2. tech-talk
  3. 1195683975

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
albertporter's avatar
Ag insider logo xs@2x
albertporter
4,949 posts
 
Unless I entered data into Google maps incorrectly, you are only 16 minutes from Audioport in Shawnee Mission, KS.

They did repair on a VTL 750 for me and the work was excellent. It may be they work only on equipment they sell, I don't know.

As close as they are you can call and ask.
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.
More to discover