Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blind Lemon on October 07, 2021, 04:20:36 pm
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First build in quite a while. Got to thinking about a single channel AB763 so I dialed up the Hoffman layout and used Pro Rev transformers.
Got it all finished up and the startup went really smooth, that great BF tone. (Thank you Mr Fender and Mr Hoffman)
Except ................ At about half volume it sounded gravelly and a little farty. Blocking distortion??????????
Don't know if that's right but that how I proceeded. (Changed out the tubes first to known good ones, didn't help.)
Did almost all the Aiken's suggestions. No help.
I have a 3 way NFB switch on it and had been running it on the normal 820 and for whatever reason decided to change it to different setting and the
less NFB I fed it the clearer (relative) the sound got. Removed the NFB and the gravelly, farty sound went away.
So my question is, "why doesn't my amp like NFB?"
BL
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Hey Blind Melon! Good to see you. Maybe you just need to swap the OT primary plate leads.
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Thanks, been doin great, been retired a year last June and don’t miss it a bit.
New even occurred to me to swap the plate leads, usually it’s the squeal when you know to swap’m.
I’ll give it a try in the morning.
BL
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WOW that was a loud squeal.
I don't think swapping the plate leads from the OPT is the thing to do.
Let me get a few thing s together and gather my thoughts.
BL
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Haha! Well now you know for certain. I always swap the OT plate leads on a build with NFB just to be certain. It's usually obvious with AB763 amps but some amps it's much more subtle.
Since your amp sounds better without NFB, I'd start by closely examining the PI cathode circuit for wiring error or incorrect valur component. A very popular mistake to make on this amp is to use a 47K or even 470K resistor rather than a 470Ω resistor for the cathode resistor.
Are you using Hoffman's layout?
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Where on the chassis did the NFB switch get put?
How about just disconnecting all wiring to the switch, and putting it stock?
To ensure NFB rather than PFB, just need a sig gen and scope.
The primary leg that’s in the same polarity as the secondary ’hot’ output/s connects to the ‘upper’ output valve on the schematic (make sure not to swap the phase splitter outputs around).
Other than the AB165, all regular valve guitar amps follow that same pattern.
https://el34world.com/charts/Schematics/files/Fender/Fender_pro_ab763_schem.pdf
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Here's a couple pics of what I got. AB763 with a SS/Tube rectifier switch and NFB multi tap switch. (Robinette)
Put in a dropping resistor because in SS mode the voltages were too high. It makes the same noise both ways so I jumped the dropping
resistor and leave it on the tube rect for the stock Pro voltages. Hope the way I explained that made sense.
When I was chasing the blocking distortion issue (?) I swapped out some caps and grid resistors. (Aikens)
Everything is back to the way the amp started out life.
When I got it back together and disconnected the NFB switch and sub'ed in an 820 NFB resistor it still made the gravelly farty soundand if I remove NFB altogether it still made that noise.
Soooo, I think I went down the wrong rabbit hole
the noise is most noticeable at half volume playing an F note at the 13th fret.
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Oh, and I pulled the tubes for the trem and reverb so they weren’t contributing to the sound
BL
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Could a recording of the noise be made?
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Give me a bit, getting a haircut.
BL
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Not sure how to upload it, can't do a .mov.
BL
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Try YouTube then copy a link to the thread.
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Not sure you'll get anything out of this.
Toward the end on the low E you can hear it.
Its different in person.
https://youtu.be/NNMqbsX7UlY (https://youtu.be/NNMqbsX7UlY)
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Part of that sounds like vibration from something in the amp?
I can hear some fizz/fuzz also, but much more vibration sound.
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It sounds more mechanical than electrical to me. Maybe a baffle board rattle?
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Ditto
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Chassis is isolated setting on the bench and happen in two different areas of the room.
I thought it might be a speaker at first but played through two different speaker cabs.
Same story.
BL
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What is the switch by the rectifier tube socket? Is that for switching between tube/SS rectifier? I can't really see it, but it looks like you have 4 x 1N4007's on the socket?
And you have 2 switches on the front of the chassis on the power amp end, power and standby?
Are you sure you have all that wired up correctly?
If you haven't seen this yet, here's a great way to double check your builds wiring from Doug;
https://el34world.com/Forum/index.php?topic=17701.0 (https://el34world.com/Forum/index.php?topic=17701.0)
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"What is the switch by the rectifier tube socket? Is that for switching between tube/SS rectifier? I can't really see it, but it looks like you have 4 x 1N4007's on the socket?"Bingo
"And you have 2 switches on the front of the chassis on the power amp end, power and standby?"Correct again
"Are you sure you have all that wired up correctly?"Have a look, I am filled with al sorts of doubts right now.
On "off" it tube on "On" its SS. Its my understanding with the switch to ON the diodes have a lower resistance path.
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The yellow wire going from the rectifier pin 2 to the switch, that should be moved to pin 8.
Pin 8 is the rectifier tubes cathode connection.
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I'll move it, but electrically it the same??
BL
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I'll move it, but electrically it the same??
BL
What’s the rectifier type?
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maybe a dumb question but: are your phase inverter ground and output jack ground on the same ground node?
