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Heathkit W-5M Rebuilds

trobbins said:
Imho, increasing the 20uF caps for PI and driver stages, or the common cap that is separated by the 100 ohm dropper, are not a concern.

Increasing the input stage supply cap may be a concern as it may have been designed to tailor the low frequency response for stability. That concern would be diminished if you added a low frequency shelf network for improved stability.

Increasing the CLC filter caps would I suggest require some design assessment, such as by using PSUD2, to see their effect on rectifier current.

Any such pcb should allow the grounds to the caps to go to their separate circuit sections. You may want to consider adding a fuse to the pcb - eg. in the CT feed.. You may want to consider adding ss diodes in series with the rectifier anodes, and placing them on a pcb.

One article with some relevance is: https://www.dalmura.com.au/static/Power supply issues for tube amps.pdf Another article with some stability references is: https://www.dalmura.com.au/static/Williamson design info.pdf
Click to expand...


Thank you very much!

Unfortunately, I only know enough to be dangerous. Which is to say that I'm pretty much out of my league when it comes to doing the various calculations necessary to redesign the power supply. While the schematic says "140 ma" at the high tension outputs to the rectifier tube (which I assume is a max power rating), I'm somewhat flying blind because I don't have actual specs on the transformer. Though assuming a max, continuous output of 140 ma is probably a good working assumption. But I have no idea how much the transformer could tolerate on initial startup, when charging the capacitors creates high initial demand. I like your idea of a fuse on the center tap, but I have no idea what rating I should use. And sadly, I have no idea what "low frequency shelf network" means. But it seems that that would apply to to the 20 mfd caps that supply the 2 12AU7 tubes.

The thing that's confusing/perplexing to me is this. Having read a lot of threads about folks revamping older power amplifiers, it seems like increasing power supply capacitance is pretty standard practice. Heck, in October, I completed and installed a capacitor board kit for my ST-70, which increased the capacitance vs the old "can" capacitor from 30/20/20/20 to 40-195-135-135, and it made the amplifier sound better. Certainly the first stage of the Pi filter saw little increase, but the downstream capacitors were increased substantially.

So I thought this was a relatively easy question, but it's increasingly appearing that it isn't. So maybe I'll just go with spec, and get on with my life, hehe.

Thanks again, and I'm going to read the links you provided.

Cheers,

Kim G
Boston, MA
 
My 2 cents: a lot of mods have and are being done based on what has been seen to be done by others, and the original mod can be based on no detailed technical assessment (either as a design or performance assessment), and given the amp has continued to work (well at least for the time a forum thread was active) then future readers just take the mod as valid and worthwhile, especially if some audiophoolery comments are included in the thread.

There is nothing wrong with people installing each and every mod that can be found on the internet, or perceiving that the mod improves their aural experience or the aesthetic appeal of their tube amp when shown off in the loungeroom - as that is a joy that many experience and want from making mods in the first place, and is part of human nature, and why anything audio related is prone to commercial exploitation.

It is not easy or simple to undertake technical assessments - it takes skill, experience, equipment resources and a lot of time and incentive (as shown in Dave Gillespie's threads on particular amps in this forum).
 
@trobbins

Indeed. Unfortunately, I don't know enough to do a proper technical analysis. I've been emailing back and forth with Sheldon Stokes of quadesl.com, who has designed a capacitor board for a number of old amps, including the W-5M. He gave me a few pointers, but it appears there's no simple "just increase Cx to xx mfd and you'll be good" kind of answer.

I'm thinking I'm going to increase capacitance modestly and just get going on the actual build.

Thanks for your input; I really appreciate it.

Kim G
 
My two cents: Keep first input cap at 10uF. Your rectifier tube will thank you. Second cap can be 50uF or even 100uF with no harm. For the rest, stick to Heathkit's 20uF.
 
zackthedog said:
My two cents: Keep first input cap at 10uF. Your rectifier tube will thank you. Second cap can be 50uF or even 100uF with no harm. For the rest, stick to Heathkit's 20uF.
Click to expand...
I think first cap value can be increased if diodes added as Tim suggested. PSUD2 should help determining safe capacitance.
 
I like the Pomona binding posts which I buy at Mouser. I feel more certain of product quality with these. I have used this double, p/n 6883 and singles from the 3750 family. They have a neat 3 post base which lets me put the black in the middle so I can use dual banana plugs for either 4 or 8 ohm connections. I don't think I have ever owned a 16 ohm speaker. The base is p/n 4619-0. The 3770 family is tellurium copper which I would like to try someday but they are a bit more expensive although not like the audiophile brands made of copper alloys.

Brass isn't a very good conductor, about 25% the conductivity of copper, so the gold plated copper alloy should be a much better connection.

