Metropoulos Forum

On the Metro PTP 2203 replacement boards they installed some 2 watt metal oxide resistors. Is the only reason for these just for the over all wire lenth? I was thinking of changing the NFB to a 2 watt cc but I don't know if I can locate any for sale anywhere. I've seen 1/2 watt 100k but no luck with 1 or 2 watt. What is the smallest wattage I can use for NFB?

You might get away with 1watt. A lot of the early Marshalls actually have 1 watt resistors especially the carbon composites(although most people think they are 2watt from their physical size.)

I'm not sure why you'd want to use CC's here though.

demonufo wrote:You might get away with 1watt. A lot of the early Marshalls actually have 1 watt resistors especially the carbon composites(although most people think they are 2watt from their physical size.)

I'm not sure why you'd want to use CC's here though.

I believe Ben on this forum claim that CC's in the NFB sound better but I have never done a A/B comparison myself. I have to admit I'm not so sure you could here a difference but maybe it makes the amp feel or react better. I guess if I can't get proof positive maybe I should just stay with the metal oxides.

You don't need 2 watts for the NFB resistor. 1/2 watt is plenty because of low voltage present there. I have an Allen Bradley CC for NFB in one of my 100w amps and I think those are 1/2w resistors.

technical explanation of NFB wattage:
http://forum.metroamp.com/viewtopic.php ... 94#p323194" onclick="window.open(this.href);return false;

The "modern" resistors simply have shorter legs than their NOS- counterparts...

Regards

Robert

Another thing I noticed is that my original Marshall printed circuit board has (2) 10k B+ decoupling/dropping resistors and the Metro Amp 2203 replacement board has (1) 10k and (1) 8.2k in those positions but the Metro Amp 100 watt 2203 chassis layout drawing shows (2) 10k so which one is correct?
The amp did have KT-88's when I purchased it used if that would have anything to do with it and I'm running EL-34's now.

Strange thing I noticed related to this subject, and i don't really know why.

A while back I put a 2watt cc resistor in the negative feedback spot because that's all I had at the time. Recently I got a 1/2 watt cc so I put it in there thinking that it would be better than the 2 watt. Well the warmth and wooly sound that I had with the 2 watt went away. I put the 2 watt back in and the warmth was back again. Both resistors were Allen Bradley's about the same resistance just different wattage.

Anyone have any idea what it going on there???

Hi cdawg

my (a bit hairspitting) theoretical interpretation is:

Lo- wattage CC = a bit more 2nd harmonic distortion (see Randall Aiken's fantastic site) than CF or MF.
Hi- wattage CC = less 2nd harmonic distortion...

Therefore, one must think "reverse", because the resistor is in the NFL, means (at least to me) this tiny distortion is cancelled out.

Regards

Robert

TJB wrote:So do you think there is any difference in sound or overall response of the amp when NFB is cc verses metal oxide?

I don't know for sure because I installed a bunch of CC's at the same time. I have since removed almost all of them because of noise and loss of dynamic "punch". Most of the "mojo" theories of CC's indicate that they can make the most difference in positions where they drop a lot of voltage, like on V2b cathode. I doubt they have much effect in the NFB position.

I don't know if it was just the Allen Bradley NOS resistors but I didn't have good results with CC's. When I first installed CC's I definitely noticed a warmer, more "vintage" tone. They also added a lot of hiss. However, after about 6 months of use my amp lost a lot of dynamics and seemed lifeless. I put CF's back in and it came back to life with much more "punch". Some of the CC's probably drifted high in value, which may have contributed to the problem. Personally, I think they should be used sparingly in specific places because their potential benefits can be outweighed by the problems they can cause.

I tried some AB CC in the tonestack and a few other spots in my 68' build some days ago. I think the amp became alittle lifeless and I feel it became cleaner/less gain. Im gonna put the metal films back in. Ive never had good results with CC resistors either.

IT'S NOT THE SOUND QUALITY, IT'S A RELIABILITY ISSUE.
The 2 watt withstands the constant expansion contraction of off/ on cycles in a lifetime of usage.
When the ambient temperature is higher, you are going to require a thicker gauge.
We can't just compute what the tube draws in terms of wattage, we have to build it so it will last for a very long time in rough usage. Over time a smaller resistor experiences more physical stress, is more likely to crack, split, develop structural defects. This problem is more likely to occur in a thinner construction than a thick one.
Sure a 1/2 watt works, but a 2 watt lasts.
OK the best built amps have 2 watt or larger, with leads like coat hanger wire.
The larger resistor has more dissipation and therefore, less thermal noise.

Now look at how the thing is made. Carbon resistors have glued in leads. No wonder the builder wants to use a bigger resistor.
What's going to happen to the glue when you solder the thing in? A bigger resistor will experience less deterioration from solder heat.

