OT - Firefly, why connect center tap to R13 and not ground?

[sir Franc of SOSCASTOA]

Hi,

I am about to start the wiring of my firefly. I was reading about filament wiring in Gerlad Weber's book and was wondering why you would connect the center tap (I use a Hammond 369EX tran) to the cathodes of the 12AU7 (=R13) instead of ground? You normally connect a center tap to ground because that's where centertaps are for aren't they?

Thanks,

Franc.

[Jim Jones]

Hi Franc,

Tying your filament center tap to the cathode voltage "elevates" your heaters with respect to ground.  This can really help keep things quiet without having to resort to powering your filaments with DC.

Jim

[Paul Marossy]

Just for the record, you can also run your heater filaments on DC as well. Convention seems to be running them on AC, for various reasons.

Read this article for the pros and cons of heater voltage and AC vs. DC:
http://www.tubecad.com/july2000/

Hope that helps.

[sir Franc of SOSCASTOA]

Thanks guys, that explains a lot.
I learned from Weber's book that rectifying the heater power can be a problem due to components needed.
I 'll try Doug's method first.

[Doug H]

Just for the record, you can also run your heater filaments on DC as well. Convention seems to be running them on AC, for various reasons.

Read this article for the pros and cons of heater voltage and AC vs. DC:
http://www.tubecad.com/july2000/

Hope that helps.

The general consensus from the amp building community seems to be that using DC heaters is a PITA that's not worth it unless you are building something *really* high gain like an SLO or triple rec clone.

Doug

[puretube]

AC heating can be more PITA for beginners...

[Doug H]

AC heating can be more PITA for beginners...

How so?

Doug

[puretube]

"routing", high current 50/60Hz wires/cables near high-impedance grids...
and the science of grounding/de-humming.

[Paul Marossy]

Well, that may be true about designing DC heater supplies. I don't know enough about that to talk intelligently about it.
I do know that my Seymour Duncan Convertible uses DC tube heating, and it actually looks pretty simple they way they went about it. But, they also had SS modules, so I guess they had to design it that way, at least that's what I think.

[puretube]

the real early tube apparatus only knew DC;
only later, "after the invention of AC", transformers
were used to provide heating;
they didn`t have high current rectifiers at that time
to get some DC for the filaments, so they invented
the split heaters to (phase-) cancel the AC-hum.

[Doug H]

"routing", high current 50/60Hz wires/cables near high-impedance grids...
and the science of grounding/de-humming.

In my experience, even for beginners, those issues are not difficult to contend with, esp. with some good simple & basic layout skills which are a normal part of the learning process anyway. IMO, it's not worth the complexity of building another DC supply and then finding the room in the chassis to put it. If anything, when it comes to noise, I've had a lot more problems with rectifier buzz than I have ever had with heater hum.

But everything is a tradeoff I suppose.

Doug

[Paul Marossy]

"IMO, it's not worth the complexity of building another DC supply and then finding the room in the chassis to put it."

Maybe so. It's not that hard to get a DC heater supply. All you have to do is get the right voltage (which could be done at the power transformer), put in a bridge rectifier, filter it if necessary and perhaps use a voltage regulator. OK, so it's a few more parts... I guess it's a matter of opinion, isn't it?  

[Jim Jones]

I'd think you'd have to have a pretty high gain amp before DC heaters were necessary.  Besides, the fewer silcon demons we end up with in our tube amps the better off we are.  

Jim

[Doug H]

It is a case of personal taste.

DC heaters sound like real good idea to an EE like me. But pragmatically speaking, from what I've been able to gather, a lot of experienced amp builders feel it is a case of diminishing returns- unless you are building a 5150 or something.  That was my only point to begin with.

Doug

[puretube]

btw.: I don`t propagate DC for power-tubes!
(though I do use it sometimes with 2 series-heated EL84s...)

[Paul Marossy]

I hear you jim, Doug, puretube. I'm only playing devil's advocate as usual.  :wink:

It makes a lot of sense to use AC for the tube heaters for a lot of reasons, especially from the standpoint of simplicity and cost when you are a manufacturer of amps. But, if you wanted to make an ultra-clean sounding tube amp, DC heaters might be a consideration, IMO.

[puretube]

just for the record (though it doesn`t at all give an answer to the question this post was about... ) :
in fact I`m referring to my designs, some of them having an internal gain capability of over 1 million with high input impedance, and that in turn are hooked up before an 5150 or the like...