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View Full Version : Re: Hum ?


Paul Sanchez
July 29th 03, 05:35 PM
The negative side of the DC filament supply is grounded. Although when I
"lifted it", it didn't make any difference.
The hum seems to be centered around the pre-amps(6sl7 & 12au7).

"Ronald" > wrote in message
...
> Is your heater circuit floating ? (not connected to anything else?)
> Does it still hum when the (output-)tubes are removed ?
>
> "Paul Sanchez" > schreef in bericht
> ...
> > RDH4 says(page 1196) ",in fairly high gain amplifiers, it is advisable
to
> > use a potentiometer of 100>500 ohms across the heater supply."
> > I'm having a problem with hum in a "high gain" amplifier. I've tried
> > everything I know of to remedy the situation. Since RDH4 mentions this
> > problem with high gain amps, my question is: why are high gain amps
> > susceptible to hum?
> > I'm using DC on the filaments. So, I would think, this is a non-issue.
> > The circuit is a fairly normal Fender preamp circuit with an added 12au7
> > gain stage(only one side is used).
> > Any insights are appreciated.
> >
> > --
> > No Tubes? You Lose!
> > www.redironamps.com
> >
> >
>
>

Ronald
July 29th 03, 05:47 PM
Are the tube to close to the power tranny ?
A 6SL7 is a bit sensitive ......
Try to experiment with a grounded shield between them .

"Paul Sanchez" > schreef in bericht
...
> The negative side of the DC filament supply is grounded. Although when I
> "lifted it", it didn't make any difference.
> The hum seems to be centered around the pre-amps(6sl7 & 12au7).
>
> "Ronald" > wrote in message
> ...
> > Is your heater circuit floating ? (not connected to anything else?)
> > Does it still hum when the (output-)tubes are removed ?
> >
> > "Paul Sanchez" > schreef in bericht
> > ...
> > > RDH4 says(page 1196) ",in fairly high gain amplifiers, it is advisable
> to
> > > use a potentiometer of 100>500 ohms across the heater supply."
> > > I'm having a problem with hum in a "high gain" amplifier. I've tried
> > > everything I know of to remedy the situation. Since RDH4 mentions this
> > > problem with high gain amps, my question is: why are high gain amps
> > > susceptible to hum?
> > > I'm using DC on the filaments. So, I would think, this is a non-issue.
> > > The circuit is a fairly normal Fender preamp circuit with an added
12au7
> > > gain stage(only one side is used).
> > > Any insights are appreciated.
> > >
> > > --
> > > No Tubes? You Lose!
> > > www.redironamps.com
> > >
> > >
> >
> >
>
>

Steve Robertson
July 29th 03, 07:27 PM
High gain circuits are susceptible to hum preceisly because they are high
gain - Any small amount of hum introduced anywhere will get greatly
amplified.

The hum is not likely comming from the heater circuit, since it is DC.

First thing to do is pull the preamp tube(s). If the hum is still there, then
the hum is comming from the power amp, possibly a gound loop. However, the
power amp is the least likely place for the hum to originate.

If it's not the power amp, then your preamp is picking up hum from somewhere.
Do you have multiple input jacks? If so, it's a good idea to use the
"shorting switch" type jacks to short unused inputs to ground. Use tube
shields on the small-signal tubes. Place a grounded sheet metal shield
between the power transformer and the rest of the circuitry. If you have to,
fabricate a grounded sheet metal shield that covers all of the preamp
circuitry. Do you have a flourescent light fixture or a light dimmer in the
room? They can induce hum. Put the amp in another room. Is it quieter? If so,
then something in the original room is inducing the hum. I have a guitar amp
(just so happens to be a Vox AC-15 clone power amp, but with a
Silvertone/Danelectro clone preamp) that hums if I place it too close to the
wall near the wall socket.

Best of luck,

Steve Robertson

Paul Sanchez wrote:

> RDH4 says(page 1196) ",in fairly high gain amplifiers, it is advisable to
> use a potentiometer of 100>500 ohms across the heater supply."
> I'm having a problem with hum in a "high gain" amplifier. I've tried
> everything I know of to remedy the situation. Since RDH4 mentions this
> problem with high gain amps, my question is: why are high gain amps
> susceptible to hum?
> I'm using DC on the filaments. So, I would think, this is a non-issue.
> The circuit is a fairly normal Fender preamp circuit with an added 12au7
> gain stage(only one side is used).
> Any insights are appreciated.
>
> --
> No Tubes? You Lose!
> www.redironamps.com

Choky
July 29th 03, 10:53 PM
"Paul Sanchez" > wrote in message
...
> The negative side of the DC filament supply is grounded. Although when I
> "lifted it", it didn't make any difference.
> The hum seems to be centered around the pre-amps(6sl7 & 12au7).
>
> > >
> >
> >
>
>
if hum is heater related,try to elevate it to some DC potential (via R
divider from Ub supply);
but-first check what other Rodents tell ya already.

--
Choky
Prodanovic Aleksandar
YU