Carlsbro replacement bits

[Shakey 1047]

I have just got an old Carlsbro Cobra Lead amp, I believe made in the 80's and one or two bits need replacing. The capacitors in the power supply are 3300uf, would it make any difference if I were to replace them with 4700 or 6800uf caps? Also the op amps are RC4136N, is there a better replacement for these? Thanks Dave.

[Jazz P Bass]

I wouldn't double the capacitor value.

The power supply was designed with a 3300 in mind.
Without studying the power supply, I would say the 4700uf should be o/k.

As to the RC4136N IC, it is still in production, so a 'newer' one may be better.
Better than what I don't know.

The RC4136N has a very odd pinout, so you will not be able to pop in any old opamp.

RC4136N - TEXAS INSTRUMENTS - IC, OP-AMP, 3MHZ, 1.7V/ us, DIP-14 | Newark element14 US

[Shakey 1047]

Thanks JP. I don't have a schematic for this amp but I have a few 4700's so I'll use them. As for the op amps I did notice the odd pinout, that's why I wondered if anyone knew of a better quality replacement something like an OPA4132 but with this wierd pinout. Thanks any way. Dave.

[J M Fahey]

If they work, leave as is.
Old ones were very good, new ones will be exactly the same, not better.

Passive parts may be improved along time, such as electrolytics having better dielectric chemistry which means larger capacity in smaller cases, or metal film resistors having less noise and improved dissipation compared to carbon film or ... ugh!!! ... carbon composition but integrated circuits are made using the original design, using the original process and specially the original photo litographic masks.

Nobody will "improve" them, but plain make new designs which get a new , different number.

So, say, a TL071 will be better than an LM741 because it's a completely new design, just pin compatible with the oldone, but a 2015 RC4136 will be the exact same as a 1975 one, hope you understand why.

Besides that, "circuits won't work better because you use better parts" , except, maybe, if you modify design to take advantage of improved behaviour ... which usually nobody does.

Only case it might be worth doing so, without redesign, is when original design stretched part capabilities to the limit and it barely met specs.

Normal good quality Engineering means not reaching that point, but staying within the comfort area.

Anyway, as said before, RC4136 it will be, unless you want to invest in a Brown Dog adapter and fit some modern SMT chips there.

And doubt any improvement will be heard in a guitar amp.

In fact, MXR Distortion + have been improved by replacing LM741 with TL071, RAT same thing replacing LM308 with TL071 or Tube Screamers have been improved by replacing RC/JRC4558 with TL072 ... all 3 cases have been reported as sounding harsh and brittle, because of the improved high frequency response

As a side note, proper design does not mention "Brand" or "manufacturing year" in the schematic , just model name so it can be bought from any supplier which offers them.

Way back when I started making amps (don't ask ) , RCA 2N3055 were preferred ... with later Motorola 2N3055A being the substitute, but in that case those 2 models were different than the rest, they used "obsolete but robust" Hometaxial process, which was later replaced by faster and 3X cheaper Epitaxial .... a weaker design.

But unless there is such huge downgrading, or similar huge upgrading, it's not worth replacing what works as originally intended and in fact may even introduce problems.

[Chuck H]

In fact, MXR Distortion + have been improved by replacing LM741 with TL071, RAT same thing replacing LM308 with TL071 or Tube Screamers have been improved by replacing RC/JRC4558 with TL072 ... all 3 cases have been reported as sounding harsh and brittle, because of the improved high frequency response

Not sure if you're being sarcastic (as in, the word "improved" should be in quotes?) I just know the LM308 is sought after for improving the sound of a few vintage boxes. Incidentally, the market is now inundated with fake vintage chips that are said to sound horrible. Member tests here haven't proved that out but do concur that the actual vintage units sound best. In the case of the fakes the TL071 does sound better IIRC from the thread.

[Jazz P Bass]

Again, it comes down to what 'improvement' the OP expects.
The amp was designed around that specific opamp.
Heck, it's good to 3Mhz.

So, yeah, see how the amp sounds first.

The original opamp was supplied in 1978.
There is a newer datasheet from 2002.
So they may have improved the slew rate of whatever.

I have seen 4558 ic's from the late 80's distort wonderfully.
A newer version does not 'drop in' as a replacement (Tube Works TA 752).
Something is different.

[doctor]

Some remarks on J.P.B. and J.M.F. replies.

1. The increase of power capacitors will only lead to a decrease in the low-frequency distortions.

2. The TL071 and LM741 opamps are quite different, and cannot be replaced in the correctly designed circuits. For this reason, we cannot say that one is better than another.

[J M Fahey]

Quote Originally Posted by J M Fahey View Post
In fact, MXR Distortion + have been improved by replacing LM741 with TL071, RAT same thing replacing LM308 with TL071 or Tube Screamers have been improved by replacing RC/JRC4558 with TL072 ... all 3 cases have been reported as sounding harsh and brittle, because of the improved high frequency response

Not sure if you're being sarcastic (as in, the word "improved" should be in quotes?) I just know the LM308 is sought after for improving the sound of a few vintage boxes. Incidentally, the market is now inundated with fake vintage chips that are said to sound horrible. Member tests here haven't proved that out but do concur that the actual vintage units sound best. In the case of the fakes the TL071 does sound better IIRC from the thread.

1) Not sarcastic at all, that's why I didn't use quotes.
Improved means improved meaning that TL071 or even better and more modern ones have improved the way circuit works, both in a measurable and audible way.

As in they do pass a blind test.

That said, Musicians in general have disliked the (technical) improvement and called it brittle/harsh, etc.
Their words, not mine, and I don't argue that, being a matter of taste.

