes, V1 socket, pots and checked solderpoints to no avail. All caps and resistors seem to check out ok.

What I have found:
The hiss increases when my hand is near the junction of resistors (3.3M, 220k, 470k) running to input of V3A.
When I connect the 68k resistor going to V1A input, directly to the input of the volume knob instead, the hiss remains, but the signal problem is gone. This also happens when I connect to the input of the tonestack.
When connected normally, the hiss and signal is not affected by the tonestack, but reverb is added, when reverb knob is increased.
I interpret this as a problem occuring before the volumepot, and after the input of V1A, possibly after the tonestack.

But when I connect the 68k resistor directly to V3A input, the master volume exibits the same symptoms. The sound plays when the knob is down...

I'm scratching my head on this one.. Help

Kind regards
Christian

[John]

the signal goes through to the master volume knob, even when the regular volume knob is turned all the way down.

To me, that means your pot is not grounded properly. Connect your ohm meter to lug 3 and the chassis. It should read zero. If it doesn't, there's your problem. If it does, then connect lug 1 (signal in) to chassis and turn the knob. It should go from about a high reading - 500k or 1M depending on your pot - to zero.

[cboysen]

the signal goes through to the master volume knob, even when the regular volume knob is turned all the way down.

To me, that means your pot is not grounded properly. Connect your ohm meter to lug 3 and the chassis. It should read zero. If it doesn't, there's your problem. If it does, then connect lug 1 (signal in) to chassis and turn the knob. It should go from about a high reading - 500k or 1M depending on your pot - to zero.

Hi John, Thank you for the response

It reads zero on ground lug, and anywhere from 0 to 350k on lug 1, depending on the other tone control knobs. Disconnecting the wire from lug 1, it reads 960kOhm.

[John]

That was my only idea.    BUT, if your signal is getting through to the MV no matter what, it's not going to ground at some point where it should be going to ground. That's all your pots do really, is send a portion, or all of, your signal to ground instead of to the next stage.


This is where a schematic really helps for troubleshooting. ;-)

[cboysen]

That was my only idea.    BUT, if your signal is getting through to the MV no matter what, it's not going to ground at some point where it should be going to ground. That's all your pots do really, is send a portion, or all of, your signal to ground instead of to the next stage.


This is where a schematic really helps for troubleshooting. ;-)

http://sluckeyamps.com/tdr/tdr.pdf


Yeah, that's the way I understand it too. I resoldered the pots and ground points, and validated good connection afterwards, without any resolve.

What makes this particularly strange, is the added noise. The amp was dead quiet, and then suddenly it hisses, and starts playing sound when the volume is down. I firstly assumed some sort of interstage coupling, but grid stopping every stage, and resoldering their connections made no difference either.

Furthermore, both the volume and master exhibits these "features", and when soldering the input connection directly to anything after V1A, the signal stops, but the hiss persists.

[John]

I'd take a wooden or plastic stick, and start poking around with the amp turned on. If you hit something that crackles, pops or thumps, that's a place to look. Almost has to be a cracked connection or bad component. Lightly whack all your resistors & caps, too.

[sluckey]

Show us some hi-rez pics.

[cboysen]

I'd take a wooden or plastic stick, and start poking around with the amp turned on. If you hit something that crackles, pops or thumps, that's a place to look. Almost has to be a cracked connection or bad component. Lightly whack all your resistors & caps, too.

The 3 resistors leading to V3A thumbs when tapped with a little force. Also the same resistors, where the hiss increases when in near approximity to my hand.

[cboysen]

Show us some hi-rez pics.

On it.

[cboysen]

2/5

[cboysen]

4/5

[cboysen]

5/5

[cboysen]

These are the resistors that thumps/hisses.

[cboysen]

Tapping Anode, cathode and grid wires near their respective tube pins on V3, yields a crackling/thumbing sound. Indepedant of the tubes in the socket... Faulty socket?

[shooter]

Faulty socket

could be, I'd re-flow everything in the area 1st, I don't know the turret board, are there under-wires in that area?

[cboysen]

Faulty socket

could be, I'd re-flow everything in the area 1st, I don't know the turret board, are there under-wires in that area?

I just don't see how a tube socket could be faulty - I mean, isn't it just metal pins and ceramics?

