ax84 SEL filament center tap

[apache]

I'm confused as to wether or not to add an artificial center tap to the SEL I'm going to build. Its not in the schematic, all I know is some voltage elevation thing by connecting the filament to one of the Kt88 pins?

Does it need an artificial center tap?

[Colossal]

I would add a 100R/100R center tap or a 500R 2W pot (with the wiper to ground or) elevate the heaters to DC by tapping off the KT88 cathode and connecting to your new center tap. Single ended amps can benefit from additional techniques to reduce hum and buzz such as increased filtering, etc, more so than their push-pull counterparts.

[surfsup]

apache, I believe the artificial center tap is beneficial when the power transformer's filament winding is not perfectly balanced to its centertap. By taping off the centertap on the PT, and using two 100R resistors, you artificially balance the winding better, hence the name "artificial center tap". It doesn't cost much but a little wire and a couple resistors so it won't "hurt" to do this, unless I suppose your 100Rs are way off spec.

What I did is buy 20 100R resistors and just measure them, when I find two that are almost the same (say 98.4 ohms and 98.5 ohms) I use those two since they are closely matched. this gets me more resistors to play with and I don't need to spend more money on the 1% versions.

[ToneMerc]

I'm confused as to wether or not to add an artificial center tap to the SEL I'm going to build. Its not in the schematic, all I know is some voltage elevation thing by connecting the filament to one of the Kt88 pins?

Does it need an artificial center tap?

Apache, Colossal hit the nail on the head. Another way to reduce hum is to add a single 100 ohm resistor to the existing center tap of the PT. Then you connect that resistor to the power tube cathode. Either scheme just adds some DC voltage to the filament suppy.

TM

[labb]

It all depends on what Power transformer you have. If it already has a CT for the heater supply then you do not have to add one. Attaching the CT, whether it is an artificial one you add or is built into the transformer, to the Cathode of the power tube is a method of reducing hum. Sometimes it is better that just grounding the CT. Sometimes not. I just always connect to the Cathode and don't worry about it.

[rogb]

I think you need 2 x100R to balance it on each 3.15v tap.
If you have a Hammond tranny, it may have a green/yellow heater CT wire, I did and this is what I used to reference the heaters to DC, via pin 3 of the EL84. That would be pin 8 of the KT88.

[apache]

Ok, thanks for the help guys. Why didnt they add any center tap to the schematic? Also, why use 100ohm resistors, I've heard of people using two 47ohm resistors instead.

Sorry for the slow reply, the Chinese government blocks ampgarage! I have no idea why!!

I can easily get some 1% metal film Chinese military stock 100ohm resistors here, they're incredibly cheap, about $0.2 for 20.

[surfsup]

That is cheap! Send some over...

[M Fowler]

Yes 47R to 100R are used as artificial center taps.

[Markusv]

Not to hijack but please remind me if adding a 100 ohm resistor to the center tap and connecting to the cathode of a fixed bias, PP pair is also OK?

Markus

[ChrisM]

Not to hijack but please remind me if adding a 100 ohm resistor to the center tap and connecting to the cathode of a fixed bias, PP pair is also OK?

Markus

The cathode of a fixed bias amp doesn't have much voltage on it so you don't get much "elevation".

You'd be better off making a voltage divider off the B+ and elevating the heaters by connecting the CT to a 50V or so node.

[rp]

the Chinese government blocks ampgarage! I have no idea why!!

- Amp porn?
- We are always pooping on Chinese tubes and the like? For a super duper power they sure got a thin skin. There, now I did it, no chance of TAG in China now

Don't worry about filament hum till you get it. I never got it even when I didn't know anything. If you have a CT use that. If not use 2x100R. If not then try tricks like elevating off the cathode. DC? just more parts, complications, and something extra to fail IMO. Always start simple, then do what you gotta do.

[apache]

What differences are there between a 47ohm and 100ohm artificial center tap?

Even Chinese tube dealers here think Chinese tubes aren't very good!
You can get an incredible range of 50s and 60s Chinese and Russian tubes though, generally they are considered to be pretty decent. I can't believe the prices people on ebay charge for 6n2s and 6n1s, you can get them for $1.5 here. I just wish there were more foreign NOS tubes in China.