Blowing Fuses in a Tube Amp

Keith...,

I suspect that the problem is not in tubes.
My friend has QuickSilver M60 mono amps that use EL34 chinese tubes. He did not replace them for more than 1.5 years and they still read to be in good shape. I already change tubes in my VTL MB100 and the following change is upcomming to be. So even if you get yourself some Mullard quads they might last longer and with less problem BUT they will certainly blow faster than they have to serve.

Amp Biasing:

If you have on your amplifier a dedicated potentiometers that are located next to each tube than you can adjust the bias. This is very neccessary part on tube power amplifiers as tubes do wear out and need to be bias-adjusted approximately every 2...300hours. If such pots do not exist you should check the grid voltage(pin#5)with no tubes on your amplifier for each and it has to be the same(can't tell the value but probably between 35...60V)
OVERBIASING means excessive quiescent current on the cathode circuit. The bias current can be measured off the connected 10Ohm resistor by measuring DC voltage and than deviding by resistance i.e. 10Ohm. Normal bias current(usually 25...40mA) must be defined in the specification of your amplifier or you might ask the manufacturer about it.
If overbiasing occurs than the tube is getting over-heat and blows. Biasing is also neccessary when new tubes are placed.

The reason of unstable or jumping bias current in addition I can say can be a shortage between the plate and cathode that further on can be cause by malfunctioning coupling capacitor.

If you're onto DIY and able to safely perform on high-voltage circuits than I can probably help you to step through the amp circuitry -- just punch me some note.

If you do not possess this knowlege, you should bring your amplifiers to tech for the sake of YOUR own safety -- f_ck the amp since 500V can fry you real quick.

I do suspect that the built quality on your amplifier is desired to be much more better, but probably if there are existing upgrades from the manufacturer, they might help you to eliminate these current problems.

Hang arround some DIY forums and sites such as www.audioasylum.com where you can grab much more technical information on your amp and even about possible upgrades and the ones who do them.

Wow thank you all. My amp is a Jolida 302a with 2 matched pairs (4) of 6CA7/EL34 power output; 2 pcs. 12AX7 pre-amplifier; 2 pcs. 12AT7/ECC81 power drivers. I have not replaced the EL34's they are still stock Chinese. The smaller tubes I did replace. With Phillips and military grade mullards. I would very much appreciate comments on testing and biasing. I did the biasing as per the specs when I set-up my unit. What would 'overbiasing' mean?

I also have made sure that the fuse type is exactly the same as speced by Jolida.

I've blown 3-4 tubes in the last 4-5 months.

Thanks again

there are lots of variables on that issue to check but this value is finite.
1.the unstable bias supply can be due to the rectifying tube or ss diode or bridge; replace with higher quality rectifying diode(a-must) or ss-diode or the whole bridge.
2.malfunctioning coupling capacitor that needs to be replaced. try to find the better model than manufacturer-installed and do same for the functioning channel as well.
3.malfunctioning trim pot if such exists. simply that one needs to be replaced. plese note that potentiometers tend to wear-out and to prevent it vacume your amps with tubes unplugged approximately twice per week. meanwhile you can also clean the tube sockets(not so often but...) and tube pins.
4.malfunctioning self-bias supply that has a complicated circuit and if no or slight experience dealing with different models of power supply i'd recommend this amp to be serviced by tech.

Enzo,
In your cituation the biggest problem is not tubes but just poor design of a power supply and bias supply of most CJ amps believe it or not. Sound has to be = to the built quality.

Gs...,
Even if you test the tube going bad you still have to check the rest of routine I mentioned above since the blown tube is only a result of malfunctioned electronics. xfmr winding must be probably checked last but completely agree with plate resistor and coupling cap.

You also could have an output tube with an internal short. If you have a tube tester, you can find out right away, or you can simply replace the tube.

Or, you may have the wrong fuse type. If your amp requires a time delay (slow blow) fuse and you have an instantaneous peak type installed, that could do it also.

There are a lot of other unlikely possibilities like a bad capacitor, plate resistor or xfmr winding.

I have a CJ MV-50. It was blowing fuses, I think it was the Svetlana EL34's. I replaced them with old Mullards that BARELY tested good and have not had a problem. So I think maybe you should try different tubes. The Mullards also sound better.

Does the amp have a tube rectifier ? If it does, it is a good place to look.

bias can be the main problem. what tubes your amp is using?
i can tell you what tube pin you should check for grid voltage in order to have a correct bias before you place the new tubes there. also i can direct or perform an installation of the bias measurement circuit almost on any tube amp with potentiometers for a neccessary adjustement when needed.

Can you adjust the bias? Mine was blowing fuses constantly, took it for repair, found the tubes were overbiased.