Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on February 20, 2018, 05:32:21 pm

Title: 1969 Soundtronics M-A200A bias Wiring
Post by: plexi50 on February 20, 2018, 05:32:21 pm

I have a 1969 100 watt M-A200A monster amplifier. I have built the normal fixed bias circuit. The PT has no bias tap that i can find. I found one wire but it only has +14.00 volts DC on it so that's out of the question. I have a 200K resistor tapped from the SS bridge rectifier to one leg of the PT high voltage lead. From there to the bias circuit using a 10K ohm trimmer pot. I can not get any voltage to pass through the 1N4007 diode / DC after the 200K tap resistor.  I remember years ago Steve taught me to use a ,047 or .1 capacitor in place of the resistor but that has no results either. What in the heck am i doing wrong here? The diagram below is what is how i have it wired minus the tube rectifier.

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: sluckey on February 20, 2018, 05:52:47 pm

As you know, what you have won't work with a bridge rectifier. Look at the bottom of page 5 of this pdf for the solution...

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

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: plexi50 on February 20, 2018, 06:00:21 pm

Yes thanks Steve. Been a long time since running into this. I cant upload any other pics because they are to large a file size.

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: plexi50 on February 20, 2018, 06:45:02 pm

Got it done and working Steve. Bottom left corner of page 5. Thankyou!

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: plexi50 on February 20, 2018, 06:59:04 pm

Want to post a pic of the work but my file is over 1050 kb. Did Doug change the file upload size?

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: PRR on February 20, 2018, 07:25:37 pm

No. Probably your camera got bigger.

Image size-How to reduce it (http://el34world.com/Forum/index.php?topic=464.0)

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: plexi50 on February 20, 2018, 09:14:25 pm

I ran into that problem using this new LG K20 plus camera that takes 13 megapixels so I quit using that for pictures but I went back to using my old camera which should be the one I used to use for all my amplifier picture uploads. I'll have to play with it and see.

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: plexi50 on February 20, 2018, 09:29:03 pm

I converted the 13 mega pixle camera pic's. I forgot i had to convert them with Free Image Converter. Dah!

Title: Re: 1969 Soundtronics M-A200A bias Wiring
Post by: sluckey on February 21, 2018, 07:36:53 pm

Good to see you still kickin'. Is this gonna be a big plexi?   :icon_biggrin: