Cub 12 new valves re-bias with only a multimeter

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Howzee
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Joined: Fri Oct 12, 2018 5:39 pm

Cub 12 new valves re-bias with only a multimeter

Postby Howzee » Tue Nov 13, 2018 11:08 am

Hi. I bought a new set of JJ el84’s for my Cub 12. Looking at the schematic it says -12.5v with a plate voltage of 300v from pin 7. When I checked the bias point between the resistors near the bias pot I got a reading of -7.7v and the plate voltage was 309v. When I adjusted the pot, I got to -10v, my plate voltage went up to 319v! Not down. If I was to go to -12.5v then the plate volts would go up way past 300v max. I don’t have a bias probe to check the mA. Is there a test resistor on the board to get that measurement? Or is it best to buy a bias probe. From the reading I’ve been doing it seems that the bias volts is not important. It’ll be what ever is need to get plate voltage to a max of 300v and to get a mA reading to get the plate dissipation to between 60-70% and tweek between that range to suit my taste/ear? Thanks in advance

TeraWales
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Posts: 1
Joined: Tue Apr 09, 2019 5:41 pm

Re: Cub 12 new valves re-bias with only a multimeter

Postby TeraWales » Tue Apr 09, 2019 5:46 pm

Hi...there are instructions how to bias my amp. I appears that there is a test point built into the amp and bias pot. Basically, black gator clipped to amp ground and Red on test point, measure and turn bias pot accordingly.
I am somewhat worried about doing this for some of the reasons you've outlined, mainly the death part. do you know of anyone in the Ventura or Los Angeles, CA areas who would be nice enough to show me how to do this safely, preferably in person?
I get the just of it, but don't want to rush into something that could potentially be dangerous, without some good background and electronic's basics info.

assembly and pcb

Howzee
MortalMortal
Posts: 9
Joined: Fri Oct 12, 2018 5:39 pm

Re: Cub 12 new valves re-bias with only a multimeter

Postby Howzee » Mon Jul 29, 2019 2:54 pm

TeraWales wrote:Hi...there are instructions how to bias my amp. I appears that there is a test point built into the amp and bias pot. Basically, black gator clipped to amp ground and Red on test point, measure and turn bias pot accordingly.
I am somewhat worried about doing this for some of the reasons you've outlined, mainly the death part. do you know of anyone in the Ventura or Los Angeles, CA areas who would be nice enough to show me how to do this safely, preferably in person?
I get the just of it, but don't want to rush into something that could potentially be dangerous, without some good background and electronic's basics info.
assembly and pcb


Hi I never found a test point in the amp. I did end up buying a plug in tube tester from eBay that had leads to connect to a multimeter. You need to take the grill off the back and pull the amp out. You then pull one of the tubes out push the bias tester in, and push the tube into the tester. You can then use a multimeter to bias your amp, using the bias pot.


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