Re: Ferrograph 4A: Restoration advice needed
Hi Uncle B,
Other more experienced members here will probably correct me but there are two reasons that come to mind why you may be seeing higher voltages in this valve circuit than in the manufacturer’s service data. The first is that you are probably measuring the circuit using a modern DMM with a very high input resistance whereas there is a good chance the figures quoted were as measured by an AVO with a typical resistance of 20k/volt.
Secondly, if you have replaced the HT smoothing and reservoir capacitors with new, their lower leakage currents take some load off the power supply leading to a higher HT voltage measured at the rectifier cathode and consequently higher voltages throughout the set - I noticed this in my recently refurbished Vortexion. It has not lead to any noticeable operating problems though.
Bill
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