Resistors types in original early Marshalls

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wombat
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Resistors types in original early Marshalls

Post by wombat » Thu Oct 03, 2019 12:16 am

Hello Marshall experts and historians

I'm wondering if anyone out there knows why Marshall used a combination of carbon film and carbon composite resistors in their early amplifiers?

I have been pouring over gut shots of as many 65 - 67 era amps as I could get my hands on. Based on what I what I've seen, it looks like the positions that carbon comp resistors show up in vary somewhat, however, there does seem to be some sort of underlying "theme" as to where they were used (ie some positions always seem to be CC). Also, that "theme" seems to change slightly from year to year. That suggests to me that there was at least some kind of conscious reasoning behind the placement of CC resistors by Marshall.

Nowadays, there is school of thought that carbon film resistors in certain positions will help give the amplifier's tone more of a "brown" sound. Could that have been what Marshall were trying to do in the 60s? Personally, I would consider that to be unlikely. Marshall were a relatively small and expanding business at that point and I would be surprised if they had the time and resources to invest in that kind of research and experimentation (particularly given that the end result does not achieve a massive difference in tone).

One known factor of the early Marshall days was an inconsistent supply of parts. It has often been discussed that Marshall frequently subbed out parts for whatever components they had available at the time. This certainly could account for some of the differences - but not all of them. I have seen a number of examples where the same value resistor shows up in both CC form and CF form within the same amp. That to me rules out parts shortages as the sole cause and implies a more deliberate reasoning.

Was unwanted noise a consideration? Are carbon film resistors noisy in some positions but not others?

If anyone can offer any insights into this it would be greatly appreciated.

RockinRocket
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Re: Resistors types in original early Marshalls

Post by RockinRocket » Thu Oct 03, 2019 11:26 pm

From what I have noticed is that the CC resistors were used where the longest reach was needed. Why this is I don't know as the Iskra and Phier I have bought always had leads long enough to reach any position.

820. Cathode CC resistors spanned the width of the board.
100k Cathode Follower CC resistor had the longest reach of any resistor but the other plate resistors were Phier.
27k NF CC resistor. Again spanned the width of the board.
56k Bias resistor spaned the width of the board. This is why the Slope reisistor was also a 56k CC. They already had stock.

Yep I think I cracked the code :D

wombat
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Re: Resistors types in original early Marshalls

Post by wombat » Tue Oct 08, 2019 12:05 am

That's interesting, I never really thought about it that way. As you say, I've never come across a Piher that wouldn't stretch the whole width of the board.

I don't think I've ever seen any of the bigger 1 watt resistors (10k , 8.2k) in carbon film (until they started using Iskras).

I'm really starting to wonder if wasn't all just down to cost cutting after all?

I have no idea what resistor prices were in the UK in the 60s. But I think it is a reasonable assumption that carbon comp would have been cheaper than carbon film (due to the simpler production methods involved). Marshall were known to be frugal when it came to costs. Maybe it just came down to trying to get away with the cheapest components that they could get away with (so long as it didn't introduce any unwanted noise perhaps????)

Remember too that after a period of using Piher (Spanish) and Morganite (British) resistors, they switched to all Iskras (which were Yugoslavian). The exchange rate in post-WW2 Yugoslavia would have made Iskras fantastically cheap compared to Spanish and certainly British products - and when Marshall made that change there was not a single carbon comp resistor in sight.

Maybe that's all it was? Maybe they just wanted to save a few quid?

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