AB763 sounds awesome but reverb will not dial down

[zfiles1701]

It sounds great.  Very fendery.  But the reverb will not dial down.  It seems to be on 100% no matter where I have its pot set at. Checking the schem and layout and will keep checking them.  But so far, I can't find any error.  I checked my pot value, ranged it with my DMM, and it seems ok too.  Any input would be much appreciated.

PS  I also found, but fixed a high freq oscillation coming from the vibrato.   Both the layout and the schem show one of the pot wires connected to the bias pot's wiper.  But when I looked at a gut shot of Doug Hoffman's build, the wire was actually connected to the tail end of the bias diode instead.  Moved mine to that position and no more awful oscillations.  Would like other builders to know this.
Now for the reverb prob.

[PRR]

.

[LooseChange]

Nice, PRR.

[zfiles1701]

Reverb pot is grounded.  But, will for continuity to ground when it's dialed down.

[eleventeen]

LMAO, PRR!

[zfiles1701]

LMAO, PRR!

Well that wasn't much help. So, ICTFPOOYP!

[sluckey]

Both the layout and the schem show one of the pot wires connected to the bias pot's wiper.  But when I looked at a gut shot of Doug Hoffman's build, the wire was actually connected to the tail end of the bias diode instead.  Moved mine to that position and no more awful oscillations.  Would like other builders to know this.

The pics you looked at were Doug's "Super Vibrolux". It's a totally different amp from the amp as pictured in his AB763 layout. Similar in ways, but very different in others. If you built your amp according to his AB763 layout, and then moved the Int. pot wire to the bias diode, you probably just lost the ability to adjust the bias of your amp.

Well that wasn't much help. So, ICTFPOOYP!

I don't know what that means. Can you give us some more info? Maybe some hi-rez pics?

[zfiles1701]

If you built your amp according to his AB763 layout, and then moved the Int. pot wire to the bias diode, you probably just lost the ability to adjust the bias of your amp.

Well that wasn't much help. So, ICTFPOOYP!

I don't know what that means. Can you give us some more info? Maybe some hi-rez pics?

Same as this Hoffman pic:
http://www.el34world.com/Hoffman/images/IMG_0633.jpg

It will still bias.  Though when I checked it after your post it was 30ma on the cathodes.  I had it at 45ma and after I moved the wire it went down to 30ma.  But thanks for the info.  The squeel might have been coming from some where else.  Will move it back and see what happens. 

Reverb will now dial down.....And I did not do a thing to it.   I think the pot is bad.  It did get a little jolt that it was not suppossd to get when I fired it up.  I think it's one of those intermittent things.  It will just go to a higher than 1 setting when it feels like it-sometimes near full but not quite.  Happens after it warms up.  The amp?  Sounds awesome-sozo caps and everything I sould think of.
Sort of made up the slang-means nothing.  I had to look up the LMAO.  Hey man.  He was LHAO.

[eleventeen]

Who's the maker of your sheet metal?

[LooseChange]

You got a "JOLT". Like in a shock? Not good!

[zfiles1701]

I did not get a jolt.  There was a slight bizzed in the area.  Forget what happenned.  Something in the area was loose.  Fixed it, but the pot might have been zapped with more juice than it was designed for.

Sheet metal?

Who does Number Two work for?
HKLLSLEO!!!

[HotBluePlates]

When Sluckey asks for hi-rez pics, he meant of your actual build. So that we might give your work a visual inspection to turn up problems.

[zfiles1701]

When Sluckey asks for hi-rez pics, he meant of your actual build. So that we might give your work a visual inspection to turn up problems.