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64 - 5U4
Chassis ground different points.
BL
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I'll move it, but electrically it the same?
Only on some rectifier tubes.
Some rectifier tubes have an indirectly heated cathode, 5AR4/GZ34 and 5V4. They have a heater and a cathode. A 5Y3 and 5U4 have directly heated cathodes, the heater is the cathode.
With the 5 volt indirectly heated rectifier tubes we use, they use pin 8 as the cathode connection. Pins 2 and 8 are the heater connections. But pin 8 is shared by the cathode and 1 end of the heater. So you want to pull the B+ current from pin 8, not from the heater, pin 2.
If you try and pull the B+ current from pin 2 with an - indirectly - heated rectifier tube, then your trying to pull all the B+ current through the heater, instead of from the cathode. And that heater is not made for that current draw.
So if you want to be able use different rectifier tubes, then the B+ should always be taken from pin 8.
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Put in a dropping resistor because in SS mode the voltages were too high. It makes the same noise both ways so I jumped the dropping resistor and leave it on the tube rect for the stock Pro voltages. Hope the way I explained that made sense.
So now it makes the same noise when your using either the tube rectifier or the SS rectifier?
Does it make the noise with the tube rectifier with that gator jumper removed from across the metal heat sink R?
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And now that your sure you have the phasing correct on the OT's primary wires, I'd shorten that brown OT primary wire a good inch or so, to get it away from the other power tube socket and the heater wires.
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I'd also try lifting the grid stopper R's on the power tubes, pins 5 and 6, up in the air above the tube sockets.
Then lift those 2 blue input grid wires up in the air to get them away from the screen grid wires and away from the yellow -FB wire going to those 3 -FB R's.
See if that helps any.
Edit; Those power tube grid wires are pretty long, grid wires are sensitive to picking up noise and causing problems. You did twist them up nice and that will give them some protection, as they are out of phase from each other. This is good.
But input grid wires should be kept away from output plate wires because the input is out of phase from the output of a tube. And screen grids are also out of phase with the input grid, screens are in phase with the plate. The screens have way less current than the plates, but I still would keep the grid wires away from the screen grid wires. And running them parallel to each other is the worst way to run them for lead dress. The least interaction with each other would be at 90 degrees.
You might end up having to use shielded cable for those 2 grid wire runs.
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When I got it back together and disconnected the NFB switch and sub'ed in an 820 NFB resistor it still made the gravelly farty sound and if I remove NFB altogether it still made that noise.
So at 1st the noise went away when you disconnected the -FB, but now it it makes the noise with or without -FB hooked up?
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Since your amp sounds better without NFB, I'd start by closely examining the PI cathode circuit for wiring error or incorrect valur component. A very popular mistake to make on this amp is to use a 47K or even 470K resistor rather than a 470Ω resistor for the cathode resistor.
Did you do this yet?
Are you using Hoffman's layout?
:dontknow:
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Put in a dropping resistor because in SS mode the voltages were too high. It makes the same noise both ways so I jumped the dropping resistor and leave it on the tube rect for the stock Pro voltages. Hope the way I explained that made sense.
So now it makes the same noise when your using either the tube rectifier or the SS rectifier?
Does it make the noise with the tube rectifier with that gator jumper removed from across the metal heat sink R?
Makes the same noise it all Rect configs. Have not moved to pin 2 yet.
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I'd also try lifting the grid stopper R's on the power tubes, pins 5 and 6, up in the air above the tube sockets.
Then lift those 2 blue input grid wires up in the air to get them away from the screen grid wires and away from the yellow -FB wire going to those 3 -FB R's.
See if that helps any.
Edit; Those power tube grid wires are pretty long, grid wires are sensitive to picking up noise and causing problems. You did twist them up nice and that will give them some protection, as they are out of phase from each other. This is good.
But input grid wires should be kept away from output plate wires because the input is out of phase from the output of a tube. And screen grids are also out of phase with the input grid, screens are in phase with the plate. The screens have way less current than the plates, but I still would keep the grid wires away from the screen grid wires. And running them parallel to each other is the worst way to run them for lead dress. The least interaction with each other would be at 90 degrees.
You might end up having to use shielded cable for those 2 grid wire runs.
Will do
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Since your amp sounds better without NFB, I'd start by closely examining the PI cathode circuit for wiring error or incorrect valur component. A very popular mistake to make on this amp is to use a 47K or even 470K resistor rather than a 470Ω resistor for the cathode resistor.
Did you do this yet?
Yes PI values are correct, double checked
Are you using Hoffman's layout?
:dontknow:
Yes, Hoffman layout except moved bias pot and stretched board in the middle to get componentsmore in line with tube and to keep wires running to tubes shorter.
BNL
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Not sure you'll get anything out of this.
Toward the end on the low E you can hear it.
Its different in person.
https://youtu.be/NNMqbsX7UlY (https://youtu.be/NNMqbsX7UlY)
Does it just happen when playing on the low E string? Have you tried playing through a different amp? It sounds like a mechanical rattle to me. Maybe string or bridge saddle rattle? There wouldn't by chance be a drum kit in the room? I've heard similar sounds coming from a snare drum.
