Help needed the Pre-3 CF CSS Config.

[G Willis]

Hi,

I decided I wanted to run my CF in CSS mode as I have rearranged my room and I now need faily long interconnects.

I am getting some odd behaviour and I wanted to check the orientation of the MJE340 is correct which I believe it is according to the manual.

I am pretty sure the other BPJ aand the LEDs are in the right way.

I cannot seem to get anything except a faint light from the LEDs and I cant get the voltage across the pot and the R7 above about 200mv. The readings are very unstable and are effected by the action of the screwdriver. In addition there does not seem to be any heater glow in the valve. I have checked all the input power and its fine. Pictures here.

https://drive.google.com/file/d/1-1-...ew?usp=sharing

https://drive.google.com/file/d/1-2E...ew?usp=sharing

Thanks, Graham

[bob orbell]

I can't get into the pictures . Bob

[G Willis]

Hi Bob,
Can you try again now please?

[bob orbell]

Well Graham, have you measured the heater voltage at the brown and grey wire entering the board, it should read 6.3 volts DC and is it present at the valve pins. The first picture of the MJE340 looks as if the pins are not soldered, and without causing any offense, the soldering in general is very poor, I hope someone can confirm if you have the LED;s fitted correctly. Bob

[G Willis]

I have reverted it now and its working fine. The heater is Ok, 6.3V, its just there is no visible glow.

I will check the soldering on the transistors. Thank you for your frankness. The LEDs have the larger internal piece, the cathode, through the square-marked hole.

[bob orbell]

Graham, a cathode follower in any one of its many configurations, will drive long cables to a power amp, it's main attractions are that it has very high input impedance and low output impedance, it also has current gain but no voltage gain, plus it can be a very simple circuit, I use an Aikido CF as my pre, this is a modified all valve CF with active load, am sure you will find yours works fine as you have it configured. I would also like to see the circuit diagram, if possible, with the two options. Bob

[G Willis]

It looks like somehow I put an ECC83 in instead of an ECC88. To be honest it's been working fine, so it was with some surprise that I discovered that I had the wrong valve.

I swapped in an ECC88 and set it up for CSS operation. It works fine now and is well behaved but I cannot get the voltage dropped across VR2/12 any higher than 470 mv. The manual says to set it to 850 mv, so I am going to set it back to standard operation for the moment I think.

For all that I am not sure I quite like the sound of the correct valve, so a couple of questions: is there any problem with continuing to run with an ECC83 and any ideas about the voltage across VR2?

Many thanks for any help.

(BTW I checked all my soldering and it's absolutely fine.)

[John Caswell]

Hi Graham,
I am surprised it worked with an ECC83 as the heater voltage will be seriously wrong because the heater pins on an ECC83 are 4,5 and 9 so you can run them at 12.6v across pins 4&5 or at 6.3v across pins 4&5 strapped and pin 9.
I am sure the correct valve is an ECC88/6922/6DJ8/E88CC so stick with it.

John

[bob orbell]

It would have worked with the ECC83 in or out, this valve would have done nothing, as John said, the heater elements would not be working, well not at full voltage, and the volume would be very low, I can offer to tidy your work up and sort the CCS for you if you wish, I charge nothing for working on it, only payment for any parts and return postage. Bob

[bob orbell]

Just so you know Graham, CSS is incorrect, It is CCS this stands for, in your case, constant current sink. Bob