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Vox-Korg 9310 Cambridge 30 upgrades/mods

Started by PoorOtis, October 29, 2013, 01:51:21 PM

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My grandsons using using one of my guitars and my Vox Cambridge amp (his gear was stolen) until he desides what he wants to replace what he had. I bought the Vox amp new in 2000 and have never had any issues with
it...he wants me to change out the chip to get more output and to change the optocoupler..??
Is all that really necessary? Will the mods add to the amps performance or output..and if it adds more output to the amp, do I need to upgrade the LB10-1615 speaker also??
Grandson says the amp can put out more with the mods and there will be a noticeable improvment..

Is it worth the $$ for the mods..or..leave it alone?


J M Fahey

I think your Grandson spends too much time on the Internet, on "the other" Forums, often source of many wild unbased ideas. ;)
Replacing, say, a "18W" TDA2030 with a pin compatible "32W" TDA 2050 won't give him extra power.
Unless you also replace the Power Transformer with a (much) beefier one, diuble the heatsink area, beef up the PSU, etc. , which amounts to building a new amp.
As of the optocoupler .... well, maybe there's a track history showing them as not quite reliable.
Ok, replace it ... when/if it fails.
And anyway you will restore normal function, "improve" nothing.
Justy curious, if at all possible, try to get and post here the links about those "mods".

J M Fahey

Well, being curious, didn't wait for you and googled myself.
Straight from the Vox Showroom "official" page:
QuoteAfter the amp started to get into customer's hands, rumors circulated about a "hot rod" output amp chip that could be installed in the Cambridge 30 to double the output power to 60 watts. I know that some people changed the standard TDA2040 output amp chip to a TDA2050, but I would not recommend this modification. It is my opinion that the Cambridge 30 power supply does not have enough current to support the additional power draw from this chip. It is mentioned it here only as a part of the history of this amp.
Well, it treats it as a rumour.
Besides, what's written in red is simply wrong, the amp just won't pull one dime extra power from the PSU.

As of:
QuoteA common Cambridge 30 service issue involves a severe loss of power and increased distortion caused by the failure of the tremolo opto-isolator. This gets a little technical, but I will try to explain it in simple terms.

Most amps that have tremolo utilize an opto-isolator. An opto-isolator has two components. It has a small light bulb or LED and a photo resistor. The light bulb faces the lens of the photo resistor and is in a sealed case.

A photo resistor varies its resistance to an electrical current depending on the amount of light striking it.

The tremolo circuitry in the amp pulses the light bulb in the opto-isolator. These pulses of light cause the resistance in the accompanying photo resistor to rise and fall from 0 to as much as 2 million ohms. This varying resistance is applied against the guitar signal in the preamp, causing the signal to "pulse" in level.

If the opto-isolator fails in the Cambridge 30, the resistance in the photo resistor rises to the high limit for the part, causing the amp to severely lose power. The amp is basically "stuck" in the quiet part of the tremolo cycle.

This problem can be remedied by replacing the stock opto-isolator with either a Fender Opto (part number 16282) or a Vactral Opto (part number VTL53/2). I don't consider this a "home hobbiest" sort of repair. I suggest your local amp tech handle the "opto" exchange.
well, *if* it fails, replace it; otherwise ....
And the long years of service probably mean you got one of the "good" optos.
Don't fix what's unbroken.


Otis, I totally agree with Juan.  Your grandson needs to work on playing and forget the internet chatter.

The output IC is really just a controlling part for the power supply.   It is an electronic valve that allows the power supply voltage to come through to the speaker.   Installing a higher power IC will not increase the amp output anymore than using a heavier extension cord will make your TV set brighter.   Or getting a larger wallet will make you richer.

And the less experienced musicians worry about power, but the physics of it are that doubling the power of an amp - going from 30 watts to 60 watts for example - only increases the loudness by 3 decibels.   That is enough to notice side by side, but not much more.

As to the optocoupler, those did have a higher than normal failure rate.   As a Vox service center, I recall replacing a lot of those in brand new amps.  But your amp is not 13 years old and has worked all along.  Any "infant mortality" failures would have occurred LONG ago.   It works, so leave it alone.

If he borrowed your car, and came home telling you you really needed to install a tuned exhaust, and a rally package suspension, would you seriously consider doing it?  Or would you ask for your keys back?


Thanks to Juan & Enzo for both your info on the upgrade on my old vox amp. I've have never had any problems with this amp,and I bought it new in 2000 and have been using it on and off since! I have to agree with you both
that he spends too much time on the net..I really do not see any reason to make changes on something that works as well as the old Vox! Its not a powerhouse,but it does have a good voice for a solid state amp of its size! I have No plans to change anything on the amp..the Grandson came over tonight and pulled up a website pertaining to Vox-Korg Modifications..but I'm a old player,not a Tech..it sounds like a pile of ...to me,but anyway heres the site that he has been driving me nuts about...https://sites.google.com/site/voxcambridge30repair/vox---korg-cambridge-30-and-cambridge-30-twin-modifications

Thank You Both for the info and opinions on this matter..

J M Fahey

I had already found it but didn't want to add gasoline to the fore , but, since you already brought it up, I'll comment on some of the points it mentions.

Quotethe output bumped up to 68 watts.

Aready mentioned before, but the power amp does not "create" power but feed the speakers what the power supply has to offer, so ... no power increase.

QuoteIt would be nice to show schematics of the changes but out of respect to all parties I'm not going to.

The *first* thing he should supply is a before/after schematic .

QuoteAn easy modification to achieve a higher output is to replace the TDA2050 with a LM3875 made by National Semiconductor and increase the output to 52 or 56 watts.
Pure and simple LIE.
This guy has no clue, so i doubt his being an EE.

QuoteIf for some reason you are using different speakers or running a separate cabinet you can bump up the power to 68 watts. This involves much more work though. The TDA2050 can be replaced with a LM3886
Same as before, only the lie becomes bigger.

QuoteThe Cambridge, of course, was also (in)famous for "Cambridgitis", which was caused by the choice of an inadequate stock optocoupler, which would fail when the amp was driven hard for 15-20 minutes.
Not true.
Some poor quality optocouplers failed, period.
Absolutely unrelated to how loud they were played.

6) he posts an incomplete Tremolo adjustment procedure .... which doesn't even apply here, it's for entirely different late sixties Thomas Vox amps.
Just to fill an extra page with something.
QuoteFor Thomas Organ  amps but may be useful
Sorry but no.

7) by the way he pulled the idea (so itisn't even his own) from a page called "Crazy Bob" or "Bob's Crazy Amps", which doesn't exactly inspire confidence.

And even HE does not claim  it raises power. :lmao:


Sure..it would be great to double the power out of a solid state amp by just changing out the chip!
If it sounds too good to be true..it is!

Thanks again for all your help on this screwball idea  :loco


To paraphrase Will Page, changing the gearbox in your car for a higher rated one will give you twice the horsepower.  Basically Mr Page is full of it and talking techo-drivel.
If you say theory and practice don't agree you haven't applied enough theory.