2) LM308?
I guess by a few vintage boxes you mean RAT pedals, I'm not aware of other significant users.

In that very particular case, the LM308 is needed and a better one does not sound quite the same, because it (like a few other MI designs) was the result of a "happy accident" and the pedal designer fully acknowledges that.

He claims something like (just quoting from memory) : "I was Protoboarding it, grabbed a 4k7 resistor to add a slight top boost (combined with a series capacitor) and by mistake grabbed a 470 ohms one .... as soon as I plugged it in I noticed my (gross) mistake but at the same time I noticed this weird, wonderful distortion flavouring/spicing the guitar sound"

And he was right: the circuit started distorting by itself (without diodes or slamming the rails) and he was baffled as to what was happening.
Yet he left it as is, he liked it.

Only much later he noticed that the circuit he created pushed the LM308 beyond what it could physically achieve and it was slew rate limiting.

A better/improved Op Amp or one with different compensation (all Op Amps are compensated, nowadays internally) will simply "amplify" and leave the clipping task fully on the clipping diodes shoulders.
Not bad but then it will sound like a million others.

So that's one specific case where a specific Op Amp is needed.

I'm talking internal Op Amp design, outside labels are irrelevant.

As I said before a 2015 LM308 (if somebody makes them today, which I doubt) will be same as a 1975 one, because ICs are made py a photolitographic process and they still use the old masks (or photocopies, literally).

Nobody will waste tons of $$$$$$$ to improve (no quotes) an old , obsolete design , making new masks and going through a complex design and testing process just to sell a few dozen or even hundred chips a year so either a new batch is made using original masks or simply a few other type modern ones are being unscrupulously relabelled to sell on EBay .

A backyard or over-the-kitchen-table operation at best.

[J M Fahey]

The original opamp was supplied in 1978.
There is a newer datasheet from 2002.
So they may have improved the slew rate of whatever.

Maybe, why not?
It's their child and afaik none other is making them .
It's different when some design becomes the standard (741, TL07x , etc.) where many make them and are supposed to be used interchangeably, by definition they must "be the same".

In that case, a new/improved version deserves its own name , of course it can be sold as "new improved Op Amp, compatible with old xxxx standard"

It looks funny, but many (most) Op Amps, for ages, claimed "741 compatible" right in their datasheets ... even when they were much better .
Delights of Marketing.

By the way, do you have links for both datasheets?
I'd love to read and compare them.

I have seen 4558 ic's from the late 80's distort wonderfully.
A newer version does not 'drop in' as a replacement (Tube Works TA 752).
Something is different.

Don't know that Tube Works, care to share a schematic?

That said, a fresh crop 4558 from an established manufacturer will be the same as an old one for economic reasons; an EBay sourced one will most certainly not.

In both cases, economy of scale applies.

It's very stinky the way obsolete parts appear in any amount needed coming from China.

Why stinky?

I can understand that some Christmas Carol singing card Factory miscalculated sales, just sold 800000 instead of the Million he expected and now he has a slight excess inventory of, say, 200000 stamp size and thickness ICs, so they pop out in EBay all over the place, for peanuts.
Same with any other modern electronic component, say MP3 decoder chips, WiFi/Netbook/Tablet parts, etc.

What I can not believe is that tens of thousands of obsolete transistors or ICs appear there ... simply because 20/30/40 years ago there was no significant China Electronics Industry

To have surplus of anything, you first need some Industry which uses it in large amounts.
Very simple math and Logic .

2. The TL071 and LM741 opamps are quite different, and cannot be replaced in the correctly designed circuits. For this reason, we cannot say that one is better than another.

They are different, no doubt.
But not that different so as to make replacement impossible.

As in:
I'm designing and making MI and PA amplifiers since 1969, and have been using LM741 since 1970

In fact, I actually went to buy LM709 and was suggested "this new improved Op Amp, which does not need compensation".

To boot, since my construction technology was based on eyeletted boards (the standard construction system in that era, think old Fenders ... which I cloned way back then) , I could not even dream of using DIP Op Amps, so I used an "octal" one (how approppriate in a MI amp ):



Many years after that, TL071/81/LF351 appeared on the market, I used all 3 in my designs, replacing original 741 in the same PCBs , whether as new production or in servicing, with no problems at all.

Of course, in new designs. I could push them further (3 X higher usable gain) and benefited from lower noise.

Not surprising, because by definition "all Op Amps are the same" ..... or are supposed to be.
Google the definition of "ideal Op Amp" and what parameters must be met by real world ones to be acceptable.

Of course, real world ones have practical limitations, but if used within their limitations, interchangeability still holds true.

Practical example:
old as the glaciers 741 has 1 MHz unity gain bandwidth, meaning in Guitar amplification it will run out of gain at 100X gain, at 10kHz , so that is the practical limit.

Yet lots of early 70's Pro and semi Pro equipment used them, although they have to be flat to at least 20kHz
How come?

Relatively simple: if you limit stage gain to 10X or 20X , you have respectively 100kHz or 50kHz bandwidth, you still have spare gain to apply distortion and response correcting NFB all over the Audio spectrum, etc. .... they are quite usable.

Plugging a better one there in principle will work the same , only in a more relaxed way ... because now you have 3MHz unity gain meaning now you have 3X (or 10dB) extra gain available for what you want to do.

Usually it's a free bonus; in some cases which were critically stable it can bring problems.

To be more precise: in general in low level circuits you will have no problems ; while it will be iffy or plain dangerous if the Op Amp is part of a Power Amp.

Why?

Because Power amps are made of many cascaded stages, big die driver and power transistors have large capacitance, that produces important phase shifts and compensation becomes critical.

In that case an improved wider bandwidth input Op Amp may destroy the amp or make it unusable.