Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 67polara on July 16, 2011, 11:50:56 pm

Title: 50 / 100 watt switch
Post by: 67polara on July 16, 2011, 11:50:56 pm

How is this accomplished?  When doing this by removing grounds to cathode the impedance will change, however I read somewhere a while back about grounding  grids to get half power with out the impedance problem.  Anyone have a circuit for that?  How does it work and is it effective?

Tony

Title: Re: 50 / 100 watt switch
Post by: LooseChange on July 17, 2011, 07:56:43 am

I have done that. Lifted the cathodes of two of the four power tubes. Works great. Stop worrying about small impedance mismatches.

Title: Re: 50 / 100 watt switch
Post by: kagliostro on July 17, 2011, 10:26:48 am

If I'm not wrong Geezer had some positive experiment with grounding grids of final tubes

other way is to put a potentiometer between cathode and ground in the PI tube

(I had a schematic but I'm not able to find it anymore, someone has it ???)

one other interesting way (on cathode biased amps) to lower max power is here

http://www.el34world.com/Forum/index.php?topic=4401.50 (http://www.el34world.com/Forum/index.php?topic=4401.50)

Kagliostro

Title: Re: 50 / 100 watt switch
Post by: eleventeen on July 17, 2011, 03:06:28 pm

I had (and still have) a very early BF Pro Reverb #138 that was "twined" in the mid to late 70's. I bought it that way. I installed a cathode-lifter switch to pull 2 of the 6L6's out of the circuit, and ran it with only two 6L6's in it for at least 15 years. Nothing to worry about.

Title: Re: 50 / 100 watt switch
Post by: 67polara on July 17, 2011, 08:15:39 pm

Thank you for your answers.  Could someone show me how to wire the switch for the grid grounding one.  I think I would like to try that one instead of the cathode lift.  Do we need to sever the connection and ground the pin or do we just ground the pin?  Is a cap necessary to keep it from popping?

Tony

Title: Re: 50 / 100 watt switch
Post by: kagliostro on July 18, 2011, 01:17:36 pm

Quote

Is a cap necessary to keep it from popping?

I think a cap will help on popping more

don't think shorting grids to ground will have a pop effect

hope someone that has do that comes here and explain we better

Kagliostro