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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

Arkless Electronics
15-09-2016, 16:58
Nice job Alan;)

I fixed a Croft power amp many years ago which IIRC had 4 x EL84 per channel and was push pull ultralinear but it was more conventional looking for a valve power amp in as much as valves, transformers etc on top of a chassis. Repair completed I give it a try in my system but was unimpressed I'm afraid... rather mushy and indistinct from memory (it was about 20 years ago at least) and I well recall that my Leak Stereo 20 thrashed it on everything but max power.. I can't recall ever seeing anything similar again so it may have been a prototype or a model that was quickly dropped maybe...

struth
15-09-2016, 17:06
Well done Alan... nice walkthrough too...

Jimbo
15-09-2016, 20:18
Thanks for enlightening us with your interesting days work on a poorly Croft. Glad it sounded good after you had sorted it.

Rob Muggleton
18-09-2016, 12:25
Indeed a most interesting day!
The chance to hear Alan's system, with his lovely sounding Firebottle monoblocks, was a real treat and the sense of stabilising the rather skittish circuit of my Series VC was also deeply satisfying.
In the best sense of the phrase, it was all a bit of a "hi fi adventure" for me and has given me back a well-loved power amp.
We've had a bit of subsequent correspondence, and the final improvement is probably to replace the 5963 driver valves (250V versions of ECC82's) with full spec '82s, since that might also help obviate any potential issues.
But anyone needing a really open, transparent and fast amp set-up should try to audition the Firebottle pre/monoblocks - they really are something special.
Cheers
Rob

CageyH
18-09-2016, 17:21
The pre and power amp do a good job too.
I do need to have a listen to the Monoblocks at some point though.