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It was first noticed using another guitar and was reproduced through a couple different speaker cabs.
No drums.
BL
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I've seen chokes and transformers develop rattles. When I was messing around with cars, I would use a long socket extension to isolate noises such as stuck valves/lifters to the offending cylinder by holding one end next to my ear and the other against the engine rocker cover. Maybe you could try something similar with a non-conductive material such as a wooden dowel rod to rule out or isolate any extraneous mechanical noise.
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Wish I had a scope to isolate it to a certain part of the circuit. I made the changes suggested above to no avail. Even rigged up a listening amp to try and find it and it didn’t show where it was.
God, I don’t want to have to pull the board to check the underside wiring.
BL
Hey and I haven’t said it yet but I really do appreciate all the input.
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We've seen several amps that had a bad silver mica treble cap in the tone stack, or had a bad ground connection that acted like yours.
I'd try changing out the 250pF TS treble cap. Clip a heat sink on the caps lead when soldering and get in and out quickly.
Do your chassis grounds have a good internal star washer between the chassis and the terminal strip?
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Swapped out 250pf and checked grounds. No love.
Going to peel the board back and see if this could have something to do with it.
BL
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WAIT!!! Don't pull the board yet.
See attachment. You used a 10Ω where a 1M should be. Looks like you did the same for the two 1Ms in the PI circuit too. Confirm and replace.
If we had hi-rez pics like your last one we would have been through days ago. :icon_biggrin:
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I don't know Sluckey that third band looks green to me. But at the top of that 1M resistor is there a connection underneath the board?
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Yeah, probably is. :embarrassed:
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I see Brown first band, Black second band, Blue third band, and Gold fourth band. This would correlate to a 1 Meg, 5% resistor. IDK as to underboard connection.
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Looks green to me when l enlarge it, kind of blue when not.
It’s usually a simple error in these cases and sometimes can be very hard to find.
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I see Brown first band, Black second band, Blue third band, and Gold fourth band. This would correlate to a 1 Meg, 5% resistor. IDK as to underboard connection.
Brown/Black/Blue is 10Meg
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I see Brown/Black/Green.
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Will someone please put an ohm meter on this resistor? :l2:
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I see Brown first band, Black second band, Blue third band, and Gold fourth band. This would correlate to a 1 Meg, 5% resistor. IDK as to underboard connection.
Brown/Black/Blue is 10Meg
So it is. I get confused with multipliers and the second digit.
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I don't know Sluckey that third band looks green to me. But at the top of that 1M resistor is there a connection underneath the board?
Brown/Black/Green 1 Meg, no under board. Under board is between the .1 and 47K.
BL
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Will someone please put an ohm meter on this resistor? :l2:
.992M
BL
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:thumbsup: I really didn't think you messed that up.
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Hell, I messed something up. :laugh: This thing is kicking my but.
I was able to make that change without lifting the board.
Here is the change.
I'm going to post some highres. of the guts.
BL
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First set:
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#2:
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#3:
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Ok the gold 25W dropping resister is bypassed because I wanted to take it back to stock voltages through the tube rectifier.
The amp makes the same distorted noise in any rectifier configuration.
The amp sounds clean with no distorted noise with guitar volume full up and the amp at about 10 o'clock.
Found the distortion with reverb and Trem operational, removed those tubes to rule those areas of the circuit out.
Same distortion whether tubes are in or out.
BL
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For good measure here's the transformer side.
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> that third band looks green to me.
Go solar! Take it out in the sunlight! No indoor light is full spectrum (even the ones that claim they are). Incandescent is blue deficient ("is it green or is it blue?"). "Warm LED" is tailored to mimic incandescent, but has different flaws. Most fluorescents are really just a couple color spikes. A mix of incandescent and "daylight" fluorescent is not too bad, but both technologies are going out of style. I had large skylights in my last work place and it made a REAL difference in color reading.
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Some Voltages:
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I don't see any red flags in your voltage chart.
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I didn’t think so either, nothing that would cause concern.
I have another OPT I’m going to slave in tomorrow.
BL
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Not the OPT.
BL
Starting to get frustrated.
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hey, took a short break to check in. It sounds to me like it could be ultrasonic oscillation. A scope would confirm. In lieu of that. Try connecting a 100pF capacitor across the phase inverter plates and see if that helps.
It would be best to actually compensate the output stage/feedback loop, but this usually helps as a generic value for audible instability
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Gave it a shot, thx for the suggestion.
BL
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Wha' happened?
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In the middle of the picture, the black wire coming in from the bottom right at the filter cap ground terminal looks like it might be a cold solder joint...maybe reflow that and double check all grounds for solid connection. I've had cold solder joints do very strange things. Also, the black wire wrapped around the white and yellow wires in the tone circuit looks like it is grounded at two places, or is that in the Bright circuit? Maybe you've got a ground loop issue?
(https://el34world.com/Forum/index.php?action=dlattach;topic=28060.0;attach=94511;image)
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Sorry guys, I took a little break from this. Had some other things to get done and thought I might have a fresh perspective when I got back.
Premiumplus: Thanks for taking a look at the pics, I’ll reflow that joint.
