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Old 6th May 2012, 10:16 AM   #1
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Default raising filament voltages above cathode 6v6 please check.

Hi,
I have built a fender champ 5f1, I am fine tuning it and currently looking at the filament wiring. I have read on another forum that I can connect the filament CT to pin 8 on the 6v6, is this true? I don't understand how that can work.

If anyone can confirm explain or even point me in the direction of some literature, that would be much appreciated.

Cheers

Last edited by magnetman; 6th May 2012 at 10:21 AM.
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Old 6th May 2012, 10:22 AM   #2
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here is the schematic, sorry!

Schematic removed by moderation. Please see rule 13 of the forum rules.
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Old 6th May 2012, 11:12 AM   #3
M Gregg is offline M Gregg  United Kingdom
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Hi,


They are trying to lift the heater wiring to the same potential as the cathode..I have never seen it done others might..

Looking at their idea the cathode is at a potential ..IE voltage by connecting to pin 8 you are connecting the heater to the cathode voltage so they should be at the same potential..however the O/P tube is biased at this point so any heater fault could burn the tube out..

I would either lift the heaters or balance /ground the Ct..

I don't know what effect linking the heater to the cathode would cause. In theory the heater is an AC signal so you may get Hum...

I personaly would not link them togeather..each to his own..

Regards
M. Gregg
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Old 6th May 2012, 11:34 AM   #4
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[QUOTE=M Gregg;3013507
I would either lift the heaters or balance /ground the Ct..
[/QUOTE]

So from what I understand to balance I would Disconnect the CT and use a 100 ohm resistor from each filament of the wires to ground, at the lamp on this amp.

If I wanted to lift the heaters would I just put a resistor 'say 22 ohm' from the CT to ground? is this right?

cheers.
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Old 6th May 2012, 11:41 AM   #5
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By connecting the ct to pin 8 of the 6V6 you are raising the heaters potential to that of the output tubes cathode. This reduces hum by eliminating the diode-effect between the heater and the cathode in indirectly heated tubes. This technique was commonly used in Magnavox amps back in the day and is a way to accomplish raising the heaters without adding to the parts count ( and cost) of the amp. The ac from the heaters goes thru C6 to ground and the dc potential is at whatever the cathode of the 6V6 is, somewhere around 12v give or take. I say go for it, worst it can do is reduce hummmmmmmmmm.
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Old 6th May 2012, 11:47 AM   #6
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by magnetman View Post
So from what I understand to balance I would Disconnect the CT and use a 100 ohm resistor from each filament of the wires to ground, at the lamp on this amp.

If I wanted to lift the heaters would I just put a resistor 'say 22 ohm' from the CT to ground? is this right?

cheers.
I use 220 ohm to balance from each side of the heaters, You could just Gnd the CT...or you could use a voltage divider across the B+ to lift the heaters..You can use something like 680K-100K to give a lift..But you have to remember that the heaters are now up at voltage. IT depends on how much hum you have..

A real easy way is to connect the CT to Gnd with a polyropylene 600v 0.1uF and it will charge up from leakage.. its quite effective for trouble hum..It will charge up so again the heaters can be at a voltage..above Gnd

Reading the above seems like it might work..then again only in a mono amp because if you did this with two channels the cathodes would connect togeather Via heater supply...

Regards
M. Gregg
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Last edited by M Gregg; 6th May 2012 at 11:53 AM.
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Old 6th May 2012, 11:49 AM   #7
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So what do you think with regards to M Greggs comment "...the O/P tube is biased at this point so any heater fault could burn the tube out." What are the risks of this if there is a fault. Is there a way to protect my tube?

I am keen to try this technique if it will work.
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Old 6th May 2012, 11:53 AM   #8
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hey I like the capacitor idea.

with regards to the hum issue there is enough to make a fuss. I have honed the grounding a little more. I have a star ground at the transformer this takes the mains the CT for HT and filament, also the power caps. At the other end of the chassis i have all preamp parts connected with ground buss to the input jack.

Ideally I would like to keep this as simple as possible.

cheers

Last edited by magnetman; 6th May 2012 at 11:56 AM.
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Old 6th May 2012, 11:56 AM   #9
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IF there was somehow a heater fault to ground then the output tube could be in trouble, but if properly designed then the fuse would blow anyway cutting the power. A fault where the heater goes open would have no effect other than the affected tube would not work. So in reality there is not much danger.. This is a good method for raising the heater to a reasonable voltage without extra parts or expense. Methinks the engineers over at Magnavox were no dummies so if it was good enough for them in a commercial product ( many of those amps are still playing) then it is a good solution here IMHO.
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Old 6th May 2012, 11:59 AM   #10
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by mctavish View Post
IF there was somehow a heater fault to ground then the output tube could be in trouble, but if properly designed then the fuse would blow anyway cutting the power. A fault where the heater goes open would have no effect other than the affected tube would not work. So in reality there is not much danger.. This is a good method for raising the heater to a reasonable voltage without extra parts or expense. Methinks the engineers over at Magnavox were no dummies so if it was good enough for them in a commercial product ( many of those amps are still playing) then it is a good solution here IMHO.
The heater supply should be isolated anyway so it might work OK.

I have found the cap to Gnd works well its worth a try if you get stuck..
It has to be a polypropylene HV..600V or higher..

Regards
M. Gregg
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