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Doz 4th October 2010 08:27 AM

CCS - Help!
I have a problem.

I have built, and had great fun modding the ultra-compact amplifier, based on EL84's as found on

However, I got power-hungry, and decided to build a version with 6550's in the output stage.

The CCS was originally the venerable LM317, and performed well in the EL84 amp, however it's not going to cut the mustard in a 6550 version, because of the elevated cathode voltage, so I have swapped to the TL783C. This is where it all goes wrong.... I'll be damned if I can set a sensible cathode current. I've pulled the CCS out, and substituted a Resistor and big bypass cap, and the amp works.

The CCS on the bench (set to 50mA) works fine in isolation, until the simulated cathode (IE a power supply and resistor) gets to about 22v , then it "runs away" and allows too much current to flow. The TL783C isn't getting hot, it just stops regulating.

I've been round it with a scope to see if there's anything oscillating, shouted at it, scratched by head, shouted at it again, checked the ADJ resistor a million times, both the maths and electrically and now I'm losing sleep....

Any thoughts?

Doz 6th October 2010 02:49 PM

<TAP> <TAP> .... is this thing on?

DF96 6th October 2010 03:44 PM

Coming through loud and clear. Sorry I can't help, as I don't use CCS myself.

Brit01 6th October 2010 04:09 PM

Do you have a circuit diagram?

This is the data sheet. Looks suitable for the job.

Rod Coleman 6th October 2010 04:16 PM

We may be able to help, but a circuit with the operating voltages attached are the minimum to be able to see what's going on. A sketch on an envelope, photographed or scanned, is fine.

Stability is always going to be a problem for IC regulators pressed into CCS duty. OTOH, it may be the 6550 going unstable. I would add a 10K G1 stopper and a 1.5K(3W) G2 stopper to the Beamer until you find what this is.

22V is right on the corner of the TL783 current-limit characteristic, so some kind of transient overcurrent, due to instability, needs to be checked out. could also try adding resistance into the cathode-to-783 connexion, & see if the behaviour changes.

Doz 7th October 2010 12:03 PM

<a href="" target="_blank"><img src="" border="0" alt="Photobucket"></a>

Excuse the hastily scribbled diagram, which I scribbled hastily ...and trhe fact I've got the number on the TL wrong... which I can assure you is the right part ;)

I'm going to try a 0.1 across the thing to see if it behaves. There's a 10K Grid stopper in the circuit BTW.

Thing misbehaves, even if there's no 6550's in there , and just a resistor.

Thanks for your comments. I've ordered some more regulators just incase I've damaged these, but RS have delivered them to my ex-wife's house... I've ordered some more ;)

Rod Coleman 7th October 2010 12:19 PM

Looks OK, but is that a 100-ohm pot ? Try Some fixed resistors [wiper to ends] to reduce its sensitivity [2x 22-ohm?] You could then put 50-100-Ohm in the tail (near to the TL783 end), and see if that quells it!

Doz 7th October 2010 12:22 PM

I've tried a 15ohm across the pot, that didn't help, but by the time I got to that stage, I may have abused the reg so much as to reduce it to a jibbering mess ;) ... much like the designer at this stage :D

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