400C preamp power supply

As previously mentioned, I have about 1 2/3 Fisher 400Cs hanging around here. One of them is missing the power transformer, which is a truly odd beast running the heaters (except for the 6X4 rectifier) from a 30V secondary through a bridge transformer for DC power, and the five light bulbs and the 6V4/EZ80 rectifier on a 6.3VAC tap.

On the other hand, a 560VCT secondary coupled with a fairly hefty 6.3V filament winding wouldn't be hard to find. The Citation IV ran the heaters from a 12V DC supply, but it doesn't seem like it would be particularly difficult to set up a 6.3V DC circuit for the six 12AX7s, the 6X4, and the five bulbs. The 12AX7s each draw .3A with the heaters in parallel at 6.3V, the 6X4 heater draws .6A, and each of the bulbs is .1, but only two should be lit at any given time. If 4 12AX7s were on the positive side with the array of .1A function bulbs, and 2 12AX7s and the 6V4 were on the other side with the power indicator bulb, it all should balance out fairly well. Am I missing something here?

I found an online store that has this for $22: Pri 120v, 50/60 MHz; Sec 580vct @ 28ma, 6.3 vct @ 5 amp; 12 vct @ 2.2 amp. The first secondary is a bit higher than stock, but that 6.3V @ 5A winding has almost double the amperage capacity needed to run everything.

The 12V winding would produce close to 30 VDC at 300 mA with a voltage doubler using 1N5822 Schottky diodes. The 6.3V winding really should be separate to keep line noise out of the low-level heater supply.

That makes sense, I didn't think about the possibility of the rectifier's heater picking up noise from the rest of the rectifier.

That said, I don't really see a reason to use the screwball Fisher 30V heater supply - I could simply use the 12V secondary with a copy of the Citation IV heater circuit. 300mA wouldn't be sufficient anyway, the stock transformer was spec'ed at 430mA. Maybe I could even rig up an LED supply circuit to get rid of the stock incandescent micro bulbs.

Yes, 750 mA at 12V should be do-able - it's only about an amp RMS from the transformer - about the same as the doubler would draw . Use Schottky diodes for the bridge - regular silicon rectifiers will drop about a volt apiece.

Or use the doubler and wire the 12AX7s in series pairs for 20-24VDC - you can drop the excess voltage a lot easier than you can deal with voltage that's too low. Sounds like that transformer will work out nicely if it fits.

It's 1/8" too tall, but I can simply add a bit of clearance for it when I build a wood case.

Sam -- A couple of things to consider:

1. Make sure the proposed transformer has a Faraday Shield included. These are the wide copper straps you often see running around the outside of the lam core in a power transformer -- sometimes they're painted black from when the transformer was painted, but you will want a transformer that includes that shield. Transformers without one make it very hard to get hum under control.

2. If your transformer has a 12 volts 2.2 amp secondary, then consider letting it power a voltage doubler circuit to get 24 vdc. This would allow the original wiring scheme of 3 parallel sets of two tubes in series to be maintained, and allow you to use 28 volt bulbs for the lamp array and power indicator as I did in my rebuild effort with this preamp. That keeps the 6.3 vac of the original design from getting up near the selector switch which controls the indicator lamps, further minimizing hum levels.

In that configuration, the heater and lamp circuit pulls about 550 ma, which a 12 vac @ 2.2 A winding would be more than able to deliver through a voltage doubler, leaving room for adequate voltage drop for filtering. The other huge benefit of this approach is that a 25 vdc 550 ma supply can use much (physically) smaller filter caps over those for a supply that must filter 6.3 volts at 2 Amps -- and that's not counting the rectifier tube.

Regardless of what configuration you use, you would not want to run the rectifier tube off of DC as there is no value in it, and it will just add to your woes of filtering. With the proposed transformer, use the 6.3 volt winding for the rectifier, and the 12 volt winding to power the DC doubler.

