Resistor upgrade help

[Psilonaught]

I'm looking to get Essex Amp Repair, my fantastic local valve amp company to upgrade the 20 resistors in my Atma-sphere MA-1 OTLs.

The current ones are here
https://www.google.com/amp/s/www.westfloridacomponents.com/wfc/amp/product/R051PD.html

Ralph recommends Caddock but I was thinking Audio Note Tantalum, please can someone recommend what would be a good drop in replacement from the hificollective ?

Unlike capacitors I'm not au fait with specs of resistors.

 

[Psilonaught]

What does R number relate to ? I need 0.47 ohm but hificollective shows all their resistors as R10 etc!?

 

[zeta4]

I think you need R47. R is the decimal point. This kind of marking is commonly used on SMD resistors

 

[Tony_J]

"The letter "R" is used to indicate the position of a decimal point for resistance values lower than 10 Ω. Thus, 0R5 would be 0.5 Ω and 0R01 would be 0.01 Ω."

So your 0.47 ohms would be 0R47

 

[Psilonaught]

Ok thanks.

 

[rabski]

The originals linked to are 5 watt, but Audio Note do a maximum 2 watt. If they're cathode resistors (which seems highly likely with that value, then you'll probably need 5W. With a quick check, the lowest value AN do seems to be 10 ohms anyway.

HFC list Mills 0R47 5W, but get your wallet out. They list a few others, but most I'd avoid, though the Jantzen and Ohmite may be decent (never tried them).

 

[Psilonaught]

The originals linked to are 5 watt, but Audio Note do a maximum 2 watt. If they're cathode resistors (which seems highly likely with that value, then you'll probably need 5W. With a quick check, the lowest value AN do seems to be 10 ohms anyway.

HFC list Mills 0R47 5W, but get your wallet out. They list a few others, but most I'd avoid, though the Jantzen and Ohmite may be decent (never tried them).

Thanks Richard.

So the Mills are considered a high quality resistor then? Ralph (the designer) uses Caddock in this upgraded version but I can't for the life of me find exactly which ones.

As these are pretty much end game amps for me that I can't see myself ever selling, I don't really care about the cost per say, I just want the best.

 

[Jazid]

Probably best to leave well alone then IMHO. Presumably the designer made some effort to get the amp right before sale?

Wire wound resistors can be non-inductive if wound with summat called "Ayrton Perry winding". This makes a difference at very high (near radio) frequencies. Excluding those frequencies, one bit of wire is much the same as another, thusly one w-w resistor vs another that are otherwise similar in wattage, resistance, and construction.

Someone will drop by to explain that this is nonsense presently

 

[Psilonaught]

Probably best to leave well alone then IMHO. Presumably the designer made some effort to get the amp right before sale?

All atma-sphere amps and preamps come with either "standard" resistors or the Caddocks. Presumably to keep initial cost of ownership slightly less eyewatering.

 

[Radioham]

I would be very reluctant to contemplate such major surgery on the amplifier. The chance of introducing faults and/or poor connections are very high. It would be OK for a new build but not for a retrofit.

 

[Radioham]

Probably best to leave well alone then IMHO. Presumably the designer made some effort to get the amp right before sale?

Wire wound resistors can be non-inductive if wound with summat called "Ayrton Perry winding". This makes a difference at very high (near radio) frequencies. Excluding those frequencies, one bit of wire is much the same as another, thusly one w-w resistor vs another that are otherwise similar in wattage, resistance, and construction.

Someone will drop by to explain that this is nonsense presently

Low value resistors are often wire-wound around a ceramic former or core. If the wire is wound in a continuous direction the result is both resistive and inductive, which will have an effect in high frequency circuits.

To create a non-inductive wire-wound resistor, half of the resistance windings are wound in one direction (say clockwise) then the wire is folded back on itself and the rest is then wound in the opposite direction (say anti-clockwise) These two windings are inductive but the two inductances so created "cancel" each other out (from an inductance viewpoint) whilst the resistance is the value determined by the full length of wire.

 

[rabski]

I suspect availability is going to be your issue. A quick run through the usual suspects suggests that unless I've missed it, RS, Mouser and Farnell stock Caddock, but don't list a 0.47 ohm variant at any wattage. If that's the case, then you'll need to buy from the US, which is going to mean time and cost. Any alternatives are going to be pure guesswork in terms of any sonic effect.

This is the sort of territory where incredibly small differences involve a great deal of critical listening, time and cost. In your case, it would mean relying on audio memory and acknowledging that combined with cognitive bias, it is going to rule out objective assessment.

As the originals are wirewound, I wouldn't personally bother with any alternative wirewound. In all the years I've been mucking about with this stuff, I can't ever recall any wirewounds having any (different) effect at all. Generally, there is some sonic 'signature' between wirewound, different metal films and different carbons. Nevertheless, it's incredibly small.

Basically, a shed load of cost here for potentially no difference or even making it worse.