John
 
Wow! $35 for two pairs of binding posts seems like a lot. I hope they include the sound-enhancement module. ;-)
 
zackthedog said:
My two cents: Keep first input cap at 10uF. Your rectifier tube will thank you. Second cap can be 50uF or even 100uF with no harm. For the rest, stick to Heathkit's 20uF.
Click to expand...

The first set of capacitors off the rectifier is a pair of 40 mfd caps in series, which means that the effective capacitance is 20 mfd. Given the relative scarcity of 40 mfd caps, I've decided to replace them with 47s, which means an effective increase of 3.5 mfd, which shouldn't cause any problem.

I am seriously considering going to a bridge rectifier instead of the tube. I know you prefer the tubes, but a bridge rectifier would have lower internal resistance and a faster response to transient power demands. Yes, this would make the amp sound different, but I believe it'd sound more accurate too, as voltage sag would be reduced. And without the need for the 5v filament power to the rectifier, the transformer should also run cooler. Of course I'd need a hefty dropping resistor of unknown resistance to produce the correct voltage drop. And if I did install a bridge rectifier, I'd also combine that with an NTC thermistor on the B+ line to moderate the startup demands on the power transformer, and also give the tube filaments some time to come up to temperature before high voltage gets applied to the plates. But that's probably a down-the-road project. For now, I'm eager to get the amps running at more-or-less original spec.

Thanks for your input,

Kim
 
Defeño said:
The first set of capacitors off the rectifier is a pair of 40 mfd caps in series, which means that the effective capacitance is 20 mfd. Given the relative scarcity of 40 mfd caps, I've decided to replace them with 47s, which means an effective increase of 3.5 mfd, which shouldn't cause any problem.

I am seriously considering going to a bridge rectifier instead of the tube. I know you prefer the tubes, but a bridge rectifier would have lower internal resistance and a faster response to transient power demands. Yes, this would make the amp sound different, but I believe it'd sound more accurate too, as voltage sag would be reduced. And without the need for the 5v filament power to the rectifier, the transformer should also run cooler. Of course I'd need a hefty dropping resistor of unknown resistance to produce the correct voltage drop. And if I did install a bridge rectifier, I'd also combine that with an NTC thermistor on the B+ line to moderate the startup demands on the power transformer, and also give the tube filaments some time to come up to temperature before high voltage gets applied to the plates. But that's probably a down-the-road project. For now, I'm eager to get the amps running at more-or-less original spec.

Thanks for your input,

Kim
Click to expand...
You might then consider going for a quasi-choke input PS to drop the voltage a bit since the OEM arrangement already stresses the opts pretty hard.
 
Defeño said:
Wow! $35 for two pairs of binding posts seems like a lot. I hope they include the sound-enhancement module. ;-)
Click to expand...

That's cheaper than the copper alloy posts at Mouser and quite a bit less expensive than others. I might worry about the structural integrity of pure copper since it is soft. That is probably why they are encased in a plastic sheath for support. I don't want to give the impression that the expensive posts are what I use. I have a mild cheapskate streak and have always used plate brass binding posts but just lately have been thinking about the good stuff. I would never consider using brass wire in an amp so now wondering if I should spring for the good posts. I also make sure I don't get passive components with plated steel lead wires. It also forces the question about what am I putting into the jacks on the speaker wire side. Just my meanderings.

John
 
I doubt doubt less than an inch of large gauge brass instead of copper is going make a meaningful difference in the results.
 
Defeño said:
The first set of capacitors off the rectifier is a pair of 40 mfd caps in series, which means that the effective capacitance is 20 mfd. Given the relative scarcity of 40 mfd caps, I've decided to replace them with 47s, which means an effective increase of 3.5 mfd, which shouldn't cause any problem.

I am seriously considering going to a bridge rectifier instead of the tube. I know you prefer the tubes, but a bridge rectifier would have lower internal resistance and a faster response to transient power demands. Yes, this would make the amp sound different, but I believe it'd sound more accurate too, as voltage sag would be reduced. And without the need for the 5v filament power to the rectifier, the transformer should also run cooler. Of course I'd need a hefty dropping resistor of unknown resistance to produce the correct voltage drop. And if I did install a bridge rectifier, I'd also combine that with an NTC thermistor on the B+ line to moderate the startup demands on the power transformer, and also give the tube filaments some time to come up to temperature before high voltage gets applied to the plates. But that's probably a down-the-road project. For now, I'm eager to get the amps running at more-or-less original spec.

Thanks for your input,

Kim
Click to expand...

