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Thread: Debugging a nice 60/120 hum in a reverb circuit

  1. #71
    Bent Member Chuck H's Avatar
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    Well to be perfectly honest Mike, I'm having some trouble following along at this point. I don't have an interpretation of the scope trace you posted. I'm afraid no one else does either because the context is a little confusing and there's probably not a precedent reference. That is, what would a scope trace look like for that measurement on another amp? I'm trying to digest info about the extra power supply node and now the added diodes. There have been a couple of things that MAY be part of the issue, but haven't been fully defined and I'm starting to think there may yet be other deviations not disclosed. Not to mention that you are now reporting hum in operating conditions other than the original condition that we worked hard to isolate for. Basically tossing every premise we had out the window.

    60Hz could be a harmonic artifact of 120Hz caused by improper implementation of the diodes on the rectifier, I dunno. Otherwise 60Hz has to be inducted (into???) via the power transformer EMF or coming through the filament circuit.

    You can always remove the extra diodes to rule that out. The amp doesn't need them so it's safe. If they aren't the problem then put them back in if you like.

    I actually don't use a scope much for chasing hum or buzz issues. I rely on my ears to tell me the frequency and start with those possible noise sources, tracing back through the associated circuits examining lead dress, grounding scheme and any filtering/decoupling. So if you do indeed have 60Hz hum there are limited possibilities for where it's coming from.

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  2. #72
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Maybe bad GZ34 out of the box?
    It's been known to happen. Also a common misteak to wire the filter bank to pin 2 instead of 8, imposing a 5V AC wave on top of the rectified voltage. Who knows, that kind of error might have been made inside the tube but I can't say I've ever seen it. How about subbing in another GZ34, or for that matter tube rectifier of any variety, or even a solid state replacement just to see if the hum level drops. FWIW our man Stan - not heard anything of late but he was a valuable MEFster - recommended Ruby's 5AR4C as top of the pops for currently made GZ34/5AR4 types.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by mikepukmel View Post
    If I have AC coupling turned on on the scope, what is the safest DC voltage I can probe with this old 7613 scope? For the AC coupling setting, they must have some kind of capacitor switched in to filter out the DC, but the switch, the cap and other components have some voltage rating. I tried looking in the manual could not find it right away.
    I have used a very low value 600v capacitor attached to my scope probe to probe higher than 300v. Film cap with one end fishhooked and the other end done in a wrap fashion so probe clips on it. Then using the fishhook on whatever I want to probe.

    nosaj

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    It's been known to happen. Also a common misteak to wire the filter bank to pin 2 instead of 8, imposing a 5V AC wave on top of the rectified voltage. Who knows, that kind of error might have been made inside the tube but I can't say I've ever seen it. How about subbing in another GZ34, or for that matter tube rectifier of any variety, or even a solid state replacement just to see if the hum level drops. FWIW our man Stan - not heard anything of late but he was a valuable MEFster - recommended Ruby's 5AR4C as top of the pops for currently made GZ34/5AR4 types.
    Have to put that one in my bank of common mistake to not overlook.
    nosaj

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    Lifetime Member Enzo's Avatar
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    How exactly would that work? The 5v is AC, but has no reference to ground. The B+ is referenced to ground, so how would the 5vAC add and subtract from B+?

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    Old Timer Tom Phillips's Avatar
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    Also remember that some rectifiers, such as the 5U4, have directly heated cathodes. Therefore, the output is always connected to the 5V AC heater supply.

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    Sorry, Chuck, really sorry, didn't mean to be a pest. First amp build, thought I remembered everything in there, but had a few rewiring rounds and such. I though it was pretty much stock except for the extra cap+resistor for teh reverb but forgot about the 'immortal mod' diodes. I don't think anything else in there.

    Thanks Leo, I will go back again and check the socket wiring. I went through the main board and all wires from preamp through to the reverb, think everything looks right, but will check again. I have another GZ34 for the next amp build, will pop that in and see what happens. I read a whole bunch before buying this rectifier tube and arrived at Ruby as well. Maybe I screwed it up somehow.