BL
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Here we go, got a little time, have messed with it a little to no avail.
Still has a trailing distortion at about 1/3 volume and above.
Seems kind of drastic ....................... RIP THE GUTS OUT
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I feel your pain. :cussing:
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Blank Slate. Not going to use any of the old components.
Going to use more spacing and the bias supply will be more traditional and off board.
BL
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More
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I see from your pics that you are pressing the turrets the hard way. Put the pointy tool in the vise and mount the other piece in the drill chuck. Now you can load every turret into the board without them falling out. And you can go lickity split with the press. Watch Doug's short video...
&t=290s&ab_channel=EL34XYZ
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Blank Slate.
fwiw;
did you do continuity tests on your material?
sneaky currents can cause complex problems
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slucky: I've seen that before and need to change my setup.
Shooter: Man, I've built an awful lot of amps out of this stuff. I would really doubt it. But thx for the
the suggestion. All suggestions are welcome.
BL
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Boards ready to populate:
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I used under board wiring in my amp. Without easy access to the underside of the board, i will never do it again
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I used under board wiring in my amp. Without easy access to the underside of the board, i will never do it again
If you do it like Blind Lemon did you never need access to the underside again.
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Populated:
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I’ve always used under the bd wiring, usually solid core, 20g. If you bend the wire over the turrets, you lock it in, like in BM’s new bd. If l have a low clearance bd like in an AO-43 chassis, l use a few dabs of silicone if l think a chance of snagging under the bd wires.
I feel for BL, probably something simple that can be very frustrating and difficult to find. I came close to doing what BM’s doing but so far found the culprit, usually something simple. I feel for ya..!
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On the under board wiring I usually weep a little thin CA under the wire to hold it down (up).
BL (not POOP :l2: )
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On the under board wiring I usually weep a little thin CA under the wire to hold it down (up).
BL (not POOP :l2: )
You got me, "Bowel Movement", I was thinking Blind Mellon, :laugh: Just showing my age, sorry about that
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WELL HELL :sad2:
Same issue. All new board and components. New wiring to the tubes except heater wiring. I use 18g heater wiring and I re-flowed all solder joints. Pulled all the pots to double check values and wiring, all controls work as they should. I am thinking of rewiring the rectifier tube socket and rerouting the wiring, although I have wire rectifiers like this before. Disregard the diodes at this point. Probable move the grounds around on the power transformer.
Don't know if I mentioned it previously but I have also slaved in a new OPT.
BL
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Also, this is built in a Twin chassis, I have an old Pro Reverb cab and wanted to be able to use it. Pro and Twinn reverb have the same size chassis and I couldn't come up with a suitable Pro Reverb chassis so I got the Twin chassis.So if you look at the way the tubes are laid out it is V1,2,3 and then reverb transformer then V4 and V5. Could the placement of the reverb transformer be causing issues? I'm grasping here.
Would lifting the power to the reverb transformer be a way to test?
BL
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I'm using Hoffman's single channel AB763 layout as a reference.
A couple suggestions to help narrow down the problem...
1. Totally bypass the reverb circuit. Follow the red wire from V1-6 to the board. Disconnect this red wire from the board. Now disconnect the red wire from V3-6 socket. Connect the two dangling red wires together and tape the exposed connection. Now you have V1 connected directly to the PI. Remove V2 and V3. Play guitar. Is the problem still there?
2. Totally bypass the tremolo circuit. Remove V5. Disconnect the wires from the INT pot wiper and right lug. Connect the two dangling wires together and tape the exposed connection. This connects the bias supply directly to the two 220K grid resistors. Play guitar is the problem still there?
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It's going to be a bit, got to go out for a couple hours.
BL
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Yes, after removing the reverb and trem circuits the noise it still there.
I hadn't replaced the first 47uf cap in the power supply so I replaced it. That didn't help either.
Even change speaker cabs thinking that maybe in all this I hurt the speaker in the 1X12 so swapped to a 2X12.
I had replaced the tubes with new ones after I replace the board also.
BL
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Slaved in a new choke, didn't help. :think1:
Lifted the ground to the tone stack, didn't help. :think1:
Heater wiring?????
Rewire rect tube socket????
Power transformer?????
BL
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are we still here;
Removed the NFB and the gravelly, farty sound went away.So my question is, "why doesn't my amp like NFB?"
if so a scope should see it.
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@All,
I have nothing to contribute but encouragement. This is an epic battle for the soul of BL's amp. Don't give up. We're all rooting for you. As my granny said, "it's always in the last place you look." Yeah, hokey but--she was not wrong... Keep at it, I know you'll figure it out.
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Shooter: No and don't have a scope.
Lectroid: Thx for the support.
I have a new Bandmaster power transformer, it doesn't have tube rect taps but I can make it work.Going to slave it in.
BL
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find yourself an old, slow, boat anchor scope. absolutely LIFE CHANGING when it comes to troubleshooting things like these, and probably would've cost less than you've already thrown at this build. i'd say a scope is third on the list to anyone looking to troubleshoot/build (which frankly is about 80% troubleshooting anyway) amps to a meter and lightbulb limiter. find your local ham radio club/electronics surplus store and you're likely to walk away with one for less than the cost of a tank of gas
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:cussing: :cussing: :cussing:
No luck
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what about OT, it's "inside" the NFB loop, you already eliminated everything else inside the loop.
