Heathkit W5M rebuilds

ab0ez · May 23, 2008

Ok, here goes the start of another thread. I am rebuilding a nice pair of Heathkit W5M monoblock tube amps for a friend of mine. The goal will be to bring the amps back up to specs and use modern components. Basically, we are talking gut everything but the transformers, but keep the chassis and appearance as original as possible.

The amps are a few years apart. One had the surgistor slow start and the other did not. The various capacitors were also from different manufacturers as can be seen in the photos of the underneaths.

The top view photo was typical of both amps.

ab0ez · May 23, 2008

Step 1: Replace the sockets!

This one will get you, from time to time.

I had a Bogen DB130 amp (a real sleeper) in which I could not get the regulated voltage for the screens on the output tubes to stabilize, no matter where I set it. Changing all the components in the circuit did nothing. Changing the socket did. Lesson learned. Although it looked fine to the eye, the dielectric in the tube socket was breaking down.

Here are some before and after shots of the rectifier socket replacement.

ab0ez · May 23, 2008

Output sockets replaced

Two shots of the KT-66 output tube socket replacement.

GordonW · May 23, 2008

Eek, I see at least 15 capacitors per amp that gotta go! That's a lot of stuff...

BTW: Which output transformers do those W5s have?

Also, just as a note: if I were re-doing these, I'd be inclined to swap out the 12AU7s for 12BH7s, or possibly even better, 6CG7/6FQ7s. Since you're replacing all the passive components, it'd be pretty easy to re-optimize the circuit. And since the amp doesn't have to drive an outboard preamp, you should have enough heater capacity on the power transformer, so that changing the input tubes won't over-stress anything...

Regards,
Gordon.

ab0ez · May 23, 2008

Ah, you are very perceptive and on the same track as I was. Every cap and resistor was replaced, along with dumping the 12AU7's for 6CG7's. Keep watching the post for, as Paul Harvey would say, the rest of the story!

The output transformers are the 16309's in both amps.

Two shots of the front end 9-pin tube socket replacements.

ab0ez · May 24, 2008

New input jack and adding a power switch

So, the original W5M amps did not have a power switch. They were swtiched on and off via the matching preamp by bridging a connection on the AC primary side of the power transformer via the preamp power socket on the chassis. This was no longer desired. In fact, all the connections and related wiring were removed from this socket.

But where do we add a power switch without altering the chassis? Well, in addition to the preamp power socket no longer being used, the AC accessory outlets were no longer needed nor desired so they were removed. An appropriate on/off switch was added and just fit into the existing hole now vacated by one of the now defunct AC accessory outlet holes in the chassis.

A vampire RCA input jack was also added. Note that the insulating washer was used and it was verified via an ohm meter that there was no continuity from the ground side of the connector to the chassis.

Andyman · May 24, 2008

Great thread!!
I have 3 of these I need to rebuild this summer. Two have the 16458 trannies and the third the 16309. I'm planning on consolidating the "best" iron and chassis for the keepers, and using the rest on the third with the 16309. I'm pretty sure that I'll keep the circuit stock as I don't want to compromise any "collectibility" of the amps if and when I decide to sell them.

Where did you source your caps at? I've made some half hearted attempts to find some, but haven't had a whole lot of luck, especially on the multistage ones. Are those new sockets Teflon or ceramic; I can't tell from the pics.
Keep this stuff flowing; it looks great so far!! :thmbsp:

ab0ez · May 24, 2008

I left the original can multi-section caps in place in order to help preserve the original appearance. They are not in the circuit, but keep following the post and all will be revealed!

The tube sockets are ceramic.

Cheers!

Andyman · May 24, 2008

ab0ez said:
I left the original can multi-section caps in place in order to help preserve the original appearance. They are not in the circuit, but keep following the post and all will be revealed!

The tube sockets are ceramic.

Cheers!

I suspected that was the case with the cans.
Keep it coming!!

ab0ez · May 24, 2008

First filter cap

I took a step beck at this point and read all I could about the W5's and some of the typical problems that they were having both after decades of use and decades of sitting.

The problem of dead power transformers was often mentioned as well as problems with the rectifier tube socket. Well, that socket was already replaced, so problem dealt with.

But what about these power transformer problems? Heath did modify the design a few years down the road to include a surgistor to give the amp a soft-start characteristic and lessen the turn-on strain on the transformer. Both amps have now been modified to include new surgistors placed in the primary path of each power transformer.

Looking a bit further in the design, you have two 40uF 450VDC electrolytic capacitors in series to effectively create a 20uF 900VDC cap that operates as the first filter cap in the power supply. Electrolytics get leakier and leakier over the years and you also have the added 100K bleeder/equalizing resistors in the circuit.
So, I decided to use a 4uF 1000VDC GE Pyranol oil-filled cap mounted in the space below the power transformer. With today's higher line voltages, I wasn't too worried about the B+ being much lower than if the original 20uF value was used. Additionaly, since the B+ is taken after the choke, I also wasn't too worried about excessive ripple voltage on the B+.

This should help lessen the initial turn-on stress.

After some further reading, I connected the center tap for the 6.3VAC heater line to the elavated ground grid. This was recommended by Heath if the amps were not going to be used to power a preamp.

ab0ez · May 24, 2008

I attached a copy of the schematic to help follow along.

