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Topic: trying to fix noisy amp |
Al Carey
From:
Dublin, NH USA
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Posted 12 Apr 2013 11:12 am |
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Hi,
I'm working on this old Sano tube amp, trying to eliminate a very noticeable hum. The hum seems to be coming from the reverb circuit(I remove its tube and the noise goes away)
The amp has a separate power amp and preamp chassis. The schematic portion shown is the reverb tube(6U10 triple-triode), and the final pre out to the power amp.
If I ground the point noted by the green arrow, the noise goes away. Grounding any other spots in the circuit(i.e. pin 9 of V2b or pin 11 of V2a) has no effect, altho I won't ground the other side of coupling caps where the plate voltage is.
I've replaced the plate resistor and the coupling cap on V2c, as well as the .001/300K feeding V4b, and tried 2 different tubes. The voltages all over the amp are low by ~10%, but consistent.
Any ideas?
Thanks,
Al
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Ken Fox
From:
Nashville GA USA
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Posted 12 Apr 2013 1:11 pm
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| Does the hum go away when the reverb pot is turned off? If not I would look at the filtering of power supply "B" that powers the reverb recovery portion of the tube |
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Al Carey
From:
Dublin, NH USA
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Posted 12 Apr 2013 1:38 pm
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Thanks for the reply, Ken.
The reverb control doesn't affect the volume of the hum, altho it does slightly affect the tone of the hum.
That same leg of the B+ supplies several other tubes, including V4b. The filter caps are all new, what should I look for in the B+ supply?
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Ken Fox
From:
Nashville GA USA
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Posted 12 Apr 2013 2:49 pm
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| Is there a filter choke just before that supply line? I had one bad on a Fender amp and it caused hum in the reverb. It had overheated and was acting like a capacitive path to ground. |
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Ken Fox
From:
Nashville GA USA
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Posted 12 Apr 2013 2:50 pm
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| Is the hum there with the tank unplugged. If so that may mean the source could the tank, cables or reverb drive side. Make sure there is no DC present at the tank input side of that .47 cap. You should also check any audio coupling caps for DC leakage. Very common in older amps. |
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Al Carey
From:
Dublin, NH USA
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Posted 12 Apr 2013 3:51 pm
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There's no choke in the amp.
I still get the noise with the tank unplugged, I've replaced the tank cables and don't see DC getting thru any of the coupling caps.
I tried to jumper from the tank return to another triode, but got a horrendous hum that way. I suspect it was an impedance mismatch or something, I just put everything back the way it was.
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Ken Fox
From:
Nashville GA USA
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Posted 12 Apr 2013 6:29 pm
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| Make sure the 500K reverb return pot is grounded per the schematic and that it is in fact a good pot. It should read 500K across the outside legs. If the pot is open to ground the input would be noisy, as it should have a ground reference at all times, from shorted to ground to 500K ohms above ground as the pot is rotated. |
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Al Carey
From:
Dublin, NH USA
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Posted 12 Apr 2013 6:48 pm
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Thanks, Ken. I'll look at that in the morning.
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 12 Apr 2013 7:31 pm
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The schematic doesn't show shielded wiring etc for the tank... the wires should have grounded outsides i.e. shields. Only one of the shields should connect to the tank chassis... if you unplug both wires from the amp you should not find ground conductivity between the two shields, this can cause a ground loop. OTOH, the tank chassis should be grounded to the amp chassis... but not twice.
Try unplugging the tank return from the tank and shorting it... this way you're isolating the circuit from the tank... just unplugging the wire is not enough, you have to short the return signal (remember a guitar cord hums when you unplug it from the guitar).
The tank needs to be far from the power transformer... if it's too close you'll get hum.
_________________
Too much junk to list... always getting more. |
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Ken Fox
From:
Nashville GA USA
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Posted 13 Apr 2013 4:04 am
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Apparently the tank is not the issue, per his comment above:
"I still get the noise with the tank unplugged, I've replaced the tank cables and don't see DC getting thru any of the coupling caps." |
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Jack Stoner
From:
Kansas City, MO
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Posted 13 Apr 2013 5:34 am
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| Do you have a Scope where you can actually see what is happening? |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 5:36 am
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| Ken Fox wrote: |
| Make sure the 500K reverb return pot is grounded per the schematic and that it is in fact a good pot. |
Looks like you're on to something there, Ken. I checked the pot, and it only measured 28K across the outside terminals. I replaced it with a pot I had in my parts bin, which turned out to be very noisy. I still got the hum, but also scratchiness when I turned it.
Should this be a linear or audio taper? The original reads "PB3516" and "1376619", which doesn't tell me anything.
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 5:40 am
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| Jack Stoner wrote: |
| Do you have a Scope where you can actually see what is happening? |
I do have a scope, unfortunately I don't have the level of electronics knowledge one would expect to accompany it. 
What should I look for, and where? I'm reluctant to put the scope on plate voltage, altho it's rated for 600v combined AC/DC so it certainly should be safe.(Tektronix 453)
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 5:43 am
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| Stephen Cowell wrote: |
| The schematic doesn't show shielded wiring etc for the tank... the wires should have grounded outsides i.e. shields. Only one of the shields should connect to the tank chassis... |
Only the shield of the return cable is grounded. It's connected along with the signal to a footswitch jack. I've jumpered from a chassis ground to that point and the noise is unaffected.