I'm going to use 1 watt metal film for plate and cathode minimum. In the signal path, where there is very little current (after the decoupling cap) I'm going to use 1/2 watt minimum. (Carvin would have used 1/8 watt or 1/4 watt).
A big deciding factor is really the price. Manufacturers use smaller parts to increase profits. But that doesn't assure maximum reliability in the long haul.
If the budget calls for the best I'm using 2 watt. Or I'm using bulk metal foil resistors, like z foil. In fact I'd rather Z foil for sure, it has like 1/10th the noise. But those cost $20 each.
See where this is going? Do you have unlimited funds, are you trying to build something that will last forever?
Or does your budget have a limit?
You will notice in an amp, like a blues junior, that uses 1/2 watt carbon resistors, the performance deteriorates waaaay faster, say 5-6 years before the thing is really going to crap. Probably less than that if the thing is played very often.
And in a silver face black face that used 1 or 2 watt, the thing may go 20, 30, 40 years, before you start to notice the snap crackle pop of stale resistors.
In a very high gain amp, the noise is going to become unbearable, like steam escaping. The higher the gain, the less noise I want, the more expensive the resistor becomes.
Also resistors are inductive, non inductive resistors cost more, and I will have way less noise.
So if I go to all the trouble of building a custom made amplifier, I don't mind spending a lot of extra money to get less noise and longer life.

But then, less noise and longer life often sounds as boring as all hell.

Vintage Marshalls sure as heck wouldn't have sounded anywhere near as exciting with near perfect components.

And smaller resistors run harder (depending on their type and quality) can often add pleasing overtones to an amp. Whilst those overtones (distortions) may not be desirable from a technical design point of view, nobody was thinking about technical perfection when creating those early guitar amps. The designers simply wanted to achieve certain tonal goals. Since all they were doing was copying (and altering slightly) age old proven circuit designs, it's hardly any great surprise.

There's far too much drama about right and wrong that the bigger picture gets lost a little somewhere.

True, but Marshalls started out with "fender type" gain structure.
Now we amp builders are pushing the limit, of the number of stages, and we are trying to get much, much, more gain out of a guitar amp, compared to how the Marshall started. Even Marshall added a gain stage.
And the more gain, the more noise.

Then, the Marshall burned the tubes base down...
But now we are building a lot of high gain amps with the tubes base up. And that makes the amp chassis hot as Hades.
The old style components don't survive that kind of heat over the long term. Hence, "aerospace grade" parts is how we answer.

The old Marshall sounded great, when it was cranked to stadium volume. My neighbors love that.
But Now, we are trying to get that effect at a very low volume level. My neighbors will undoubtedly miss that lovely noise.

And so invention is the mother of necessity.
We can either increase the bias voltage, and the amp runs cooler, the parts last longer... but the amp sounds like ca-ca.

Or we crank it hot to make it sound good, and install parts that are flame proof.

Many years ago, in the 70s, I was undoubtedly one of the first to suggest that we install aerospace grade parts in guitar amps. At the time, many people were critical of the suggestion, it appeared unnecessary.

But now, the suggestion seems borderline intelligent.
How do you think Randall Smith could deal with all the heat noise oscillations, in his amps, with the parts all crammed together, and the tubes hanging upside down? He installed military grade parts.

You may be getting it now, the unfortunate, none- the- less present reality.

soundguruman wrote:IT'S NOT THE SOUND QUALITY, IT'S A RELIABILITY ISSUE.
The 2 watt withstands the constant expansion contraction of off/ on cycles in a lifetime of usage.
When the ambient temperature is higher, you are going to require a thicker gauge.

Thicker gauge of what? Please specify.

soundguruman wrote:We can't just compute what the tube draws in terms of wattage, we have to build it so it will last for a very long time in rough usage.

What exact tube are we talking about? and what tube draws in wattage?

soundguruman wrote:Over time a smaller resistor experiences more physical stress, is more likely to crack, split, develop structural defects. This problem is more likely to occur in a thinner construction than a thick one.

So a higher rated in watts resistor is physically larger than a smaller rated one?

soundguruman wrote:Sure a 1/2 watt works, but a 2 watt lasts.

What are the other ratings of either.

soundguruman wrote:OK the best built amps have 2 watt or larger, with leads like coat hanger wire.
The larger resistor has more dissipation and therefore, less thermal noise.

So if a lead is made of coat hanger wire, but made of steel it is better than a "not so Coat hanger wire: lead that is made of cladded copper? Also the larger the resistor the more dissipation rating? Have you been paying attention in the last 20 years? Thermal noise also is not as one sided as that.