Iskras 0.5w vs 0.25w

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neikeel
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Iskras 0.5w vs 0.25w

Post by neikeel » Sat May 07, 2011 6:04 am

I just bought some Iskra resistors off ebay.

They were in UPM050 labelled packets, but when arrived are 8mm long x2mm wide, ie much smaller than the ones in my Marshalls (10mmx4mm). One of my 1970 JMPs had the smaller ones, but I always assumed that they were 1/4w.

Can anyone give reliable info on this, thanks.
Neil

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Re: Iskras 0.5w vs 0.25w

Post by wrea398 » Sat May 07, 2011 3:17 pm

I may have bought from the same guy. They havent arrived yet. I went back to look at the ads and you can barely make out the size on the label. 8mm x 2.5mm. Ad says 1/2 watt. I will be sending mine back if thats the case. I spent 40.00 on a few values. The pic below shows the size difference. (Thanks Vintage Charlie..."I modified one of his pics".)

I have some 1/3 watt Pihers (68ks) on my 67/68 build. Your size 8mm equals .315 inches and the 2mm equals .078 inches. My 1/3 watt Pihers are .375 x .125 (3/8 inch x 1/8 inch). I think you may have 1/4 watt. Hope this helps.
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Re: Iskras 0.5w vs 0.25w

Post by steve1 » Sun May 08, 2011 5:25 pm

I am the seller in question and am posting mainly to say that I sold these resistors in good faith, genuinely believing them to be 1/2watt and ultimately if it turns out they are *definitely* not 1/2watt then I'll refund everyone who purchased them. I also withdrew them from sale immediately when Neil (who started this thread above) wrote to me expressing his doubts about them.

I wrote to all the people whose orders I had not yet sent out and explained the situation and refunded their payments, I also refunded Neil who issued the first complaint. I have since had a few people write back to me saying they still believed them to be 1/2watt and asking me to still ship them.

I can be 100% certain these are type UPM050 as they came out of sealed packets labelled accordingly and I have a few packets that are still sealed if I ever needed to prove this.

I cannot be so sure however that ISKRA type UPM050 is 1/2 watt, I just have the word of one person I consulted before listing them on ebay and two other people who I questioned further after they said they still wanted to buy them. I have quoted below extracts from their most recent replies:

"....if you find them in the sealed bags with the identification paper,then your resistors are 1/2W Iskra sized 8x2.5mm,the factory could not made a mistake in packing them,and that is the other people which are wrong(including myself)because we though that 11mm long Iskra were 1/2w and in fact they are probably 3/4w"

"I think it is 0.5w, there is written UPM050.....It has to be known that the first Iskra "yellow" resistors like yours that were made before 1969-70 where smaller than the next of the 70s.......I have just seen the dimensions of the resistors written on the Iskra pocket 2.5x8mm, and all is ok, it is 0.5w resistors"


I've now asked them if they have access to any evidence (catalogue, datasheet, old magazine or book) that type UPM050 was indeed 1/2watt regardless of the size and await their replies. I also pointed out this thread and encouraged them to post here, and would appeal to anyone reading this who has any such evidence to please post it.

I admit the majority opinion seems to point to it being almost impossible that any 1/2watt resistor could ever have been manufactured in size 2.5x8, but feel I should at least pursue the possibility (however unlikely) that these may have been the exception to the rule.

Another thing I don't understand which may be relevant is Neil above mentions some smaller sized resistors in one of his amps, where as Bill mentioned to me in one of his emails that "No amplifier manufacturers used less than 1/2 watt" putting those two points together suggests maybe there were some smaller than expected 1/2watt resistors? just a thought.

Anyway sorry for the long message and I ask that those who purchased the resistors would please be a little patient until we have a definitive answer one way or the other, I am happy to refund but if they should turn out to be correct after all it would not benefit the buyer either.

Thanks,

Steve

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Re: Iskras 0.5w vs 0.25w

Post by neikeel » Tue May 10, 2011 5:05 am

Thanks

It is a shame for me as I need 0.5w in this instance

68k or 470k would work on preamp board as they are genuine Iskras with full length leads.

Seller is very straight and sold them in good faith gave instant refund.

Oh well - keep looking :(
Neil

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Re: Iskras 0.5w vs 0.25w

Post by wrea398 » Tue May 10, 2011 11:27 am

Yeah he is a nice guy. He did the same for me, refunded. I had a chat with Karlis and he thinks he might have some UPM050 (.5 watts) that are thinner but not shorter. As far I have has discovered 10mmx 3.5 to 4mm is correct for 1/2 watt Iskras from 60/70s.

