How critical are the two 1k27 resistors?

[mcandmar]

The two 1.27ohm resistors i received look like they are from a different batch/manufacturer and vary a little in value. The purple line on one of them looked black so i measured them to be sure they were correct and found they are only just within 1% tolerance, but at different ends of the scale from one another.

Given the unusual and rather specific value how critical are they in the circuit, will a change in value effect channel balance or what is their purpose in the grand scheme of things?

My 2nd question is if i was to replace them what working voltage do they see?  Is it 320v or 160v?

Thanks,

Mark

[Paul Birkeland]

Visually looking at a pair of resistors won't tell you much.  They could have been made in the same batch, but the machine that coats the outside of the resistors may have been refilled during production, leading to slightly different coloration.

A 1% match is within the specified parameters of the part value.  I would very strongly advise that you build the kit with the parts supplied.  Taking care of the repairs at Bottlehead, I end up spending a lot of time removing "upgraded parts" that have render kits inoperable. 

(Also, the voltage across these resistors is a couple of volts)

[Doc B.]

A 1% tolerance resistor in this application is overkill. Being at "different ends of the scale from one another" is insignificant with respect to the amp's performance. By the way they are 1.27K ohms. That K is waaaaaay more important than the 1%.

[mcandmar]

Ok i forgot they are K and not ohms so therefore they are way outside the 1%, infact now i have looked a them again i am 99.9% sure one of them is wrong. i.e. (brown/red/black/brown/brown) instead of (brown/red/purple/brown/brown). Measurements are 1.262k for the purple, and 1.196k for the black.

Closest i have in my box of spares are 1.2k 1/4w 200v rated items, or should i wait and get the proper 1k27 items?

[Paul Birkeland]

Closest i have in my box of spares are 1.2k 1/4w 200v rated items, or should i wait and get the proper 1k27 items?

I would do neither - I would use the resistors provided with the kit.  When it comes to voltage checks, you can measure the voltage across each of these resistors and check is against the reference voltages to determine if it is within 10% (which it will be).  70 Ohms of difference in this position will shift the bias about 1/4 of a volt (down), and move the plate voltage down a little bit, assuming that the two 6DN7's have exactly the same emission. 

[mcandmar]

Fair enough i will use them to get the amp up and running but i do want to change them for a matched pair for peace of mind.  It just seems fundamentally wrong to me to have two different values on each channel, i like things to be symmetrical.

What is it you have against replacing them?   Is there something specific about their spec/construction that i am not aware of?

Cheers,

Mark

[Doc B.]

Our concern is that users might be replacing parts without knowing all of the pertinent reasons that part was chosen and thus putting in a part that may be "premium" according to the seller, but is in fact substandard for the job at hand. PB explained that the difference you measured in the parts provided will have a negligible effect.

Our obligation is to advise customers based upon our level of certainty that the parts will function as required. We know that resistor will work, so the clearest solution for us is to suggest you use the parts provided. The next logical alternative is that we send you more resistors that we have matched so some level that you find acceptable.That will cost us time and money that isn't figured into our pricing structure, it won't make the amp work any better, but it will perhaps offer some relief for the issue of your need to feel the circuit is somehow symmetric. I do understand well that a lot of the positive effect of building these amps is psychological.

The third potential solution is to advise you on acquiring some alternate replacement. We don't know your level of experience in this, so it may take some back and forth to make sure you are acquiring a part that we feel fulfills the requirements.

[debk]

I agree with what the Bottlehead team says.  Build it stock first before you start upgrading parts. When you do upgrade, understand what you are upgrading and it's possible effect on the circuit before you do anything.

In respect to the OP's resistor question. If the color codes are as he states in the post, then he probably does have two different resistor.  The br/red/blk/br/br would be 1.2K 1% and the measured 1.196K wold be within the 1% tolerance.  The br/red/pur/br/br would be 1.27K 1% and the measured 1.262K would be within it's 1% tolerance.

I don't have the schematic for the SEX amp to look at, but If Doc and Paul said this difference is not an issue then I would trust their judgment and use the parts.

Debra

[Doc B.]

OK, I am sensing that you guys and gals are asking for some education on the whys and wherefore of this.  I am not disputing that the values might be a tiny bit different. Sometimes a batch of resistors can get an odd value mixed. 1% tolerance is not necessary for this component, it just happens that the resistor we chose for other characteristics is a 1% tolerance part. It would be OK if we used 10% parts. And those parts are only differing in value by about 6%. The thing that we are concerned about is not what exact value the resistor is, but what voltage and current the tubes operate at. Either of these values will put the tube into the proper operating region, and there can easily be more voltage and current difference between the tubes than the tolerance difference resistors we are talking about could create. That is simply the nature of the beast. Remember that we are concerned about getting the best sound possible, not mathematical precision.

To give this a little perspective those highly sought after McIntosh and Marantz tube amps were jam packed with 20% carbon comp resistors that are often as much as 30% or 40% off value by now.

All that said we are happy to send another pair of resistors that are matched if it will give you some additional peace of mind, Mark.

[Grainger49]

Adding to Dan's post.  As CC resistors age the value wanders.  That is what he means by they can be 30% or 40% off by now.

[braubeat]

I would have to get the resistors the same. It's just one of those things for me.

Michael

[mcandmar]

You have a PM doc.

Cheers,

Mark