    THanks for the tip, Nosaj, working up the nerve to try looking at some of the HV traces. Not sure it will lead anywhere, at this point.

    Thanks Tom, I think that is what this amp has. One side is one leg of the heater + HV output, pin8, what Leo is talking about mis-wiring?

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  8. #78
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Sorry, Chuck, really sorry, didn't mean to be a pest. First amp build, thought I remembered everything in there, but had a few rewiring rounds and such. I though it was pretty much stock except for the extra cap+resistor for teh reverb but forgot about the 'immortal mod' diodes. I don't think anything else in there.
    NBD. Really If it were I probably wouldn't even have posted. As it is now I'm probably just going to lurk unless a symptom that offers a clue (to me?) is given.

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    Thanks Chuck, Id hate to wear out my welcome.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Thanks Chuck, Id hate to wear out my welcome.
    You? Not a chance! There have been a few. I was probably one once upon a time You're easy comrade More important, you're clearly enjoying this stuff rather than being angry about it.

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  11. #81
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Heck of a time getting a photo of the scope trace. scoped hooked up to speaker output inside the chassis. Kinda stinky but this is it:

    Attachment 49868


    Peaks are about 17ms across, so about 59hz (within the calibration state of the old scope). Reverb knob on 10, about 150mv. Reverb knob on 1, just some noise.

    This was taken with the short jumper soldered from pin 2 of the reverb recovery socket to nearby ground.

    Maybe I screwed up the cap board wiring?

    Could this be a leaking filter cap but the reverb recovery just picked it up and maybe its there on the other channels at a lower level?
    That looks a bit like the current that flows to the primary filter caps. Here's a couple of suggestions. First move your wire to between the grid and cathode bypass cap ground of the reverb recovery. Keep your wire short. If this eliminates the hum then it coming from the ground. In that case a gut shot might help.

    Les likely but should be checked if that didn't help, try disconnecting the wire to the plate of the reverb recovery tube. The objective being to leave the 100K connected to the supply and the coupling cap in place. This will tell us if it's coming from the supply. Have you checked that the 100K really does connect to node "D" of the power supply?

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    Quote Originally Posted by Chuck H View Post
    You? Not a chance! There have been a few. I was probably one once upon a time You're easy comrade More important, you're clearly enjoying this stuff rather than being angry about it.
    Chuck, the most enjoyable thing I've ever done!

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    Quote Originally Posted by nickb View Post
    That looks a bit like the current that flows to the primary filter caps. Here's a couple of suggestions. First move your wire to between the grid and cathode bypass cap ground of the reverb recovery. Keep your wire short. If this eliminates the hum then it coming from the ground. In that case a gut shot might help.

    Les likely but should be checked if that didn't help, try disconnecting the wire to the plate of the reverb recovery tube. The objective being to leave the 100K connected to the supply and the coupling cap in place. This will tell us if it's coming from the supply. Have you checked that the 100K really does connect to node "D" of the power supply?
    Thanks Nick. When you write "move your wire" do you mean the shorting jumper? So, short the grid to the ground side of the bypass cap on that tube?

    Ah, ok and second test, just turn off the tube. but this also keeps the supply out of the equation at that point in the circuit.

    Re node D Yes, I checked a couple of times, think its OK. Will check again. I put 4 photos covering the amp guts viewed from the top, post #46, should be fairly high res, on the previous page. I didn't get shots of the cap board, though, will do that when I get home tonight. The photos show the extra cap and dropping resistor that I have removed while going through the suggested tests.

    Todo:
    - different routing of shorting jumper on reverb recovery grid.
    - disconnect reverb recovery plate lead
    - try a different rectifier tube

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    In the photos, post #46 on the previous page, you can see my "jungle gym" attempt at a star ground. I took 4 lugs, sanded and put nokorode, bolted them together and soldered them all together so they would not lose contact. Then I bolted them to the chassis and put some more solder on so that they are soldered together and to the chassis.

    It turned out much more ugly than I thought it would.

    Two things that bothered me about the way it turned out were
    1) the long wires that need to be routed from/to the ground lugs, to various points in the circuit
    2) not sure if it mattered which wires should be soldered to the same lug, e.g. should power tube cathode and power transformer center tap be on the same lug? Its not really a 'star' if various pairs of wires are soldered to different lugs, so they are still separated.

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    One more variable: I didn't solder grounds to the back of the pots. (many threads on MEF about why we should not do that). Instead, I ran a wire from the pot lug to the ground bus wire. I twisted all of the wires coming from pots to the main board. Should I have ran the ground wire separately, i.e. not twisted it in the bundle?

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    The corrosion inhibitors used in house wiring have silicone in them I believe from the way they feel. Kind of a silicone caulk that doesn't set up. If that's what you're talking about using before you soldered your ground connections, that could be a concern.

    (Nokorode on surfaces not letting solder bond)

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    Last edited by ric; 07-30-2018 at 02:26 PM. Reason: typo/ clarify

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    Quote Originally Posted by ric View Post
    The corrosion inhibitors used in house wiring have silicone in them I believe from the way they feel. Kind of a silicone caulk that doesn't set up. If that's what you're talking about using before you soldered your ground connections, that could be a concern.

    (Nokorode on surfaces not letting solder bond)
    Thanks Ric! I checked with the gurus on MEF, and this is what they suggested for soldering to the steel chassis, like fender and clones. With a big soldering iron, works great. It's really a flux, rather than a corrosion inhibitor. I think the name means that it is not acid core, like some plumbing solder fluxes.

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    Supporting Member loudthud's Avatar
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    Quote Originally Posted by mikepukmel View Post
    So I have this old throw away TEK 7613 scope. It came with one set of 10x probes. If I use Ac coupling, can I probe the plates in the preamp? that would be 200v range dc, but curious what AC might be there, and what the trace would read (if it won't do something terrible like sparks or fire)
    The TEK 7613 can use several different input "amplifiers" that plug into the scope below the screen. The 7A18 is a common dual trace plug-in. The AC coupling cap in that plug-in is rated at 600V.

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    Quote Originally Posted by loudthud View Post
    The TEK 7613 can use several different input "amplifiers" that plug into the scope below the screen. The 7A18 is a common dual trace plug-in. The AC coupling cap in that plug-in is rated at 600V.
    Thanks! Yes this one has a 7A26 and a 7A18. So, with 10x probe, the cap would 'see' 1/10 of whatever the probe touches? The rectified output of the GZ34, with about 760vac rms input, (330 - 0 - 330) or about 330 *1.414 less 40 volts or so lost to the tube rect, something like 420v. So the scope end of the probe sees about 42v dc? So, I could probe that without blowing up the scope, or causing a fire, or other horrible thing? These are really old probes, should I clean off the wires with something maybe alcohol?

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  20. #90
    Supporting Member loudthud's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Thanks! Yes this one has a 7A26 and a 7A18. So, with 10x probe, the cap would 'see' 1/10 of whatever the probe touches?
    No, the cap sees the full DC Voltage at the probe tip.

    Quote Originally Posted by mikepukmel View Post
    The rectified output of the GZ34, with about 760vac rms input, (330 - 0 - 330) or about 330 *1.414 less 40 volts or so lost to the tube rect, something like 420v. So the scope end of the probe sees about 42v dc? So, I could probe that without blowing up the scope, or causing a fire, or other horrible thing? These are really old probes, should I clean off the wires with something maybe alcohol?
    I don't recommend cleaning any part of the probe unless you know what you are doing. Parts of the probe might not be able to stand the liquid. Some plastics might absorb the liquid and change their high frequency characteristics.

    All the circuitry that changes the Volts/Division (called the Deflection Factor) is accomplished by passive circuits before any active circuitry on the 7A18, slightly different on the 7A26. At the 5mV/Div position, the signal goes straight thru to the input JFET. The active circuitry is protected against excessive Voltage, but, like anything that might be subject to abuse by a customer, you can't protect against everything. One of the toughest things to protect against is the charging and discharging of that input coupling capacitor. It's no accident that the AC/DC switch has Ground as the center position. To protect the input, Switch that switch to GND, connect the probe to the energized circuit, then move the switch to the AC position. To remove the probe, switch to the GND position, disconnect the probe from the circuit and touch the tip of the probe to ground to discharge the cap. Then switch to DC and you can use probe as usual.

