Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Oh_noes117 on March 07, 2016, 06:10:37 pm

Title: Fender Princeton Reverb Negative Bias voltage HELP
Post by: Oh_noes117 on March 07, 2016, 06:10:37 pm: EDIT: Problem solved! Cold solder joint :laugh:

Hi everyone.
been lurking for a bit but made an account to get some help with my Homemade Fender Princeton Reverb.
When i try to bias my amp the Negative voltage on the Power tubes goes up to -235V, and the 10k pot only will change that value half a volt. This is done with the Power tubes removed and the HV secondarys unsoldered. (No Standby Switch) and with my meter on pin 5
My bias circuit is exactly like this one http://el34world.com/charts/images/biascircuit2.gif (http://el34world.com/charts/images/biascircuit2.gif) except the 27k is a 30k and the 100k is a 150k. Its just what i had on hand.
Ive already triple checked the wiring.
Heres the schematic Ive been using http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf (http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf)
Any help is appreciated!
Title: Re: Fender Princeton Reverb Negative Bias voltage HELP
Post by: octal on March 07, 2016, 06:40:46 pm: Is the centertap of the HV winding of the transformer grounded?
Title: Re: Fender Princeton Reverb Negative Bias voltage HELP
Post by: Oh_noes117 on March 07, 2016, 06:44:07 pm: Quote from: octal on March 07, 2016, 06:40:46 pm
Is the centertap of the HV winding of the transformer grounded?
Yes it is grounded with Star grounding on a Transformer lug
Title: Re: Fender Princeton Reverb Negative Bias voltage HELP
Post by: octal on March 07, 2016, 06:49:06 pm: Verify that the bias adjust pot and the resistor at the end of it are properly grounded and that the pot and the resistor measure OK. If they are open, or not grounded properly
it can work like a charge pump circuit and build up extreme high voltage.
Title: Re: Fender Princeton Reverb Negative Bias voltage HELP
Post by: Oh_noes117 on March 07, 2016, 07:02:44 pm: Quote from: octal on March 07, 2016, 06:49:06 pm
Verify that the bias adjust pot and the resistor at the end of it are properly grounded and that the pot and the resistor measure OK. If they are open, or not grounded properly
it can work like a charge pump circuit and build up extreme high voltage.
Grounds are good. 30k reads about 28k and pot is perfect from 0 ohms to 10k. Saw a cold solder joint on the resistor to the pot so I fixed that and im going to try it again hold tight.
Title: Re: Fender Princeton Reverb Negative Bias voltage HELP
Post by: Oh_noes117 on March 07, 2016, 07:06:52 pm: IT WORKS!
I think there was a bad connection between the pot and the resistor to ground. Now im getting -33 to -25 volts on each tube. Thanks for the help octal!!!
Title: Re: Fender Princeton Reverb Negative Bias voltage HELP
Post by: eleventeen on March 07, 2016, 10:09:28 pm: Congrats! You may need a few more negative volts. Might need to adjust a resistor value or two in the bias rectifier chain but that's no big deal.