Two final benefits of this overall heater configuration are that then:

1. You can strap the rectifier cathode to the transformer winding powering the rectifier heater. This then puts the heater of the 6V4 rectifier tube at the same potential as the B+ voltage appearing at its cathode, which will cause the tube to virtually last forever.

2. You can also then positively bias the audio tubes about 70 volts above ground -- not only to help minimize noise, but also to cause the design to be a little kinder to the 12AT7 tubes it employs. These tubes have a heater/cathode voltage rating of 90 volts, yet operate the cathodes of these tubes at as much as 150 volts above ground, which the heaters (basically) reference. By biasing the heater system at 70 volts above ground, these tubes then operate with their heater/cathode voltage within the rating for the tubes. This helps to preserve their lives by not inviting any heater/cathode damage (shorts) due to exceeding the cathode sleeve insulation rating within the tube.

These are just some of the modifications I developed and discussed in my rebuild thread for this unit. Since it sounds like you're going to have to modify the unit somewhat anyway for use with an aftermarket transformer, you might consider them from the get-go before you get started!

Good luck with it!

Dave

Edit -- I see Tom covered some of the same territory I just did. It's still food for thought anyway.

Dave,
I've read your threads and posts on the 400C with great interest. I plan to rebuild the earlier 400C in my possession as a basically stock unit with a few tweaks. The second, later unit (with the dual concentric pots) is going to be hot-rodded. That's the one that needs a transformer and both rotary selector switches. And a big portion of the sheet metal...

Also, there are 12 (really 14) volt E5 bulbs available, but your suggestion of the voltage doubler on the 12V circuit makes sense.

I have no idea if this transformer has a Faraday shield, but if it doesn't, there should be a few ways to build a shield around it.

If you can see the lams on the proposed transformer, see if there is a wide copper strap running across them in a perpendicular fashion-- usually in the middle of the lam stack. That will be the Faraday shield. Fisher used them almost exclusively on all of their power transformers -- be it amplifiers, tuners, or receivers.

Dave

I know what you're referring to, I just don't have any pictures of the transformer. Just the dimensions and voltages.

They make LED's in E5 bulbs. They are offered in 12v. I have them in mine. They replaced the stock incandescent without a hitch. Several places to tap them for power, could even use the 6.3v heater line, they're excessively bright anyway. No need to make a custom circuit, just move the wire for the power light somewhere else and the rest will follow.

I have to either make a custom circuit or have someone like Heyboer build a new power transformer. As Dave pointed out, there are good reasons for keeping the heater circuit as-is.

Got the Kenyon transformer from Surplus Sales of Nebraska today. I've noticed two things so far: I don't have a wiring color code chart, and this thing rather oddly has the wire exits on the top edge of the bells. I'm going to have to pull those and drill holes in the bottom edges before using it.

I can figure out the wiring chart with a variac and some time, but it would have been nice to have a spec sheet or something. I can't find a Kenyon catalog from later than about 1940. Tom Bavis has a few sheets from 1953, but they don't have this one listed. ST-1360. 6.3 vct @ 5 amp; 12 vct @ 2.2 amp; Pri 120v, 50/60 MHz; Sec 580vct @ 28ma.

On one side I have two green wires with what appears to be a center-tap wire, green with a yellow tracer - these three are a bit heavier than anything else on the transformer. The same side has a pair of smaller red wires. The other side has a pair of brown wires with an apparent center-tap wire, brown with a yellow tracer. On the same side is also a pair of black wires, a grey wire, and a red wire with a yellow tracer. I'm guessing that the green / green-yellow trio and the brown / brown-yellow trio are the 580VCT and 6.3VCT connections, but I'm not sure which is which. It seems that there are also a couple of extra wires that wouldn't be strictly needed for the specs listed above.