And no, the only "under-wires" you see, are those from the board to the tube sockets, they all connect underneath, at their respective turret lug. I have checked continuity from all wires to and from the tubes, and everything seems to be A-Ok. I'll try to replace the socket, and reflow the area.

thanks

[shooter]

don't see how a tube socket could be faulty

I've replaced 5 or 6 sockets and problems went away, I don't like to over think things, it just hurts my brain 

[sluckey]

I just read through all your other posts about this amp. I didn't get the idea that this amp has ever worked right. So, did you fix this problem earlier this month and now it has suddenly started acting up again?

[cboysen]

I just read through all your other posts about this amp. I didn't get the idea that this amp has ever worked right. So, did you fix this problem earlier this month and now it has suddenly started acting up again?

Yes. I'm sorry, I'm from Denmark, so my english may not be that clear at times. I'll try to be more precise.

You helped me a lot with mods and what not, which I'm really grateful for, but once I had done all the initial startup tests, and fired the amp up, I realized the sound was quite bland. I removed the post phase inverter master volume, which helped on the tone. Also removed the boost knob, since I didn't find it as useful, as I believed it to be. So I put it back to stock/your diagram, and it worked perfectly. In fact, it sounds/sounded incredible. Then after moving it around a bit, It suddenly started to act up. I spent hours trying to figure out what was wrong, I even moved the entire filter cap section closer to the power transformer. At one point I resoldered nearly all the points on the board, since all the poking around had made them look quite crude. Fortunately it had the side effect of making the amp work again for a few days, when the same problem occured.

When the amp works, it's dead quiet, the tone is tight, clear and very lush, but when the problem occurs, the tone becomes muddy and indistinct, and it hisses quite a bit. 

I've tested all the pots, resistors and capacitors, and everything seem to be fine. But still, when tapping near or at v3 tube socket, it becomes microphonic. I just swapped the tubes AND the tube socket, and yet, the problem remains.

I have never heard of components being microphonic..

What I can't wrap my head around, is how both the volume and master volume knobs test out fine, yet exhibit the same symptom of sound coming through when at zero, as if they are grounded poorly. Also, how come the "wrong" sound doesn't get affected by the tone stack, but is affected by the reverb, yet it's R13, R21 and R20, and V3 cathode, anode and grid wires, including tube and tube socket, that are microphonic. It baffles me.. 

Sorry for being such a noob at this, and again thank you for all your help, it really means a lot.

Kind regards Christian

[sluckey]

The problem with the volume pots is probably a poor ground connection somewhere. You said the wires connecting the tube sockets to the board are soldered to the turret on the bottom side of the board. That's a bad idea. With all the re-soldering you have done on top of the board, it's possible that a connection on the bottom of the board has come loose or is no longer a good connection. I recommend you undo any wire that is soldered to the bottom of the board and reconnect to the turret on the top side of the board. Wrap the wire around the turret for a good mechanical connection, then solder it in place.

[cboysen]

The problem with the volume pots is probably a poor ground connection somewhere. You said the wires connecting the tube sockets to the board are soldered to the turret on the bottom side of the board. That's a bad idea. With all the re-soldering you have done on top of the board, it's possible that a connection on the bottom of the board has come loose or is no longer a good connection. I recommend you undo any wire that is soldered to the bottom of the board and reconnect to the turret on the top side of the board. Wrap the wire around the turret for a good mechanical connection, then solder it in place.

I made the exact same conclusion, once I began meddling with the amp. I just haven't had the guts to redo all those wires, but I will do that then. Unfortunately, I'm running out of solder, so any more work will have to wait.

[sluckey]

Just be patient. I know it's a lot of work. But, you may fix a lot of problems or potential problems. IMO the effort is worth it.

[shooter]

This is just chasing rabbits down holes, but I spend many hours tacking down micro-phonic parts, R's C's, wires,  the problem, the bias was set 2X what it shoulda been, adjusted bias and walla, I don't know why, or care, but I always verify bias 

[cboysen]

Just be patient. I know it's a lot of work. But, you may fix a lot of problems or potential problems. IMO the effort is worth it.

I did what you asked. Wired all ground points above board, and the whole preamp section. A few wires in the power amp section is still under board. Sadly it made no difference.

This is just chasing rabbits down holes, but I spend many hours tacking down micro-phonic parts, R's C's, wires,  the problem, the bias was set 2X what it shoulda been, adjusted bias and walla, I don't know why, or care, but I always verify bias 

I just tried turning the bias down to zero and back up to the set point before. Around 20mA. It made no difference.

