Cary Audio CAD-280SA V12R

To @jngrausr the V12R is a unique amplifier and I regret selling mine.  A really nice gentlemen bought it from me, and he tried with this his friends' speakers too. A few others on this forum have explored running the amp with fewer tubes as well, removing all the tubes except a few pairs on each side. 

Fuses, and Biasing:

My local 50yr in business tech repair friend helped me, and we had the former amplifier engineer at Cary Audio on calls together a few times to address fuse-blow issues once, and biasing.  Two things came about.  

Bias, per side:

Bias at 260-280ma per side [as stated in the old non-updated manual] was deemed as being "too high, and unnecessarily burning up the EL34 tubes prematurely." 

Fix: back the bias down to a low of 200ma to a high of 230ma per side. "no need to go higher" according to my local tech friend.  Sounds good in this range, saves your EL34 tubes. 

Fuses:

The Cary tech at that time had restored a few V12Rs that came in on trade, and stated if the EL34 tubes are all good, the power transformers are good, then running a 5am slow blow fuse was okay, and the original manual use to show 4amp IIRC. 

Procedure:

If you can try the procedure above, and rule out the transformers are okay, and don't blow the as a result of transformer deterioration, next is to test the tubes. That amplifier made me come close to buying my own tube tester rig and instead I'd rely on my local tech to test tubes for me. The entire time I owned my V12R, I ran the same EL34 tubes over several years.  Maybe reach out to a local shop/tech, and have all your EL34 tubes and little input tubes tested first, if you are not sure. 

Good Luck.  

Tubes are fine. Left channel is fine and biases easily.

Right channel pushed >500mA all down the line and cooks the EL34s. The fuse blows when I have 3-4 tubes in the chain. I did not smell or see anythibg on that side. May the bias circuit has a malfunction? Any clue?

I had one of the amps, (actually, I still have it in a box in the basement).

It was probably the sweetest sounding amp I have ever had.

I thought maybe the best thing about it was that you could run it in UL mode, or Triode mode, OR WONDERFULLY in two different hybrid modes.

For each channel there are three switches to choose between between UL or Triode mode, but they all don’t have to be set to the same mode (and you can switch them on the fly with the amp on). 

So for instance you can put the front and back switch on UL and the middle switch on Triode.  This configuration gives you 83 watts per channel. 

Or, you can put the front and back switch on Triode and the middle switch to UL.  This configuration gives you 67 watts per channel. 
 

It depended on the music which of these two configurations I would choose. 2/3 UL for more drive and dynamics, or 2/3 Triode for more warmth and smoothness.
 (I never ran it in all UL—too clean and sterile.  I never ran it in all Triode—too soggy and bloated.) 

I loved it. 

All fixed up. I ran this all winter and really enjoyec listening to the big, round sound that never fatigues. I am sure the Cary SLP-98 preamp is a a major factor. Put on the shelf for now. Replaced with a little synthesis ensemble integrated. Like swapping a Duesenberg out for a Maserati.

"@jngrausr Right channel pushed >500mA all down the line and cooks the EL34s. The fuse blows when I have 3-4 tubes in the chain. I did not smell or see anythibg on that side. May the bias circuit has a malfunction? Any clue?"

Looking at some different notes on this, one item called out that it could be. Many of us who owned these amps proactively replaced some of the more common failing parts due to heat and age. Here is one thing to to look at or have a tech check for. 

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"Coupling Capacitor Leakage (very common)", 

• Each output tube grid is fed through a coupling capacitor from the driver stage.

If this capacitor leaks DC voltage, it pushes the grid positive and the tube conducts heavily.  Symptoms are:

• Bias cannot be lowered enough

• Tubes run hot

• One channel only

This is a classic failure mode in older Cary amps if caps are aging.

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