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    Thanks!

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Thanks Nick. When you write "move your wire" do you mean the shorting jumper? So, short the grid to the ground side of the bypass cap on that tube?
    Correct

    Ah, ok and second test, just turn off the tube. but this also keeps the supply out of the equation at that point in the circuit.
    The point is to disconnect the tube but leave the supply in place. I don't really suspect that supply but never say never.


    Re node D Yes, I checked a couple of times, think its OK. Will check again. I put 4 photos covering the amp guts viewed from the top, post #46, should be fairly high res, on the previous page. I didn't get shots of the cap board, though, will do that when I get home tonight. The photos show the extra cap and dropping resistor that I have removed while going through the suggested tests.

    Todo:
    - different routing of shorting jumper on reverb recovery grid.
    - disconnect reverb recovery plate lead
    - try a different rectifier tube

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    Quote Originally Posted by mikepukmel View Post
    Thanks Ric! I checked with the gurus on MEF, and this is what they suggested for soldering to the steel chassis, like fender and clones. With a big soldering iron, works great. It's really a flux, rather than a corrosion inhibitor. I think the name means that it is not acid core, like some plumbing solder fluxes.
    Gotcha, the flux in a little tin. I actually have some in with my welding equipment box.

    I was thinking of the stuff I've been using lately to rewire the house, a completely different thing using a similar name.

    Ah, the joys of aging.

    Hope you get the hum tracked down. You can always figure the first build is what it is and use imperfections as a reason/ excuse to build version 2.0...

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    Quote Originally Posted by nickb View Post
    Correct



    The point is to disconnect the tube but leave the supply in place. I don't really suspect that supply but never say never.
    Progress!! I got home, dead tired as usual, but threw some water on my face and got enough steam to try some of the changes out (didn't want to wait all the way till the weekend).

    1) Move ground short wire from input pin2-nearby ground, to input pin2 to ground side of cathode cap/resistor. Check connections, power up. Turn the reverb knob to 10, no hum at all. The only thing that increased was the 'johnson noise, a little white/pink noise. Turned off, drained filter caps. Triple checked, looks like this is "it".

    So, the nasty signal is making its way into the tube through my nightmarish grounding wiring? I did something dreadfully wrong. Maybe several things.

    Now the ground bar is wired to the lugs soldered to the chassis. So, what I don't understand is how this signal could get in that way.

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    My all time favorite quote from the old TV show "Mash", maybe my all time favorite quote ever. By Colonel Potter:
    "You're askin' a man who graduated 246 out of a class of 248 to do his BEST?"

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    So, in the original amps (the ones that work ) there are 3 wires leading from the - side of the cathode cap/resistors to the brass plate. Then there is one black ground wire coming from the cap board, through the grommet and over to the brass plate. But its inbetween the rightmost cathode ground and the next one over.

    There is one more ground wire coming from the cap board that is grounded soldered to the chassis, over by the power transformer. The main difference I can see in my ultra-humluxe is that instead of running a ground wire from the bus bar right down to the chassis, I ran a wire from the middle of the bus bar over to where the PT CT, reservoir caps (-) and bias are all 'grounded' to the chassis. Is the problem that this 'star' ground is badly constructed?

    Click image for larger version. 

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  27. #97
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by mikepukmel View Post
    ... Is the problem that this 'star' ground is badly constructed?...
    Yes. You have used the star ground point you created with multiple solder lugs at the PT end of the chassis for too many connections. If you are going to follow the approach used in the old Fenders that point should include only the PT center tap and reservoir (First stage filter cap) connection. That is a very "dirty" & noisy circuit point and it is best not to connect other circuit grounds directly to that point. There is much to say about the merits and pitfalls of various grounding schemes. Sorry I don't have time to add more details about your situation at the moment. Maybe later. Meanwhile, there are lots of good existing MEF threads on the subject. Here is one suggested thread to read http://music-electronics-forum.com/showthread.php?t=33322

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    Thanks for the info Tom, and thanks for the link. (understood regarding time, and this thread is getting pretty long ) I've been reading myself blue re grounding schemes. I got it pretty wrong. This morning, backed out most of the mistakes. Starting to understand more bits and pieces re Fender multiple ground points, what they put at each point. I'll get it right sooner or later!

