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Jolida JD1000A 1 tube will not Bias

I have a jolida JD1000A tube amp that i bought a few years ago, and replaced all the tubes and found that when i was biasing them all one tube will not bias, reads only .001V anywhere you adjust the pot.

I swapped tubes around thinking it was the tube, but it does the same with a different tube, I pulled the amp board out to inspect solder points, traces and ohmed out some resistors and the biasing pot and all looked good.

Posted this problem several months back and had someone recommend checking the tube socket, i bent all the contacts in a bit so they would get a tight fit, and it still fails to bias.

I noticed that when you turn the bias pot clockwise or "Bias down" on that bad tube sector the amp puts out a mild humming out of the speaker that is hooked to that side and when you turn the pot CCW the humming goes away at about the 10 oclock position (fully CW would be about 5 Oclock and fully CCW would be about 7 oclock) all the while the volt meter still does not read over about 1mV on its test point.

also, just to make sure that im on the right Biasing pot and Biasing test point, all the other 7 power tubes are biased and are reading the correct voltage around 30 mV DC i think it was.

Any ideas? i havent found any local tube audio repair shops in the Kansas City area.
archangelluke
dwr
113 posts
 
I had exactly the same problem in a JD1000A that I owned a few years back. Turned out to be a bad resistor in the biasing circuit. A fairly inexpensive repair.
archangelluke OP
14 posts
 
RE: Audioport

yes i called them yesterday and the guy i talked to stated that they do not repare tube amps, i even tryed sound FX and no go with them too, thanks for looking tho, i do have a phone number that a rep at funky munkey gave me, for a company called QTS somewhere in the 816 area code,

There used to be a QTS (Braught.com) in eudora KS that was really good at fixing tube equipment, but they have gone out of buisness. thanks for looking tho.

RE: Dwr

Excellent! thank you, i will check the transistor, is it near the tube socket? i have a scematic of the amp, will trace it out and test it to see and check back.
archangelluke OP
14 posts
 
Ok, i did more testing, there are in fact no Transistors in this amp, however there are 4 diodes that look similar to transistors,the 400v diodes/rectifyers and they are putting out the needed 400 Vdc, pulled out the scematic and traced out the whole biasing paths on the bad tube and checked all the components and even did voltage tests and comparisons to other tube biasing paths and found everything to be good, i even tested the bad tubes biasing potentiometer powered on and it is putting good voltage that varys with the potentiometer movement to the tube,checked the voltage after the capacitor that is in the biasing path its good,ohmed out the tube base and checked for continuity on all contacts, partially placed the tube in the socket and checked for continuity between the base and the tube itself, i swapped tubes again to double check, same problem, not even reading .001 Vdc at the bad tubes biasing test point, checked the biasing on the other 7 tubes, there still fine.

wow, this has to be something strange with the tube socket, i may swap 2 sockets out to see if the problem moves!

It would be virtually impossible for it to be anything other then the tube socket! i checked everything that has to do with the bad tubes biasing circuit.

will keep posted.
archangelluke OP
14 posts
 
Ok found the problem, R20, a 100 Ohm 5 watt resistor, outside of the biasing circuit,schematic was incorrect on the resistor numbers.

thank's guy's, will try and order this resistor and see if this takes care of it.
dwr
113 posts
 
I think if you talked to the people at Jolida they could tell you exactly which resistor to check. I don't want to give you any wrong info on that, as it was a few years ago, I did not do the repair myself, and I can't remember exactly which resistor it was.
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