Tired Valve? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Techman · 8th Feb 2019, 1:40 pm

It's not so much the resistors as the capacitors. What the previous poster meant was that the coupling capacitors were so bad that they'd virtually turned into resistors. I'm not one for replacing original capacitors when there's no need to, but in an amplifier such as yours, with the symptoms that you describe, they'll need to be replaced before any further attempt is made to power it up. If you don't do so, then your new valves will be ruined in about 60 seconds flat and you'll be ordering yet another set! There'll be a couple of capacitors coupling to the grids of the output valves and also there's likely to be a couple more around the phase splitter that'll need changing before doing anything else.

Try to identify where the smoke came from, there's a chance that the smoothing section of the 'can' capacitor has started to leak and the associated resistor could be burning out, but this is just a suggestion and it may well not be faulty at all. It goes without saying that a general check of resistor values is always a good thing to do while you're at it.

PJL · 8th Feb 2019, 5:00 pm

New valves is the right decision. The one without any gettering has gone to air and that is probably the cause of the blue glow.

Have you checked continuity of the output transformer primaries?

When you have replaced the capacitors and get the new valves in, I suggest you check the bias on each valve by measuring the cathode voltage and calculating the combined screen+anode current. If it's drawing too much current you can increase the cathode resistor.

8th Feb 2019, 1:40 pm	#21
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Tired Valve? It's not so much the resistors as the capacitors. What the previous poster meant was that the coupling capacitors were so bad that they'd virtually turned into resistors. I'm not one for replacing original capacitors when there's no need to, but in an amplifier such as yours, with the symptoms that you describe, they'll need to be replaced before any further attempt is made to power it up. If you don't do so, then your new valves will be ruined in about 60 seconds flat and you'll be ordering yet another set! There'll be a couple of capacitors coupling to the grids of the output valves and also there's likely to be a couple more around the phase splitter that'll need changing before doing anything else. Try to identify where the smoke came from, there's a chance that the smoothing section of the 'can' capacitor has started to leak and the associated resistor could be burning out, but this is just a suggestion and it may well not be faulty at all. It goes without saying that a general check of resistor values is always a good thing to do while you're at it.

8th Feb 2019, 5:00 pm	#22
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Tired Valve? New valves is the right decision. The one without any gettering has gone to air and that is probably the cause of the blue glow. Have you checked continuity of the output transformer primaries? When you have replaced the capacitors and get the new valves in, I suggest you check the bias on each valve by measuring the cathode voltage and calculating the combined screen+anode current. If it's drawing too much current you can increase the cathode resistor.