Super newbie question about resistors

[Michael415]

Apologies for the very green question. I have built kits before and know I prefer certain parts in specific locations if at all possible but to be honest do not understand the math/science behind it. This amp will be a stock (but I do intend to upgrade to the C4S, unsure about the iron yet). I was wondering what ill effects there would be if I used a few resistors that were slightly rated different than what comes with the kit (watt rating will match or be higher in the replacements that much I do understand). Specifically these:

Kit:                Possible substitute:
1.27k ohm      1.3k ohm
249k ohm        250k ohm
620 ohm         600 or 680 (prefer the 680's brand if possible)

C4S
Kit:                Possible sub:
270k ohm       330k ohm

[Doc B.]

Hi Michael,

I'll let PJ coach you on the resistor values, but I just wanted to mention that the first name/three numbers format of your username is now being used by a lot of meatware spammers as they try to develop less obvious ways of posting junk links. We tend to key on that username format when we cruise through the member list, so you might want to think about changing it to avoid being accidentally banned by us overzealous mods.

Also, we request in the forum posting rules that members sign their posts with their real names. Not picking on you in particular, there have been a lot of new members lately and some have probably missed the requirement in the forum rules, as it is not a common one. I am just using this opportunity to remind everyone that posting our real names helps to keep this forum as civil as it is.

[Michael415]

Sorry about that, corrected the signature line.

[JC]

I believe that 620 Ohm actually helps to set the negative bias Voltage for the output, so the amount it drops is rather specific.  Not so sure I'd want to vary from the 620 Ohm value too much.

Having said that, I also don't think that the type/brand will have much appreciable effect on the sound.

In general, I'm favorably impressed with how much attention to picking the proper type for its intended service that Bottlehead evidently pays to its resistor selections.

[Paul Joppa]

Apologies for the very green question. I have built kits before and know I prefer certain parts in specific locations if at all possible but to be honest do not understand the math/science behind it. This amp will be a stock (but I do intend to upgrade to the C4S, unsure about the iron yet). I was wondering what ill effects there would be if I used a few resistors that were slightly rated different than what comes with the kit (watt rating will match or be higher in the replacements that much I do understand). Specifically these:

Kit:                Possible substitute:
1.27k ohm      1.3k ohm
249k ohm        250k ohm
620 ohm         600 or 680 (prefer the 680's brand if possible)

C4S
Kit:                Possible sub:
270k ohm       330k ohm

OK, the 1.27k is actually a 1% resistor but it does not need to be better than 5% - I like metal film for stability and low noise in this position.

The 249K is a grid resistor and can be anywhere from 150K to 500K, so again the fact that it is a 1% part is not critical. It does not carry any DC current so carbon or carbon film are reasonable alternatives. (Metal oxide has a poor reputation for sonics so I'd stay away, though I've never done the experiemnt myself.)

The 620 ohms is indeed the bias resistor. It's a 5% part and should be rated at least 2 watts; it dissipates some heat so stability is important. The value is critical; I would not use a 680 ohm part, and in fact would not stray more than 5% (and even then, only if you can get a 1% part to replace it).

The 270K in the C4S is the LED bias and can deviate 10% without problems, however it does drop 350 volts and must be rated to withstand at least 400 volts DC, working voltage not dielectric strength. We have had some bad experiences pushing resistor working voltage ratings, so a 500v part would be more confidence-inspiring.

[Michael415]

Thanks so much for the information. In the past I have used Shinkoh Tantalum resisters and had great luck and was looking to repeat history with the new project. It sounds like I should leave the 620 and the 270k alone though just to be on the safe side as I am not sure how much voltage it can withstand.

I am also likely to upgrade the caps to Jupiters (HT's as you recommended for longevity). I am thinking with these changes and knowing the type of sound I like I should be extremely content.
I have been reading up on the iron upgrade but need to continue reading more since this amp will only be used for phones not sure its a worthwhile investment.

[ssssly]

I'm a Shinkoh fan as well. I have been using Takman for the values I can't get in Shinkoh with good results.

[Jim R.]

Michael,

Yes, definitely use the 499/500 ohm shinkoh in the CCS boards -- this is one of the places where a really good resistor will be most audible.

This has been discussed a lot in the past, and I have both matched pairs of shinkohs and caddocks for the r1 on the CCS boards.  I'll try both andkeep whatever sounds best to me.

-- Jim

[tsingle999]

The iron upgrade is definitely worth it even for headphone use only.

[Michael415]

Thanks for all the great input. I will be checking out Takman as well. I had totally forgotten about caddocks and they could also be a viable option.

I am curious why you think the iron upgrade is worthwhile for just phones? I was under the impression it was mostly to give more umph for speaker applications. BTW this amp will be running low ohm cans, 25-32 primarily if that is a factor w/the iron upgrade.

Jim, quick question about the CCS boards - I did a quick search but didn't find much about them in regards to the SEX amp. Care to elaborate a bit more on them?

[Paul Joppa]

...
I am curious why you think the iron upgrade is worthwhile for just phones? I was under the impression it was mostly to give more umph for speaker applications....

I can offer a theoretical explanation - distortion. Iron distortion is quite complex; the papers by Partridge offer probably the best explanation even though they are more than 50 years old. While most nonlinearities in electronic components are reduced at smaller signal levels, iron-cored components have a broad power region where the hysteresis distortion is substantially constant. That means it is equally audible at small or large signal levels, i.e. headphones or speakers. A generous amount of inductance (usually meaning quality core materials, carefully stacked) reduces that distortion; quality materials also have less hysteresis distortion in the material to begin with. At high frequencies, the capacitance of the winding comes into play, and better transformers use materials with better dielectrics, again reducing distortion - basically a better transformer is a better capacitor at high frequencies.

[Michael415]

Thanks so much for the information and clarification!