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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Heathkit W-5M rebuild

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Johan,

No apologies necessary. I appreciate advice - just because it may be basic doesn't mean I don't need to be reminded. Due to the wiring mess I found a complete rebuild seems the best way to go.

Many of the components had an oily film on them - sort of like the residue from acid flux solder. Also a cap had a hole on one end - the edges were mushroomed out so whatever caused the hole did it from the inside out. I am going to replace ALL the parts except for the transformers and choke. As I have already stated, this is going to be fun!

Changing the subject, I am going to start a new thread for information about a Shuguang integrated amp I picked up on eBay.

Kevin
 
HollowState said:
Still I'm at a loss to explain your OOT numbers. Perhaps I have not checked enough of my own to get the full picture. Or they may not be as old as yours. Don't know.

I suppose that it has to make a difference if a carbon composition resistor were made in 1980 or 1940. I'm guessing that many of my NOS carbon comps were made when FDR was still in the White House!

HollowState said:
Another reason would be in keeping an amplifier original when restoring it. In much the same idea that one would restore a classic car. You wouldn't want air bags or tinted windows in a model T Ford would you?

Well, yes, I might, if I really intended to use the Model T for daily transportation and wanted a safer, more drivable car. Of course we're stretching that analogy too far, as Johan says. If I were worried about historic considerations and appearances only - then yes, I'd use only vintage parts. But if I wanted an amplifier to sound its best, and to do so for a long time, I would consider modern parts where they could make demonstrable improvements.

For example, if I had the good fortune to own a Marantz Model 7C (see recent thread), and I planned to unload it on eBay for $6K, I would go to some effort to find vintage parts if needed to bring it up to working condition. For a less dear older amp that I wanted to use in a working music system, such as the fine Heathkit W-5M, I wouldn’t have any hesitation mixing in newer parts. Just MO of course.
 
Firenewt,

From one Hoosier (now a Floridian) to another - for what very little it may be worth, my much-abridged views on using various resistors types for audio:

Wire-wounds: most plate loads, any critical power resistor use in the audio path. Low short-term voltage coefficient of resistance, low excess noise and modulation noise. Inductance is not usually a problem, as Morgan Jones found in tests. Also useful in power supplies of course. These actually sound demonstrably better as plate loads to me than any other resistor type.

Metal films: low power, low DC current applications in the audio path, wherever high accuracy is needed (RIAA filters, feedback resistors, etc.). For all grid “bias” resistors where there is no current. Rarely used for plate loads due to excess and modulation noise. With prices very low these days, I also use these in less critical applications where a carbon comp might have been used in years past. For example, bypassed cathode resistors may often be either metal film or carbon comp since the cap swamps the audio characteristics of the bypassed resistor. But I’d generally use a metal film for stability of value.

Carbon composition: for low criticality use only, especially outside the main audio path and where reliability under high power surge might be an issue. Examples: power supply dropping resistors, bleeder resistors, voltage dividers for biasing floating heater supplies. The only use in the main audio path might be for small-value grid stoppers. I measure all resistors before using them, but especially carbon comps to make sure that values have not drifted outside my requirements.

Carbon film: I treat these much like carbon comps, but where a touch more accuracy might be needed. I don’t generally bother to buy them with metal films so cheap.

Metal foil, tantalums: expensive boutique parts that I rarely use, but have employed for very critical use: feedback resistors, RIAA and other filters. Season to taste.

Metal oxide: for less critical power uses.

I know everyone has his preferences, and I may regret posting such an abbreviated set of uses. There will certainly be exceptions to what I said above.
 
Brian,

You will not be regretting that because of any comment from me. But perhaps a question: You seem to veer towards low current (low heat?) applications only for metal film resistors. I have used them quite hot, within the dissipation and voltage limitations also as plate loads. I am using 3W metal oxides as such which get pretty hot (7mA through 33K). Did you have experience that should steer one away from this?

Here I must add that local availability plays a role. I would use WWs but locally 5Ws are not available up to 33K, and 10Ws are uncomfortably bulky. (Yes, one could import, but effort, quantities, cost .....) My tests did show that metal film (and WWs) have less of a temperature co-efficient than carbons, but not seriously so.

