Please help me interpret this 5F1 mod.

[Kregg]

From Marsh Amps:
"A mod to the filament circuit designed to have it inject less noise into the signal path of the
amp. This requires you take 2 pieces of green filament wire twist it tightly into lengths of 4.5
inches (for connections between 6V6 and 12AX7) and 4 inches (for connections between the
pilot light and the 6v6 filament socket pins).
Then take the two green wires from the Power Transformer (6.3VAC supply) twist them and connect one wire each to the pilot light tangs.
The pilot light and the filament circuit is no longer grounded on one side,instead you take two 100 Ohm resistors connect one end of each to the pilot light tangs with the filament supply (from Tranny) and Feed (to tube) the other end of each resister get connected together and then to ground."

The problem I'm having is this sentence, "... take two 100 Ohm resistors connect one end of each to the pilot light tangs with the filament supply (from Tranny) and Feed (to tube) the other end of each resister get connected together and then to ground."

Did they mean to say instead, "... connect one end of each resister to the pilot light tangs with the filament supply (from Tranny) and Feed to tube. The remaining ends of each resister are connected together, then to ground." ?

[Jana]

Yes, it is not clearly worded but it is the standard faux center tap that is created with the 100 ohm resistors.

[Kregg]

Thanks Jana, I just found a clearer explanation on Doug Hoffmans site, http://www.el34world.com/charts/CommonHookups.htm

[Jana]

A picture is worth a thousand words!

[Bob-I]

A picture is worth a thousand words!

That's one of those running jokes at work, the technical writer I work with always wants to use 1,000 words, I draw pictures.

Why use a picture when a thousand words will do.

Hoffman's site is a wealth of information, excellent pictures and explainations.

[rfgordon]

Another benefit from using the 1/2 watt resistors to ground is that they form a fuse of sorts. Should you have a tube, particularly a power tube, that shorts plate-to-filament, then the two resistors will open up, saving the PT from a violent, stinky death.

I had this happen in my Dragon amp, which runs one KT88 single ended. After the short, the tube measured 1k5 ohms from plate to filament! Yikes!

[paolojm]

You can achieve even further noise improvements by connecting the centre of the 100 ohm resistors to a positive DC voltage instead of ground. I hooked it up to the power valve cathodes in my build and have virtually noise, with 4 gain stages. There is only slightly more noise when connected to ground the difference is noticable, in my amp anyway.

[Andy Le Blanc]

That's a hum-dinger, it works. I have a 50w with a pair of 6l6, where the
first two tubes in the pre have their heaters wired series and form the bias
resistor for the pair of 6l6. This works simply like this with a pair of 6l6, EL34,
or a quad of 6v6.

[tubeswell]

That's a hum-dinger, it works.

Well yep it is - altho' a humdinger is more commonly thought of as (say a 200R) pot with each end of the pot going to one each of the heater winding ends and the pot wiper going to ground, so that you can dial-in the perfect balance.

For a bit of useless knowledge, the idea of the two 100R resistors, is that if they are 1% tolerant, then you will get a balanced ground reference point for both sides of the heater winding. The only advantage of the pot is that you can fine-tune it a bit. A heater winding centre tap will also work in 95% of cases, and although some centre taps aren't perfectly in the middle of the winding, they don't slurp any current either (like the resistors/pot method do)