*assuming the symptoms are still; works fine with NFB disconnected*
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How is the secondary of the OT grounded?
If it's just through the speaker jack to chassis, try a separate wire
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From post #61
Not the OPT.
BL
Starting to get frustrated.
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Shooter: I know this is a long thread, see below from post #3.
Here's a couple pics of what I got. AB763 with a SS/Tube rectifier switch and NFB multi tap switch. (Robinette)
Put in a dropping resistor because in SS mode the voltages were too high. It makes the same noise both ways so I jumped the dropping
resistor and leave it on the tube rect for the stock Pro voltages. Hope the way I explained that made sense.
When I was chasing the blocking distortion issue (?) I swapped out some caps and grid resistors. (Aikens)
Everything is back to the way the amp started out life.
When I got it back together and disconnected the NFB switch and sub'ed in an 820 NFB resistor it still made the gravelly farty soundand if I remove NFB altogether it still made that noise.
Soooo, I think I went down the wrong rabbit hole
the noise is most noticeable at half volume playing an F note at the 13th fret.
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Also, what's going on with the blue wire coming off of the wiper of the bias pot?
Nevermind, I see that goes to the intensity pot, as it should
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SG:
What post and what pic are you referring to?? The bias supply has been completely redone and the tremolo and reverb have been bypassed.
There are no blue wires on the bias suppl, the the bias adjustment pot is connected to the pair of 220k resistors.
BL
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I would still try a separate ground connection for the speaker jack, just to rule it out.
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I may have a line on a couple of scopes I can borrow, he said he had a Ballentine 1010A and a Tektronix 455.
He not sure of the shape either is in. Going to pick up in a bit.
BL
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SG:
What post and what pic are you referring to?? The bias supply has been completely redone and the tremolo and reverb have been bypassed.
There are no blue wires on the bias suppl, the the bias adjustment pot is connected to the pair of 220k resistors.
BL
Sorry, BL.
I got hit with a couple phone calls and wound up editing my post while you were posting. I was referring back to Reply #6 for pics.
And I'm just here to throw darts that you haven't yet.
I can't believe this is still going on. Good luck.
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the noise is most noticeable at half volume playing an F note at the 13th fret.
that sounds like a rabbit hole of it's own :laugh:
might figure out what frequency F on the 13th fret equals and use that as your input signal for scoping.
your description sounds like a harmonic distortion which will be hard to "see" on a scope.
when volume get to near max does the amp become normal?
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Scopin;
Put ch 1(or A) on the tip input jack, ground to chassis
Put CH 2 (B) on the Speaker +, ground to chassis
Set your input signal amplitude to 100mV rms (everyone has their favorite #)
Leave it alone
Set tone pots to 5. Bring volume pot from 0 to 10 real slow.
See any oddities? Get a screenshot, image and post
If nothing jumps out
Vol 5 tone pots 0 to 10
Oddities?
The 2 pics are from my last build. The 1st is what you should see, both in and out the same.
The 2nd pic is an intermittent harmonic distortion when volume gets ~~ 7-8
It’s still there a year later, I just don’t “warranty” volume >7 :icon_biggrin:
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Hey guys, got the scopes. Will mess with them in the morning after I get the slaved in choke and power transformer out and the original parts back in.I'm going to configure the circuit without the trem and reverb as slucky suggested and try to find the distortion from there.
Since this is kind of a new start with this issue and this tread is getting so long, and not everyone has read all the post I was thinking of starting anew thread restating the issue, in the first post, as I have become educated with what it is and when this distortion happens.
What do you think, new thread or keep this one going?
Either way nothing is going to happen till tomorrow morning. :icon_biggrin:
BL
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Why wound you even consider a new post…? Work out the issue right here. You have the greatest minds helping you. I know this is very frustrating for you but a component or wire mishap, or whatever is is the culprit, you just need to find it. You’ve rebuilt the amp, obviously you repeated the problem..? Maybe not, but…, keep the post here.
You just need to step aside for a day or so, go over every wire again, check components, there’s a mistake somewhere..?
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Ok I'll keep it here.
Working putting the original choke and PT in the circuit.
Also the scopes work, but the 455 I can only get channel 2 to read anything. The Ballentine works on both channels.
Back to the amp.
BL
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Have you tried another guitar with this amp?
What other amps do you have?
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Yea, I've tried 3 different guitars with 3 different cables, 3 different instrument cables and the same number of speaker cabs.
Amps: Starting left to right.
TW Rocket no cab
EL86 Plexi no cab
Mongral no cab
5881 Princeton /correct transformers to support the 5881s - on floor
Stack:
Revibe
6V6 Plexi
TW Express like amp with 6V6s
EL34 Plexi w/1 input
50w HP Tweed Twin w/BF Super OPT
Fender Reverb unit
And one that I didn't build in black tolex behind the tweed is a hot rodded Princeton.
Not showing off, but these are a few of the amps I've built. Mostly scratch built with new parts and transformers.