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Ken Fox
From:
Nashville GA USA
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Posted 13 Apr 2013 7:03 am
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You might also want to replace the 1.5K cathode resistor.
If you ground the grid does the noise go away? I would use a linear taper for the reverb pot (same as Fender does on their tube amp reverb circuits).
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Ken Fox
From:
Nashville GA USA
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Posted 13 Apr 2013 7:34 am
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| I looked at the complete schematic. The "B" power supply is only for that reverb recovery section of the tube. You might want to replace the B+ dropping resistor in that power line as well. Also does the hum balance control seem to work (for the tube heater balance)? |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 8:01 am
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| Ken Fox wrote: |
You might also want to replace the 1.5K cathode resistor.
If you ground the grid does the noise go away? |
No. I'm going to go buy some 1.5K resistors, and hopefully a 500K pot.
I did check the value of the cathode resistor and it's ok, but it could still be noisy?
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 8:04 am
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| Ken Fox wrote: |
| I looked at the complete schematic. The "B" power supply is only for that reverb recovery section of the tube. You might want to replace the B+ dropping resistor in that power line as well. Also does the hum balance control seem to work (for the tube heater balance)? |
I replaced that 12K dropping resistor, altho I could only get 15K.
The hum control does work, but the noise it effects is more of a buzz(higher pitched) than the noise I'm getting from the reverb circuit.
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Tim Marcus
From:
San Francisco, CA
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Posted 13 Apr 2013 8:07 am
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I'd suspect two other things:
Lead dress (reverb can be really finicky)
The 300k/.01 reverb mix resistor
The reverb circuit might have always been that noisy. Might just have to add a few caps in to filter it out or re-wire it to Fender style reverb which is easy to get very quiet
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Milkmansound.com |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 8:13 am
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I've replaced the 300K/.01 altho, I couldn't get 300K so used a 330K.
| Tim Marcus wrote: |
The reverb circuit might have always been that noisy. Might just have to add a few caps in to filter it out or re-wire it to Fender style reverb which is easy to get very quiet |
Could I do that using the existing 6U10 tube? Would I need to add a choke?
The reverb right now is very strong, cool sounding, altho it's got something of a "howl" if turned up more than 1/2 way. Almost like it's too strong.
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Ken Fox
From:
Nashville GA USA
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Posted 13 Apr 2013 10:25 am
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| Yes, just like a plate load resistor it can get noisy. Had one in an amp that was very noisy a while back. Last thing I would have suspected. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 13 Apr 2013 10:39 am
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| Ken Fox wrote: |
Apparently the tank is not the issue, per his comment above:
"I still get the noise with the tank unplugged, I've replaced the tank cables and don't see DC getting thru any of the coupling caps." |
R.E. my comment about a guitar cord being unplugged causing hum... you have to short the RCA end of the cable to confirm that the stage is humming, not the tank/cords. When you unplug a guitar cord from an amp the jack shorts, for this reason.
_________________
Too much junk to list... always getting more. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 13 Apr 2013 10:46 am
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| Al Carey wrote: |
What should I look for, and where? I'm reluctant to put the scope on plate voltage, altho it's rated for 600v combined AC/DC so it certainly should be safe.(Tektronix 453) |
Your scope is not a problem.. but your probe might be. The probe would be the weak link here... find its rating before probing HV. If your probe is x10 then your probe would see 6000v before your scope saw 600v... as long as you ground your probe end. And where you ground your probe end can actually cause *more* hum.
Using a scope to probe for hum often complicates matters... is the scope injecting the hum? Your ears are plenty sensitive. You should be able to short out the signal and find where the hum comes in... don't short any DC, of course.
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Too much junk to list... always getting more. |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 11:56 am
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I replaced the cathode resistor on V2c and the 500K Reverb pot, and it's much better!
I still get some hum/hiss from the reverb- when the pot is off it's a noticeable hum, at 1/4 turn it's almost silent, then as I turn it up more it brings up a hiss which gets louder as I turn the pot. Does that make any kind of sense?
Thanks so much for al the help guys! I really enjoy doing(and learning!) this stuff, even when it has me banging my head against the wall!
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc... |
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Al Carey
From:
Dublin, NH USA
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Posted 13 Apr 2013 12:02 pm
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| Stephen Cowell wrote: |
Your scope is not a problem.. but your probe might be.
Using a scope to probe for hum often complicates matters... is the scope injecting the hum? Your ears are plenty sensitive. You should be able to short out the signal and find where the hum comes in... don't short any DC, of course. |
I have an X10 probe, but I've found it pretty much futile to try to track down the noise with the scope, and that's probably more my lack of knowledge than anything to do with the scope.
I also have a BillM audio probe, but that hasn't been any help either, it seems to pick up noise everywhere.
I have the utmost respect for B+ voltage, and am very reluctant to touch any circuit before a coupling cap for fear of DC. So that always leaves me with a big question mark of what I'm missing in those parts of the circuits...
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