Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Deric on September 03, 2017, 02:21:29 am
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Working on a 67 Super Reverb. Local guy bought it and loves it but it would start to crackle/pop after 20 minutes or so. Opened it up and it's been worked on quite a bit... PT has been replaced, filter caps, most bypass caps, grounded cord.
I replaced V1 and V2 plate resistors and the cathode resistors/bypass caps on both input stages. Rebuilt the bias board (cap, resistor and diode). New screen and grid resistors on the outputs. Cleaned up the wiring on the replacement PT and power cord. Removed the death cap.
Powered it up and it's nice and quiet - no more crackle/pop even after 20-30 minutes. Thought I was done, but... Plugged in a guitar - sounds great but there is a static sound as low notes decay. Only noticeable on the E A and D strings. High G B and E are fine. It's effected by the volume - does it quiet or loud. It's on both channels. Disconnected reverb, pulled reverb driver tube and trem tube with no change. Swapped all tube (including outputs) no change. Haven't swapped the rectifier but I can't see that being the problem (will try it anyways).
This is not a loud pop or crackle - it's more a static sound as the note decays.
I've now rebuilt the entire phase inverter section - no difference.
Any suggestions?
Thanks
Edit: I did try a different speaker too...
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Have you tried cleaning and tightening all of the tube socket pins? Swapping known good tube in the PI position?
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Does is sound like a sizzle, like faint 'bacon frying"? Only on the decay/tail of certain notes, and will become less noticeable if the volume is turned down enough?
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I have cleaned/tensioned all sockets and tried another PI tube. Will try a couple more...
It does sound sort of like "faint bacon frying" and it is only on the decay of some notes - anything on the G string or higher seem fine.
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It does sound sort of like "faint bacon frying" and it is only on the decay of some notes - anything on the G string or higher seem fine.
Could be a mechanical vibration interaction with the amp circuit. Most combo cabs have a resonance in the 80hz to 150 hz (Low E to Low D notes in standard tuning). Sound waves and vibes from the speaker can cause a sympathetic vibration in the circuit, and presto, sizzle.
Since the sizzle is faint, I would suspect a grounding cold solder joint or perhaps a loose grounding wire somewhere. In my experience, many combos have a slight grounding sizzle, but it tends to be masked if the amp has enough background hum or hiss. Since you just fixed the amp, maybe it is now quiet enough for it to be heard.
If the sizzle were louder, then I would suspect something in the signal path. Things like a microphonic tube, loose tube socket, or cold solder joint in the signal side. In my experience, signal path sizzle can be heard pretty clearly on the pick attack too, and then lingers as the note decays.
Possible fixes:
a) Make sure all grounding nuts are tight
b) Use a soldering iron to re-flow any connection that is reachable (ground or signal)
c) Check any hum shielding (metal) on chassis cover, cab sides, etc. Make sure that the contacts are solid.
Lots of knowledgeable people on this forum, so others probably have good ideas/advice.
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Thanks for the input. Chassis is on the bench so it's not vibration from speakers/cab...
I did just do a little more testing/listening. Pulled all preamp tubes but V1 and V6. Plugged in a guitar and listened. The static is not just on the decay - it rides on the note the hole time just becomes slightly more obvious as the note decays. And still, only on the E A and D strings.
Chop sticked all around V1 and V6 thru the speaker jack with no change. Will reflow everything I can as well as pull the dog house and check the grounds on the filters (filter caps were done before I got it).
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Plugged in a guitar
what kind of toys do you have? got a sig-gen that you can inject specific frequencies.
scope to "see", two input r better than 1 :icon_biggrin:
if you only have a meter, try clipping it across the "last" cap, the one that filters V1, set it to VAC, and just watch it for "normal", hit an E and watch for a noticeable change, for more fun, repeat with VDC
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I had the same issue, although this type of trouble can be a few things.
It goes with what the other fellows say... grounding
My fix was a loose speaker jack. I tightened it up and no more weirdness on the low strings.
Good luck
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Pulled the doghouse and checked components and relfowed all solder joints. Reflowed all ground connections including all the chassis grounds. Tightened all input and speaker jacks. Swapped speaker cable. No change...
Plugged guitar directly into reverb return - static is still there.
Hooked a meter across the last filter cap - I think all is normal. See an occasional flicker of AC at idle (less than .020 vac) see a little more when playing (less that .050 vac). DC is steady at idle, sags ~20v when I bang an E chord.
I do have a scope but not very good at using it... For a signal generator I have an iphone with an app.
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Reflowed PI tube socket pins. No luck. Had a OT sitting on the bench so tacked that in quick just to eliminate the possibility. No luck...
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Soooo...... Speaker. All this time.
One of the 10" speakers in the cab has a rattle. When I plugged in another cab sitting next to it - it caused the same rattle. Sitting side by side I couldn't tell the rattle was coming from the cab that wasn't connected to anything.
Finally built a listening amp. Even testing the signal at the input jack the "static" was still there. I finally moved the stock cab out of the room and the noise was gone. Brought the cab back for one last test. Sure enough...
The very first thing I tried was a different speaker.... Anyways, Thanks for the input!!!!
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Thanks for reporting back with your success!
Chassis is on the bench so it's not vibration from speakers/cab...
Sympathetic vibrations are infamous! I've heard stories of a room rattle being mistaken for amp rattle, but never another speaker cab getting into the mix. Good to know.
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Pulled the speakers out of the Super. Turns out it was a dust cap wedged between the grill cloth and baffle. First test I set a closed back 112 cab in front of (facing way from) the Super cab. Didn't catch on till I fired up the listening amp. Still had the same static but the listening amp was on the other side of the room. Still thought it was the amp at first. :BangHead:
Anyways...lesson learned. For now.
Thanks again for the input/tips!!
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That reminds me of the time I had a tremolo effect in an amplifier with no Tremolo effect. Drove me nuts for a few minutes until I realized it was coming from the ceiling fan in the shop. :l2: