TECH HELP: Guitar amp buzzes with no guitar plugged in

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lolzgreg

lolzgreg

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I have an Engl Savage 120 guitar head. The amplifier makes a very clearly audible and nasty 60hz+overtone buzz on the clean and crunch channels that exists even when there is no guitar plugged in at all. The amplifier was recently looked over by a guitar tech and recently retubed. Does anyone have any idea what may be wrong with my amplifier?
 
I will run a frequency analyzer on the noise tomorrow. Thank you.
 
lolzgreg":vv2bx0bm said:
I will run a frequency analyzer on the noise tomorrow. Thank you.
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dont need to - 60hz is a deep bass frequency, 120Hz is more buzzsaw-ish.

does the hum increase with volume?

pull the preamp tubes one by one - see if the hum goes away.

if not, its in the powertube section.

if that is the case, then try another set of tubes just to make sure it isnt the tubes that are currently in the amplifier. if that solves the problem - order another set and have the amplifier rebiased.

if that doesnt fix the problem, then its component level oriented and needs to be serviced.
 
Jason_86_951":3ksasw07 said:
It could be that the power tubes (in pairs) that are not matched well to each other. This is why Fender in days of old put a "hum balance" control on their amps. To help with hum when power tubes are to far apart in bias to each other.

Just because the tubes are new doesn't rule out this being the issue. Do you have means to check bias on each of the power tubes?
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the "hum-balance" control you speak of is a virtual ground on the AC heater taps using 100 ohm resistors and is an old school trick to balance AC hum on all heater filament connections when no center tap is available.

a simple bias should not have thrown such a design out of whack, and this is an ENGL savage model - i dont recall engl's using such techniques for the heaters. however, no ENGL 120 schematic has ever been leaked - and i have no clue to know exactly what design they are using. the ones floating around for download are so riddled full of mistakes its almost a wonder why they didnt just put a picture of a whale. it amounts to the same thing as far as being able to use them.

the savage is also a single tetrode/kinkless tretrode push-pull design. from what i can research on the amplifiers they are not 120W push-pull setup but 50W push pull setup. so you cant pull a tube expecting to get an output.

it does make you wonder though - the amplifier was fine when greg purchased it and had it shipped to him - but began to have a problem as soon as a retube and bias was done by the tech. so tubes and proper bias would be the first to check. but theres no telling if anything else is at fault without having voltages to measure and a proper schematic to look at :dunno:

if the bias is too far apart individually you will get distortion of the original signal - but unless you are running the tube beyond the voltage heater-to-cathode maximum, then no hum should be present with or without signal. if you are close to this maximum, then an increase in volume and a presence of signal would throw it over and cause the humming and possibly even oscillations during playing - but those are all symptoms that he is not describing.
 
I have a 100% correct schematic for the Savage. It has a hum balance pot, but it does absolutely nothing. I had the AC heaters converted to DC for this amp to make it less noisy.

The amp had this problem when I got it, by the way :( It has actually improved a bit.

It's definitely more buzzsawish- like 120hz.
 
i sent you a PM - ill look it over and see what else i can find for you.
 
how old is this model?

was it sitting for a long time before you purchased it?

did it hum before or after the DC filament conversion?
 
I'm not sure how old it is, to be honest. It is in good shape on the outside and inside, though.

It was played by the person I bought it from; there was one owner before that.

It hummed BEFORE the DC filament conversion. It has gotten much quieter since the change, but it is still bad.

Thanks again,

-Greg
 
i know what the problem is then. sent you a PM with instructions :thumbsup:
 
All the tubes are DC heaters now or just the preamp? If it's just the preamp and the hum balance has no effect when you turn it I would suspect the hum balance pot. If the amp has seen any high voltage damage (wrong load, no load, used with an attenuator, shorted power tube) the hum balance pot gets smoked. I've seen it on many of the newer Marshall amps with the hum balance pot.
Jerry
 
Jerry,

I believe all of the tubes. I have to check with the people who did the work. He told me all the heaters were converted to DC.

Thanks for the advice. I just need to find someone who I can drive to, have this issue repaired, and bring my amp home in 100% functional shape.

The hum pot WAS fried when the amp was brought for service the first time. A new hum pot was installed, and did relatively nothing to lessen the severity of the buzz.
 
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