Hi All,
Having completed the work on this little amp I thought that I should add a write up. I have for many years wanted to buy a small sixties amp and this one, a 68 Bronco clone of a Fender Vibrochamp, appeared a good project to start with.
I bought the amp via eBay from the US (a guitar shop in Houston, Texas) in the knowledge that it had problems and was sold as a ‘project’ described as making a very loud hum. It arrived well packaged although it was noted that it was missing a power valve (6V6GT), but had its original 12ax7s (Telefunkens too – both tested as good using an Avo mk1 VCM).
Through my naiveness, it became immediately obvious that even if it had been working it was US spec amp and was powered by 115v 60Hz supply so the transformer could have been a potential problem as it might overheat using a step down transformer for UK voltage. Therefore, I decided from the start of the project that it was going to be worth using suitable replacement transformer. Having looked around for a suitable replacement, I opted for a custom built Ted Weber replacement (sorry Ed) which was available from the US for $39 (although it cost me $50plus p&p and import tax!!!). This postage was twice as much as I paid for the whole amp to be transported from the US (I still cannot figure this one out?).
First examination of the circuit indicated that the Mallory electrolytics had seen better days and that the mains voltage smoothing cap was also going to need replacing (all tested with the ESR built from a David Taylor project). I also checked many of the carbon resistors which were now borderline towards the high side of their specification. Having taken good advice that these may well have been high to start with, I decided to leave well alone otherwise this could have led to changing the majority of the circuit (would this then be considered a restoration??).
The Mallory electrolytics were al replaced with Sprague ‘atoms’. I don’t know whether it’s me but it appeared very hard to source replacement low voltage axial capacitors. Even the ones I ordered were well in excess of the 25 volts needed for replacement of the Mallorys (the ones I obtained were 3 x 25uF at 50v and 1 x 10uF at 150v)
I settled for a JJ Tesla replacement smoothing cap which was ordered from Germany. This was slightly shorter than the original Mallory but 1.5 times the diameter. It was at this point I realised that the chassis was going to need modification to accommodate the new cap. The chassis was drilled and cut to take the new JJ smoothing cap.
A new dtpt switch was fitted to accommodate 240v connection as no suitable slide switch could be sourced. This was mounted at the rear of the chassis (as with many of this amps bigger brothers). The slide switch was left in place for purely cosmetic reasons.
At this point the new transformer arrived – this proved to be almost twice as big as the original.
It was indicated at the time that I bought this that it would need new bell ends so that it could be mounted axially upon the amp chassis and allow the primary and secondary tap leads to hang below the body of the transformer to the underside if the chassis where the circuit board was mounted. It was recommended to me to use the 240v primary and the 600v secondary’s (300v -0 – 300v) for the 5Y3 full wave rectifier but to leave the 660v secondary’s (330v -0v – 330v) on standby should the first not be enough. The leads for the heaters etc were also selected with all remaining unused taps carefully cut short, had heat shrink added to the lead end, bent over and sealed tight using self annealing tape. The unused ends were all carefully packed inside the transformer bell ends before tightening the transformer bolts up. The transformer was then mounted and the leads connected.
I cut out the ‘death cap’ attached to the chassis from the 1amp slow blow fuse for obvious reasons. At this point valves were added (albeit these were all valves that were selected as second choice ones so it didn’t matter if they were damaged), the speaker connected and the amp powered up. All appeared to be working although this was without adding an input signal i.e. a guitar. The chassis was carefully remounted back into the cabinet and tried for the first time with a guitar. To my disappointment it initially sounded poor lacking any ‘punch’, the break up harmonics at higher volume was awful.
I then decided to add ‘better’ valves including a 6V6TA (unusually a Mullard GB one) which apparent has a higher output/electron saturation due to barium doping of the plates. This made the sound worse, with the speaker rattling if though it was ‘blown’.
At this point I cannot thank my ‘mentor’ for this project (David Taylor) enough as he suggested that I go back to first principles and measure the voltage readings at certain points on the circuit. The original schematics are quite detailed and give test point indication with recommended readings. This was duly undertaken it was found that this amp was nearer to the voltage spec to the original Vibrochamp ‘blackface’ amps (AA764 circuit) than the Bronco circuit (AB764). A couple of minor tweaks were made; the 0.2uF disc cap feeding pin 6 of the pre amp valve (12ax7) was changed for a Wima polypropylene cap as was the one from pin 8 of the tremolo circuit cap (12ax7) and the 330pF cap across the 6V6gt was removed. The amp was now revalved using a military spec 5Y3 rectifier, a Tung-sol 6V6GT and one of the other Telefunken 12ax7 as valve 1 (pre amp). Any type of 12ax7 can be used in the tremolo circuit as its not part of the signal amplification side.
The amp was now tried again – wow! I can see why these are so sought after. Firstly, with no signal input and the volume turned to 10 there is no noise through the speaker whatsoever. I was expecting to hear something but no, this was even the case with a guitar lead added which usually causes a lot of noise but this amp is uber quiet. Ok, with the guitar added the amp tone is clean to about 6 on the volume control and overdrives beyond this. It not the loudest of amps (8” speaker?) but it has enough volume to ‘feel’ the sound so its really good for home use. The tremolo is pure 60s which was great then but too much for my taste above 3 one speed and intensity.
Having spent a lot of time on this amp it has made me realise how much my playing has suffered so don’t expect any sound clips just yet.
I shall now save up my pennies for my next import project……..Fender Silverface deluxe reverb or twin??