I enjoy building and and playing.
BL
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Nice stable! My question was a leading one. I'm hoping that you can rig up to be able to patch your AB763 preamp into another power amp, maybe the 6V6Plexi (or another amp with a LTP PI). And also patch the 6V6Plexi preamp into the AB763 power amp. Still trying to determine if your issue is in the preamp or power amp. Look at this schematic for an idea how to easily do the patching, or just sit the amps side by side and use gator clip leads.
http://sluckeyamps.com/november/november.pdf
On another note... I know we've been over this before but I gotta ask again since you have all new components on the board. There are five 1M resistors on the board, but my eyes only see two. The other three look to be 10Ω to me. I've attached your board with the resistors circled. Resistors 1, 4, and 5 look like brown/black/black (10Ω) on my monitor and phone. Resistors 2 and 3 look like brown/black/green (1M) to me. I'm probably just seeing them wrong but will you please verify they are all 1M resistors? See attached pic.
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sluckey:
Yes I could slave in another amp if need be. Was hoping the scopes would be of use before all that. :dontknow:
Yea, those one meg resistors have a sort of forest green color to them.
Here is a pic of the 1meg color and probably what you are use to seeing.
Going to eat a quick bowl of venison stew and be right back. Hell was hoping to get out to do some bow hunting today. Not happening.
BL
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So here is where I am right now.
The amp is back with the original components, but the tremolo and reverb are jumped and removed from the active circuit with those tubes pulled. As suggested slucky post #83.
What I am hearing now that I have some history and have done some critical listening. At lower volumes (~8:00) there is a fuzz/grrr on top of the note then as the note kind of dies and a feedback bloom starts it sound like a box of rocks shake. I had a 5751 in it and put a 12ax7 in and it does it more.
So gain seems to have a hand in it.
BL
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In these photos of the scope display the bottom trace is the input and the top trace is the probe @ the output of the 220k riz going to the input grid of the power tube.I am no expert at this, but is that fuzz on the top trace distortion? BTW it happens at the output of both 220K riz, 180' out from each other. I wonder why.......... :laugh:
I'm generating the freq. with a plucked open low E.
Help me understand or confirm what I am seeing.
BL
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your input signal (bottom trace?) looks like it's missing a ground reference
and ya, those RF bursts are not good. measure again at the "left side" of the coupling caps that feed the PI
MAKE SURE the scope is set for AC coupling. there will be biggish DCV there
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Got a few screen shots.
I'm using a freq. generator of of my iPad.
82hz (low E) @~ -4db
amp has volume control at about 8:00, using a speaker for load, I also have a bigass (technical term) 8 ohm load I could use.
Bottom trace is always the input.
First photo @ output jack.
BL
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After PI.
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Before PI
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PI input good. PI output bad. Looks like the problem is in the PI. Do both PI outputs look like crap?
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yes
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Disconnect NFB wire and post new pics of PI outputs. Instead of calling them PI 1 and PI 2, call them pin 1 and pin 6.
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Coming your way
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Here you go
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Much better without NFB. I seem to remember that you have modified the NFB to some fancy robrob circuit? If so, can you bypass that and just use the standard AB763 circuit, ie, 820Ω and 100Ω (or 47Ω) voltage divider network? Also, how is your speaker jack grounded. I like to run a dedicated ground wire from the jack over to the power ground point, even if I'm using a Switchcraft 12A jack that automatically grounds to the chassis.
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Not a great pic.
Thats the NFB to circuit wire flying up.
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Disconnect that NFB switch from the speaker jack and connect a 820Ω resistor from the speaker jack directly to the 100Ω onboard resistor, just like Hoffman did. Also run a dedicated ground wire between the speaker jack and your power ground point.
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Did it.
Jumper wire to PT ground and 820 to the 100 on the board.
Same, same.
When I lift the 820 it cleans up.
BL
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I'm replacing jacks.
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With the scope on the output does the signal amplitude change when you connect the NFB versus no NFB?
I'm beginning to suspect power grounding scheme. Can you take some more closeups that show every filter cap and it's ground connection. Probably take 3 or 4 pics to show it all.
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Jacks didn't help
Give me a couple and I'll get pics
BL
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Take a look at the ground path through the bias pot body, if that is still the way you have it set up.
Not only does that ground the bias circuit, but it handles the output tube grid leaks as well.
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Here they come.
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Nodes B,C
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Preamp end
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SG: Checked it a couple days ago and made sure it had one of the big lock/grounding washers under it.
BL
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If the jacks don't work;
try putting a 500pF to .001uF across both PI plate R's (one cap on each R)
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With the scope on the output does the signal amplitude change when you connect the NFB versus no NFB?
I'm beginning to suspect power grounding scheme. Can you take some more closeups that show every filter cap and it's ground connection. Probably take 3 or 4 pics to show it all.
Sorry, I missed this.
Yes volume decreases quite a bit.
BL
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Where is the PT HT center tap???
I don't like the B and C ground point. It requires current for those caps to travel through the chassis to get back to the HT center tap ground point.
