Vox Cambridge 15 and Cambridge 30 reliability upgrades, mods, and tweaks

[Tone Meister]

I wasn't sure whether to post this here or in Mods and Tweaks, but since I have a Cambridge 30 with Cambridge-itis I chose to post it here. I am an absolute novice guitar amp tech as compared to many members of this forum, but I do have a complete electronics workbench and a full understanding of standard practices and safety procedures drawn from my many years as a tech in a related field.

What I have is both a Cambridge 15 and a Cambridge 30 from the 90's that I'm interested in tweaking and upgrading for the kids and grandkids to enjoy. The 15 really doesn't have any problems except the Celestion Bulldog speaker is tiring and the preamp tube is just about shot as evidenced by the tone dulling over time. The 30 is starting to suffer from Cambridge-itis, so I want to use it as an opportunity to tweak both amps without reinventing the wheel so to speak.

I really want to keep things simple as possible without compromising the end results; here are the things I'd like to accomplish:

Cambridge 15: I'm pretty much set on placing the tube and pulling the speaker baffle and installing a 10" alnico speaker. Tremolo has always worked decent, but now would be the time to assess the optocoupler and perform the VTL5C3 upgrade that is talked about so often. I'd be interested in upgrading the output chip if I can do so without performing major surgery such as upgrading the PT. Finally, now is the time to perform any filter cap upgrades or other reliability or tone mods that have proven to be worthwhile without creating a dreadfully poor cost/benefit result.

Cambridge 30: Replace the tube, upgrade the optocoupler to get the output stable again, perhaps upgrade the output chip up to what the current power supply will support. I'm not interested in pouring a great deal of cash into the amp just to get marginal results. Finally, I would like to perform any filter cap upgrades or other reliability or tone mods that have proven to be worthwhile without creating a dreadfully poor cost/benefit result.

Until I open up both amps, I am unable to say exactly which version of each amp that I have, so I realize that could possibly slow down the flow of information at first. I will need schematics and layouts for each amp and will welcome any information, suggestions, and caveats you experienced techs are able to offer.

Thanks fellas, have at it.

-Stan

[Jazz P Bass]

What is an 'output ic upgrade'?
And what is it supposed to do to a 15 or 30 watt practice amp?

[Tone Meister]

What is an 'output ic upgrade'?
And what is it supposed to do to a 15 or 30 watt practice amp?

That is exactly the type question for which I am seeking answers myself. Presumably the output IC can be upgraded to increase wattage output and thus headroom, but I am only repeating what I have read here and there. I do know this forum is frequented by some exceptionally astute amp techs, so I am hoping several of them will chime in soon.

[Jazz P Bass]

You cannot increase anything if you do not first increase the supply voltage (ie: the power transformer)
The TDA ic's that Vox used are used in many amps.
They have a built in thermal overload shutdown that should save the chip if it is overdriven for hours at a time.
As to the Vactrols, if there is a symptom of them failing, replace them. (amp is stuck in a low output mode)
I would look at them first, they may be the newer type.
The old number was LT9912.

[Steve Conner]

The Cambridge 30 runs its TDA2050 chip right at the limit. A few people have swapped in a LM3876 or LM3886 after the original chip blew. You won't get any more power by doing this unless you also swap out the transformer for a higher voltage one, but it'll be more reliable.

This thread gives some details. http://music-electronics-forum.com/t13531/

[Fragger]

Some useful information by Will Page - https://sites.google.com/site/voxcam...emap/hierarchy

[Jazz P Bass]

I do not understand where the 'they ran it at overvoltage' thing is coming from.
Page 1 of the datasheet states that the TDA2050 is a 50 volt device.
ST Microelectronics - datasheet pdf

[Steve Conner]

This is certainly useful, but it claims that swapping out the chip will increase the power, which is false as we said above. It may give a slight increase if the TDA2050 had been overheating and limiting its output, but it won't give you 56 or 68 watts.

JPB, the 50 volt rating is for the sum of the positive and negative rails. +/-25V.

[Tone Meister]

Seems like we have become focused solely on the output chip, and I definitely understand the points that have been made thus far. Please bear in mind that I have a couple of amps here and simply want to show them some love while I have them open. You can be sure that I am not some mad scientist who insists on ramming a square peg in a triangular hole.

If there are reasonable upgrades, especially with regard to reliability, then I want to know about them. I'm quite surprised there hasn't been any mention of upgrading filter caps yet since that seems to be one place where most entry-level amplifier designs fall pitifully short. If there are none that are worthwhile, then I will simply swap out the parts mentioned in my original post and be on my merry way.

Thanks for the replies from everyone so far.

[g1]

I'm quite surprised there hasn't been any mention of upgrading filter caps yet since that seems to be one place where most entry-level amplifier designs fall pitifully short.

You say designs so I take it you are not talking about the quality or reliability of the filter caps. So are you saying most entry level amps suffer from inadequate filtering? Will increasing the value of the filter caps in these amps offer significant improvement to the sound quality? Your use of the term "pitifully short" implies to me that there are very substantial improvements to be made, is this the case?

[Steve Conner]

Electrolytic filter caps have a finite life. If you're servicing a 20+ year old amp it won't hurt to replace them.

I wouldn't say that they are "pitifully short". If anything, modern entry level amps have better filtering than the highly regarded classic tube amps did. Electrolytic caps have got a lot cheaper and more compact over the years, and solid-state rectifiers can tolerate a lot more capacitance than the old rectifier tubes.

[Tone Meister]

...I wouldn't say that they are "pitifully short"...

I've sure learned that one should choose his or her words carefully, because it tends to take a thread off topic with folks focusing not on the original questions but on a single ill-considered statement. That said, when one opens an amp with unmarked electrolytic capacitors whose measured ESR is in double digits, my initial thought would be to replace them, and upgrade them if worthwhile. When I mentioned the word "design" it was a misstatement.

The question remains: if anyone with experience with either of these two Vox Cambridge amps has upgrade suggestions, I am open to advice.

[DrGonz78]

That said, when one opens an amp with unmarked electrolytic capacitors whose measured ESR is in double digits, my initial thought would be to replace them, and upgrade them if worthwhile.

I would like you to be more specific here... What do you mean exactly "unmarked electrolytic capacitors." Just curious as to exactly what you mean...

[Steve Conner]

Yes, you might have chosen a more careful word than "Cambridge-itis" to describe your initial problem. Inflammation of the Cambridge?

[Enzo]

In my shop, Cambridge-osis is more of a problem.


Tone M, you are dealing with technicians and engineers here. We will scold someone for telling us they got "no reading" on their meter at some point in a circuit, when what they meant was zero volts or open on resistance. So best to leave out editorial comments about circuits until folks have a chance to know your writing style.

If a cap is tired, yes, by all means replace it. The concept of upgrading is the issue. These are pretty basic little practice amps, and in my own opinion, I don't see what upgrading there might be. You could increase the capacitance, but what would that do? The existing filtration is enough for the amp. You could pay more for some sort of premium cap brand, but again, this isn't hifi, so no one will be bragging that his soundstaging has opened up but for a bit of graininess in the 6k-8kHz range.

As to increasing power, as was said above, all power comes from the power supply, so putting a heftier output IC in there won't increase power or headroom. Steve mentioned running the TDA2050 at its limit. But as a Vox repair station, I have seen many of those come through with failed opto, but very few with failed power amp IC. You mention these amps are for the kids and grandkids, so we might expect them not to be used all the way up, so much of the potential stress on the IC doesn't materialize.

Just my opinion, but for an amp like this, all the upgrades I could hang on them would be like installing a racing handling package on my mom's sedan.