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Voltage on Rectifier output- Heath W4?

GordonW

GordonW

Speakerfixer
Subscriber
Need to know the output DC voltage at the output of the rectifier (before it goes into the choke) on a Heathkit W4. Got one here with bad caps. Wondering if I can use a polypropylene 10uf cap instead of the series'd 20uf caps... that'd be nice, if it'd work...

Anyone got one they can check, or have the spec from a builders manual? I think the W4, W4M or W4AM would all be applicable...

Thanks!

Regards,
Gordon.
 
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GordonW said:
Need to know the output DC voltage at the output of the rectifier (before it goes into the choke) on a Heathkit W4. Anyone got one they can check, or have the spec from a builders manual? I think the W4, W4M or W4AM would all be applicable...
Click to expand...
I used to have five Heath W-4M amplifiers. After going through all of them, I measured the voltages at different points of the amplifiers once they were up and running and settled in.

Measurements from the five amplifiers:

Voltage at the first B+ capacitor.

485V
486V
473V
485V
483V

Voltage at the capacitor after the inductor

462V
464V
453V
467V
463V

Please note that the voltage chart that comes with the amplifier suggests that 440V DC should be on the first capacitor. All my amplifiers were consistently higher and the voltage on the other side of the inductor exceeds the voltage rating of that capacitor. Also, please note, that when the amplifier is turned on the B+ will rise to a much higher voltage until the output tubes begin to draw current. Make sure your first capacitor has plenty of headroom. The W-4M amplifiers have a reputation for blowing power transformers.
 
I would expect about 600V with output tubes pulled - it probably won't get that high at startup, since the rectifier is a slow a warm-up type.
 
OK, so at LEAST a 630v cap, or two 350v in series (like they did it originally). Got it.

I think I'm gonna go overkill... since it's easy (I got a BUNCH of the caps for a REALLY GOOD price)... three 33uf 400v caps in series, to make an 11uf. When the caps only cost 20 CENTS each (I bought a HUGE BUNCH)... using three of them suddenly doesn't seem like much of a waste. :D

I'll put 100K ohm bleeder/balancer resistors (same value as the original bleeders), one across each cap... should be just fine...

Thanks!

Regards,
Gordon.
 
Anybody tried running one of these WITHOUT the first cap (ie rectifier -> choke, without a cap between), in an effort to reduce the voltage at the first section of the can cap? I'd guess it oughtta drop it a bit, right? Maybe enough to get it back below 450V at the output of the choke?

Regards,
Gordon.
 
GordonW said:
Anybody tried running one of these WITHOUT the first cap (ie rectifier -> choke, without a cap between), in an effort to reduce the voltage at the first section of the can cap? I'd guess it oughtta drop it a bit, right? Maybe enough to get it back below 450V at the output of the choke?

Regards,
Gordon.
Click to expand...

I wouldn't recommend that way. Jim McShane has a pretty good set up to control inrush at startup using a varistor. Here's where you can read about the "limiter" function he installs in Citation IIs for a similar reason. This approach uses 2 CL-145s on on both legs on the primary side of the power transformer to limit the inrush current into the first cap during power up. Scroll down the page a bit and you'll see his reworked cap bank with the 2 varistors on the far left side - the 2 little black things that look somewhat like ceramic caps.

You can also do as the McIntosh amps did and use one varistor on the primary side of the power transformer winding. The MC225/240/275s used this method. Either should work but the McShane approach allows the bias to come up first rather than having B+ on the plates before bias charges up.

One last thing, most of these old amps were built back when nominal AC line voltages in US residences was around 110vAC. This means that you will read higher B+ voltages on these amps when powered from AC lines that now average more like 120-125vAC. Some of the old amps may have an extra tap on the primary side to allow for this, such as the Mac MC-30s.

Cheers,

David
 
GordonW said:
Anybody tried running one of these WITHOUT the first cap (ie rectifier -> choke, without a cap between), in an effort to reduce the voltage at the first section of the can cap? I'd guess it oughtta drop it a bit, right? Maybe enough to get it back below 450V at the output of the choke?

Regards,
Gordon.
Click to expand...
Using a choke input will drop the voltage more than a bit. I will not hurt to try. Let us know what voltage you get.
 
dshoaf said:
I wouldn't recommend that way. Jim McShane has a pretty good set up to control inrush at startup using a varistor. Here's where you can read about the "limiter" function he installs in Citation IIs for a similar reason. This approach uses 2 CL-145s on on both legs on the primary side of the power transformer to limit the inrush current into the first cap during power up. Scroll down the page a bit and you'll see his reworked cap bank with the 2 varistors on the far left side - the 2 little black things that look somewhat like ceramic caps.

