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fixed bias Ampeg b-15

9/26/1998 6:13 PM
fixed bias Ampeg b-15
What resistor and cap should I play with to change bias....tubes are burning hot and are about 50 volts over output specs. Pin 3 of the 6l6 states v should be 450v it is  
the cathode current is 55ma should be 35 to 40ma
9/26/1998 8:17 PM

Rick:I have a few suggestions, and questions.Which B-15 is this, S-or-N,The reason I ask is, the S model uses 7027s and could have been changed to 6L6s,this model runs higher plate voltages. If it is the N model with 6L6s,you should first check the resistor to see if is, what it is supposed to be, and not went out of tolerence,I would just replace the cap, and use the same value.Are your tubes new?If not, it just might be the tubes are not holding up.According to the schematic you should have about -50 volts on pin 5. [and depending on the tubes] I would reccomend you put in a bias pot, to be able to dial in the bias,without changing resistors back and forth untill you get the right one.[lots easier]I think Reid is an ampeg fan,maybe he will put in a post for more info on what he would suggest.But the main thing is, be sure the amp don't have other problems first, before trying other things to fix it. I would also check or change the caps in the driver section,ampeg is bad about caps and resistors going bad,or way off spek. Good luck [Richie]
9/29/1998 5:39 PM
Reid Kneeland

>I think Reid is an ampeg fan,maybe he will put in a post for more info on what he would suggest  
Thanks for the vote of confidence, but I don't know too much about B15s. Docs advice sounds good.  
9/28/1998 9:48 AM

Does your amp have cathode bias or fixed bias? The earky version was cathode biased, later versions were fixed.  
May I suggest that, before you begin trying to set the power stage tube bias current, replace the coupling capacitors between the phase inverter plates and the power tube signal grids. Often these are leaky and will allow positive plate voltage from the phase inverter to "poison" your negative bias, causing heavy, uncontrolled conduction of the power tubes. Use Sprague orange drops, or any high grade film caps rated at 600vdc.  
Once you know that the power tubes are truly DC-isolated from the previous stage, you're ready to work on bias.  
If the version you have is cathode biased, I would replace the electrolytic filter cap in parallel with the 250 ohm cathode power resistor. This may be leaky, reducing the effectiveness of the resistor to limit current.  
If it's fixed bias, I would replace the 100uf/100v bias supply filter cap. Now check to see if you have appx.-50vdc at the negative end of the bias filter cap (junction of two 270k resistors?) Check voltages at each power tube socket pin 5, the signal grid. Voltage should be similar, maybe a volt lower than the raw supply.  
If all these voltages check out, and your tubes are still conducting more current than you'd prefer, it's time to modify the bias supply resistors.  

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