Citation 12 trouble-new transistors

[lasser]

Could this amp be working now?

Hello,

Just matched and installed two new transistors (KSA 992) as diff-amp-
( I bought 40 units and the two first matched within 0,01 uA and were hFE 310 and 312)

The voltages are as shown below and the output offset is abt. -40 mV just as the "healthy" channel.

I plan to let the amp be on power for a few hours and re-measure but what do you think? If this is stable, is there any other measurements to do or can I let the amp go on full input voltage?

Best regards//lasse

 

[PB2]

Voltages look fine and I would try it on full power now.
Let it settle down for output idle current, then you can try different output
powers into the 8 or 4 ohm load. See where it clips.
I would not drive it higher than 5 KHz at high power.

 

[lasser]

Hi,

Thanks for the advice.

The amp has been on low voltage for abt three hours and the values are stable, with one exception: the bias adjusting voltage, the values seem to drift with temperature.

Which is better? to adjust when the amp has reached working-temp or in cold state?

Regards//lasse

 

[PB2]

Hi,

Thanks for the advice.

The amp has been on low voltage for abt three hours and the values are stable, with one exception: the bias adjusting voltage, the values seem to drift with temperature.

Which is better? to adjust when the amp has reached working-temp or in cold state?

Regards//lasse

I'm not sure that I follow, are you saying that for example the Vbe of the "Bias" transistor
is changing with warm up? This is completely normal and is required to keep the output
stage bias constant with temp.
Does the voltage at the bias test points change once it is warmed up?
You should adjust the bias when the amp is warm and you might have to touch it up a
few times. The exact bias point is not very critical +/-20% is not a problem as long as
it stabilizes.

 

[lasser]

Voltages look fine and I would try it on full power now.
Let it settle down for output idle current, then you can try different output
powers into the 8 or 4 ohm load. See where it clips.
I would not drive it higher than 5 KHz at high power.

Hi.
I get easily 60V p-p as in post 66 type display in 10 ohms load, both channels. I tried 5 ohms ( two ten’s in paralell) but the heat melted the sonder

I would say that this amp is now ready for service in a system.

I cannot thank you enough for the invaluable support and help on this Endeavour and hope to return this some day!

Perhaps this thread also can serve as a kind of tutorial for other people, trying to get their old Cit12 back to life.

Please, hold your thums for me when I connect this beast to a pair of speakers.......

The very best Regards//Lasse
Stockholm, Sweden

 

[lasser]

Thanks and a Happy New Year!!

Hello,

The citation 11+12 combination is now in service and they sound very well (First impression!)

Finally, I get to hear this 70ies amp with the new electrocaps (Panasonic FC), that started this business.

Once again, Thanks for all the support and I wish you all a very good 2018, in DIY and all other domains!

Regards//lasse

 

[PB2]

Great news, and have a happy and safe new year!
I always liked Citation equipment and they are nice looking gear!

 

[lasser]

And up until now, the amp has been running for abt 30 hours,,,,

and sounding great!!

Regards//lasse