• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Need help with amp

After 4 years I decided to use again my Copland CTA-401.
It started working good, but after an hour the Main fuse(3,15A) burnt. I replaced it, but it burnt again, immediately.
I decided to control, looking at all parts of the amplifier.
This is the schematic: http://www.audiokit.it/ITAENG/UpElettr/COPLCTA401/Sch401-1-1.jpg

R34 seems to be "cooked". the other 3 resistances of same purpose(R33, R133, R134, in the other 3 tubes) have the red band visible. The red band of R34 became "darky", and also silver and gold bands are darker. I decide to measure: R34 indicates 819 ohm. the other 3(R33, R133 and R134) measure 812ohm.

What happened? Do you think I can simply change that resistance and try again, or is better to control every part? Maybe there are problems with the tube or with the output transformer? :eek:

Thanks to everyone can help me!
Joined 2003
Without component values, a diagram isn't a huge help. Nevertheless, I wouldn't expect an overloaded screen grid resistor to be part of the chain of events that blows a 3.15A mains fuse. As a starter, I might try removing the EL34 connected to R34 and seeing if that allows the 3.15A fuse to survive.

I'd expect the fault to be in the power supply. Perhaps the mains transformer, or the bridge rectifier/reservoir capacitor.
To expand on what EC8010 has said - appologies if "preaching to the converted" and stating the obvious. Debug procedure (also a set back to work after repair procedure).

1/ Pull all the EL34 tubes keeping track of V6 - the one with the stressed screen resistor, actually best to keep track of which socket each of them came from
2/ replace fuse F1 with 3.15A and check F2 the High Voltage Fuse (it should be about 500mA).
3/ Turn on power - If the 3.15 Amp fuse blows straight away then check the following:
Diodes D5 to D8 for short circuits
Capacitors C21 and C22 for short circuits

If fuse stays intact then:
4/ check the bias supply at each EL34 tube socket by measuring from Grid 1 (pin 5) to Cathode (pin 8) these should read about minus 40 =/- 5 volts - If not then check:
D9, D10 for short or open circuits
C19 for short circuit.
Also check C10 and C11 for short circuits/ high leakage

If Bias Suppies OK
5/ Check High Voltage at each EL34 tube socket by measuring from Anode (pin 3) to cathode (pin 8). This will be 40 or 50 volts higher than the 430V figure on the schematic because your not drawing output tube current yet.

If all that is OK the turn off - plug in the EL34s to the original sockets.

Apply power - if fuse blows means a stuffed EL34 probably V6
If fuse OK measure cathode voltage to ground for each EL34 that is voltage across each of R31, R32, R131, R132
These should all be 0.48V
If voltage across both R31 and R32 are too high or low adjust with P3
If they are out of balance adjust with P1 (or P2 for other channel)
HINT: to set balance its not actually necessary to use any test equipment. Put your ear agains the speaker and adjust for minimum hum.

At this stage you should have glorious music

Thank you very much for help.

I pulled all the El34s, but fuse blows again.
I checked for all Diodes and capacitors(with tester in ohm position). No shorts seem to be there.
Maybe there is a short in signal tubes...(I finished my fuses and I need to buy a box of them before I can try if fuse blows also without signal tubes in sockets).
I think that if fuse will blows also without signal tubes, the CtA401 probably has a problem in the "torry" transformer...
...or there are also other checks to do before testing/changing "torry"?

Thanks again for help. ;)
Joined 2003
You need a very good quality short-circuit to blow a 3.15A fuse, so it's unlikely to be in the signal valves. I'm afraid I would suspect a dead mains transformer, or, if you're lucky, some chafed mains wiring shorting down to the chassis. Have a good look inside (with the amplifier unplugged from the mains!).
I disconnected all sec of tranformer from the rest of ampli, to test the transformer alone. (In this situation the 220VAC go through main switch, main fuse, C43 and C44 and "pri" of transformer. "Sec" are without any kind of load).
The fuse blow immediately, with a great "flash".

I think I need a new "torry" transformer... :(
...hoping that it burned for self internal problems, and not for other problems concerning the amp circuit.
BTW I think it is a good idea to put a new resistance for V6, and new tubes. I can't risk to burn also the new "torry" I'll buy.

Thanks again for the help.
I made some translations(colours of torry wiring are indicated in swedish...)
2 "pri" 0V-110V in series.
1 "sec" 0V-330V - 0,5A
1 "sec" 0V-36V - 0,3A
1 "sec" 0V-6,05V - 8A
1 "sec" 0V-6,85V - 2A
(there are also 2 unused "sec" 0V-10V. Bah...)

Maybe the 6,85V - 2A is not enogh, since the 5 signal-tubes are 2xecc83(for phono), 1xECC83 and 2xECC82.
So 0.3A * 5 = 1,5A, and using special or mil-spec tubes(like 5814 for example) the If is 0.35A. 0.35 * 5 = 1,75A...
In my amp there are 2 x 5814 and 3 X C492. total = 1,6A

Btw I'll try to order a new torry(not so simple to get soon, because of holidays...). I'll change the sec 0V-6,85V from 2A to 3A.

With all tubes and F2 and F3 removed there is virtually nothing connected to the secondary windings.
So if F1 still blows or the transformer gets warm after some time you probably have shorted windings.
There is nothing to protect the primary insulation in case of pulling the power cord or other attacks from the 230V..
Its fairly unusual for the transformer to fail. I would be doing a very careful check of all of the mains wiring, including the switch and fuse holder first. With the switch in "ON" position and fuse installed but power cord unplugged from the wall socket check for shorts between active and ground and neutral and ground. If you find such shorts, disconnect the transformer and repeat the test. If you can't find any shorts (or at least low resistance paths to ground) then you will have to conclude it is probably the transformer.
Ok, it was the torry.
So I decided to change it with a conventional transformer.
I went to my friends of Satel ( http://www.satelsnc.it/ they make also nice output transformers ;) ) and they made for me a grain oriented transformer, with additional external copper screen, in a day.
A nice work. We measured togeter the sections of every wire in the old torry and every voltages of new tranformer are the same of old torry with great precision. And also I saw every phase of transformer construction.
Now the Copland work very fine, and it seems to have also better sound performance than ever.
Here is the photo of copland with new transformer:

An externally hosted image should be here but it was not working when we last tested it.

Thanks to all, and also thanks to Satel.