Opportunity to buy an old valve guitar amp

[Bushbaby]

Lawrence
You have the two schematics. I searched a number of websites and forums relating to these and apparently Fender did have some mistakes in their schematics taking figures from other circuits by mistake. I have also read that they also allowed for differences in expected mains voltages too.
I would thing that vibrochamp circuit is likely to be the most accurate (dont quote me on this though please.)

Regards

Rob

[Robert Darwent]

Rob,

Assuming Lawrence is correct about all the voltages throughout the schematic being higher due to a higher input voltage from the mains transformer, then it is improbable that they are all 'mistakes' even allowing for the differences in the expected mains voltages you mention.

The two schematics may appear virtually identical (ie; the symbol for the mains transformer is the same on both) but the likely explanation is that the mains transformer is physically different in the two models, with one wound to give 315 volts whilst the other is wound to give 365 volts.

If this does turn out to be the case, then you will need to change the mains transformer as well as halving the value of the reservoir capacitor, not just change the capacitor alone.

IMHO you would better restoring the amplifier using like-for-like component values and get it working reliably first before experimenting with trying to convert it to another model straight away.

Regards

[pwdrive]

Lawrence
You have the two schematics. I searched a number of websites and forums relating to these and apparently Fender did have some mistakes in their schematics taking figures from other circuits by mistake. I have also read that they also allowed for differences in expected mains voltages too.
I would thing that vibrochamp circuit is likely to be the most accurate (dont quote me on this though please.)

Regards

Rob

It could well be a mistake Rob, as said earlier the anode voltages quoted on the valves are well up on the bronco as compared to the vibrochamp and those excess voltages would be over the limit possibly for the 6V6, I guess you will know what's what when it arrives.

Lawrence.

[Bushbaby]

Hi Rob,
They are the same model and use the same transformers (the same part no for both TR1 and TR2). The only differences are the valve designations and the smoothing cap. The Bronco was rebadged as it was sold as a 'kit' with a matching Bronco guitar - its physically the same amp as the vibrochamp. This is why I cannot understand the change in the cap value other than the thoughts that have been indicated on some of the forums that its value was changed due to US mains voltage fluctuations. How true this is I cannot really establish.

Best Regards

Rob