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Re: Iskras 0.5w vs 0.25w

Post by VintageCharlie » Thu May 19, 2011 1:43 pm

Hey guys,

just noticed this post - i belong in this club too :D

I had bought these too, before even seeing pics of the resistors themselves - saw the code, which is the right one and got them. Seller refunded the money immediately and is really a nice guy, so it's fine.
My best guess is that it is a factory error. I have seen and heard of (as an example) RS packets being mislabeled. Why shouldn't this be possible for the Iskra factory or whatever place they were put into these smaller packets?
Also, the UPM050 code really makes sense for 0.5w, doesn't it? 0.33w is ... you guessed it, UPM033. Haven't seen 1w's in packets, but probably these say UPM100.

Yup, so i'm stilll looking for a darn 10k 1/2w iskra too :D

Best,

VIntageCharlie

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Re: Iskras 0.5w vs 0.25w

Post by VintageCharlie » Thu Jun 30, 2011 8:08 am

Hey guys,

i think a little correction is needed and we also owe Steve an apology!

I now think he was right - the small 10k's could have been 1/2w indeed and the UPM50 labeling was right. As i'm looking for some Iskras, i have had this problem over and over again that when asking for "1/2w", i get the small ones and all of them have been labeled upm50 - some of those i got from people who had huge stash of these, so they should know.

I have gotten upm50's in the size that Marshall builders are after though. I suspect that at some point, Iskra just reduced the size of their 1/2w resistors - they are still labeled upm50, but are of the size of the former upm33's. So upm50 MIGHT be the 1/2w in the size we are hoping to get, but, if it's a later production, they will probably be of the smaller size. Now i'm wondering what the size we are after is called? Maybe it's the upm75. I think that a guy who sent me one 10k in the proper Marshall size, told me that it was labeled as upm75, not upm50.

I'm not sure if this is right, neither when the smaller sizes appear, from which year, IF any of this is correct, but it starts to look like it. Maybe this helps a bit with the confusion, or maybe it will just increase it - please take it all just as a guess, which is what it is.

Regards,

VintageCharlie

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Re: Iskras 0.5w vs 0.25w

Post by wrea398 » Sat Jul 02, 2011 10:22 am

Interesting... He stated in his ad they were 60s production and I have a friend who burned one at under 300 volts. This would lead me to believe they are less than 1/2 watt. Most Carbon Comps/Films are rated at 350v working voltage. I think my buddy said it burned at 295v +/-. He is still a decent guy and tried his best to take care of us. Whatever the situation it was not his fault.

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Re: Iskras 0.5w vs 0.25w

Post by Xplorer » Fri Jul 22, 2011 8:01 am

just wondering : what is the size of the Iskra 2w, in mm ?
thanks.

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Re: Iskras 0.5w vs 0.25w

Post by wrea398 » Sun Jul 24, 2011 7:56 am

They are 20mm x 8mm . Alberto on Ebay says this for his 2W 10K Iskras.

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Re: Iskras 0.5w vs 0.25w

Post by Xplorer » Sun Jul 24, 2011 8:50 am

ok, thanks. i was wondering if the 10k and 8k2 are 1w or 2w on the superleads sometimes.

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Re: Iskras 0.5w vs 0.25w

Post by demonufo » Sun Jul 24, 2011 4:46 pm

Xplorer wrote:ok, thanks. i was wondering if the 10k and 8k2 are 1w or 2w on the superleads sometimes.
Given the sizes and the timelines, I'm fairly (not 100%) convinced that Marshall were mostly fitting 1W resistors, rather than the specified 2W.
So I like purple, okay!!!!!!

83.7% of all statistics are made up on the spot!

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Re: Iskras 0.5w vs 0.25w

Post by VintageCharlie » Mon Jul 25, 2011 2:19 am

+1

only possibility wuld be that at some point the sizes changed, but i have never seen a single pic of a marshall that looks remotely stock that would have something as big as 2w iskras or pihers in the power supply - they just wouldn't ook right.

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Re: Iskras 0.5w vs 0.25w

Post by demonufo » Mon Jul 25, 2011 3:15 am

Now the Carbon Composition resistors that were approximately the same size, that Marshall were fitting around the same time ARE 2W I believe, but I could be wrong about this too. :wink:
So I like purple, okay!!!!!!

83.7% of all statistics are made up on the spot!

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Re: Iskras 0.5w vs 0.25w

Post by VintageCharlie » Mon Jul 25, 2011 3:33 am

From what i know the cc's are 1w too - 2w are NOT as large as the Iskra?piher counterparts, but still very big. It might be that there are some originals who used 2w though, but standard seems to be 1w in cc's too.

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