Vox Cambridge 30 Reverb Twin keeps blowing TDA2050

[Robert M. Martinelli]

"+1 on the 3886.... I actually used a 3876 ( 56 W ) in my C30, together with an oversized toroidal PT ( to properly feed the "current hungry" 3876 ), an oversized ( 8A ) bridge, PS "overfiltering", socket mounted 5532s and an VTL5C3 as the "trem roach". 56 W(RMS) are more than enough for almost every situation.... I didn't use the 3886 ( 68 W ) to be able to keep the original speakers, otherwise the mods would have cost me more than what I paid for the amp..."

how would you modify the C30 to take the 3886? or can you just put it in there?

The 3886 is nothing but a "beefed up" 3876 ( or you can think about the 3876 as a "beefed down" 3886 ) same case, same pinout.

Once you have provided the correct voltage/current ( which I did by using an oversized toroidal PT, bridge and PS filtering ) they need, you only have to worry about proper thermal dissipation, putting in a bigger heatsink or adding a small fan on the original one....

The C30's original speakers barely cope with the impressive performance of a 3876 though, so, if you're planning to throw in a 3886, be prepared to change the speakers as well.

Oh, I just realized that in my previous post on this subject I forgot to mention I also switched to VR ICs ( 7815/7915 w/heatsinks ) for the preamp section, leaving the original Zener arrangement on the 9.1 V rail only ( which supplies the trem osc only ).

Hope this helps

Best regards

Bob

[vipersassasin09]

cool, yeah, i was wondering... the transformer in the C30 i have is a 066-CVU, is there anything that can replace it? and still get the same voltage? b/c i have a TDA2050 in my amp, and the heatsink, kinda doesn't seem to work... but, idk... maybe i am not using enough of it... anyway, mainly is there another transformer i could use and get the same results?

[Robert M. Martinelli]

cool, yeah, i was wondering... the transformer in the C30 i have is a 066-CVU, is there anything that can replace it? and still get the same voltage? b/c i have a TDA2050 in my amp, and the heatsink, kinda doesn't seem to work... but, idk... maybe i am not using enough of it... anyway, mainly is there another transformer i could use and get the same results?

Of course there is, only, I used an Italian-made toroidal PT which I don't think it's available abroad....so you'll need to find something similar.

If you plan to throw in a 3876 you'll need a PT capable of supplying some 44 VAC ( 22-0-22 with center tap ) 2A. This way you' ll have some +31/0/-31 VDC out of the bridge rectifier/filter section.

Now, about your heatsink....What do you mean by saying it doesn't seem to work? Does it stay cold while the IC gets hot ? In this case you have "thermal contact" problems ( the heat mostly remains on the IC and only a little goes to the heatsink ).

As I said, if planning to throw in a 3876, get yourself a bigger heatsink or add a small ( PC ) fan on the old one. I installed a small thermal switch close to the IC, its N.O. contact "closes" at 50 deg C bringing the fan on.

Hope this helps

Best regards

Bob

[vipersassasin09]

okay, what if i want to keep the original TDA2050 in it... what transformer would i use to replace the 066-CVU? or what is a place that sells them? and the price on it... i know of marshallparts, anywhere else?

[Robert M. Martinelli]

okay, what if i want to keep the original TDA2050 in it... what transformer would i use to replace the 066-CVU? or what is a place that sells them? and the price on it... i know of marshallparts, anywhere else?

I would advise you to use a different transformer, the original PT has a 19/0/19 VAC secondary winding, this makes for some +27.1/0/-27.1 VDC out of the rectifier/filter section, which is way out of spec for a TDA 2050. Get a PT with some 17/0/17 VAC at its secondary ( 1+1 A ) so that you'll have +24/0/-24 out of the rectifier/filter section.

Another idea would be to simply replace the PT with another 066-CVU and to throw in a 7824 ( on the positive rail ) and a 7924 ( on the negative rail ) voltage regulators. They can handle currents up to 1 Amp ( 1.5 if mounted on a proper heatsink ), and 27VDC ( -27 VDC ) at their input is enough to get +24VDC ( - 24 VDC ) out of them. This way your 2050 will be operating within its limits and the risk of "burning" it will be greatly reduced.

Should you choose this last solution, be careful as the 7824/7924 pinout is different, for the 7824, with the IC face towards you, the three terminals are I - 0V - O ( Input - 0V - Output, metal tab internally connected to 0V ), for the 7924 the pin arrangement is ( always with the IC face towards you ) 0V - I - O ( 0V - Input - Output, metal tab internally connected to the input ).

Hope this helps

Best regards

Bob

[Steve Conner]

That 066-CVU part number won't be a thing you can buy off the shelf. It'll have been custom made for Vox. I doubt the transformer is blown, but if it is, it shouldn't be a problem to buy a similar transformer, or even rip one out of an old stereo.

I don't think the 7824/7924 thing is a good idea, on account of line voltage variations and sag. I'd replace the TDA2050 for a higher rated chip as discussed, or buck off some line voltage with one of RG's Vintage Voltage adapters.