I have noticed something interesting, if I do the following:

1) Turn the amp on, with volume off, and master cranked. The looper pedal signal comes through rather full, including a lot of background hiss and microphonic tube/components.
2) Turn the master down/off, crank the volume; The sound is now distorted, obviously, affected by tone controls. Hiss and microphonics are inaudiable/gone.
3) Now the interesting part, when I again turn the volume off and crank the master, the hiss is the same, but the looper signal is faint, like you have turned the bass and mid knobs down.
4) If I turn the amp off, drain the caps, and turn the amp on, we are back to step 1...

Any ideas?

Btw, the hiss/microphonics disappear if I ground the guitar by touching the strings.

[davidwpack]

Lacing between turrets? I've missed a few before.

[cboysen]

Lacing between turrets? I've missed a few before.

I'll check, but considering the amp was functioning correctly before, I doubt it's any miswiring.. but right now anything is on the table..

[davidwpack]

Mine was intermittent for long periods. One day it would sound great. Several days later it would be ear-piercing treble. I gave it the "Fonz" one day with the palm of my hand and the EQ was like"EYYYY. I missed a couple lugs in the tone stack with solder.

[cboysen]

Mine was intermittent for long periods. One day it would sound great. Several days later it would be ear-piercing treble. I gave it the "Fonz" one day with the palm of my hand and the EQ was like"EYYYY. I missed a couple lugs in the tone stack with solder.

Hmm.. I'll look towards the tone stack again.. All the points have been soldered.. several times by now ... The lacing checks out by the way.. So does the entire amp according to sluckeys TDR diagram.


It passes signal to the speaker, even when pulling V3  and cranking the volume with master down. It shouldn't be able to. It is as if the ground wires are being used as a signal path somehow..

[davidwpack]

You can still hear your guitar after pulling V3? I don't know. Pins touching other pins on V3? Arcing?

[cboysen]

You can still hear your guitar after pulling V3? I don't know. Pins touching other pins on V3? Arcing?

Doesn't appear so, I have replaced v3 socket and carefully resoldered all the connections. Didn't solve it. :/

[davidwpack]

I don't know. I looked at the schematic. I would have suspected something on or around V3 or the master volume. I figure some of these guys with more practical experience will have a better idea. I traded most of my IQ points for booze and cigarettes this morning.

[shooter]

guessing you don't have a scope?  So....
try this unsolder whatever is on the treble wiper, unsolder whatever is on the Mvol (top), now temp a wire from treble wiper to MV top, you'll loose gain & tone, but that doesn't matter, yet

[cboysen]

guessing you don't have a scope?  So....
try this unsolder whatever is on the treble wiper, unsolder whatever is on the Mvol (top), now temp a wire from treble wiper to MV top, you'll loose gain & tone, but that doesn't matter, yet

Sadly I don't own a scope.. It would've made this a lot easier.

I actually have the master volume wired as a variable resistor, and not a divider. It just shuffles the signal to ground. So theres just a wire connected from wiper of the master volume to the intersection between C13 and R23.

Anyway, lifting C13 and connecting the wiper of the treble pot to input of master volume as you suggested, turned the master volume into a regular volume knob, with tone controls functioning, no surprise there. Sadly though, turning the master volume all the way down, and sticking my head against the speaker grillcloth, I could still hear sound coming through. I did try to change the master volume pot early this week, it made no difference.

edit: Connecting C13 to Mvol input, wiper to R23, and pot ground lug to bus bar, yields the strangest thing. First of all, with the Mvol down, turning the volume up, makes the signal go through quite a lot. Furthermore, there is hiss, and the best part.. When moving C13/The wire from Mvol Input closer to R13 and C14, the hiss AND the volume increases. .. How in the world can the volume increase by the range of one capacitor to another.. That's some serious coupling..

[sluckey]

How is that buss bar for the pots connected to chassis?

Why did you use a 220K rather than a 100K for V3-1?

[cboysen]

How is that buss bar for the pots connected to chassis?

As per your suggestions/drawings on the Tweed Deluxe lite, the buss bar is connected near the guitar input. Cap D and the preamp ground board also.

Why did you use a 220K rather than a 100K for V3-1?

Just changed it back to 100k.. It was a leftover mod from when the post phase inverter master volume was installed.. 100k sounds better.. My bad

[sluckey]

As per your suggestions/drawings on the Tweed Deluxe lite, the buss bar is connected near the guitar input. Cap D and the preamp ground board also.

