Princeton RVB Filaments.. Virtual CT or Real CT???

[Govmnt_Lacky]

Thinking about the center tap for the filament wiring on a Princeton Reverb. Which option is best/better? Using the dual 100 ohm resistors on the Indicator lamp as a virtual tap -OR- using the PT's 6.3V winding CT?

Thoughts and reasons...

[Justin Thomas]

As far as which one "works better" for noise, I've heard the artificial one; sometimes the tap isn't really in the center. I tend to look at it as a failure thing - in case of a fault, would I rather risk toasting a tranny winding or some resistors?

That's just me. I'm poor so I like to do whatever I practically can to save my trannies.

Jusrin

[mozz]

Once you have it finally built, try both ways. Put a meter on the speaker, you will measure AC millivolts of hum, one way will be lower. Or install a hum balance pot of 250 ohms and tweak to lowest hum.

[pdf64]

I suggest to DC elevate the CT.
It will mitigate a potential hum source, make the amp less fussy about the valves used in V1 and V3 sockets.
See 'heater elevation' http://www.valvewizard.co.uk/heater.html
It only needs a couple of resistors (I typically use 220k and 47k) and a decoupling cap.

[catalin gramada]

Wiring in line have resistance and not quite equal for balance purposes. The most important noise coupling is at the first stage. Make an artificial CT at the end, mean first preamp socket with two 100ohm resistors soldered at the first preamp socket. And ground it at the input jack ground point. Usually this works best for me. For elevation should try elevating with positive and negative voltage to see what work best. From unknown reasons for me regard isolation between heaters and cathode I found for some tubes work best in term of noise with negative voltage as heater reference.

[R.G.]

Good advice Catalin.
In addition , the artificial center tap offers the somewhat-advantage of letting you sometimes cancel hum by offsetting filament hum in either of two directions.
@ Justin: if you're into saving transformers, consider whether you can fuse each separate winding section. The complaint I get every time I mention this is that it needs a lot of fuses. That is correct. But fuses are cheaper than transformers, so it's a balance of PITA fuses versus buying replacements.

[mozz]

Have to disagree with this.

"Wiring in line have resistance and not quite equal for balance purposes."

No way you are going to measure any difference between a pair of twisted wires going from point A to point B. Do you have lab grade equipment, let us know what the acceptable limits of resistance difference is. Then measure a Fender which used the chassis for one leg of the filaments and the comparable hum increase.

It's build technique and use of nice heavy wires you will have no hum. You can't say center tap or artificial center tap is going to be better, way too broad of a statement. A balance pot is the best without going to elevated. Throw decent made tubes in there also.

[Justin Thomas]

@ Justin: if you're into saving transformers, consider whether you can fuse each separate winding section.

I did this once, in the lone amp so far that started as a plain chassis instead of a salvage job. I do it when I've got the space; most of the time I just use the original tube complements & trannies so don't worry about too much in the way of abuse.

@RG: on the subject of fuses, the mains fuse in an amp's MAIN job is to protect your house from your gear, not to protect your gear from itself, correct? Any protection to your gear that a mains fuse offers is incidental, correct?

Jusrin

[catalin gramada]

Have to disagree with this.

"Wiring in line have resistance and not quite equal for balance purposes."

No way you are going to measure any difference between a pair of twisted wires going from point A to point B. Do you have lab grade equipment, let us know what the acceptable limits of resistance differ.

There is completly wrong statement to say. It well depends by currents and ussualy parallel distribution lines are used for this purpose. There is a general practice you may found references in Morgan Jones book. From my point I always notice differences in terms of tens of mV even with 12AWG wire for circuits who demand 9-10A or so. The best current balancing propose using wiring in line found from Tungsol recommendation. Supplying the line from opposite side. Very clever.
https://frank.pocnet.net/sheets/127/6/6336A.pdf pag.5, fig.3 Those sketch is a schematic, Various practical layouts may be imagine to run the wires twisted for canceling noise purpose aso.

[mozz]

Still shows nothing but normal wiring. That tube draws 5A . Millivolts difference between 1 tube and the next is not going to contribute to hum.

[catalin gramada]

Still shows nothing but normal wiring. That tube draws 5A . Millivolts difference between 1 tube and the next is not going to contribute to hum.

It well depends what tube you use. If cathode is bypassed or not, voltage gain etc...There are certain tubes which have big appetite to couple the ac heaters into cathode. 6H8S, 6H9S russian equivalents for 6sl7, 6sn7. I have bunch of these...hundreds...Nice tubes, I use for my projects but completly unusable if don't show to the heaters some respect.
Just thinking the purpose of pot balance is just for this reason : to balance the heaters in most sensitive tubes of the circuit, .. Of course for certain circuits it doesn't matter but that don't mean it doesn't count.

[R.G.]

I did this once, in the lone amp so far that started as a plain chassis instead of a salvage job. I do it when I've got the space; most of the time I just use the original tube complements & trannies so don't worry about too much in the way of abuse.

@RG: on the subject of fuses, the mains fuse in an amp's MAIN job is to protect your house from your gear, not to protect your gear from itself, correct? Any protection to your gear that a mains fuse offers is incidental, correct?

Jusrin

G'don'ya, mate.

Yes, the mains fuse is required by the majority of national electrical safety codes. It's not there primarily to protect the equipment itself. The fact that sometimes it does protect the equipment is a lucky accident.

There are some scenarios where a short, especially a soft short on a low voltage winding will burn out a winding inside the transformer without popping the mains fuse. Or at least, the mains fuse will not clear before the transformer is already dead and the destruction has spread to a winding that will break the mains fuse. If each winding is protected by its own fuse, rated for its own maximum currents, it is far more likely that a heater short (for instance) will open the heater winding before the transformer dies.