Cary Audio CAD-280SA V12R

@jngrausr is yours the 100-120v or 220-240v version V12R?

My former V12R was the 120v version, and ended up finding out the manual was later updated and I moved from a 4v to 5 volt SLOW BLOW. Found out why though, there was a bad hexfred diode from a former upgrade, had to be found and replaced. There can be a cause if it keep blowing fuses.  1) is an EL34 output tube going bad, 2) is the power transformer and/or rectifier going bad?

BEFORE replacing the fuse and just trying again randomly, it's best to troubleshoot in steps.  

Try this procedure carefully:

1. Remove all tubes, insert a correct new fuse, and power up.

   • If the fuse does not blow, the power transformer and rectifier are likely fine.

   • If it does blow, the issue is in the power transformer or primary circuit.

2. Reinstall rectifier and driver tubes only — test again.

3. Add output tubes one at a time while monitoring fuse and bias current

Best NOT to power back up quickly after each test.Apparently if the main power caps are not discharged a little and you power it back up fast after a test, that alone  can blow a fuse. Take your time with each step above. The next owner who acquired my V12R had to replace the main (large) power transformer after playing the amp super hard for many hours on some large demanding electrostatic speakers.  Best to go through the checks, or find a tech to help if not sure.  Good Luck.  

Hello. Thank you so much for the guidance and encouragement. It is the U.S. 100-120v. I see that you have provided a great deal of info on this amp over the years. I will peruse. 

Will report back if I dont electrocute myself.

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.