Firebottle
15-09-2016, 16:48
Some time back I offered to look at a Croft VC power amplifier for Rob (Muggleton).
Well today Rob made the trip up from rather further south in France. I had not seen or heard a Croft VC before so it was interesting for me to find out what made it tick.
For those that don't know it is EL84 based, with parallel '84s in ultra linear fixed bias guise, giving about 30W per channel.
Here is a photo, basically there is nothing to see from the front:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/P1050737.jpg
Being fixed bias it was prone to excessive current flow when the valves aged or decided not to play the game regarding published specifications.
So after hooking up the necessary test gear it was discovered that the quiescent current was jumping about all over the the place, with nothing to control excessive spikes, this was what was causing the HT fuse to blow.
After a break for lunch (we are in France after all), Rob was then given a demo of the Firebottle amplifier set-up through the ESL57's, and was impressed by the air and clarity.
After returning to the workshop it was decided that some auto bias needed to be added to the fixed bias of the VC in order that balance could be achieved between the parallel output valves.
Resistors of 16.4 ohms were fitted to each valve cathode and satisfactory balance was achieved, with only a very small loss of output power.
On testing in the lounge driving the ESL57,s the VC definitely didn't like the reactive load and promptly blew a fuse and one of the cathode resistors. After fitting a Zobel network to each channel output, peace was reinstated and a very smooth and open sound was the result.
So a good day in so far as I could bring a problematic Croft VC back to proper operation, plus meeting up with a fellow music lover.
Cheers,
Alan
Well today Rob made the trip up from rather further south in France. I had not seen or heard a Croft VC before so it was interesting for me to find out what made it tick.
For those that don't know it is EL84 based, with parallel '84s in ultra linear fixed bias guise, giving about 30W per channel.
Here is a photo, basically there is nothing to see from the front:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/P1050737.jpg
Being fixed bias it was prone to excessive current flow when the valves aged or decided not to play the game regarding published specifications.
So after hooking up the necessary test gear it was discovered that the quiescent current was jumping about all over the the place, with nothing to control excessive spikes, this was what was causing the HT fuse to blow.
After a break for lunch (we are in France after all), Rob was then given a demo of the Firebottle amplifier set-up through the ESL57's, and was impressed by the air and clarity.
After returning to the workshop it was decided that some auto bias needed to be added to the fixed bias of the VC in order that balance could be achieved between the parallel output valves.
Resistors of 16.4 ohms were fitted to each valve cathode and satisfactory balance was achieved, with only a very small loss of output power.
On testing in the lounge driving the ESL57,s the VC definitely didn't like the reactive load and promptly blew a fuse and one of the cathode resistors. After fitting a Zobel network to each channel output, peace was reinstated and a very smooth and open sound was the result.
So a good day in so far as I could bring a problematic Croft VC back to proper operation, plus meeting up with a fellow music lover.
Cheers,
Alan