ab763 reverb squeal and more

[nateflanigan]

Once again, I can't find anything out of the ordinary.  Looking over the bias circuits page in the library I noticed that the hoffman board smooths the - bias voltage after the bias pot while the typical fender circuit smooths it before the pot.  I can't imagine this is causing any problems.

I'm thinking I want to disconnect the bias from the intensity pot and got straight to the 220k resistors.  Maybe even pull all the tubes from the vibrato channel.  I hate to be a wuss, but I wanted to make sure this doesn't seem dangerous to any one.

 

[Fresh_Start]

Nate - what signal are you injecting at the input?  Keep the gain (Volume) low and match the signal voltage coming from your guitar.  Try 1KHz, 50KHz or 100Khz.  

Start signal tracing at the input jack and trace the signal path.  Do not "listen" to points where DC is present (i.e. cathodes & plates).  Listen to input grids, between the preceding coupling cap and the tube's grid.  I don't think you should try "listening" anywhere past the power tube's grids.  IOW stay away from power tube plates // OT primaries.  There are really big voltage & current swings there you don't want to mess with.

I can't tell what's going on at the PI from your description.  To check the input to the PI, test at the junction of the two 220K resistors and the 0.001uf coupling cap.  There are caps on both sides, so no DC.

For the power tube inputs (also PI output), check on pin 5.  That's the control or input grid.  Forget pin 4 (screen grid) and pin 8 (cathode).  Avoid pin 4 (plate) for reasons stated above.  If the signal is clean on pin 5 of each power tube you should be OK.

"Crackle" sounds like there's a problem.  Bad tube, bad solder joint, component burned up by too much heat with soldering iron.  Did you use a heat sink when you were soldering in your components, especially caps?  See the "References" sticky and go through the material on soldering. 

Soldering seems to be like shooting a gun - every American man thinks he's born with the ability and knowledge needed to solder (and shoot) well.  I have learned from experience that it is not true in either arena.  Most of us have to study, learn and practice to perform either task well.

To de-bug the Vibrato channel, you don't have to bypass the Intensity pot.  Just disconnect the lead from the tremolo circuit to the pot.  (I don't connect that lead until initial testing is done.)  Also, ground the grid (pin 2?) of the reverb recovery triode.  That's just what the reverb pedal does.  Now you're listening to the Vibrato channel with no reverb & no tremolo.  Fix those later.

Sorry but I don't have time to re-read this entire thread.  Are you using "known good" tubes?  IOW do they work properly in another amp?

I strongly suggest that you start over again with matching the schematic to the layout with a highlighter, checking every single component and connection in your amp one-by-one.  Yes, it's tedious.  

Use your multimeter to systematically verify continuity at EVERY point in the circuit which should be grounded with the negative lead attached to the safety ground.  If the resistance is more than a fraction of an ohm, there's a grounding problem.

BTW if the Normal channel preamp sounds clean, what's the sound coming out of the speaker(s) like?

HTH

Chip

[nateflanigan]

Nate - what signal are you injecting at the input?  Keep the gain (Volume) low and match the signal voltage coming from your guitar.  Try 1KHz, 50KHz or 100Khz. 

I used a looper pedal strumming an open E

If the signal is clean on pin 5 of each power tube you should be OK.

When using the normal channel it is.

To de-bug the Vibrato channel, you don't have to bypass the Intensity pot.  Just disconnect the lead from the tremolo circuit to the pot.

To clarify, in the Hoffman AB763 old diagram that would be labeled "10".  Or in other words, the wire that connects one of the side terminals of the intensity pot to that last .1 cap in the trem circuit.

Sorry but I don't have time to re-read this entire thread.  Are you using "known good" tubes?  IOW do they work properly in another amp?

My god, that would be absurd, especially on Labor Day.  I can verify the 12ax7's, but right now I don't have access to an amp that uses 12at7's or 6v6's.  I realize that's an important thing to be absolutely certain about, I'll work on it.

BTW if the Normal channel preamp sounds clean, what's the sound coming out of the speaker(s) like?

Well, when using the listening box, clean all the way up through pin 5 on either power tube.

Here's where it get's "interesting"...

Normally, when using either channel the output is very distorted and quieter than I know a medium powered fender should be.

But...

I went ahead with the a fore mentioned plan, that is using the normal channel and hooking the bias directly up to the 220k resistors.  Everything sounds great.  Clean, clear, loud, and very very un-noisy.

So, it seem's the next step is verifying all the tubes to eliminate that variable.  Then hook the bias back up through the intensity pot but otherwise disconnect the trem and reverb.  I'll do that then do a detailed report on the signal trace through the vib channel.

Thank you!

[nateflanigan]

I took my tubes over to a freinds to verify them in his DRRI, all is good.

I disconnected the tremelo and shorted the reverb, then set up my listening box situation again, I think that there's more than one problem to sort out on the vib channel but the first is the preamp stage.

Listening at pin 7, and all the lugs on the various tone pots I have a nice clear signal.  But at pin 2 of the reverb driver (or any of the caps in between) there's a problem.  I would describe it as a sort of clipping, meaning I hear the strum of the chord from the looper pedal starting nice and clear and then it abruptly distorts.  Sort of like "strummmmBZZZ!!"

All I can really think to do is replace everything between that side of the preamp tube and the input of the reverb driver.

Thoughts?

