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Replacement PT question

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I have a Knight KN-728B that needs the power transformer replaced.

The original PT is: 640VCT 130ma
6.3VCT 5.5a

All I can find are a couple of Edcor's that are close but not exact.

- 600VCT 200ma
6.3VCT 6a


- 660VCT 200ma
6.3VCT 6a

Can I use either of these?

These are both very close... -6% on one hand, and +2% on the other... These are tiny differences when compared to normal line and load variations, component tolerances, power output into different speakers, and so on. Either one should be fine.
Frankly, no. :cool:

A few percent plus or minus in voltage is not something you would hear. You might ask someone if that manufacturer rates their transformers to be on the conservative side, or at least look up what primary voltage they specify (meaning what voltage do they use to rate their product).

For example, if they use 115 VAC, I would choose the lower spec, because the line voltage in a typical home is higher than that most of the time, so your actual output will be proportionally higher. If they use 120VAC, I would choose the higher spec.

However, these are tiny differences when it comes to audio power supplies which are almost always rated very conservatively. In either case, you are buying a transformer that is rated at a higher current, so may be a bit bigger, and I would check the dimensions too.
The warning about making sure a replacement fits mechanically is quite correct.

Go with the 300-0-300 model and switch to SS rectified B+. The 150 mA. 6CA4/EZ81 is "in over its head" powering a PP 6BQ5/EL84 stereoblock. Forward drop losses in SS diodes are considerably less than those found in vacuum rectifiers. That way, a rectifier winding that's somewhat low in the voltage dept. gets compensated for.

BTW, the 130 mA. OEM part is undersized for the job at hand. That may account for the failure observed.

P.S. Post the schematic if you have it.
The schematic shows that quite a few corners were cut. :( Besides the undersized power trafo and rectifier, the power supply takes O/P tube B+ from the 1st filter capacitor. No doubt about it, this is a cheap design.

Another penny pinching thing that was done is sharing a single bias network among all 4 O/P tubes. That requires a very closely matched quad and, of course, more expense to the unit's owner.

1108, how far do you want to go in getting this "beast" in proper working order? There's a "laundry list" of changes that can be made, besides correcting B+ PSU flaws. Modernization of source selection capability is 1 of them.
Well Eli, I think I'll put this back on the shelf for now and start the next project on my list.

Thanks for your time and advice.

1108, I wouldn't worry too much about the basic design. It's from the early 60's, there are a lot of them out there playing music just fine. It probably wouldn't be designed that way today, and certainly not by most folks rolling their own amplifiers - but that doesn't take away from its usefulness for playing music.

The transformer may have been damaged by the electrolytic caps in that unit. If the amplifier has the original electrolytics in it, they are likely WAY past their use-by date. If they are leaky (in the electrical sense, not the sense of leaking fluids :)) then they will load the transformer and cause it to overheat.

Take a look under the bottom and see if the caps are the old paper-wrapped electrolytics (or if the original can caps are still in it). If so, then investing a small $$ by replacing them all will save you trouble when you replace the power transformer.

If the wires from the PT look well-cooked - especially near the transformer - it's a good indication that the transformer has been running hot for quite some time. (for some reason I though you had posted photos of the underside - musta got this thread confused with another...)

Look at it this way - the design worked fine for nearly 50 years, then something got tired and went blooey. It happens - if you're happy with how the amplifier performs, then maybe it's worth spending some money on a replacement transformer and some relatively inexpensive electrolytic capacitors to get it back on the air.

While you're at it, you might consider upgrading the carbon comp resistors in the signal path as well. Another relatively inexpensive and easy upgrade.

Good luck with the project! :cool:

~ Sam
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PS - if you decide to "go under the hood" and fix this amp, I would also seriously suggest you install a 3-wire plug and get a good chassis ground on the amp. The older designs all had 2-wire non-polarized plugs, it was a different time...

Run the green wire in the new cord directly to the chassis using a lug and a bolt - preferably a star-lug that will get a good "bite" into the chassis. This is the most important thing - make sure the chassis can never get to a dangerous high voltage if anything inside goes bad and creates a short to chassis.

The transformer, when it failed, could easily have shorted to chassis and not necessarily blown the fuse. The chassis could well have been at high voltage when plugged in (switch didn't need to be on - the non-polarized plug could have put the hot side of the wall switch directly to the transformer...) so you can see how this could have become a lethal little amplifier.

As long as you're at it, run the black (Hot) wire to your fuse and then re-wire the fuse in series with your power-switch (the fuse in the schematic is in the unswitched leg - again, different times...) and finally run the white (Neutral) wire directly to the other transformer AC lead.

A simple bit of re-wiring that will make this puppy a LOT safer in the future.

Adding the chassis-ground might cause ground-loop hum-problems in your system, which will be something else to sort out - but better sorting out ground-loop problems than calling next-of-kin, IMHO.

