Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: isaac_teller on June 12, 2019, 09:55:16 am

Title: UL mode
Post by: isaac_teller on June 12, 2019, 09:55:16 am

Hello. Is there a more sophisticated way to switch an amp from pentode mode to UL mode other than a regular toggle switch? Thank you.

Title: Re: UL mode
Post by: shooter on June 12, 2019, 10:21:19 am

Quote

more sophisticated way

absolutely, I can probably get a 10 part count circuit but we usually strive for KISS  :icon_biggrin:

use the switch for experimenting, you'll probably find what most have, it's either hardwired UL or Pentode, no real advantage in otherwords

Title: Re: UL mode
Post by: pdf64 on June 12, 2019, 10:25:48 am

Bear in mind there are massive back emf voltage spikes on the plates of overdriven power tubes; so depending on the HT voltage, maybe 2 to 4kV between the terminals of the poles of a dpdt ul-pentode switch. I don’t think it’s good practice to introduce switches / anything there, just use direct tidy wiring, use a switch in the design phase only preferably.

Title: Re: UL mode
Post by: sluckey on June 12, 2019, 10:42:02 am

Quote from: isaac_teller on June 12, 2019, 09:55:16 am

Hello. Is there a more sophisticated way to switch an amp from pentode mode to UL mode other than a regular toggle switch? Thank you.

Sure. Use an extra set of sockets. Wire one set for pentode and wire the other set for U/L. Swap tubes back and forth.

Or, you could wire two sets as above but put two sets of tubes in. Use a SPDT switch to connect the cathodes of only one set to ground.

I've never tried either of these Rube ideas, so proceed with caution.   :icon_biggrin:

Title: Re: UL mode
Post by: isaac_teller on June 12, 2019, 11:05:11 am

Thank you everyone!