Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Captain chunkulus on August 28, 2017, 10:05:44 am

Title: Bias Question.
Post by: Captain chunkulus on August 28, 2017, 10:05:44 am

Okay, I have a bias related issue that I just can't figure out. Okay, so my power transformer doesn't have a center tap on the secondary 350v tap. So I am taking my bias circuit off of  one of the power lines on the secondary just like on some marshall circuits. After the reverse polarity 1n4007 diode, I get like - .18VDC. As in not even 1 volt. Weird right? So the ac voltage hitting that diode is around 185vac. No matter what Ive tried I cannot get any bias voltage from the ht. I'm completely stumped. Nothing is grounded out anywhere. I even tried just the power wire(185vac) to the reverse polarity diode with the other end floating(as in tied to nothing) and get no negative voltage dc at all. I have tried other brand new 1n4007 diodes and still the same result. I get 185vac into the diode and no ac or dc voltage out no matter what I do. Have any of you guys ever experienced this? Any help would make my day.. LOL. I am including a schematic of the amp style I derived my bias circuit from. Thanks, Joel.

Title: Re: Bias Question.
Post by: sluckey on August 28, 2017, 10:18:58 am

That bias circuit will not work if your PT has no centertap connected to ground. You probably have a full wave bridge rectifier. Build the circuit at the bottom left side of page 5 in this pdf. The capacitor between the PT and the diode is absolutely necessary.

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf)

Title: Re: Bias Question.
Post by: Captain chunkulus on August 28, 2017, 10:35:38 am

Cool, thanks so much! I'll give it a try and let you know what happens..J

Title: Re: Bias Question.
Post by: Captain chunkulus on September 04, 2017, 12:36:47 pm

Okay, so I tried it and it works! I however have another question if you are willing. So the amp I built passes signal but I had made an earlier mistake in the phase inverter section. I had used a 470k resister instead of a 470ohm on the cathode of the pi. Obviously, my output volume was low so, I changed it to the right value. No the amp goes into a low uncontrollable woofy style feedback. The only way I can get it to stop is by replacing the 470 ohm with a much higher resister  value but that brings the over all volume way down. Any of you guys ever experienced this? I am including a picture of the pi I used.

Title: Re: Bias Question.
Post by: sluckey on September 04, 2017, 12:47:18 pm

Disconnect the negative feedback wire from the speaker jack. Does that cure the woofy? If so, switch the OT primary plate wires.

Title: Re: Bias Question.
Post by: Captain chunkulus on September 04, 2017, 01:24:24 pm

Disconnecting the NB wire got rid of the woof. So I should swap the two wires  from the ot tranny and reconnect the - feedback ? Thanks again. Sorry, auto correct..

Title: Re: Bias Question.
Post by: sluckey on September 04, 2017, 01:30:41 pm

Quote

So I should swap the two wires com mk ingredients from the ot tranny

I don't understand that sentence??? But after you swap the OT plate leads reconnect the feedback wire.

Title: Re: Bias Question.
Post by: Captain chunkulus on September 04, 2017, 01:40:47 pm

Sorry about that, that was autocorrect. I did just like you said and it works great thanks again. J