At least the schematic does not appear to show a "death-cap" from power-line to chassis which was typical in those days. If there _is_ a capacitor from the AC power line to chassis, then at least cut it out and toss it. If it fails shorted (which was common for those types of capacitors), then the chassis can be at 115VAC which is seriously not a good thing.

Best luck. :cool:
Well Eli, I think I'll put this back on the shelf for now and start the next project on my list.

Thanks for your time and advice.

Don't give the ship up! You get to pick which "laundry list" items get implemented and the associated costs. For instance, upgrading source selection is easy and inexpensive. A suitable, Lorlin brand, rotary switch can be sourced from Mouser for 5 bucks. :D

BTW, Edcor's model XPWR178-120 that you originally hinted at has a 5 VAC winding. That winding allows you to use a very high forward drop, 250 mA., vacuum rectifier (the 5R4) and get the B+ rail voltage "just right".

Please post photos of the unit. There is a fair chance that the O/P "iron" is decent, which makes refurbishment a rational choice. Expense can be controlled, while obtaining a highly satisfactory end product.
I probably should have mentioned this sooner but my goal for this project was just to see if I could bring this amp back to life and have something that sounds and looks decent. I’m not expecting this to be the ultimate in sound. The condition it’s in is the way I received it so I have no idea what it’s capable of.

My plan was to replace the PT and power supply caps, and if it fired up go on from there. Replacing all the caps, etc.

I’m fairly certain that everything is original, nothing looks like it’s been replaced and there’s no obvious damage to any of the components.

I have the Sam’s for this and it lists the OPT’s as Triad S-28X, 8500 ohm CT, 16,8 and 4 ohm taps. If that means anything. They look to be decent compared to some others I have seen on low end vintage amps.

Eli, yes the Edcor XPWR178-120 was what I was looking at. Can you explain what I would do to use the 5R4 for the rectifier?

Sam, thanks for your recommendations and instructions for the power cord, fuse and chassis grounding. I would never have thought of any of those.

I really appreciate the time you guys have put into helping me with this.
Hi 1108, glad you're going to give it a go!:)

The 5U4 that Eli mentioned, unfortunately, is a much larger tube in an octal-socket. It's a beefy rectifier tube, to be sure, but it won't fit in the 9-pin miniature socket that your 6CA4 tube lives in right now.

The Edcor transformer Eli recommended should do the job, though, as long as it is physically close to your amp (the Edcor site provides the dimensions of their transformer). Just insulate the 5Vwinding wires and roll them up out of the way for now and wire the other two secondaries per the Knight (or Sam's) schematic.

Edcor make a fine product, by all accounts, and has good pricing - but they make the transformers to order and it's typically a 4-6 week wait for them... I have a couple of Edcor output transformers on order myself, getting anxious!! :D

A possible alternate is a Hammond 276X. It has a 640VCT (just like your original) @173mA secondary, and also the necessary 6.3VCT filament winding. It is a bit more expensive (~ $100) than the Edcor, but you might be able to find one in stock at places like http://angela.com/hammondpowertransformer276x.aspx or http://www.newark.com/hammond/276x/transformer-power-150va-115vac/dp/66F7312 (the Newark price is a bit higher still)

Time is money :D

The Hammond also has a 5V winding - again, just insulate the ends and roll the wires up for later use. You may decide one day to re-purpose the power transformer into another design that would need the 5VCT winding.

Best luck on your rebuild! :cool:

~ Sam
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Hi Sam,
I saw the 5U4 was an octal not a nine pin. Taking a quick look, I thought I could enlarge the hole enough for the larger socket. But that’s ok, I can stick with the existing rectifier for now and maybe do that as a later stage.

I have always used Hammond iron in the past but have read good things about Edcor and want to give them a try. The Edcor is bigger than the original but I think I can make it fit.

Thanks again and I’ll be looking for you guys if I get stuck along the way.
Before you buy from Edcor, upload photos of the amp, showing the tube and "iron" layout from above and the front and back panels.

I suggested the 5R4 for its large forward drop. I'm trying to get very close to the OEM B+ rail voltage, which is quite high to begin with. Yes, the 5R4 has an Octal base. Use a nibbling tool and a file to enlarge the opening and install an Octal socket.

Keep in mind the fact that the OEM PSU is totally inadequate. The 5 VAC winding on the proposed Edcor replacement has a center tap. That's a good thing and you will take the "raw" B+ off that CT. Start with a 4.7 μF. part in the 1st filter position. Between the 1st filter cap. and the B+ reservoir install a (sic) Triad C-14X choke. Use a 100 μF. part in the reservoir position. You will tweak the value 1st filter cap. to get the B+ voltage right.

The only replacement O/P tube I would use is the Russian 6П14П-ЕВ (6p14p-ev), AKA EL84M, which is a genuine 7189 equivalent that's "tough as nails".

Contact Jim McShane for tubes, caps., and all around help.
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