Up next is the 2nd filter cap section, a new bleeder resistor, and a new 100 ohm dropping resistor.

The second cap filter section is composed of four 33uF 450VDC caps in a series-parallel arangement to form an effective 33uF 900VDC cap. This is a bit higher than the original value of 20uF, but since this is buffered by the DC resistance of the 7 Henry 160 ohm choke, it won't make much difference in transformer stress, but gives us a little more filtering and energy reserve right at the B+ tap point.

A new installed a new 100Kohm PRP metal film bleeder resistor across the 33uF filter cap network. Even though the value and tolerance of these resistors isn't real important, another thing is: Noise! Ever have a not real loud intermittent popping noise in an amp? Replace those old carbon comp bleeders!

The original 100 ohm dropping resistor in each amp were an old 7 watters that looked as though they had been baked in a charcoal grill. They were replaced by two 220 ohm 10 watt resistors in parallel, for 110 ohms. Why didn't I use two 200 ohm resistors? Easy enough: I didn't have any in my junk box! Besides, the original was a 10% tolerance part, so 110 ohms would have still been within tolerance and the additional 10 ohms over 100 won't make much difference. Now we have a dropping node with a dissipation rating of 20 watts vs the original 7 watts. It should run a tad cooler!

ab0ez · May 24, 2008

Cathode bypass caps gotta go!

Needless to say, those old, falling apart cathode bypass caps gotta go!

I didn't get extravagant here, I just used some chemi-con 100uF 250VDC caps that I already had on hand in my trusty junk drawer. Even though they are the same physical size as the old caps, they are rated for 250VDC vs 50VDC in the original configuration. The new caps shouldn't be strained at all!

You can also see the new Kiwamme 5 watt 270 ohm screen current limiting resistors for the KT66 tubes.

ab0ez · May 24, 2008

More old stuff replaced

In this shot you can see the old resistors from the bias network and balancing network are gone and have been replaced with Kiwamme and PRP resistors. I also replaced the old 330 ohm 7 watt wirewound resistor with a new 330 ohm 10 watt resistor.

k**rkleclaire · May 24, 2008

Hi, Great Thread!...
I've done a W-5M rebuild and am a bit of a newbie and this thread would have been a great help. Question: when I replaced my Power caps I used high end Jensen electrolyics strung togather, these are physically large and expensive. My question: do power caps affect the sound? I know the output caps do, but could I have saved some expense and space by using smaller cheaper caps? I'm happy with mine but am curious?
Kirk

dividebytube · May 24, 2008

I've heard some modified W5s where the builder added fixed bias. Negative bias was taken by using a radio shack bell transformer - using the 12V side hooked to the filament supply. The primary (er 120V side) was used as the secondary + diodes/caps to get the negative bias voltage.

They also added battery bias to the first section of the 12AU7. I would prefer the 12BH7 or 6CG7 in place of the 'AU7.

ab0ez · May 24, 2008

Power supply caps?

Do power supply caps change/effect the sound?

Great question!

The answer is yes, and no.

If you are running a crummy preamp and a set of speakers from a Kenner close & play, the answer is no!

If the rest of your system is excellent, i.e. excellent preamp, excellent speakers, good quality interconnects, good playback source and material, then the answer is a bonafide yes! However, I don't think even all that will make rap music sound good......

One thing I have noticed over time is that the better my system got the more apparent differences became between CD's with respect to sound quality. For instance, the difference in sound quality between Steely Dan's "Katy Lied" release and "Aja" is light years different. I had heard for some years about how Becker and Fagen were so unhappy with the sound of the album that they refused to listen to it for years. On the system I had when I was a couple decades younger, I couldn't tell much difference between the two.

Finally, it will also come down to your own personal preference in the sound you want.

ab0ez · May 24, 2008

Fixed Bias?

If the amps were mine I probably would have done just that and installed a fixed bias network.

The 12AU7's are history, replaced with 6CG7's. NOS Sylvania black plates, to be exact.

currituckco · May 25, 2008

Wow, rap music sure does take a beating on this here forum!

I think tube amps and Gangstarr can mix quite nicely, actually.

ab0ez · May 25, 2008

Hey, if you like it, that's cool.

But, I was referring to the practice of compressing the music so badly that there is only about 5 or 6 dB between the softest and loudest points in the music....yuch!

It seems more and more to be the normal practice nowadays and not totally confined to rap music. Take a listen to Santana's Supernatural. Terrible recording, painful. Good music, but a shame the labels think a higher average volume level is better and sells more music.

If you're a ham radio operator, listen around for the "speech processor" guys. Won't take you long to find one. Same idea, they raise the average audio level way up because it sounds louder.

But I digress, back to the W5M story.

jon_s · May 25, 2008

currituckco said:
Wow, rap music sure does take a beating on this here forum!

I think tube amps and Gangstarr can mix quite nicely, actually.

My Craftsmen 500s love Tribe Called Quest ... Much less-compressed than most new stuff, and lots of sweet stereo tricks and crystal-clear vocals. A great way to show off the fact that some tube amps do, in fact, reproduce tight bass. Against all odds, the 500s beat my Citation II hands down for rap :smoke:

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