Re: Hoffman Fender Blues Junior Conversion 6v6

[gpilcher2001]

Hello my name is greg. Im new to this forum. Ive built a few ampsbir this blues jr. Is my first scratch build. I know enough to get in trouble whichbis where im at with this build.

With that being said i've completed the hoffman blues jr build and i decided to use 6v6's. Im using the allen tp24 pt and the info above is correct, im seeing 28v on pin 5 with 8v on the ground side of the 1ohm resistor 6v6's installed. I've located sluckeys jcm 900 layout for the bias circuit  and have a couple questions. Im asumming 1/2 watt resistors are more than capable in this circuit? Is the .047uf capacitor an electrlytic (if so source?)?, and im also assuming the voltage rating on the capacitors are 100v? Lastly i would tap the the ac winding on the tp24? Red wires before the rectifier?

Thanks in advance for any help with this!

[2deaf]

Do you have the spec's for the TP24 transformer?

[Latole]

0.047 is not a 'lytic capacitor, it is signal / ordinary capacitor rated 400 to 600 volts.

[ac427v]

The transformer specs are key because Hoffman's conversion used the 20v bias winding on the PT. He did not connect to the bridge for bias voltage as shown on the Marshall drawing. Did not use a classX .047 cap either. David Allen no longer lists the TP24 or specs on his site. Can you find out more on your PT from some other source?

[gpilcher2001]

This transformer is not listed on allens site anymore. Good thing the wayback machine exists! I've attached the tp24 description. Pretty much same as the fender stock with a little higher voltage.

[sluckey]

With that being said i've completed the hoffman blues jr build and i decided to use 6v6's. Im using the allen tp24 pt and the info above is correct, im seeing 28v on pin 5 with 8v on the ground side of the 1ohm resistor 6v6's installed. I've located sluckeys jcm 900 layout for the bias circuit  and have a couple questions. Im asumming 1/2 watt resistors are more than capable in this circuit? Is the .047uf capacitor an electrlytic (if so source?)?, and im also assuming the voltage rating on the capacitors are 100v? Lastly i would tap the the ac winding on the tp24? Red wires before the rectifier?

As you have discovered, the TP24 20v bias winding and Hoffman's bias circuit does not produce enough bias voltage for 6V6s. The JCM 900 bias circuit will work. All resistors are 1/2w. The .047µF is a special "class X" cap. I would not use a regular coupling cap because of reliability issues. It will work but may be prone to failure. You may have to go to a supplier such as Mouser to find the Class X cap. 100v filter caps should be fine. Connect to one TP24 red wire before the bridge rectifier (just as shown in your pic).

[gpilcher2001]

Thank you much sluckey!

[tubenit]

You could simply change to cathode biased instead of using fixed biased.  Ground the 220k grid resistors and use a 250 ohm/10watt cathode resistor and a 47uf/50v cathode cap

Then you wouldn't have to rebias in the future when changing tubes.

With respect, Tubenit