You can also do as the McIntosh amps did and use one varistor on the primary side of the power transformer winding. The MC225/240/275s used this method. Either should work but the McShane approach allows the bias to come up first rather than having B+ on the plates before bias charges up.

One last thing, most of these old amps were built back when nominal AC line voltages in US residences was around 110vAC. This means that you will read higher B+ voltages on these amps when powered from AC lines that now average more like 120-125vAC. Some of the old amps may have an extra tap on the primary side to allow for this, such as the Mac MC-30s.

Cheers,

David
Click to expand...
Installing varistors can make a lot of sense for tube amplifiers with solid state rectifiers, particularly if one adds additional capacitors to the first location after the rectifier. I do not think that it makes nearly as much sense for amplifiers with tube rectifiers, they have a slow start because the filament does not conduct for a few seconds.
 
I'm thinking I'm just going to wire in some series-parallel caps with bleeder/balancer resistors, to bring up the voltage ratings on the first stage of the can cap, and just change the first dropping resistor as needed, to bring the other voltages down to where they need to me. With modern 6L6GC tubes in those W4s, the extra B+ on the anodes won't really hurt anything... it'll just be a matter of adjusting the bias to normal spec, and it should be fine...

I found that by paralleling a 33uf 400v cap with the 20uf first cap section, and series-ing a second 33uf cap with that combination (with 100K balancing/bleeder resistors across both caps), you get right at 20uf... but with 800v voltage handling. No way THAT won't be enough!!

Regards,
Gordon.
 
OK, I did it.

Replaced the main can cap with a 4x20uf Rubycon (450V per section, identical to the original) that I had here. Then, I did the series-parallel trick with the 33uf 400v Panasonic caps I got from Electronics Goldmine (they're the 5 for $1 caps, believe it or not.. they work GREAT!). I then wired three of the 33uf caps in series (to make 11uf) and used 100K ohm bleeder/balancer resistors (one across each cap) to make an assembly to replace the two 20uf 350V caps that feed directly from the 5U4 rectifier. This made an 11uf (close enough to the original 10uf) 1200V combo... NO WAY that there can be ANY problem with voltage, on that!

Amp now works great! Didn't have to do a single other thing, to it or the preamp! Woohoo!!

BTW: I'm reading about 475V (@ about 125V from the wall outlet) at the first cap (output of the rectifier), 465V at the next stage (first can cap section, with the series-parallel 33ufs), and 442V at the second stage of the can cap. All within voltage limits of the caps!!

Regards,
Gordon.
 
2 - 100 µf @ 350 V in series...

50 µ is the most input capacitance that is recommended for a GZ-34, and it "should" lower the hum level when used with very efficient speakers. One of John's amps didn't respond very well to me.

I did extensive work on the input and driver stages to keep them more linear.

Richard C.
 
This amp has an original (stock) 5U4 rectifier in it, which I plan to leave. For that one, the spec sheets recommend 10uf... I have 11uf, which should be fine.

If I had two of these W4s, I'd be inclined to do more to "hop them up"... but with just one, I tried very hard to stay right at stock... only change the very few things necessary to make it safe to run. None of my changes should alter the sonics, realistically...

Regards,
Gordon.
 
"Stock" rectifier is 5V4, not 5U4...big difference!

I can't find any reference to a 5U4 being used in these amps.

Richard C.
 
roger that 5V4G . . . .

Gordon, the schematic I have shows 380-0-380 Volts on the high voltage taps right at the transformer, and 130mA. The choke is 7H, 160ohm. Not sure if that shows up on the schematic for the other amp you have or not.

Anyway, you are a lot better at calculating all that stuff out than I am, that should give you enough to go on.

Don
 
BrassTeacher said:
roger that 5V4G . . . .

Gordon, the schematic I have shows 380-0-380 Volts on the high voltage taps right at the transformer, and 130mA. The choke is 7H, 160ohm. Not sure if that shows up on the schematic for the other amp you have or not.

Anyway, you are a lot better at calculating all that stuff out than I am, that should give you enough to go on.

Don
Click to expand...

Found the W4 schematic...

w4schematic.gif


Regards,
Gordon.
 
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