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Old 16th November 2007, 07:18 AM   #1
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Default Cathode Resistors

Aloha,

I'm not a DIY person, per se, but I've become curious to experiment with resistors in my reference amplifier, perhaps learning to solder in order to try various simple modifications. I spoke with a known technician in the business who's come up with some modifications. Essentially, he replaces the stock 10ohm/2W cathode resistors (wire wound) with 2.7ohm/2W metal oxide resistors. He also changes the 10a fast-blow fuse to a 6a slow-blow.

Well, upon having these mods installed, the amp didn't improve on its characteristic sound at all and, instead, sounded closed, cranky, shrill, edgy. I wrote and called several friends and acquaintances, one of whom wrote that the 6a fuse "changes the parameters under which the power tubes operate and alters the sound." He owns a tube business. I changed the fuse back to a 10a fast-blow and the sonic signature of the amp returned.

Meanwhile, I got to thinking about the cathode resistors, having discovered that there are several kinds and that each type, even used as a cathode resistor (for the bias circuit), could affect the sound. There are carbon composite (Allen Bradley), carbon film (Shinkoh, Riken, Audio Note, Kiwame), wire-wound (Mills), metal-oxide, and metal foil (Yageo).

Many DIY posts simply state that carbon-comp, Riken, Shinkoh, or wire-wound resistors are "the best," but without distinguishing these for function. My concern is, were I to replace the wire-oxide resistors in my amp now (current mods) with one type or another, I might be placing the amp at risk--that a transformer might blow or something more serious than a resistor.

I've been assured that, with 2.7ohm/2W resistors, I shouldn't be in any danger here, but, were I to change these out for Riken carbon film resistors, say, or for Mills wire-wound resistors (both reputed to sound "the best"), I could be placing the amp at some unknown risk.

My inclination is to restore the amp to its stock 10ohm/2W wire-wound resistors, but the entusiast in me would like to explore what carbon film and carbon comp resistors might sound like.

The amp just misses being completely open in the treble range, sweet and transparent. It's still a shade closed on top and massed treble sounds--orchestral strings, choirs--and dynamic, sustained trebles (opera sopranos) can be thin, strident, and hard at times. It was so with the stock resistors too.

Question: Does anyone have experience with these kinds of mods and circuits who can advise me what the implications are for any of these contemplated modifications? For example, though I was assured that Shinkoh .25W or .5W resistors would be "entirely enough" to perform as cathode resistors, I was also told that the implications regarding failures were unpredictable and that, were a tube to blow out, it might be possible that the resistors would, though not blow themselves, nonetheless pass the stress along to other parts of the amp. This alarmed me. Therefore, my thought is to explore sonic changes with resistors of the SAME VALUES AS STOCK, but of different materials--carbon film, foremost (i.e. Kiwame or Riken).

Please advise. And thank you very much for any thoughts.
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Old 16th November 2007, 10:56 AM   #2
SY is offline SY  United States
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Quote:
Does anyone have experience with these kinds of mods and circuits who can advise me what the implications are for any of these contemplated modifications?
I've built or modded at least a hundred tube power amps and I am still amazed at what these sorts of mods can do, working directly on the part of the human brain that latches on to implanted notions of "difference." Unless you do something bizarre like use inductive wirewounds, there will most likely not be any real difference in the sound between any of those resistors, assuming they actually are cathode resistors and (from their values) part of the output stage. The degeneration involved is miniscule and the signal level is high. Now, varying the value of these resistors could potentially change the sound, since it's likely to increase the distortion. But 10R to 2.7R? Miniscule, less than the difference between warmup to warmup.


Quote:
dynamic, sustained trebles (opera sopranos) can be thin, strident, and hard at times.
If this is the case, you've more likely got excessive high order distortion or some instability. That will require a more thorough analysis; changing cathode resistor brands is not going to help.


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I wrote and called several friends and acquaintances, one of whom wrote that the 6a fuse "changes the parameters under which the power tubes operate and alters the sound."
Run, as fast as you can.
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Old 16th November 2007, 11:20 AM   #3
Nordic is offline Nordic  South Africa
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6A fuse can cause 75mV forward drop according to most datasheets I saw... this goes up for medium and fast blow fuses.

But I'd still disregard this.... You can use Carlos's trick of soldering one filament from a fine braided wire across the fuse... btw If I unnderstood him correctly he also didn't like fuses in amp too much...
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Old 16th November 2007, 11:38 AM   #4
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It's a mains fuse. You have more than 75mV drop turning a light on in the next room.
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Old 16th November 2007, 06:15 PM   #5
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Aloha e,

Yes, Tetusujin is right--it is a mains fuse (10a fast-blow) that I changed (to 6a slow-blow) and then changed back again. Thank you all for posting replies.

The sound of the amp DID change with the restoration of the original 10a fast-blow fuse--for the better and pretty much to what I was used to the amp sounding like.

I also had a tech short across the resistor in the bias-circuit as described above. This at first altered where the bias had to be set, but, once I restored the 10a fuse, the amp once again biased normally (in terms of settings).

So--about HALF my advisors are correct, then? In that changing the types and values of resistors (within the 2.7-10ohm/2W range) SHOULD NOT affect the sound of the amp? The other half are off-base?

I suppose the only way to find out is to try it, but Tetsujin sounds definitive.

Mahalo,
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