Regards
Rob
Having completed the work on this little amp I thought that I should add a write up. I have for many years wanted to buy a small sixties amp and this one, a 68 Bronco clone of a Fender Vibrochamp, appeared a good project to start with.
I bought the amp via eBay from the US (a guitar shop in Houston, Texas) in the knowledge that it had problems and was sold as a ‘project’ described as making a very loud hum. It arrived well packaged although it was noted that it was missing a power valve (6V6GT), but had its original 12ax7s (Telefunkens too – both tested as good using an Avo mk1 VCM).
Through my naiveness, it became immediately obvious that even if it had been working it was US spec amp and was powered by 115v 60Hz supply so the transformer could have been a potential problem as it might overheat using a step down transformer for UK voltage. Therefore, I decided from the start of the project that it was going to be worth using suitable replacement transformer. Having looked around for a suitable replacement, I opted for a custom built Ted Weber replacement (sorry Ed) which was available from the US for $39 (although it cost me $50plus p&p and import tax!!!). This postage was twice as much as I paid for the whole amp to be transported from the US (I still cannot figure this one out?).
First examination of the circuit indicated that the Mallory electrolytics had seen better days and that the mains voltage smoothing cap was also going to need replacing (all tested with the ESR built from a David Taylor project). I also checked many of the carbon resistors which were now borderline towards the high side of their specification. Having taken good advice that these may well have been high to start with, I decided to leave well alone otherwise this could have led to changing the majority of the circuit (would this then be considered a restoration??).
The Mallory electrolytics were al replaced with Sprague ‘atoms’. I don’t know whether it’s me but it appeared very hard to source replacement low voltage axial capacitors. Even the ones I ordered were well in excess of the 25 volts needed for replacement of the Mallorys (the ones I obtained were 3 x 25uF at 50v and 1 x 10uF at 150v)
I settled for a JJ Tesla replacement smoothing cap which was ordered from Germany. This was slightly shorter than the original Mallory but 1.5 times the diameter. It was at this point I realised that the chassis was going to need modification to accommodate the new cap. The chassis was drilled and cut to take the new JJ smoothing cap.
A new dtpt switch was fitted to accommodate 240v connection as no suitable slide switch could be sourced. This was mounted at the rear of the chassis (as with many of this amps bigger brothers). The slide switch was left in place for purely cosmetic reasons.
At this point the new transformer arrived – this proved to be almost twice as big as the original.
It was indicated at the time that I bought this that it would need new bell ends so that it could be mounted axially upon the amp chassis and allow the primary and secondary tap leads to hang below the body of the transformer to the underside if the chassis where the circuit board was mounted. It was recommended to me to use the 240v primary and the 600v secondary’s (300v -0 – 300v) for the 5Y3 full wave rectifier but to leave the 660v secondary’s (330v -0v – 330v) on standby should the first not be enough. The leads for the heaters etc were also selected with all remaining unused taps carefully cut short, had heat shrink added to the lead end, bent over and sealed tight using self annealing tape. The unused ends were all carefully packed inside the transformer bell ends before tightening the transformer bolts up. The transformer was then mounted and the leads connected.
I cut out the ‘death cap’ attached to the chassis from the 1amp slow blow fuse for obvious reasons. At this point valves were added (albeit these were all valves that were selected as second choice ones so it didn’t matter if they were damaged), the speaker connected and the amp powered up. All appeared to be working although this was without adding an input signal i.e. a guitar. The chassis was carefully remounted back into the cabinet and tried for the first time with a guitar. To my disappointment it initially sounded poor lacking any ‘punch’, the break up harmonics at higher volume was awful.
I then decided to add ‘better’ valves including a 6V6TA (unusually a Mullard GB one) which apparent has a higher output/electron saturation due to barium doping of the plates. This made the sound worse, with the speaker rattling if though it was ‘blown’.
At this point I cannot thank my ‘mentor’ for this project (David Taylor) enough as he suggested that I go back to first principles and measure the voltage readings at certain points on the circuit. The original schematics are quite detailed and give test point indication with recommended readings. This was duly undertaken it was found that this amp was nearer to the voltage spec to the original Vibrochamp ‘blackface’ amps (AA764 circuit) than the Bronco circuit (AB764). A couple of minor tweaks were made; the 0.2uF disc cap feeding pin 6 of the pre amp valve (12ax7) was changed for a Wima polypropylene cap as was the one from pin 8 of the tremolo circuit cap (12ax7) and the 330pF cap across the 6V6gt was removed. The amp was now revalved using a military spec 5Y3 rectifier, a Tung-sol 6V6GT and one of the other Telefunken 12ax7 as valve 1 (pre amp). Any type of 12ax7 can be used in the tremolo circuit as its not part of the signal amplification side.
The amp was now tried again – wow! I can see why these are so sought after. Firstly, with no signal input and the volume turned to 10 there is no noise through the speaker whatsoever. I was expecting to hear something but no, this was even the case with a guitar lead added which usually causes a lot of noise but this amp is uber quiet. Ok, with the guitar added the amp tone is clean to about 6 on the volume control and overdrives beyond this. It not the loudest of amps (8” speaker?) but it has enough volume to ‘feel’ the sound so its really good for home use. The tremolo is pure 60s which was great then but too much for my taste above 3 one speed and intensity.
Having spent a lot of time on this amp it has made me realise how much my playing has suffered so don’t expect any sound clips just yet.
I shall now save up my pennies for my next import project……..Fender Silverface deluxe reverb or twin??
Regards
Rob
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