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Biasing instructions for Kora Jupiter EL84 amplifier

Hi i'm trying to locate the biasing instructions for a French manufactured amplifier made by Kora Audio.
It utilises 12 EL84's in total cross the two channels but the method and amount of biasing is not obvious from the physical layout.

I was hoping someone on the forum might own or have experience of biasing this amplifier.
 
Hi i'm trying to locate the biasing instructions for a French manufactured amplifier made by Kora Audio.
It utilises 12 EL84's in total cross the two channels but the method and amount of biasing is not obvious from the physical layout.

I was hoping someone on the forum might own or have experience of biasing this amplifier.
Does'nt kora audio has some instructions for this ?
 
Unfortunately not as the original company is no longer in business.

Someone purchased the brand name and is producing amplifiers with the Kora name they have no knowledge or connection with the original products and company :(
Good to know for anyone that considers purchasing a kora amp, "no support", and a perfect
reason to never buy an amp without also obtaning schematics.


How difficult could it be to sift through the assets of a company you bought ???
 

Complin

Member
2007-01-30 4:47 pm
Derby
Good to know for anyone that considers purchasing a kora amp, "no support", and a perfect
reason to never buy an amp without also obtaning schematics.


How difficult could it be to sift through the assets of a company you bought ???

The new outfit just purchased the name thats all no other assets

Usually schematics can be reverse engineered if needed except those manufacturers who remove information from components or encase them in resin to hide whats there! Never buy a product like that.

Similarly try getting modern surface mounted electronics repaired! :mad:
 

6A3sUMMER

Member
2016-06-07 6:50 am
Sounds like 3 Push and 3 Pull EL84s per channel.

1. Self Bias
If it were my amplifier, I would try and find the room for individual cathode RC self bias networks (12 for stereo). Then get 12 fairly matched tubes (does not have to be super matched)
A little less output power than using fixed adjustable bias.
But when power mains voltage varies, the tubes will still have roughly balanced currents; and will be roughly balanced with aging too.
The output transformer works better with balanced current, especially so when there are Bass notes present, no saturation, and no intermodulation distortion Bass/mid or Bass to treble.

2. Fixed adjustable bias:
That would take less room than 12 bias pots, and 12 cathode sense resistors.
After you get all the "matched" tubes bias balanced, just watch what happens with time (and also as the power mains voltage varies).
More power out, but Lots of trouble.

Purchasing 6 exactly matched EL84s per channel is not what I would do.
But if there are not either 12 self bias networks, or 12 fixed bias adjustment pots, the tubes need to be exactly matched.

3. Or, I would sell the amplifier cheap, and look for another amplifier.

That is just my opinion.
 

Complin

Member
2007-01-30 4:47 pm
Derby
Sounds like 3 Push and 3 Pull EL84s per channel.

1. Self Bias
If it were my amplifier, I would try and find the room for individual cathode RC self bias networks (12 for stereo). Then get 12 fairly matched tubes (does not have to be super matched)
A little less output power than using fixed adjustable bias.
But when power mains voltage varies, the tubes will still have roughly balanced currents; and will be roughly balanced with aging too.
The output transformer works better with balanced current, especially so when there are Bass notes present, no saturation, and no intermodulation distortion Bass/mid or Bass to treble.

2. Fixed adjustable bias:
That would take less room than 12 bias pots, and 12 cathode sense resistors.
After you get all the "matched" tubes bias balanced, just watch what happens with time (and also as the power mains voltage varies).
More power out, but Lots of trouble.

Purchasing 6 exactly matched EL84s per channel is not what I would do.
But if there are not either 12 self bias networks, or 12 fixed bias adjustment pots, the tubes need to be exactly matched.

3. Or, I would sell the amplifier cheap, and look for another amplifier.

That is just my opinion.

Well its not clear how the biasing is performed as there seems to be just a single adjustable pot for each channel or from where exactly you measure the bias setting. Hence the question!

I was hopeing another owner or previous owner might be in a postition to help.
 
If I'm not mistaken these are fixed bias. Get us a pic of the inside to confirm, but the last I saw the insides of these there's a bias potentiometer between the sets of output tubes. I'm not able to find a schematic diagram unfortunately, but if you get us some good pics we can go from there. You might have to do some iterative voltage testing to figure out where they intend you to measure from.
 

6A3sUMMER

Member
2016-06-07 6:50 am
A single pot for each channel means you really need to very carefully match the output tubes; at least match all the ones in a channel, and all the ones in the other channel.

Lots of amplifiers do not provide for separate bias adjustment, and lots of times the tubes are not matched, then lots of times one tube runs away.

And when there is fixed bias, and the max grid resistor is 300k, when you parallel 3 tubes, the max grid resistor is 100k.

Just sayin'
 
Kora

Unfortunately not as the original company is no longer in business.

Someone purchased the brand name and is producing amplifiers with the Kora name they have no knowledge or connection with the original products and company :(

Hi there,

Bruno Vander ELst, Kora chairman is able to help you.

He fixes, regularly, old Kora stuff !!!

Don't hesitate to mail ;)

Regards from France

Rock