 

[Psilonaught]

Maybe I need to just speak to Caddock directly, or try the Mills ones which are less expensive.

Appreciate the comments re improvements being small, I will continue to investigate

 

[rabski]

Actually, here's a serious suggestion. You've already built your own clock and have some experience and DIY ability.

Try and find an electronics course somewhere that includes valve amplifier stuff. Not impossible, as it's still very much a 'thing' with guitar amps as well as hifi. Learn a bit more about the practical side, and also the theoretical aspects of how valves work and the relevant aspects of valve circuits and power supplies.

Armed with that, there is nothing to stop you taking on things like this as a DIY job. It would not be a five minute job, but nor would it be that complicated. A few basic tools, some space and the relevant knowledge and this would be a pleasant weekend job.

Safety-first hat on, of course. Poking about inside valve (or solid state for that matter) amplifiers is a bad idea unless you are armed with appropriate knowledge, as there are many things inside that could go bang or that could spoil your day permanently.

 

[rabski]

Maybe I need to just speak to Caddock directly, or try the Mills ones which are less expensive.

Appreciate the comments re improvements being small, I will continue to investigate

Forget the Mills. They're high quality, but standard wirewound and will make about zero difference to what you already have. Their advantages are tolerance values and stability. The difference in that regard will be massively less than the difference between any two valves at normal operating points.

 

[zeta4]

There is an interesting thread on another forum where someone has measured certain cement wire wound power resistors and found significant distortion ie something like -70dB third thd on a -10dB 1khz sine wave. With other makes the distortion was almost absent.

The distinguishing factor was that the offending resistors were magnetic in some way. Its been suggested that this kind of distortion is always there even in say metal film resistors with steel end caps but if the current flow in them is low the distortion is very low. However at high current use like in cathode resistors the distortions can become significant.

This ties up with a similar reported problem by Bruno Putseys, with reed relays which have a magnetic reed. In the same way the distortion rises as the current handling rises.

This effect is measurable so amp manufacturers can now select suitable resistors. Maybe thats what they have been doing by ear all along without knowing about this effect. If so then its wise to stick with the resistors the manufacturer chose or if you want to change with those chosen by experienced specialists.

Of course you can always just use a magnet !

ps Ive just checked most of my power resistors and a lot are magnetic to some degree except the
silver russian ones that have been strongly recommended. Ive not got any Caddock or other specialist resistors so if you have I would be interested in what you find.

 

[Psilonaught]

Actually, here's a serious suggestion. You've already built your own clock and have some experience and DIY ability.

Try and find an electronics course somewhere that includes valve amplifier stuff. Not impossible, as it's still very much a 'thing' with guitar amps as well as hifi. Learn a bit more about the practical side, and also the theoretical aspects of how valves work and the relevant aspects of valve circuits and power supplies.

Armed with that, there is nothing to stop you taking on things like this as a DIY job. It would not be a five minute job, but nor would it be that complicated. A few basic tools, some space and the relevant knowledge and this would be a pleasant weekend job.

Safety-first hat on, of course. Poking about inside valve (or solid state for that matter) amplifiers is a bad idea unless you are armed with appropriate knowledge, as there are many things inside that could go bang or that could spoil your day permanently.

that's a good idea actually, and I do have a dedicated workshop area with bench.

I need a proper soldering iron and I need to try using flux, which I think is where I go wrong with soldering.

A course would definitely help! I am also hankering to build a 10Y preamp...

 

[Radioham]

that's a good idea actually, and I do have a dedicated workshop area with bench.

I need a proper soldering iron and I need to try using flux, which I think is where I go wrong with soldering.

A course would definitely help! I am also hankering to build a 10Y preamp...

Never use flux unless you are a plumber. Use Multicore solder with the flux built in. Flux is corrosive and will eat away at wires etc. If you have to use flux make sure you wash it away with flux remover.

Look at the Essex Ham Website they run courses where you will learn quite a lot about electronics as its a requirement to become a radio ham, they also have "skills night" which maybe worth attending

 

[dave]

that's a good idea actually, and I do have a dedicated workshop area with bench.

I need a proper soldering iron and I need to try using flux, which I think is where I go wrong with soldering.

A course would definitely help! I am also hankering to build a 10Y preamp...

Unless you are going for exotic solder that I don't know anything about, I would recommend something like Loctite C400 60EN 5C 0.7mm S. leaded, naturally and just works for almost all cases. Flux only necessary for really tough like trying to solder on to oxidised tin plate. I found unleaded rather soul destroying personally for hand soldering.

 

[Psilonaught]

Turns out Ralph has Caddock make custom ones for Atma-sphere

Message below

The Caddock resistors we use are custom built for us just

As a general rule of thumb, things to look for are the change in resistance as voltage is dropped across the part. This spec, which is not always published will tell you who has done their homework. Otherwise low inductance is good, since this can mess with the ultrasonic response of the amplifier, which may affect phase shift in the audio band. If you are not using feedback this can be important.