CHAMP ELECTRONICS -" THE VINTAGE VALVE AMP HOSPITAL"

NOTTINGHAM, ENGLAND

 

RESTORATION OF A 1970's VAMPOWER Mk III AUDIO-PACK 100watt AMPLIFIER HEAD

 

 

 

 

AN INTERESTING "LITTLE" TOY................

 

Coming through the workshop at the back-end of 2005, this VamPower had undergone some serious and very strange modifications. The top front panel had been drilled with four large holes (see photo below), to take the standard black nylon jack socket cups like the two for the retaining screws on the ends of the said panel, and these holes were fitted with 3 jack sockets and one switch.

 

 

On close examination it turns out that two of these sockets were wired as pre-amp out and power-amp (slave) in. The third socket was added as a reverb foot switch. The switch in the fourth hole seemed to mute the pre-amp section, which seemed pointless to me as the original standby switch fitted on this model also turns off the pre-amp stages, leaving the output stage running anyway.

I removed all the added sockets and switch from the front panel, and the owner has subsequently re-covered the panel using the same type of fret material that would have originally been used on the amp; as can be seen from the photo at the top of this page.. (This type of fret material is still available today, and in fact is the same as was used by Orange).

On first power-up, the performance was really bad with the HT sagging immensely and a large 100cls ripple coming over the smoothing capacitors. All four of the original Sprague HT capacitors were dried-out, and in fact were swelling at the bottom. I replaced these with just a pair of modern 330uf @ 450v, and the HT was stiffened right up.

Now that I was getting some real power out of her, I could hear some fizzling and see large peak flashers on the scope. On further investigation, I found that this noise was coming from the output transformer. I turned everything off and did a gentle pull on the three primary leads, one of which came straight out of the core!! On close examination with a spy-glass, I could clearly see that the plate (anode) lead had been wrapped around the end of the copper coil winding. Fair enough, but...... the solder hadn’t flowed properly and the joint was dry!

This amplifier most probably had had this problem from new. Fortunately, it was the plate lead that was on the last winding - on top. I carefully, with a Stanley Knife, cut away the top layers of blue varnished card to reveal the last part of the winding, found the end, and very carefully cleaned off the varnish and soldered a new lead to it - hence the white card (I had no blue left in stock at the time). I re-sprayed this side of the transformer with clear lacquer and “job done”. If the lead had been any further down the winding it would have been a transformer re-wind. That was very lucky!

 

    

 

The tremolo was non-existent and the reverb was just trying to work; it could just be heard in the back of the speaker. Once I’d gotten my head around the original circuit (a schematic diagram not being available so I just simply chased it myself), I found another very strange modification. Though all the components and the tube itself (ECC83) were still actually in place and working, someone had removed the two “stage-gains” from the reverb circuitry and path! It is of no wonder that the reverb was so very quiet. It had been mixed straight back into the phase splitter grid with no other amplification to lift it! Also, the ECC83 reverb drive tube was incorrect. It should have been an ECC81 (this amp uses a similar reverb drive to the Fender, i.e. transformer driven). Having rewired all of this back to rights the reverb worked fine.

 

       

 

There was just one EL34 missing and the remaining three were 1970’s Brimar, with Mullard coding. I found a fourth Brimar of similar dating to make up the set of four, as the fitted Brimar’s were all fine. (Far better than most of today’s efforts anyway!).

 

 

During the sixties and seventies, and in fact even up to the present day, most musical instrument amplifiers that use “fixed bias” have either no adjustment (using a fixed value resistor), or one pre-set control for all the tubes (no matter how many). If you are very lucky there will be two, one for each half of the push-pull stage. After first removing this amplifier from its case, I was impressed to see four pre-sets; one for each tube. That is a feature that I have incorporated on all the “fixed bias” amps I have ever made. (And believe me, that’s been a damn good few!). This is the best way of doing fixed bias with multiple tubes and it also virtually eliminates the necessity of perfectly matched tubes. The only down-side to this amplifier, on this issue, is the fact that although all four pre-sets have been set to the correct -37 volts, the only benefit would be that if one of the EL34’s sustained a grid one short, it wouldn’t take out its associated mate. (They are isolated from each other by the coupling caps.) A simple mod of placing a 1ohm, 1watt, 1% resistor in each output tube cathode gives a direct read-off of 1ma/V, and this can then be used in conjunction with the four (or however many) tubes bias pre-sets to facilitate equal tube currant and output stage balance.

I have done this mod and it can be clearly seen in the photo below.

 

 

Having changed a couple of tired wire wound resistors things were slowly coming together and becoming fine.

 

 

The amp is now all back as it should be, as original and certainly “kicks ass” now at about 110 watts RMS. Cool!

I have heard that the guy who started the VamPower company and designed the Vamp products was an ex-employer of Jennings Musical Instruments, aka Vox?

By the way; if you’ve ever heard any of Marc Bolan or T-Rex’s music then you have most probably heard one of these amps before! Marc used these on stage and on a lot of his recordings too.

Your interest is appreciated. John.

 

 

 

 

 


 

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