Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blind Lemon on June 20, 2017, 05:25:50 pm
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It's right on the tail end of the note. Almost sounds like a rattle or buzz, more mechanical. It's most prominent when playing an E on the G string. At first I thought it might be a speaker, it's not, can here it in other speaker cabinets also. Amp is about a 3 year old 5F6A, swapped out all the tubes, put grid resistors on the power tubes, swapped all the poly caps one at a time, played with the feed back resistor.
Looking for a suggestion or 2.
BL
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Tried a different guitar? I had a similar issue and it turned out to be a sympathetic vibration of the G string above the nut on a Squier Strat.
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Fret buzz????
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3 different guitars
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It could very well be a mechanical vibration. Have someone else play the note and try pressing on different parts of the chassis exterior and see if it goes away while pressing. Take the amp out of its cabinet and try and narrow down where the sound is coming from by putting your ear up to it... don't burn your ears on the tubes though! :icon_biggrin: Check that all your nuts are nice and tight.
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First post, through other speaker cabinets also.
I have completely isolated the chassis from vibration.
BL
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Look up Paul Ruby mod it's a couple of diodes across power tubes Grid resistors. The noise is heard at a end of a distorted power chord, a buzz or rattle sound that comes at the end of a note or chord. Just a four 1N007 diodes
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First post, through other speaker cabinets also.
I have completely isolated the chassis from vibration.
BL
That's not my point... the vibration could be IN the chassis.
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DUDE!!!!!! thanks I'll check it out.
DP not trying to be difficult, so if the chassis is out setting on a bench and is isolated from vibration????
BL
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OK I see what you are saying.
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Since you're saying it's on an E note on the G string .............. which would make it a somewhat higher note, .......... IF you have insulated alligator clipped wires, you could try an "enhance cap" across the LTPI entrance plate resistor. Maybe try a 250p to 390p range. This may not resolve the issue, but it's an easy thing to try out.
The "enhance cap" seems to eliminate some high end "hash/harshness" for me. You can do a search on the "enhance cap".
The only other things I can thing of would be to try something like a 20uf/500v electrolytic cap across each of the B+ nodes (again carefully and with insulated alligator clipped wiring). IF you have an electrolytic cap needing replacing, this may help locate that?
And lastly, try moving the heater wiring around with a non-conductive chopstick. I've seen heater wiring run too close to a signal wire and cause some odd oscillation noises.
IF none of those do it, I'd look for a poor solder joint. OR a broken bear claw clamp for power tubes. OR I would continue trying more tubes incase you replaced a rattling tube with another rattling tube (which I have done before with 12AV7 and 12AY7 tubes).
You can also pick up the chassis and shake it to see if anything rattles (yes, I remember you played thru another speaker/cab but if the sound was hitting the chassis and you had a loose bolt or part, it's possible that it could still rattle IF the speaker is directed at the chassis?) I found a bolt that was holding a terminal strip in place had come somewhat loose over the years of playing.
with respect, Tubenit
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...played with the feed back resistor
Have you actually disconnected the feedback resistor?
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sluckey - nope just changed values, 27,56,100k
Will try it
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sluckey - Yes Sir it is still there with the NFB riz diconnected.
TN - Still there with this also.
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Does what? Still distorts? No longer distorts?
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Wasn't very clear, hope that cleared up your question.
BL
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No expert here but I just looked up the schematic.
Just thinking off the cuff here.
Have you tried/are there 4 different inputs?
Also is there a spring reverb in there? Signal getting through?
Try a different guitar cord and speaker cord?
Good luck.
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Yes 4 input normal and bright, high/ low. You can hear it on both channels.
Different guitars/cords.
No reverb.
Can't really here it at low volume levels, but when you start driving the circuit it becomes very noticeable.
BL
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Can't really here it at low volume levels, but when you start driving the circuit it becomes very noticeable.
BL
Have you tried cleaning/replacing/jumping the volume pots?
Or put a 200k/470k/ or higher resistor across the volume pot to simulate different volume settings? If the problem goes away or gets better your on the right track.
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You can hear it on both channels.
If you have the means, try injecting something close to that frequency, then monitor the "common" Tube past the 2channels at the grid and see if there is biggish DC as your signal increases
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I think this is a good time to set p a listening amp. I'll do a little thread on that when I get started.
BL
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https://www.18watt.com/storage/18-watter_buzz_info_311.pdf (https://www.18watt.com/storage/18-watter_buzz_info_311.pdf)
al
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Good read. Not sure how you would test this on a fixed bias amp.
BL