I see the black wire headed up from the chassis lug but I don't actually see it connected to the buss. Be sure there is a solid mechanical and soldered connection. This is an important connection. I'd wrap it like you have some of your other buss connections.
 

[cboysen]

As per your suggestions/drawings on the Tweed Deluxe lite, the buss bar is connected near the guitar input. Cap D and the preamp ground board also.

I see the black wire headed up from the chassis lug but I don't actually see it connected to the buss. Be sure there is a solid mechanical and soldered connection. This is an important connection. I'd wrap it like you have some of your other buss connections.

It's wrapped tightly 2/3 of a turn around the buss bar and then soldered. I checked for continuity, it seems good... I'll try to swap the wire for a new one.

Edit: Changing it for a new wire and clean solder points made no difference.

[cboysen]

guessing you don't have a scope?  So....
try this unsolder whatever is on the treble wiper, unsolder whatever is on the Mvol (top), now temp a wire from treble wiper to MV top, you'll loose gain & tone, but that doesn't matter, yet

Sadly I don't own a scope.. It would've made this a lot easier.

I actually have the master volume wired as a variable resistor, and not a divider. It just shuffles the signal to ground. So theres just a wire connected from wiper of the master volume to the intersection between C13 and R23.

Anyway, lifting C13 and connecting the wiper of the treble pot to input of master volume as you suggested, turned the master volume into a regular volume knob, with tone controls functioning, no surprise there. Sadly though, turning the master volume all the way down, and sticking my head against the speaker grillcloth, I could still hear sound coming through. I did try to change the master volume pot early this week, it made no difference.

edit: Connecting C13 to Mvol input, wiper to R23, and pot ground lug to bus bar, yields the strangest thing. First of all, with the Mvol down, turning the volume up, makes the signal go through quite a lot. Furthermore, there is hiss, and the best part.. When moving C13/The wire from Mvol Input closer to R13 and C14, the hiss AND the volume increases. .. How in the world can the volume increase by the range of one capacitor to another.. That's some serious coupling..

The input to the phase inverter C14, seems weirdly microphonic too, including the input wire to the phase inverter. Nothing is touching or arcing around the phase inverter, and changing the input wire, and C14 didn't do much. I suspect it may be a cap somewhere, since when the amp starts up it sometimes makes an odd but faint "tea kettle" sound.

Shooter, did you have any follow up on your idea with attaching the wiper of the treblepot directly to the master volume?

Kind regards

[John]

These are the resistors that thumps/hisses.

If you haven't already, I'd take out those carbon comp resistors and replace with metal film/ carbon film. CC's are noisy/hissy compared to the quieter resistors to start with,and nothing is saying one of them isn't just plain faulty. Also, the cloth covered wire wrapped around the turrets: that's a nice clean connection?


You keep saying you're getting signal thru to speaker. How much? How loud? If you have to put your ear right next to the speaker to hear it, not sure that's a real issue. If it's more than that, then somehow somewhere something is wired wrong or not making good connection. Hoffman's Law: if it was wired right, it'd be working. 

[John]

I actually have the master volume wired as a variable resistor, and not a divider.

If that wiper on your MV isn't making good connection, that could be part of the problem? Maybe? Anyway, bypass the MV: unsolder the wire connected to the wiper (middle lug) and connect it to that R23. See how that sounds. Then, clip that wire directly to your chassis. It should then be impossible for any signal to get thru to speaker.


Oh, and probably it doesn't matter, but I'd wire the MV pot exactly like the schematic.

[Ed_Chambley]

You seem to have your Power Transformer filiment winding with a center tap.  It appears to be connected and you also have balance resistors.  You cannot use both it is either or.  If you are certain which is the center tap, remove it and tape it off.  Measure those Dale balance resistors and is measure ok, turn the amp back on.  If not, replace them.

Sorry, I see there is only 2 green filiment wires.

[Ed_Chambley]

Did you install a RAW pot?  If so, the Bass pot should not ground.  Lug 3 bass pot connects to 1 and 2 of the raw pot via a resistor and then lug 3 of the raw pot makes the tonestack ground.   It looks as if the bass pot is grounded, but also connected with a wire to the raw pot.  It also appears solder is missing off one of the lugs of the volume pot and it appears to have something connected to lug 1 of the bass pot.

[shooter]

Shooter, did you have any follow up

I got lost in the weeds, so no.
If it was mine, I'd make the PA section, including PI work, by work I mean, temp in a jack, connect it to the coupling cap into the pi and put music in the jack, after a couple fun hours listening to music, I'd move 1 stage left and repeat.  I'd also remove the reverb completely for now, give V3 it's own cathode R instead of sharing, before you know it, you're at the input, then add the verb and go gig

[cboysen]

If you haven't already, I'd take out those carbon comp resistors and replace with metal film/ carbon film.

I don't think it's the hissy CC resistors. The amp worked fine and suddenly not. When it worked fine, it worked amazingly well, and barely any noise.

Also, the cloth covered wire wrapped around the turrets: that's a nice clean connection?

I think so? I have checked the resistance with the DMM, and it displays near 0,X at all points from turret top to tube pin.

You keep saying you're getting signal thru to speaker. How much? How loud?

The signal was faint with the Mvol Down, and Vol up, when treble pot was connected directly to the  Mvol, bypassing V1B, V2 and V3. With the amp hooked up normally, the volume is bedroom/apartment playing volume, with the Vol Down and Mvol up, or vise versa.

I actually have the master volume wired as a variable resistor, and not a divider.

If that wiper on your MV isn't making good connection, that could be part of the problem? Maybe? Anyway, bypass the MV: unsolder the wire connected to the wiper (middle lug) and connect it to that R23. See how that sounds. Then, clip that wire directly to your chassis. It should then be impossible for any signal to get thru to speaker.

Oh, and probably it doesn't matter, but I'd wire the MV pot exactly like the schematic.

I'll try that. But it's both the volume and master volume pots that has the same "error". So I'm connecting something else is at play here.

Did you install a RAW pot?  If so, the Bass pot should not ground.  Lug 3 bass pot connects to 1 and 2 of the raw pot via a resistor and then lug 3 of the raw pot makes the tonestack ground.   It looks as if the bass pot is grounded, but also connected with a wire to the raw pot.  It also appears solder is missing off one of the lugs of the volume pot and it appears to have something connected to lug 1 of the bass pot.

I removed the Raw pot, it's just a 25k pot, as a mid control. It's wired as sluckey suggests it. From Right to left: Jack, Volume, Treble, Bass, Mid, Master, Reverb.. 

http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf

Shooter, did you have any follow up

I got lost in the weeds, so no.
If it was mine, I'd make the PA section, including PI work, by work I mean, temp in a jack, connect it to the coupling cap into the pi and put music in the jack, after a couple fun hours listening to music, I'd move 1 stage left and repeat.  I'd also remove the reverb completely for now, give V3 it's own cathode R instead of sharing, before you know it, you're at the input, then add the verb and go gig

Yeah, the thread is getting long, and without you guys having the amp in front of you, I guess it's nearly impossible to say what's wrong. The suggestions are great, and I have figured a lot on the way.

Although I want to mention, that the amp DID work at one point, with this current configuration. That is why it bothers me. I would have accepted defeat in light of my clear lack of knowledge towards amps. But since it worked, I guess I got something right, even if it was just reading robinettes pages and following a drawing.

I had long thought about separating the cathodes from V2 and V3. So I might end up doing that either way.

I'll give your "work your way backwards"-method a go.. and if all else fails, I'll travel the distance, and get an amp tech to look at my work, or lack there of.



SO.. I think I'm gonna call it guys. Many thanks for all the great tips and help, it means the world, truely.

Kind regards, Christian.

[John]

You're probably almost there. For instance, say it's the MV pot that's faulty because the wiper is making not-quite-perfect contact with the carbon strip: that accounts for your signal getting through, and quite possibly is introducing some hissing and crackling. If you take an alligator clip from the middle lug to ground, and that solves your issues, there ya are.


Of course, if it doesn't solve the issue...       But pots do go bad, it's not unusual.

[cboysen]

You're probably almost there. For instance, say it's the MV pot that's faulty because the wiper is making not-quite-perfect contact with the carbon strip: that accounts for your signal getting through, and quite possibly is introducing some hissing and crackling. If you take an alligator clip from the middle lug to ground, and that solves your issues, there ya are.


Of course, if it doesn't solve the issue...       But pots do go bad, it's not unusual.

Well, I did try removing the Master, replacing it with a 47k resistor. Didn't do it.. so .. I'll figure it out eventually :)