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Old 18th July 2008, 10:27 PM   #11
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Is this thing cathode biased? If so referencing heater to the cathode of the power tube helps with hum reduction.

mike
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Old 18th July 2008, 10:43 PM   #12
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Hi Mike,

Yes, it is cathode biased. From a practical point of view, does this mean less hum with separate heater wires referenced to ground with the virtual center tap as mentioned above or is this an argument for running one limb of the heater circuit through the chassis?

I have attached the schematic for reference as this seems to be getting a little more controversial than I first thought it might be.

Cheers,

Rob
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Old 18th July 2008, 11:08 PM   #13
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No, use twisted pair for all of the heater wiring and connect the CT (or virtual CT) to the cathode of a power tube instead of ground. No other part of the heater circuit should be connected to ground or any other reference point.

This puts the heaters at a higher DC potential than the cathodes of the preamp tubes so that hum induced in the tube itself is essentially eliminated. Careful wiring is still required to prevent picking up any hum radiated from the wiring in the input lines but this is necessary anyway.

I did this with a high gain modified Marshall 18W amp and had zero hum. I had a real CT on my heater windings so I don't know if the virtual CT will have any negative impact on this scheme. Maybe some of the other guys can comment.
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Old 18th July 2008, 11:17 PM   #14
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Quote:
Originally posted by mashaffer
No, use twisted pair for all of the heater wiring and connect the CT (or virtual CT) to the cathode of a power tube instead of ground. No other part of the heater circuit should be connected to ground or any other reference point.

This puts the heaters at a higher DC potential than the cathodes of the preamp tubes so that hum induced in the tube itself is essentially eliminated. Careful wiring is still required to prevent picking up any hum radiated from the wiring in the input lines but this is necessary anyway.

I did this with a high gain modified Marshall 18W amp and had zero hum. I had a real CT on my heater windings so I don't know if the virtual CT will have any negative impact on this scheme. Maybe some of the other guys can comment.
I spent quite a lot of time on a mixer pre amp with an ECC83 that had hum problems.

Routing of the heater wires is very important.
Passing signal wires or mains wires induced a lot of hum.

In the end I opted for a regulated 12volts DC and this turned out best in my design. This is obviously easy for a one valve design.
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Old 19th July 2008, 07:18 AM   #15
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Quote:
I had a real CT on my heater windings so I don't know if the virtual CT will have any negative impact on this scheme.
It shouldn't. I would think that they would work nearly identically.
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Old 19th July 2008, 11:51 AM   #16
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Quote:
Originally posted by Rob11966
Perhaps these guitar amp people don't worry so much about hum!

For reasons I cannot understand, this is true. I have spoken with a number of owners of guitar amps, and each time I mentioned we should work a few simple tricks (they love to mod, after all) to reduce hum, they shrug their shoulders. Just don't seem to care. Don't get it.
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Old 19th July 2008, 12:55 PM   #17
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DON'T run heater current through the chassis... I doubt Fender did - a lazy draftsman probably just used the ground symbol instead of showing the actual connections. If heater circuit is connected to chassis, it should only be in ONE place. If there's one rule for making a quiet amplifier, it's to provide a return circuit for every current - don't allow high currents to share a path with signals.
Well I cannot comment on what Fender did but I can comment on what Wurlitzer did on a particular organ amplifier they manufactured. I have 4) of the same model of the Wurlitzer amplifiers that they grounded one side of the heater. They did not do so at one spot either. It has hum and it should have been done as individual runs. It would have been nice if it were twisted and routed correctly for lowest hum.
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Old 19th July 2008, 01:23 PM   #18
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Quote:
Originally posted by zigzagflux



For reasons I cannot understand, this is true. I have spoken with a number of owners of guitar amps, and each time I mentioned we should work a few simple tricks (they love to mod, after all) to reduce hum, they shrug their shoulders. Just don't seem to care. Don't get it.

This is becasue guitar players usually like it loud and then you cant hear the hum anyway. So long as the hum isnt cutting through the guitar ssignal then its not the end of the world.

I run a mobile disco and built an ECC83 based mixer/pre amp and it was important on that to get rid of as much hum as possible as there is sometimes a short quiet period between tracks.
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Old 19th July 2008, 11:14 PM   #19
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Thanks for the replies folks.

Condensing the information, it seems that some manufacturers seem to ground one leg of the heater wiring - whether fender did or not is not absolutely clear but it seems that they might have. This does cause increased hum but they (the users) don't seem to care!

Unfortunately, I have obsessive compulsive traits, so I do care. I will follow the advice above and create a virtual ground and run the (twisted) heater wires as per normal.

Thanks again foor all the useful replies and advice.

Rob
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Old 20th July 2008, 02:08 AM   #20
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Unfortunately, I have obsessive compulsive traits, so I do care.
Not always a bad thing. Happy hunting.
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