"That Cap" - UK Vintage Radio Repair and Restoration Discussion Forum

kevinski · 30th Jan 2014, 7:03 pm

Hi all, I am right in thinking that "That Cap" applies only to valve amp players and not transistor players or am I incorrect in that assumption.

Kevin

G8HQP Dave · 30th Jan 2014, 7:22 pm

Only valves. To be more precise: only situations where a coupling cap comes from a high DC voltage and feeds a high DC resistance. This is unlikely to be found in a transistor amp, but I suppose an analogous situation could occur if an electrolytic went leaky - then the voltages and impedances are all much smaller but could perhaps be in the same ratios as for valves.

julie_m · 30th Jan 2014, 9:06 pm

It mostly applies only to valve equipment. Leaky capacitors will upset the biasing in transistor circuits, but this is rarely fatal. Remember valves are voltage-operated, depletion-mode devices, which means they default to full conduction with no bias; and also there is more than enough HT voltage, and the circuit impedances are high enough, for even a slight leak to cause problems.

kalee20 · 31st Jan 2014, 3:18 pm

In fact, leakage in "that cap" will put a positive voltage on the grid, relative to chassis - so it doesn't matter if the valve is an enhancement-mode or depletion-mode device. Either way, there's trouble.

30th Jan 2014, 7:03 pm	#1
kevinski Retired Dormant Member Join Date: Aug 2013 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 330	"That Cap" Hi all, I am right in thinking that "That Cap" applies only to valve amp players and not transistor players or am I incorrect in that assumption. Kevin

30th Jan 2014, 7:22 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: "That Cap" Only valves. To be more precise: only situations where a coupling cap comes from a high DC voltage and feeds a high DC resistance. This is unlikely to be found in a transistor amp, but I suppose an analogous situation could occur if an electrolytic went leaky - then the voltages and impedances are all much smaller but could perhaps be in the same ratios as for valves.

30th Jan 2014, 9:06 pm	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: "That Cap" It mostly applies only to valve equipment. Leaky capacitors will upset the biasing in transistor circuits, but this is rarely fatal. Remember valves are voltage-operated, depletion-mode devices, which means they default to full conduction with no bias; and also there is more than enough HT voltage, and the circuit impedances are high enough, for even a slight leak to cause problems. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

31st Jan 2014, 3:18 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: "That Cap" In fact, leakage in "that cap" will put a positive voltage on the grid, relative to chassis - so it doesn't matter if the valve is an enhancement-mode or depletion-mode device. Either way, there's trouble.