VT136 (1625) DC Testing - UK Vintage Radio Repair and Restoration Discussion Forum

David Simpson · 26th Apr 2015, 12:34 pm

Folks may recall my recent quest for a UX7 Valve Holder, to enable me to mate it to a B7 Valvebase. Thus, with the fitment of wee ferrite beads to this adaptor's internal connections, parasitic oscillations could be avoided when DC testing fearsome VT136's.
However, this new adaptor worked fine up to 60mA using a normal AVO top-cap connector lead, but exibited evidence of slight parasitics(approx 74MHz) with higher Ia.Two large ferrites were placed over the Heater supply lead, and the HT supply lead, which cleared the problem up to 80mA. So then adapting my homemade top-cap connector lead(red)with a toroidal ferrite choke cleared the problem even when testing to 110mA Ia.
Please dont ask me the characteristics(u,Permeability, Z, etc) of the choke.
Many thanks to Mark James for donating the small UX7 ceramic valve holder.
I hope this is of some interest to those adventurous guys who are building homebrew valve testers.

Regards, David

26th Apr 2015, 12:34 pm	#1
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,857	VT136 (1625) DC Testing Folks may recall my recent quest for a UX7 Valve Holder, to enable me to mate it to a B7 Valvebase. Thus, with the fitment of wee ferrite beads to this adaptor's internal connections, parasitic oscillations could be avoided when DC testing fearsome VT136's. However, this new adaptor worked fine up to 60mA using a normal AVO top-cap connector lead, but exibited evidence of slight parasitics(approx 74MHz) with higher Ia.Two large ferrites were placed over the Heater supply lead, and the HT supply lead, which cleared the problem up to 80mA. So then adapting my homemade top-cap connector lead(red)with a toroidal ferrite choke cleared the problem even when testing to 110mA Ia. Please dont ask me the characteristics(u,Permeability, Z, etc) of the choke. Many thanks to Mark James for donating the small UX7 ceramic valve holder. I hope this is of some interest to those adventurous guys who are building homebrew valve testers. Regards, David Attached Thumbnails