Ideally you want the PT center tap, first filter cap, B cap, C cap, bias board ground, bias pot ground, and that power ground buss on the board to all be connected to the same chassis ground lug. Try moving the B and C grounds IAW attached pic first.
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The 1st B+ cap's ground is going through the chassis over to the PT B+ CT ground.
The B+ CT should go directly to that 1st caps ground lead and then run a wire over to your power amp chassis ground. Don't move the cap, move the PT CT and run a wire back to the chassis power ground.
Do what Sluckey wrote 1st and see if that gets rid of your problem. If it doesn't or only gets part of it then try what I wrote.
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No luck on both accounts.
BL
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I always grounded the CT, nodes, A,B,C all to the same point with the speaker ground, bias ground. The preamp filter ground opposite end of CT, run a buss (picking up other grounds) hanging free on one end all to bolt on chassis near the input, l even run a short lead from input ground to that bolt. Never had any noise or any issues.
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This amp was never noisy at idle or low volume. But I consolidated my power grounds to one place.
BL
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Did you test it after moving those grounds with the speaker jack still jumpered over to the chassis power ground?
Are you using bias vary trem? If so that needs to get grounded with the power amp grounds.
How did you ground the power tubes K's?
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Changes sluckey suggested.
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No luck until I added this.
BL
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Sorry.......... :l2:
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The B+ CT should go directly to that 1st caps ground lead and then run a wire over to your power amp chassis ground. Don't move the cap, move the PT CT and run a wire back to the chassis power ground.
Leave the black wire you ran from the 1st B+ cap's ground, but move the B+ CT to that 1st caps ground lead. Your in this far, It may or may not fix it, but, it makes a difference.
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Guess your done for the night. :laugh:
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This took care of it.
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:blob8: YES!!!!!!
Across the LTPI's plates?
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So my first question was >>>>>> Why doesn't my amp like NFB.
I'm going to stop for this evening and take a look at it again in the morning, but the amp sound a whole hell of a lot better now.
BL
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Yes, across the PI plates. :d2:
BL
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Looks like I missed a lot while out in the shop. I've set up my bench with my Phoenix (AB763 with an additional Ampeg preamp). Anyhow, I can now do some checks for comparisons. Here's what my PI looks like. 500Hz signal adjusted for 1VRMS at PI input. Got about 30Vpp at each PI plate. I'm done for tonight but I can do this again tomorrow (just not during Alabama/LSU game).
I think your PI signal still looks crappy but I have no idea what signal level you have at the PI input. That cap across the PI plates is a band aid fix. It's just masking the problem.
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The grid wires from the PI to the power tubes are pretty long. Even though you have a nicely twisted pair of wires there and it is a stronger signal there, they could be picking up stay signals.
And since the problem seems to be at/after the PI, at this point I'd try using shielded cable for those. See if that helps. Worth a shot.
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I've started my herbal meds so;
they could be picking up stay signals.
why ONLY when NFB is applied?
agree the Cap is just a band-aid, maybe.
If it stops the tube from breaking into oscillation and doesn't have a noticeable tonal effect
I'd just leave it, play the amp till you have a good sense there ain't more issues unseen/heard.
there are plenty of "professional" amp schematics that did just that, "boss we're shipping it" tell R&D to look into it next Revision :icon_biggrin:
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At the risk of biting the hand…….
Geaux Tigers
Sooners got a bye this week..
Boomer
BL
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Geaux Tigers
Always a fun game to watch. Can go either way. IMO the game to watch this year is SEC championship, Alabama and Georgia, providing Alabama can hang in there. :laugh:
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It seems like you've tested all of the likely possibilities. Any chance that the PI tube or tube socket could be bad?
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Not saying that a socket couldn't be bad, new Beltons, but I would think unlikely. I've had 2 new JJs and a good old RCA in that spot.
Anyone have any revelations overnight on this issue? I'm pretty much still game.
sluckey, what model Siglent are you using and what are your thoughts on it.
BL
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Play it with the cap in til you are convinced there are no other "issues".
If it plays good and sound right then get another Fat Tire and enjoy
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I have the Siglent Technologies SDS1202X-E (https://www.amazon.com/Siglent-Technologies-SDS1202X-Oscilloscope-Channels/dp/B06XZML6RD/). Bought it from Amazon in April of 2020. Chinese prices were on a roller coaster at that time. I bought when the price was at the bottom of the track.
It's a very nice scope. I haven't turned on my Tek 465B since I bought the Siglent. Takes a bit of time to get used to the features and controls. If you learned how to use all the features of those wacky Fluke 190c scopemeters that FAA bought, then the 1202 will be a breeze. But if you only have experience with analog scopes be prepared to spend a lot of time pushing buttons and twisting knobs with the 1202 (or any of the new digital scopes for that matter). :icon_biggrin:
Here's the setup I used to get those scope pics... HP204D Oscillator set for 500Hz sine wave at 400mVpp amplitude connected to amp input jack. Scope triggering to external connected to sync output from HP Oscillator. CH1 connected to PI input cap. Amp volume adjusted to provide 1VRMS on CH1. CH 2 probe connected to PI plates.