Are you talking about a true bridge rectifier, or just using SS diodes in full-wave configuration in place of the tube rectifier? Unless I'm mistaken, a bridge would yield an enormous amount of voltage and would halve the current capability of the PT.
 
nerdorama said:
That's cheaper than the copper alloy posts at Mouser and quite a bit less expensive than others. I might worry about the structural integrity of pure copper since it is soft. That is probably why they are encased in a plastic sheath for support. I don't want to give the impression that the expensive posts are what I use. I have a mild cheapskate streak and have always used plate brass binding posts but just lately have been thinking about the good stuff. I would never consider using brass wire in an amp so now wondering if I should spring for the good posts. I also make sure I don't get passive components with plated steel lead wires. It also forces the question about what am I putting into the jacks on the speaker wire side. Just my meanderings.

John
Click to expand...

I think the key is sturdiness. I'm not all that fussy about binding posts but I use a heavy-gauge cable that needs to be tightened down, and flimsy binding posts can snap or twist. Plus it would be handy to have something that fits neatly into the existing jack holes without a lot of play that might risk shorting to the chassis.
 
Pio1980 said:
You might then consider going for a quasi-choke input PS to drop the voltage a bit since the OEM arrangement already stresses the opts pretty hard.
Click to expand...

Hi Pio1980,
I'm not sure what you mean by "quasi-choke input PS." The "factory" schematic takes the rectified B+ off pin 2 of the rectifier at 510 VDC (with ripple) and feeds it into the first pair of 40 mfd caps, which then feeds to a choke, beyond which is a second pair of 40 mfd caps, from which the center tap of the OPT is fed. And after that, there's a 100 ohm resistor which feeds a pair of 20 mfd caps, which then feed the 2 12AU7 tubes. So there's already a choke in place.

Thanks for your comments,

Kim
 
nerdorama said:
That's cheaper than the copper alloy posts at Mouser and quite a bit less expensive than others. I might worry about the structural integrity of pure copper since it is soft. That is probably why they are encased in a plastic sheath for support. I don't want to give the impression that the expensive posts are what I use. I have a mild cheapskate streak and have always used plate brass binding posts but just lately have been thinking about the good stuff. I would never consider using brass wire in an amp so now wondering if I should spring for the good posts. I also make sure I don't get passive components with plated steel lead wires. It also forces the question about what am I putting into the jacks on the speaker wire side. Just my meanderings.

John
Click to expand...

Hi John,

I rather doubt anyone could tell the difference between copper posts and something considerably cheaper. I too have a cheapskate streak, combined with a skeptical streak, so no $70+ USD capacitors for me, hehe.

Cheers,

Kim
 
Quasi choke input usually means using a quite small first capacitor, i.e., 1-5uF or so, which drops the voltage after the choke but still gives a little initial filtering. If you have been using PSUD, you can see the effect quite readily. This first small cap will likely need a fairly high voltage rating as it will see high ripple voltage. It's a way to tune the output B+ voltage.
John
 
zackthedog said:
Are you talking about a true bridge rectifier, or just using SS diodes in full-wave configuration in place of the tube rectifier? Unless I'm mistaken, a bridge would yield an enormous amount of voltage and would halve the current capability of the PT.
Click to expand...

Hi @zackthedog,

I was thinking of using a plain old, single-piece silicon bridge rectifier. How would that be any different (aside from easier) than wiring together 4 diodes to do the same? And I don't believe it should generate any excess voltage, beyond the difference between the voltage drop across the tube and the voltage drop across the rectifier, which I believe is somewhere in the neighborhood of 60 volts. Keep in mind that the rectifier tube is a full-wave rectifier, not a half-wave rectifier. Were the tube only a half-wave rectifier, yes, then replacing that with a full-wave rectifier would result in an increase in DC voltage. But otherwise, the main difference should be the difference between the voltage drop of the tube and the voltage drop of the silicon bridge rectifier.

Figuring out the ohms and wattage rating of the necessary, additional dropping resistor would need to be done, as well as figuring out whether the additional heat generated by that resistor would be a problem.

But if you think any of the above is wrong, please let me know. My electronics knowledge, politely speaking, is fairly basic.

Thanks,

Kim
 
nerdorama said:
Quasi choke input usually means using a quite small first capacitor, i.e., 1-5uF or so, which drops the voltage after the choke but still gives a little initial filtering. If you have been using PSUD, you can see the effect quite readily. This first small cap will likely need a fairly high voltage rating as it will see high ripple voltage. It's a way to tune the output B+ voltage.
John
Click to expand...

Hi John,

I'm still working my way through the voluminous reading that Trobbins provided on the last page. And I haven't yet dived into PSUD. Heck, I'm just trying to get my parts order straight so this can all proceed from the theoretical to the solderable.

Thanks for your comments. Cheers, Kim
 
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