Sam -- The heaviest leads will be the winding with the highest current capability. Therefore, the green set should be the 6.3 volt set since it is a 5 amp winding. Then traditionally, red leads usually indicate the HV leads, while black indicates the primary winding. The brown lead set could then be the 12.6 volt winding. Finally, the gray lead may be (hopefully is) a shield or core ground connection.

Is it possible to take the transformer apart and simply turn the end bells around 180 degrees to have the wires exit properly for mounting in this unit? As you know, the original transformer mounted by way of mount tabs on the end bells, and hopefully, this one mounts basically the same way. But sometimes, those mount tabs are brackets that are separate from the end bells themselves, allowing the bells to be installed either way.

Dave

Dave,
I have some transformers with the separate feet. These appear to be integrated, so I'm going to have to take the bells off, drill holes at the bottom, and plug the top holes.

Any suggestions on a voltage doubler circuit I might copy to get the 12V winding up to the 24-30 range for the 400C heater supply? Overall, I think this will work out well, at significantly lower cost than having Heyboer build a new one - they don't have the 400C transformer specs on file.

Edit: I checked the transformer against the chassis. It's exactly the same height as the installed cover over the bank of switches at the front, so it fits perfectly within the form factor. I'll have to drill new holes for the mounting screws, the footprint is a bit smaller than the original transformer. If it was turned on its side, they'd be about the same size. Now that I think about it, since I have to modify the bells anyway, I could cut the feet off and make new ones to mount it horizontally...

Hrrmph. Now Surplus Sales is telling me that the 6.3V secondary is 2.7A and the 12.6V secondary is .525A. I've asked for clarification, but I might have to run a voltage tripler off the 6.3V secondary to get enough current for the heaters. Assuming the new ratings are correct, would it be possible or advisable to run the voltage multiplier circuit after the rectifier heater tap, or should I drive it with a voltage divider from the 12.6V winding? I could easily get 14V bulbs for all of the lights, and run the voltage divider after them.

Edit: they've verified that the revised numbers are correct.

Also: I'm confused as to how the original power supply design ran 6 150mA heaters with a 430mA secondary. Shouldn't that have burned the transformer to a crisp?

Sam -- Fisher notoriously ran 12AX7 heaters (and its brethren) at 10-11 vdc, reducing current flow through the tubes. And, in the original design, the tubes were configured as three 150 ma series heater stings, with two 12.6 volt configured tubes per string, and the three strings then connected in parallel. Therefore, where as the three strings would normally draw 450 ma at full operating voltage, they could very easily draw only 430 ma at the reduced voltage they ran at.

With the new ratings, it would require some significant reworking to use the new transformer. You could convert the rectifier tube to a 12X4 (unless there's a 12 volt version of the EZ81) and use the 12.6 volt winding to power its heater, while the 6.3 volt winding could still be rectified through a doubler to produce a filtered 12 volts DC, with the audio tube heaters all rewired in parallel to require 12.6 volts at .9 amp -- which the 6.3 volt winding could support. The indicator lamps could then be converted to 12 volt units as well.

I hope this helps!

Dave

Unless my math is really bad, wouldn't a 4x multiplier circuit on the 6.3V secondary still have some headroom left on those heater strings? At the same time, a voltage divider could be placed on the 12.6V secondary to drive the rectifier's heater. Would there be any problem with the rectifier heater being on a separate DC circuit from the rest of the heaters? Eliminating the tube rectifier entirely is another option.

I'm going to ask Heyboer how much it would cost to wind a new PT, though I'm pretty sure I'm going to hate the answer.

Another option, if I could figure out where to bolt it down, would be to use one of the 12.6V 3.0A Radio Shack transformers to drive the heater filaments. I have a couple laying around.

One significant problem with using a quadrupler is where the heck are ya gonna put all those caps associated with that circuit?

Dave

Four caps instead of two. What size capacitors would we be talking about, anyway? 50V ratings should be fine.

Maybe you can find the space. I'd use nothing smaller than 4700 uF.

Dave