OK.  MIght not be able to get that one till early next week when schedule permits.  Amp is now working great actually.  Cannot believe how incredible it sounds.  Switched that wire back to its original spot and no problems.  Got it real quite too.  The reverb is most likely the pot-will replace it and see what happens.  But, the full on reverb prob has not reared its ugly head after hours of playing.  One thing though.  The grid where I'm at varies from 118 to 132 VAC depending on time of day of course.  I started out with about 495 VDC to my plates early today.  (measured it three times) Said Whoa, let me bias down to a cool 37ma.  Later that afternoon the grid was at about 118 VAC when I was playing.  Does the bias change when the gred changes?  And even with the grid that low my PT felt hot when I was playing full volume after about 2 hours of palying.  The last part of the power supply grounded and burned out the decoupling resistor on the end.  Could the PT now be bad?  The voltages all look good and I cannot find any grounds.  My supply caps do not have continuity across them.  If they were bad, would that cause over heating.  It's such an awesome amp I would not mind getting another PT if I had to.  Maybe one day I'll get it right.
Either way I do appreciate all the advice.

[phsyconoodler]

"Got it real quite too."
  What does 'quite' mean? Do you mean 'quiet'?

Power transformers run hot.How hot is the question.If you can hold your hand on it it's not hot enough to worry.If it's so hot it melts wax,then you've got a problem.

[zfiles1701]

"Got it real quite too."
  What does 'quite' mean? Do you mean 'quiet'?

Power transformers run hot.How hot is the question.If you can hold your hand on it it's not hot enough to worry.If it's so hot it melts wax,then you've got a problem.

Very little hum even at high volume.

I think its over 140 F, maybe closer to 200 F.  General rule is if you can keep your hand on it for about four seconds and then have to let go it should be about 140 F.  When hot, can keep my hand on for one to two seconds and then have to take my hand off it.  A friend and I have built: VOX ac30, PLEXI, AX84 p1x/SEL, and now the AB763.  The last two were on my own.  To be honest, out of all of them, I've felt warm and very warm; but never a power transformer this warm (hot).  If you or someone could tell me if this is normal or not for an AB763 I would very much appreciate it.

[HotBluePlates]

Do you have a rectifier tube in your amp? If so, radiated heat from the recto usually makes the PT seem hot.

What PT did you use? Is this a 2 or 4 output tube version? 6V6 or 6L6?

[zfiles1701]

Do you have a rectifier tube in your amp? If so, radiated heat from the recto usually makes the PT seem hot.

What PT did you use? Is this a 2 or 4 output tube version? 6V6 or 6L6?

Yes have Recto Tube GZ34.  And it's near the PT.  Would say the power trans is 160 to 180 F (possibly a little hotter), comparing it to things at work that are at those known temps.  
P-TF22723  supposed to be good for 40 watts.  Two 6L6.  OT has 4 and 8 ohm taps and it's wired to the 8 ohm tap and has an 8 ohm speaker load.  The OT is just warm after playing loud for a half hour or so.  Maybe I'll just bias it cold as safely possible, and try playing full volume awhile.  And just see what happens.  The concern is having that dreaded Bzzzd-smoke thing and having to $$$ more parts.

MOD: Compared it to a 159 F pipe at work--The PT is not near as hot, so, I would say it's about 140 F maybe a tad hotter.  Could not keep my hand on that pipe for a split second-yeow (as opposed to 1-2 seconds of the PT).  I would not make a good Kung Fu type.

[zfiles1701]

Transformer not hot as I thought.  Forgot I had a thermocouple for my DMM.  Played loud for an hour, got the PT hot as usual, put the thermocouple on multiple points and came up with 140 F max temp.  Electric pump motors run that hot at work too.  Guy who sold me the PT said it should be pretty hot to touch.
Reverb went full on again but only when amp has been played an hour or so (gets pretty warm inside chassis).  Cut right back out when I removed the amps back cover.  Made it hard to diagnose the prob. 

[zfiles1701]

FOUND IT.  The reverb pot's wire to the ground would disconnnect when hot.  It coiled around around the main gournd bus and was soldered, but not good enough.  Amp would get hot, pot would lose its ground, and full on reverb.  Soldered the pot's ground-problem fixed.   Thanks again for the suggestions-they did help.
PS And I'm not trolling for attention either.  Just don't know enough about these things.  This is my first solo AB1.

[PRR]

> reverb pot's wire to the ground would disconnnect when hot

October 29, 2010


> Well that wasn't much help.

Sorry.

[alerich]

Wanna make that reverb even better? Change the reverb intensity pot from 100K Linear to 100K Audio. Trust me on this. You'll love it.