18 watt power transformers

[kinmike]

Guys,

I built an 18 watt about ten years ago and I remember having troubles with slightly higher voltages. I think this was due to the clone PT that was designed for 110V but got 120V from the wall.

My question is, does the Classic Tone 18 watt PT make up for the modern wall voltages in its design and windings? The diagram for the Classic Tone shows 120 primary with 290 center tapped secondaries. This is opposed to the vintage PT diagrams with 110V primary with 290V secondaries. So does the Classic Tone PT shed the extra voltage by design? That would be better for the heaters when compared to the original, I think?

Thanks,

Mike

[jaysg]

I was just looking at the two they offer. That is what the spec sheet is telling you. I think we all had higher B+ than the originals. I had the EMC and having dealt with John Cerniak a bit, it was probably a clone not a redesign.

[kinmike]

Has anybody tried the trick of using the unused 5 volt tap to add 5 volts to the primary?

[CurtissRobin]

I'm sure that the 5V is a separate winding and not a tap on a winding. As a separate winding it's a maybe but if it's a tap, then definitely no. I've used separate filament transformers to reduce secondary voltages but never a winding on the same PT. It's been discussed here and it linked to another website but it'll probably take some searching to find the thread.

KennyO

[kinmike]

Here's a link on how to use the "buck windings". Seems like it would work but I'm no electrical engineer.

I see folks using zeners to lower voltage. The bucking winding seems easier at no cost.


http://www.ws.binghamton.edu/fowler/How ... rimary.pdf

[Daviedawg]

That is an interesting approach.

But personally I would stick with zeners for this purpose. They are just as simple to use and can provide the exact HT voltage that you are aiming for. Experimentation is also a simple task if you plan your layout for it.

But your approach gives food for thought in other ways too perhaps. I am glad you shared it.

Dd

[CurtissRobin]

Provided that the 5V winding is an independent winding and not a tap on the 6.3V winding, the method in the linked article would work fine. As the writer mentions, be aware of the phase of your primary connection. If the 110V and 5V windings are in phase you'll get the desired lower voltage on your secondary but if they're out of phase the secondary will be a higher voltage. This method will change your B+ by less than 5% and should work, though it may be too much.

Zener diodes are certainly a well-tested and well-liked option. Using zeners has two great advantages: it's easier to get to your desired voltage (340-345 is the happy range), even if there's no need to be as precise as many people try to be, and if you have to replace the PT down the road you have more options since some otherwise satisfactory PTs don't have a 5V winding. As to the zener approach being "just as simple," I beg to differ. Zeners need to be properly selected (power dissipation being the part that's too-often ignored), it has a greater parts count, there are more solder joints, it needs to be electrically isolated from the chassis and it creates additional heat. (N.B. The amp doesn't much care about the additional heat but the zeners themselves might.)

For safety, zeners should be installed in the B+ line, not in the ground leg (center tap).

Use whichever method floats your boat.

KennyO

[no_fwet]

Thanks Kenny, a great post ... That answers some concerns of mine, for my organ conversion wrt the elegance of the zener solution. I'll be dropping about 20-25 volts to get to a 345 sweet-spot.

[kinmike]

But zeners won't help with heater voltages.

Mike

[CurtissRobin]

But zeners won't help with heater voltages.

Mike

'tis true. Say you've got a PT designed for 110VAC on the primary, and you run it at the design voltage. The filament (heater) winding will be at 6.3VAC. Now you connect that primary to today's common 125VAC line, about 13.6% higher than design, your filament winding will be at 7.16VAC. That's well above the 6.9VAC that's the conventional upper limit of acceptable filament voltage and is sure to shorten the lives of tubes significantly. To get it down into the 5.7-6.9 VAC range your only practical solution, w/o swapping out the PT, is to buck the primary voltage down to 120VAC or less. It's true that you could install a resistor (resistance according to Kirchoff's law and power rating I-squared-R plus 25%) in the heater circuit to drop that voltage. It'll generate a good deal of heat and it will not fix your problem, only change it.

KennyO

[Striker52]

The heater voltage can easily be reduced by putting a pari of antiparallel diodes, such als the P600, in line with one of the 6.3 wires. This will generate a voltage drop of 0.8 Volt.

[Daviedawg]

Due to our high wall voltage I need diodes in the heater circuits in all my builds. I have used 6A60s I think which are high current diodes. They take up a bit of space so need to be planned for in your heater wiring layout. So far that has worked for me in smaller and bigger builds.

Dd