Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: timmyjoe on January 01, 2022, 02:38:52 pm
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My goal is to have the resistors act as a dummy speaker load if I forget to plug in the speaker. I blew the filter caps in my other amp by leaving it on too long without a speaker load. Right now I've got it set up so the 8ohm connection go to the resistor and then to the common of the output transformer. The 4ohm does the same things. I'm leaving the 16ohm output un connected.
*Is it still correct to have the ground of the speaker jack making contact with the amp case? Or should I isolate it with a rubber or plastic grommet?
*Is it OK to have the 16ohm output not connected? Or does it go to ground when it is not connected?
Thanks guys for all your help with previous projects.
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Re-thought this and here's an update diagram of what I'm trying to do.
Here's the goals:
*Ability to chance impedance output
*Ability to have switch for negative feedback, normal, and capacitor removed modes
*Ability to foot switch the negative feedback
Special notes on this:
*The board on this is a standard 5E3 with only the negative feedback mod to the board.https://robrobinette.com/5e3_Modifications.htm
*The rotary switch is single shafted so that it changes the impedance and the dummy load at the same time.
Most of this is from Rob Robinette 5E3 page. The dummy load is my creation from what others have suggested.
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My goal is to have the resistors act as a dummy speaker load if I forget to plug in the speaker. I blew the filter caps in my other amp by leaving it on too long without a speaker load.
The only protection you need is to put a jumper between the sleeve and switch lugs of the speaker jack. It's highly unlikely that your filter caps failed due to no speaker load.
Re-thought this and here's an update diagram of what I'm trying to do.
You show the NFB switch connected to pin 9 of a tube socket labeled rectifier. Rob's NFB switch is connected to V2 pin 3.
Your FS jack must be isolated from chassis. Otherwise, the OT 8Ω secondary tap will be connected to chassis ground. Hoffman sells the insulated shoulder washers (https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=MiscHardware&ORDER_ID=!ORDERID!) used to isolate a switchcraft style jack. You need two shoulder washers.