Let me also agree that, mainly as a correction to those always flinging generalities around, the inductance of WWs are negligible in audio. The highest ones I have tested (68K), had an inductive component of perhaps 5% of the ohmic value at 20 KHz. So please folks, let us just write that one off as academic. (If one wants to be pedantic, it would slightly increase the bandwidth with the Cs in the circuit, not decrease it.)

Regards
 
Johan,

Poking around the web, you can find some literature about resistor noise modulation. The URL below is one such reference. Such noise is usually specified as uV of noise per volt of applied DC. Our friend Morgan Jones has also covered this topic in his book (pages 206 and 207 in the 3rd edition). He says that a typical value for a lower value metal film would be 0.1uV/V, while the higher values that we tube designers might use could exceed 1uV/V. While quiescent conditions alone could create a noise floor of as high as -100dB, in the case of something like a plate resistor, signal voltage swings will pump the noise level up and down – noise modulation. I have seen odd noise floor modulations before on spectrum analyzers. Perhaps this was one cause. I can’t tell you if the relative lack of modulation explains why I have preferred the sound of WW as plate loads before, but it is possibly one reason.

Vishay paper on resistor noise
 
Brian,

No fine, point made!

WW seems to have a whole 10dB (3,3x) advantage over metal film. But if my calculation is correct, I find that for a 100K at 1V signal, even the m.f. resistor's noise as given by the Vishay article is only about 0,5% of the Johnson noise for audio.

This is then insignificant in all but the input stages of a phono pre-amplifier or such. But your post did juggle the grey cells (in my case inside and out!) into recapping my facts.

I would still like to know if there is anything dissipation wise that should put me off using metal film resistors, from yourself or anybody else. In power amplifiers resistor noise is hardly a factor, but I will in future regard the WWs with greater desire! What is the highest value you know of in 3W and 5W? (As said local supplies are limited.)

Thanks and regards.
 
Johan Potgieter said:
I would still like to know if there is anything dissipation wise that should put me off using metal film resistors, from yourself or anybody else. In power amplifiers resistor noise is hardly a factor, but I will in future regard the WWs with greater desire! What is the highest value you know of in 3W and 5W? (As said local supplies are limited.)

Johan,

Dissipation-wise, no problem. But voltage ratings are often limited to 200 to 350 volts (500 volts for a 1 watter) because the spiral laser cut used to tweak the values makes a thin channel across which high voltages can breakdown. Sometimes it might not be even possible to achieve the dissipation rating for a given resistor value and remain below its voltage rating. For example a Vishay-Dale CCF50 is a nominal 1/3 watt resistor rated to 200 volts. You can buy a 1 MEG value in the CCF50 line, but 200 volts across 1 MEG is only 40mW. Placing MF resistors in series is one solution, of course.

I don’t know if Mills resistors are available in South Africa, but they might be worth searching out. They have suitably high values for plate loads:

Mills resistors

Also, I have found a number of 5 and 10 watt WWs with values between 6K and 100K by scrounging at flea markets and hamfests. I’ll buy higher resistance value NOS WW resistors at a flea markets whenever I come across them. A handful for a buck? Can’t pass them up.
 
But if my calculation is correct, I find that for a 100K at 1V signal, even the m.f. resistor's noise as given by the Vishay article is only about 0,5% of the Johnson noise for audio.

But, in an extreme example, if you have 200 volts DC dropped across that same resistor, say in a plate resistor role, that's maybe 20uV to 200uV of broadband noise. That could well be more than the Johnson noise. Now if the plate swings by tens of volts of audio instead of 1V, you have significant modulation of that noise. The psychoacoustics of that situation are unknown to me.
 
But, in an extreme example, if you have 200 volts DC dropped across that same resistor, say in a plate resistor role, that's maybe 20uV to 200uV of broadband noise. That could well be more than the Johnson noise. Now if the plate swings by tens of volts of audio instead of 1V, you have significant modulation of that noise. The psychoacoustics of that situation are unknown to me.

I think plate loads are one place its recommended to use carbon comps for people after vintage sound in guitar amps...
 
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