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Heathkit W4-AM Replacement Transformer

Kasanay

Kasanay

Active Member
I have a pair of Heathkit W4-AM monoblocks and one has a defective power transformer (in the photo, the amp on the right is fitted up with the incorrect transformer). I found a Hammond (Model 274BX) that looks like it will work as a substitute and I would like some confirmation on that. Do you see anything wrong with this choice?

Also, I would like to salvage the original transformer cans and slip them over the Hammond to maintain the vintage look. Do I need to worry about this causing the Hammond transformer to over heat?

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Yes, it will work. It’s a bit shy of the original (760vct). But you’ll probably get a bit of over-voltage to compensate. Hawk Electronics has the lowest price, by far. I’m not sure I’d cover it though. I understand the concern about appearance. Maybe someone else can add their thoughts.
 
What's up with the defective transformer? Just curious as those things are bullet proof but I have heard that HeathKit runs them rather hard.

Those original power transformers are potted which means they are filled with tar so good luck getting all the tar out to get the coil out. It might be worth your time to get it out but it ain't worth mine. :D
 
century tek said:
What's up with the defective transformer? Just curious as those things are bullet proof but I have heard that HeathKit runs them rather hard.

Those original power transformers are potted which means they are filled with tar so good luck getting all the tar out to get the coil out. It might be worth your time to get it out but it ain't worth mine. :D
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so my story goes like this:
I bought the 1st W4 at an estate sale for $50.
I start a search on eBay for a mate.
After several weeks, one shows up and I grab it ($170 + shipping).
In my haste, I didn’t realize that the power trans was not original. Previous owner installed a larger unit and hacked up the chassis to install it.
I then started a search for an original power transformer and was able to buy one off eBay.
The one I bought ($75) was in good cosmetic condition, leads were nice & long and were all labeled!
That project ended up sitting on the shelf for about 6 month. When I got around to doing the swap, I found the HV readings WAY off. Measuring resistance, I think the center tap is shorted.
 
Have you checked it all by itself for voltage? Make sure you have a fuse on it and just plug it straight into the wall by itself and take voltage measurements. Unloaded, voltages will be higher than base schematic notes.

How did you come to the conclusion that the center tap was shorted? On resistance readings, the two high voltage leads should show a number resistance and each leg to the center tap will be about half that. Don't worry if either side half is uneven. They never are perfectly even. Same goes for voltage readings when powered up.

If center tap were shorted, you would see way off readings on one half compared to the other half, and it might show continuity to other taps, or the transformer may get warm or smoke.
 
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I’ve gone back and confirmed my earlier resistance readings.
5V and 6.3 taps are fine.
On the HV leads I get a constant R66.5 Between CT and good HV lead.
Measuring out to out or CT to bad lead gives inconsistent readings from R1.3K to R8.6K.

This tells me the issue would be with one HV lead, not the CT.

Good news - the new Hammonds showed up today.
 
century tek said:
Have you checked it all by itself for voltage? Make sure you have a fuse on it and just plug it straight into the wall by itself and take voltage measurements. Unloaded, voltages will be higher than base schematic notes.

How did you come to the conclusion that the center tap was shorted? On resistance readings, the two high voltage leads should show a number resistance and each leg to the center tap will be about half that. Don't worry if either side half is uneven. They never are perfectly even. Same goes for voltage readings when powered up.

If center tap were shorted, you would see way off readings on one half compared to the other half, and it might show continuity to other taps, or the transformer may get warm or smoke.
Click to expand...
 
I’m currently rebuilding a non-functional Heath W4-AM, and wonder if the power transformer is good. I get an unloaded reading on the red leads to the rectifier of 845 volts rather than the 760 volts shown on the schematic. I don’t know if that’s normal, but I don’t want to plug in the rectifier until I'm sure it won’t blow due to overvoltage or something (I don't claim to be an expert here).

Other voltages are as follows: the 5V leads show 5.8V loaded (all tubes but the rectifier plugged in) and 5.87V unloaded.

The 6.3V filament leads show 6.6V loaded and 7.2V unloaded.

I am replacing all caps and most resistors, but am hesitant to power it up until I’m sure about the PT.

Your advice is greatly appreciated.
 
Np. And your line voltage is most likely higher now than back then.
So your loaded voltages may still be a bit higher than specs.
 
Tube love said:
I’m currently rebuilding a non-functional Heath W4-AM, and wonder if the power transformer is good. I get an unloaded reading on the red leads to the rectifier of 845 volts rather than the 760 volts shown on the schematic. I don’t know if that’s normal, but I don’t want to plug in the rectifier until I'm sure it won’t blow due to overvoltage or something (I don't claim to be an expert here).

Other voltages are as follows: the 5V leads show 5.8V loaded (all tubes but the rectifier plugged in) and 5.87V unloaded.

The 6.3V filament leads show 6.6V loaded and 7.2V unloaded.

I am replacing all caps and most resistors, but am hesitant to power it up until I’m sure about the PT.

Your advice is greatly appreciated.
Click to expand...

I don't want to make you nervous, but I would recommend caution. Oddly enough, I was just restoring a pair of W4's today and the power tranny readouts were very similar, as in WAY too high. Even loaded I was getting about 418VAC per plate on the poor little 5V4, and plate voltage on the 6L6's was about 450VDC. Even worse, I just bought some CE 525VDC can caps for them and one of them snapped a couple of times on startup. Voltage at the first cap before current draw kicked in rose to 570VDC before it settled down.