[Robert M. Martinelli]

Hi Steve!
I have pulled the original PT away, and there's nothing special about it, it's a 240VAC---->19/0/19 VAC 40 VA transformer. He said he wanted to keep the 2050 in so I gave him two different "ideas" on how to lower the voltage to something the 2050 can withstand....

As to throwing in a 7824 and a 7924, can you elaborate on why you think this is not a good idea?

I though about 'em because their dropout voltage ( the minimum voltage difference between the input and the output those ICs need to work properly ) is about 1.2 V ( the internal voltage reference ), so they' ll work properly even if the input voltage drops down to 25.2 V ( some 8 % drop ) while, on the "high" side, if memory serves me well, they can withstand voltages up to 37 V ( provided the PD is no greater than 15 W ) so even a +3 to +6 V variation at the input is not a big problem. The original 2050 together with the preamp draws less than 1 A/rail, so, under normal conditions ( 3 VDC drop ), the 78/7924 will have to dissipate less than 3W each, which turns into a safety margin of 5, as they're rated at 15 W ( w/proper heatsink )

Cheers

Bob

[Steve Conner]

Well, my main worry is that 8% drop, which could easily get used up by ripple and sag under load. (A supply like this would have 10% ripple at full load.) The regulators will still function once they've dropped out, but it just seems ugly.

Also, the regulators' current limiting is sensitive to peak current, and for 20V peak/8 ohms, that's a little under 3 amps. So you'd need to add extra large capacitors after the regulators to sustain the current peaks, and then you'd need protection diodes, and it all gets complicated.

[Robert M. Martinelli]

Well, my main worry is that 8% drop, which could easily get used up by ripple and sag under load. (A supply like this would have 10% ripple at full load.)

Oh...Ok, but...in this case it would be enough to throw in some "overfiltering" ( like I did ) after the rectifier, in my C30 I used two 4700 uF per rail, and that should be enough to cope with sudden current demands....but, then again, you're right...It would become too complicated...and probably not worth the effort...as we said in our previous posts, a 3876 together with a new PT would do just fine....I took that way because he wanted to keep the 2050 in.

Have a nice WE

Bob

[R.G.]

You keep talking about firing people for making poor judgment calls. I believe that in cases like that they generally qualify for unemployment which usually comes out of your company's pocket. (That is how it is in California.)

It is much better to encourage someone to quit. Perhaps have a "friendly competitor" offer them a job at a substantially higher salary so that they give notice, and guess what- the new job was just eliminated in the latest bout of layoffs. Gee whiz, we are awfully sorry...

Steve Ahola

You are right, in practice. I was speaking theoretically.

I have managed programming and design departments in a company that was notorious for not letting anyone be fired. In practice, most of my peer managers found nonperformers another position somewhere else in the company rather than spend the time and effort to face up to a nonperformance issue.

In the long run, it's not only unfair to the company but unfair to the person being fired if the person is actually not capable and has had a fair chance to improve.

But yes, in the real world, firing costs money. So does letting people make mistakes that cause product problems. It's a multi-sided coin you have to toss in such situations, and the solution is hardly ever cut and dried.

[vipersassasin09]

okay... got the amp to turn on without blowing a fuse... only problem is, the speakers won't quit buzzing... they have a whirring sound... what could be wrong?

[afm_380]

vr11 and vr12

Can anybody elaborate a little on the calibration of vr11 and vr12 after replacing the optcoupler. twiddling with it till it sounds good has me twiddling to no end ... ;o)

[tom o]

VR11&12

Can anybody elaborate a little on the calibration of vr11 and vr12 after replacing the optcoupler. twiddling with it till it sounds good has me twiddling to no end ... ;o)

Hi there, this is something that I need to look into more deeply myself; I should have some hints to post by the end of this week, as I'm prepping my CamRevTwin for sale...BTW I put in a 3875'F' (insulated tab) chip last night and I'm really happy w/the result--almost reconsidering the idea of selling it.

tom o

[tom o]

VR11,12 Twiddle report

Hey AFM,

did you get satisfaction w/your adjustments? I spent time trying to learn from the schematic, but the trimpots didn't quite do what I thought they would based on that cursory study...I went a-twiddlin, and ended up with VR12 all the way CW; then adjusted (on clean ch, vol & trem depth set to zero) VR11 from full CCW position, clockwise a bit until the hiss was minimized... at the expense of some gain, and trem depth. I got a balance of 'hisslessness', gain, and trem depth to maybe 80% of 'King Of The Hill' throb. I'm wondering if your experience was similar.

The chip upgrade to 3875F made a great improvement BTW; also, the chip, heat sink & PT were about as warm as my forehead, after 10+ minutes of full-out bashing the amp...a slight ghosting on the E note, strng4 frt14 still exists, but less than before; I'm hoping that a filter cap upgrade will help this.

One more design complaint: C48 (1uf/50V) is across the HV rails, close to 60V between them; failure to plan, or planning to fail, Korg?

[Enzo]

KorgUSA is just the distributor for Vox products in the USA. They didn;t design the amp.

Here is the trem adjust procedure. Please don;t ask me to interpret.