6.3v with CT vs no CT

[xtian]

I"m not going to use any form of CT or hum balance on my filament coils. I'm going to implement a 60 Hz notch filter post power tubes. Shouldn't be a problem.

Sarcasm detector broken. Are you serious?

[pompeiisneaks]

I'm not sure if that's sarcasm or not...

I hope it was, and I missed it. You need either a center tap OR a virtual center tap or you'll have horrible hum... (virtual center tap includes the option of a humdinger if you want)

~Phil

[Medley]

Can we ignore the CT on a heater winding and use resistors as faux CT? I think yes, but always good to reinforce knowledge.

I know you can't do BOTH (real CT and pseudo CT) without causing trouble.

I know this is conventional wisdom, but I know someone (me) who wired his first amp build exactly this way with zero problems. There is not even a slight hint of hum; the noise floor on this amp is amazing. I'm aware of the "ground loop" thing, but I have to say, this hasn't been my experience.

I'm genuinely interested to hear the potential problems with this configuration.

[sluckey]

Sure you can do both. But why would you?

[TUBEDUDE]

The majority of heater hum is coupled from the filament to the cathode. Balancing the halves around ground is a method to reduce hum, but if you take the center tap (real or faux) and tie it to a HV reference. This biases up the 6.3 VAC to ride on 70VDC or so and eliminates the cathode coupling of the A.C. hum from the heater.

[Medley]

Sure you can do both. But why would you?

That's a bit of a story, but you haven't really answered my question, which is:

The general consensus seems to be that it will cause problems if you do so; what problems is it known to cause? I know Rob Rob says it'll create a ground loop and they are generally considered sources of hum, but that hasn't been my experience. I am interested to know what problems doing this is known to cause based on experience. I'm genuinely interested, not trying to be difficult.

[sluckey]

You missed my point. Sure you can do it, meaning I see no issues. Either one is acceptable. Seems a bit redundant to do both. Then I turned it around to you by asking "but why would you?", meaning what is your reasoning for doing both?

[Medley]

You missed my point. Sure you can do it, meaning I see no issues. Either one is acceptable. Seems a bit redundant to do both. Then I turned it around to you by asking "but why would you?", meaning what is your reasoning for doing both?

As I mentioned, this was my first build. I've owned and used, and still do, several oldies for a number of years, but I never really understood them so thought I would build one to learn more (and get a nice amp out of the endeavor). So I purchased a Mojotone 5F4 Super. This particular model doesn't come with any assembly guide; just the schematic and layout diagrams. I have a fair bit of hands-on electronics experience so wasn't particularly concerned about safety, my soldering abilities, understanding schematics, etc. I also was already fairly well equipped with the appropriate tools.

So I followed the layout primarily, comparing it against and verifying things against the schematic. When it came time to solder the two 100R resistors to the pilot lamp terminals, I followed the layout without really understanding what they were there for. But the PT in that kit also provides CT for the 6.3 along with the HV. There was no indication anywhere what to do with the CT for the 6.3 coils, but I figured that it made sense to ground it along with that of the HV. That's how I ended up with both a CT ground as well as the virtual one. My amp was dead quiet, and the performance was/is perfect once I replaced the JJ 6L6 that was DOA.

When I was doing subsequent reading I came across a number of mentions on various forums, sites, etc, of the edict to avoid using both grounding schemes for the filament supply. However I'd already built my amp, found it worked perfectly, was/is dead quiet and wasn't really interested in going in to desolder components that seemed to have no deleterious effects as far as I could/can tell.

The reason I asked my question was because I wanted to see if there could be some downsides I'm missing, but also to test whether the conventional wisdom (ie, "there will be problems") is based on facts or repeated assumptions that aren't necessarily true. I am aware of the alleged protection using only the VCT can provide to the PT in the event of a massive tube failure, but that's not the kind of thing that would tip the decision for me when I have an actual CT at my disposal.

So there you have it.

[pjd3]

Oh Sheet, I'm sorry, I was away for a while and just getting back.

Of course I was being facetious about the "no gnd/center tap - 60 cycle filter after the power tubes". I don't know what got into me.

By what you've all said I would consider using the 6.3vac-2A w/centertap for the preamp tubes and the other two 4 amp 6.3vac for the rest. This Edcor transformer is hilariously over spec'd for heater taps and current but, it gave me the HV voltage I was looking for.
I'll be back with a major change to the power section on this 3rd build of mine. I was going to use KT88's but, a fellow handed me over 2 SED winged C (st petersburg) EL34 so, Those are going into the power section. I was being a little greedy to use the KT88's now that I think of it. A pair of EL34's will be plenty of power for this application and will help to relax alot of things about this amp.

I"m glad to see so much input on this topic, this is greatly informative!

Thank you!

PJD3

[TUBEDUDE]

A major route of 60 Hz hum is radiation from inside the tube. The heater injects hum to the surrounding cathode.
This can be eliminated by biasing up the heater circuit with a D.C. voltage.
Drop some high voltage down to 50V or so, (or take a lead from the power tubes cathode if cathode biased), and attach to the heater center tap (no ground connections).
This can keep also benefit you from possibly exceeding tube ratings if you have a cathode follower in circuit.
Just a voltage divider from the HV to drop the voltage under 100V, with a small cap shunting the lower resistor (for a little filtering) and Shazam.
With this, and good wiring practices, hum is done.

[TUBEDUDE]

I run separate heater lines to the power tubes and preamp tubes. No sense in having the full heater current running thru the whole amp. This will help keep hum down also. The more current the more radiated hum. Better to have lower current lines running through the preamp tube corridor. Seperate taps, even better!