I had to install a pair of 6.3VAC bucking trannies to get the AC input down to 115V. And the HV secondaries are *still* reading a bit on the high side.

If you don't have a variac, use the toughest tubes you have to test it. 6SN7s and 6L6's will generally handle the higher voltage. A 5V4 might complain a bit but it will take it for a while. I don't know what kind of the shape the PS caps are in. A 5V4 is not as slow to warm up as one would like. It'll probably be okay. I was prepared for some over-voltage but not that much.

The W4 output tubes only run at about 50mA each, so it doesn't pull as much current as some other Williamson-style amps.
 
zackthedog said:
I don't want to make you nervous, but I would recommend caution. Oddly enough, I was just restoring a pair of W4's today and the power tranny readouts were very similar, as in WAY too high. Even loaded I was getting about 418VAC per plate on the poor little 5V4, and plate voltage on the 6L6's was about 450VDC. Even worse, I just bought some CE 525VDC can caps for them and one of them snapped a couple of times on startup. Voltage at the first cap before current draw kicked in rose to 570VDC before it settled down.

I had to install a pair of 6.3VAC bucking trannies to get the AC input down to 115V. And the HV secondaries are *still* reading a bit on the high side.

If you don't have a variac, use the toughest tubes you have to test it. 6SN7s and 6L6's will generally handle the higher voltage. A 5V4 might complain a bit but it will take it for a while. I don't know what kind of the shape the PS caps are in. A 5V4 is not as slow to warm up as one would like. It'll probably be okay. I was prepared for some over-voltage but not that much.

The W4 output tubes only run at about 50mA each, so it doesn't pull as much current as some other Williamson-style amps.
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Zack, I really appreciate your input - I am new to tube amps, so am trying to be very careful. I bought all new (or NOS) tubes for this project. Outputs are 5881s, the rectifier is a 5v4g as specified in the schematic. The PS filter cap is a new CE 475 volt- should I look for a higher voltage rated can? That was all I could find at tubesandmore.com. And should I go with 6L6s for output tubes? Tell me more about the bucking transformer - I am not familiar with that or its purpose, though it sounds like a way to reduce input voltage (for the filaments or otherwise?). Still learning my way around tube amps, so I need all the help I can get. Don’t want to turn my W-4 into a mini-Chernobyl!
 
Tube love said:
Zack, I really appreciate your input - I am new to tube amps, so am trying to be very careful. I bought all new (or NOS) tubes for this project. Outputs are 5881s, the rectifier is a 5v4g as specified in the schematic. The PS filter cap is a new CE 475 volt- should I look for a higher voltage rated can? That was all I could find at tubesandmore.com. And should I go with 6L6s for output tubes? Tell me more about the bucking transformer - I am not familiar with that or its purpose, though it sounds like a way to reduce input voltage (for the filaments or otherwise?). Still learning my way around tube amps, so I need all the help I can get. Don’t want to turn my W-4 into a mini-Chernobyl!
Click to expand...

At tubesandmore.com there is a CE multi-cap can 40/20/20/20 @ 525VDC that's perfect for the W4. Get a new mounting flat too, it's listed on the page for the cap. For the first input caps, if they need to be replaced, you can substitute any pair of 22uF/350VDC axials. I would upgrade the coupling caps to 630VDC. JustRadios sells JFX caps at 630VDC that are compact and very good-sounding.

If you haven't read the Heathkit W4 stabilizing thread, you should, it under the "Dave G's Excellent Info" heading.

Search for "bucking transformer" and you can find lots of info. I don't mean to alarm you--there seems to be a lot of healthy W4's out there, but I was taken aback at how high the voltages came out. Variacs are cheap and very good to have around.
 
zackthedog said:
At tubesandmore.com there is a CE multi-cap can 40/20/20/20 @ 525VDC that's perfect for the W4. Get a new mounting flat too, it's listed on the page for the cap. For the first input caps, if they need to be replaced, you can substitute any pair of 22uF/350VDC axials. I would upgrade the coupling caps to 630VDC. JustRadios sells JFX caps at 630VDC that are compact and very good-sounding.

If you haven't read the Heathkit W4 stabilizing thread, you should, it under the "Dave G's Excellent Info" heading.

Search for "bucking transformer" and you can find lots of info. I don't mean to alarm you--there seems to be a lot of healthy W4's out there, but I was taken aback at how high the voltages came out. Variacs are cheap and very good to have around.
Click to expand...
 
I want to thank all those who offered advice on my Heath W4 rebuild, especially Brice and zackthedog. Replaced all the caps and some resistors that had drifted out of spec, popped in replacement tubes, and voila! It lit up and worked first time I powered it up. The WA-P2 preamp works fine after a little work on the pots and switches with contact cleaner, and the whole rig sounds great through the Heath SS-1 and SS-1B speakers that came with it. I’ll go ahead and replace the caps in the preamp as time permits. Now for the rest of the story: I watched my father build this system in 1957 when I was seven, and acquired it when he passed away. It worked for a while, then quit one day. I am very happy to have it working again. Thanks again for all the help!
 
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