Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Apprentice on July 17, 2017, 11:19:23 am

Title: Fixed to cathode bias switch
Post by: Apprentice on July 17, 2017, 11:19:23 am

Can anyone help? Ive built a Hoffman BJ and its great, so thought I would tweak a bit.
I can install a cathode bias mod no problem and revert back to fixed bias no problem. However I want to switch between the two. I've seen the london power switch but am concerned that on the BJ you effectively ground the bias supply when going over to cathode bias. Any comments?
I'm eventually going to build a voltage controller so probably don't need to switch back to fixed bias. But would be funky eh

Title: Re: Fixed to cathode bias switch
Post by: Tony Bones on July 17, 2017, 12:37:40 pm

The 'a' side of the switch either grounds the cathodes or the 220k grid leaks. The 'b' side disconnects the bias supply so that it doesn't get shorted to ground when cathode bias is selected..

Truth is, shorting 15V through 15k ohm will result in only 1mA of current from the shorted bias supply, which I doubt would even be noticed, but you might as well use a DPDT switch and disconnect it completely as shown by KOC.

Title: Re: Fixed to cathode bias switch
Post by: Apprentice on July 17, 2017, 01:59:52 pm

Thanks for that Tony. The BJ does not have a 15k resistor, probably as it has no tremolo (only a guess). Do you think I should add one?
Am I just getting paranoid?ive got 5.5 watts of dissipation with a 330 ohm cathode bias resistor but without having 2 switches that 330 will be in circuit when I go to fixed bias. A bit complicated but I've tried a number of permutations and can't resolve.

Title: Re: Fixed to cathode bias switch
Post by: sluckey on July 17, 2017, 02:46:50 pm

Look at page 3 of this document...

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

Title: Re: Fixed to cathode bias switch
Post by: Apprentice on July 17, 2017, 03:37:29 pm

Sluckey that info is terrific. I'm going to give it a go
Thanks

Title: Re: Fixed to cathode bias switch
Post by: Tony Bones on July 17, 2017, 07:10:43 pm

Well there you go. If Steve say do it, the do it. Might make a pop if you switch it live, but no damage will be done as long as you don't use a "center off" switch, a.k.a. on-off-on.

The 15k was just a rough estimate of what the top of R61 might be when the bias is adjusted where you want it. I was just trying to ballpark how much current would flow if the bias supply were shorted to ground... I should have explained myself more thoroughly.

Title: Re: Fixed to cathode bias switch
Post by: sluckey on July 17, 2017, 07:23:10 pm

It's entirely possible to just use a SPDT switch. But you must have a relatively high resistance in series with the bias supply. Your circuit would not work, but this works just fine. Take a look...

     http://sluckeyamps.com/november/november.pdf (http://sluckeyamps.com/november/november.pdf)

Title: Re: Fixed to cathode bias switch
Post by: PRR on July 17, 2017, 07:33:10 pm

Put 10K between R61 wiper and C42"-". Now you can short the bias at any position of R61.

Title: Re: Fixed to cathode bias switch
Post by: Apprentice on July 18, 2017, 01:20:47 am

Thanks all. I'm going to have to digest all of this a bit and have a play.
I'll let you know how I get on.

Title: Re: Fixed to cathode bias switch
Post by: Apprentice on July 18, 2017, 12:03:17 pm

Thanks to all who contributed on this post. I have now managed to fit a cathode/fixed bias switch. I will probably warm up the el84s a bit with a lower value cathode resistor (330 ohms now will try 220 ohms)
Built a voltage controller and got B+ down to 250v. Started to get some broken up sounds below that. I think it may be the preamp stage not liking the lower volts. Will have a go at splitting the supplies and controlling the power tubes only.
Thanks again to all, it's good to get a number of ideas.