Here's the schematic of the my amp. Basically the same as yours except the added Ampeg preamp and improved bias supply. More info on my website if needed...
http://sluckeyamps.com/phoenix/phoenix.pdf
And here's my bench. I'll leave it like this while you're sorting your amp...
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Basic question... Are you using a 12AT7 for the PI IAW Fender specs?
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Yes I am, re: 12AT7
Cleaning the shop right now, going to get some pre PI and post PI readings in a bit.
Good game last night.
BL
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This time tell us what signal voltage levels you have at the PI input and PI plates.
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Good afternoon.
Got some pics coming. I not sure I understand all I know about what I'm seeing.
BL
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No signal input, input grounded through jack.
Both pics bottom trace PI input between .1 and .001 caps.
First pic top trace PI output, V5P1.
Second pic top trace V5P6.
10X probes
BL
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Heres a couple more pics.
Bottom trace CH1, top CH2
Also, this is into an 8 ohm dummy load
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That could be anything from hi freq oscillation with some 60Hz hum inside the amp, to bad probes, bad scope, poor ground connection between the amp and scope, etc.
Get some confidence in your scope and probes to be sure they are presenting the picture they should be. Be sure to use the probe ground clip. Look at the 1V cal output on your scope. Is it a clean square wave? Then look at the 6.3VAC filament voltage. Is it a clean sine wave?
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Probes showed clean 1v square wave and a clean 6.3 heater.
Probes are the same also, traces stack nicely on top of each other.
BL
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Steve, all these test today have been into a dummy load and 500hz instead of 82hz. 300pf across the output of the 2 200k resistors.
Also the reverb and Trem have been put back in the circuit also.
BL
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Swapped out that PI tube and can't reproduce the fuzz.
Oscillating tube?
BL
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Well I've had a couple buddies come by and play the amp and although the PI seems to have some roughness on the scopeit sounds pretty dam6 good to the ear. :icon_biggrin:
I really appreciate all the help and suggestions when it came to trouble shooting this problem.
First time in a looong time I've had to use a scope, for guitar or even audio amps I've never felt the need. Probably would be good to keep one and
and some sort of signal generator.
Been kind of torn between finding an old analog scope or just get one of the cheap digitals and some sort of signal generator.I've got my iPad but I would prefer to have a stand alone. Suggestions are well come. Any suggestions would be welcomed.
And this is my 700th post.
BL
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I'd rather be without coffee than my scope :icon_biggrin: (and I LOVE coffee!)
If you want to spend time tweakin, or repairing, especially signal path stuff they are worth the time n money and the young'ns think you're really cool :laugh:
at end of day, the "sound-check" with multiple guitarists IS the final say
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Been kind of torn between finding an old analog scope or just get one of the cheap digitals and some sort of signal generator.I've got my iPad but I would prefer to have a stand alone. Suggestions are well come. Any suggestions would be welcomed.
I recently tried to "upgrade" my bench with a PC based scope and I just hate it and I will be downgrading to analog in the near future.
I just don't even want to try to get used to it. I don't need to record a trace or need stupid high bandwidth or blah, blah, blah...
There's just something that feels right about the immediate response of the analog scope that makes me want to throw the PC version against a wall. There's also lots of discussion about this on the web.
I'll hang onto the PC unit for a backpack, service call unit, or a backup.
The delay with the USB throughput ruins it for me.
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> And this is my 700th post.
Congratulations!
> I'd rather be without coffee than my scope
You need to figure what is *really* important. Drugs before toys!
> I've got my iPad but I would prefer to have a stand alone.
Tracfone will sell you Android-flavor phones $29 and less. Get a refurb LG, they are decent, I've owned 4 this decade. You may have to buy "minutes" but there is a $15 starter. Do not let it register itself on the cell-net, it will stop nagging in a week. --- Ah, today's best deal may be "Samsung Galaxy A01 (S111DL) - RECONDITIONED" plus a 30-day plan, $25+$15+$5 is 45 bux. (I can't buy an iPhone cable for that.)
https://www.tracfone.com/phone/samsung-galaxy-a01-reconditioned
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Been kind of torn between finding an old analog scope or just get one of the cheap digitals and some sort of signal generator.I've got my iPad but I would prefer to have a stand alone. Suggestions are well come. Any suggestions would be welcomed.
I recently tried to "upgrade" my bench with a PC based scope and I just hate it and I will be downgrading to analog in the near future…
I got a Picoscpoe usb thingy a few years back, as my old old scope was getting a bit dodgy. Thankfully it carried on, as it took me maybe a couple of years to get conversant with the Picoscope user interface, but I’m good with it now, and can get it to do what I want pretty quickly, including stuff beyond a regular old scope’s capabilities. Some aspects of it are frustrating still though, and I’m thinking of getting a Velleman.
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We carried the Velleman line when I worked at You Do It Electronics. They’re kind of a Strange cat in that, some of the products they distributed were total garbage. But there were other occasions where they put out some pretty decent products that were much higher quality. My buddy let me borrow a small battery powered multi waveform generator, with a bandwidth of 1kHz to somewhere in the hundreds of kHz. Plastic enclosure, with a bnc jack, and a simple interface. But the output was clean! And way more accurate than I was expecting.