[67polara]

I had a bad solder joint have the same effect.  It looked good but it wasn't, not even sure which one it was I just reflowed them all on 1 tube and problem solved. 

Tony

[nateflanigan]

A few posts back I resoldered the entire board, but I didn't think to do the tubes themselves.  I'll definitely do that before messing around with components. 

Thanks for the tip!

[Fresh_Start]

A few posts back I resoldered the entire board, but I didn't think to do the tubes themselves.  I'll definitely do that before messing around with components. 

Thanks for the tip!

Look really carefully at each tab on the tube sockets too.  I had a tiny whisker of solder - almost invisible - producing an intermittent short on a tube socket that drove me crazy for a couple of hours...

Chip

[nateflanigan]

further down the rabbit hole...

I reslodered all the tube pins, and the lugs on the pots just to be sure.  No Improvement, but I really really studied the schematic and layout and traced the signal coming off of the first half of the vib channels preamp tube.  It sounds great throughout the tone stack, all the way up to the side lug on the volume pot that connects to the wiper of the treble pot. 

But then...

Listening carefully on the volume wiper I could here the clipped signal (I only had the volume at 2-3) as I turned up the volume I could hear the clipping very clearly.  Here's where I get confused, I disconnected the wire going from the wiper to the grid and suddenly I have a very nice loud clear signal at the wiper.

Seems the next step is to replace the wire and verify the signal on the other end before connecting it to the tube.  Maybe the wire got nicked or something.  Despite needing to help a friend hang a set of french doors I should be able to get to that today.  It sure would be nice to get at least one problem ironed out this weekend.

thanks again for everything!

[nateflanigan]

messed around some more with the volume pot wiper/pin 7 connection, replaced the wire, no change.  It's really weird (to me anyways) that when listening to the signal at the wiper with the lead disconnected from the tube it's clear but as soon as I connect it to the tube there's clipping. 
It doesn't seem like there's a whole lot going on in the circuit at that point...

-I'm 100% sure the tube is fine.
-I rechecked the voltages at all of the pins, everything looks good.  Meaning the two halves of the 12ax7 match and I checked the measurements against a few other members charts
-Unless I'm really really failing to read some component values, I'm positive things are set up correctly

-I think I'll pull the tube and check for continuity in the tube socket.  Maybe it got damaged or was faulty, not outside the realm of possibility.
-My only other idea at the moment is to disconnect pin 6 just to try and isolate things further.

[Fresh_Start]

Any chance you mixed up pins when wiring the tube socket?  Lord knows I've done it!

Chip

[nateflanigan]

Hey Chip, how's it going?  I really think the wiring is correct.  I mean, obviously something is wrong and nothing is off the table as far as potential mistakes.

Disconnecting pin 6 didn't get me anywhere.  If anything things sound weirder without it hooked up.  What do you think about jumpering around in the amp? IE sending the signal from the normal channel over to the reverb section and vice versa.  I'm just trying to think of different ways to isolate problems.

[nateflanigan]

Ugggh

I tried an interesting experiment tonight, I connected the output of the normal preamp to the coupling cap that goes to the reverb circuit.  I had good sound going into the reverb driver (where as before it was clipping right out of the vib channel preamp) but I had clipping coming out of the reverb driver.  Which has got to be a separate problem than the first. 

I reconnected the vib channel pre but this time connected pin 8 to the cathode resistor that normally goes with the normal channel (of course disconnecting it from the norm channel's tube).  No change, same old crap. 

I am way burnt out on this.  I think I need to start over.  I feel motivated to find the problem with the vib channel pre, but knowing that there's another problem just like at the next tube is pretty depressing.  Also, the overall construction and lead dress was less than awesome to begin with and having been messing with it, everything feels pretty wild and messy.

I'll let it sit for a awhile, maybe I'll have some new ideas tomorrow.

[Fresh_Start]

Walk away from it for a week.  Play guitar, go sailing/hiking/biking, whatever.

You don't want to hear this, but get a new copy of your schematic.  Assume NOTHING.  Start at the input jack and highlight each and every component and connection on your way to the speaker jack.  Use your DMM to verify, for example, that the Volume pot wiper goes to the next tube's grid pin.  On your way, test the value of every component that you can.  If you can't get an accurate reading with the component in the circuit, lift one leg and get an accurate reading.  You may have fried a component, more likely with a cap than a resistor, while soldering and/or re-soldering.

Did I mention ASSUME NOTHING?  Verify the pin numbers on each and every tube socket.

OH, and get a roll of desoldering braid.  Use it to get rid of as much old solder as possible before re-flowing any solder joints.  Adding new solder to an already bad joint often doesn't help at all in my experience.

You won't figure out what is wrong if you actually start over. 

Remember Hoffman's Rule:  if it were wired properly, it would work properly (or something like that).

I know this already has been a painfully frustrating experience.  It's a complex circuit and there's a LOT that can go wrong.  BTW did you use color-coded wiring or is everything "code blue" (or yellow, or whatever)?  I'm sure the old hands think I'm compulsive, but I know that blue wires are plates, yellow are cathodes, orange (or brown) are grids, red is B+, etc.  It helps me a lot.

Cheers & good luck!

Chip

[nateflanigan]

Insert Quote
Walk away from it for a week.  Play guitar, go sailing/hiking/biking, whatever.

Yeah, you're so right, I can't remember the last time I sat around for a few hours playing guitar on a saturday.