    The only thing I could not "fix" was Fender fed the reverb driver tube with the dirtiest node in the power supply, but tied the cathode cap negative side and cathode resistor to the second preamp cathode and grounded those at the same point on the chassis. I ordered a turret staking tool, will add one more turret and break those two ground points. ...

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    Hopefully bringing some closure to this rather long thread, that someone might get some use out of it at some point.

    - Took apart the failed star ground.
    - Put the power transformer center tap, and negative lead coming back from the 2 reservoir caps onto one lug.
    - Found that I had mistakenly put 2 wires running from the ground bar to the failed star ground. They were separated by quite a bit of distance on the bar. Removed both wires from the star lugs, removed one wire completely. Ran a ground wire from about 2/3rds of the way down the ground bar to the chassis. Would have been about where Fender would have ran a wire from the cathode cap/resistor to the brass plate.
    - Removed the 2 wires leading from the power tube cathode resistors to the star lugs. Shortened the wires, and soldered to the chassis right between the power tubes.
    - Ran a wire from the first filter cap (the one right off the downstream side of the choke) to where the power tube cathode wires were soldered to the chassis.
    - Ran a wire from the negative lead of the second filter cap to the ground end of the negative feedback resistor that grouds the phase inverter, then ran a wire from there to the chassis about where Fender would have put one.
    - Found 3 or 4 tube socket wires that were too long (still after 3 re-wirings!!!!), shortened, and resoldered.
    - Cut the end off the 'ground bar' that connected it to the phase inverter resistor that goes to ground.
    - Ran a wire from the last filter cap, the D node, to the ground bar about where it is grounded to the chassis (about where Fender would have run a bus wire to the chassis).

    Checked connections, powered up. Much less hum in the reverb! If I put the reverb on 10, its there, and noticeable, but at 3 or 4 (way more reverb than I usually play with), not noticeable. Amp does seem quieter overall as well.

    As mentioned elsewhere, what I can't fix:
    - The reverb driver tube is fed from the dirty end of the power supply, first filter cap node. But the reverb driver tube's bypass cap and resistor are grounded to the same point where the second preamp tube is grounded. This can't be good. I ordered a turret staking tool, will try to put one more turret in and move the reverb driver's bypass to a separate wire and run that over to where the first filter cap negative lead is attached to the chassis.
    - Maybe last thing to do, move the ground for the bias board away from the "dirty signal" ground lug.

    Thanks everyone for all the input, suggestions and help.

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  30. #100
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    Latest update. Tried several used old stock 12AT7 tubes in reverb driver spot. One was very quiet. With:
    - fiddling with the grounding
    - two new caps (off the plate of V2)
    - replace 12AT7

    Turn the reverb knob to 10, and almost as quiet as at reverb at 1. Just a little more hiss, but no hum.

    These new tubes are pretty stinky. This is the 3rd tube replaced in this amp, and its not 'done' yet. Two EH 12AX7's severely microphonic. If I tapped my forefinger on the top of the cabinet, LIGHTLY you'd hear an audible sound in the speaker. Vendor replaced those.
    12AT7 died, started crackling badly.

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  31. #101
    Bent Member Chuck H's Avatar
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    Are you saying the new at7 died now?

    1 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  32. #102
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    Quote Originally Posted by Chuck H View Post
    Are you saying the new at7 died now?
    Hi Chuck, nope, that baby is running smooth as a gravy sandwich.

    1 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  33. #103
    Bent Member Chuck H's Avatar
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    I love a gravy sandwich!

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  34. #104
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    I love a gravy sandwich!
    I like Lumpy Gravy. With a schmear of Suzy Creamcheese.

    0 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  35. #105
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    I like Lumpy Gravy. With a schmear of Suzy Creamcheese.
    May the spirit of Sheik Yerbouti smile upon you good sir

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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