I am not a tech but I can follow schematics and do repairs fairly well But
I am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in the
background
I have recapped the amp coompletely and retubed it. And tried to swap each
tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER REFERENCE"
Can anyone help me with resolving that or tell me where that may be in the
schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
Thank you in advance for any info tha can help

Kirk

www.kirkpowers.com

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In article ,
svtbass wrote:

I am not a tech but I can follow schematics and do repairs fairly well But
I am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in the
background
I have recapped the amp coompletely and retubed it. And tried to swap each
tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER REFERENCE"
Can anyone help me with resolving that or tell me where that may be in the
schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
Thank you in advance for any info tha can help

I suspect that the "heater reference" may be referring to the "HUM
BAL" potentiometer in the lower left. This potentiometer appears to
adjust the ground reference voltage point for the low-voltage AC which
is fed to the tube filaments (heaters).

This is probably a relatively hefty wire-wound pot... its static
dissipation would be around 400 milliwatts, so I'd guess they used at
least a 1-watt wirewound and perhaps something heavier.

You can try tweaking this pot to minimize the amount of hum coming out
of the amp. The pot may be in need of a good cleaning, if its wire
element has gotten dirty or badly tarnished over the years, and it's
possible that it might even need to be replaced. Also, check the
integrity of the connection from its wiper to ground - if this isn't
solid, the ground reference voltage might tend to wander.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Thank you.. i will try the Hum balance pot and check the solder... It does
seem to work as I rotate it throughout from CCW to CW...And somewhere in the
middle there seems to be less hum... it kind of has the sound of about 120Hz
and not 60Hz... it is a very mellow sound like a real smooth sine wave in
that is present in the background.. I will try what you precribed and make
sure all is OK with the connections and since I have been re-capping and all
I might a s well replace that wire wound pot... piece by piece it will be a
new amp...

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"Dave Platt" wrote in message
...
In article ,
svtbass wrote:

I am not a tech but I can follow schematics and do repairs fairly well
But
I am lost here.
I have a 1972 Ampeg SVT 300 watt tube Bass amp that has a slight hum in
the
background
I have recapped the amp coompletely and retubed it. And tried to swap each
tube one by one to see if the hum can be eliminated.
I was told that there may be a possible problem with a "HEATER REFERENCE"
Can anyone help me with resolving that or tell me where that may be in the
schematic.
The amp is OK to gig with but to record with the hum is a bit annoying.
This is a link to the schematic
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
Thank you in advance for any info tha can help

I suspect that the "heater reference" may be referring to the "HUM
BAL" potentiometer in the lower left. This potentiometer appears to
adjust the ground reference voltage point for the low-voltage AC which
is fed to the tube filaments (heaters).

This is probably a relatively hefty wire-wound pot... its static
dissipation would be around 400 milliwatts, so I'd guess they used at
least a 1-watt wirewound and perhaps something heavier.

You can try tweaking this pot to minimize the amount of hum coming out
of the amp. The pot may be in need of a good cleaning, if its wire
element has gotten dirty or badly tarnished over the years, and it's
possible that it might even need to be replaced. Also, check the
integrity of the connection from its wiper to ground - if this isn't
solid, the ground reference voltage might tend to wander.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

You could change it to a smoothed DC filament supply - then no need for
hum balancing and slightly less chance of hum crosstalk into hi-z sections
of the circuit...


geoff


Hi geoff

Wow.. if i were a tech.. i would do that. .but i have no idea how to
accomplish that.
I live in the Austin TX area but have not found an Ampeg SVT expert.
here are schematics to the amp
SVT Preamp http://www.drtube.com/schematics/ampeg/svtpre-jp.gif
SVT Poweramp http://www.drtube.com/schematics/ampeg/svt6550-jp.gif
http://www.kirkpowers.com/SVT_AmpSchem.jpg
I have tried adjusting the hum balance.. and the pot works well throughout
its travel from CCW to CW with the min hum in the aproz center.