Grid coupler change?

[Mike Watterson]

All the advice I've been given is sound, I've taken it all on board and am very grateful, not quite sure what the 1st sentence in your post means.

I think it's just an observation of people in general. Also communication isn't always clear. I can see you have been following advice and the mod to the grid connections eliminates the PCB issues.

Certainly series heaters made a big difference to cost and weight of TVs, but I've always thought it a bit penny pinching on radios. I've a big Philco with part of dropper replaced by a halogen lamp to give the correct current/voltage. It's UL84 and very unreliable. I'm nearly temped to parallel wire heaters and put a transformer. I stripped and rebuilt the wavechange switch to get it going (LW, MW & VHF, I forget if SW too) but it's silent again, though valves lit
Scale was broken and I glued the bits together. Wasted too much time on it!

[Murphyv310]

All the advice I've been given is sound, I've taken it all on board and am very grateful, not quite sure what the 1st sentence in your post means.

Who are you referring to?
If its me then let me explain.
There are some that believe they are correct in their approach to a certain repair, in many cases they could well be right, there are cases though that the diagnoses is wrong and the alleged work is never going to work. I've tried to guide them in the correct direction and in a few cases by PM when I've felt what they are doing is dangerous or a total waste of time. There is a number here and on other forums like myself has a lifetime of experience, personally I was a Radio and TV engineer, working with Valve, solid state,
video, audio (DAT, Minidisc, CD, Guitar Amps, Synthesizers, Colour TV, you name it, with big companies including Sony and my own business, plus the hobby and Amateur Radio and fully qualified.
OK sorry for sharing my CV. Yes I make mistakes as I'm only human.

The other thing though is some don't fully read the thread and advice that is freely given is then ignored, so you can get frustrated.
A recent thread on UKVRRR, went to 13 pages, some of the advice was complete nonsense, I decided to not contribute until very late and I made one suggestion, the OP decided to send it to another member who found the fault and it was exactly what I had suggested and the thread went quiet. This causes frustration and sometimes I think "why bother"!
Hope you understand.

[Danndad]

Sorry you feel that way Trevor.
Perhaps it's time to close the thread.

[Mike Watterson]

Perhaps it's time to close the thread.

No, because I think you're making progress.

[joebog1]

Thanks Mike,
                      That little decision is totally opposite to UKVRRR. Having said that, I find both an extremely good source of information and advice.
If there is wrong, bad or dangerous advice given its usually very quickly corrected without offending anybodies feelings.
There is also the problem of people not reading the whole thread, as has been suggested. That is where many problems occur.

Joe

[Mike Watterson]

I'm only giving an opinion. I just post here!

[Murphyv310]

Hi.
Connect your DMM negative to the cathode and the positive to the Control grid G1 and see what it's voltage is, it should be negative.
Also if the cathode volts are say 13v and the resistor is a 220 ohm the total current will be 59ma, which is too high IMO, if the cathode resistor is 470 ohms with the same 13v it'll be 28ma, now much lower. So we need to know what the grid volts are in respect to the cathode. Ideally to measure the grid you need a high impedance meter for this most DMMs are pretty good TBH so we know the cathode is X and we measure the grid as Y. EG, X=+13, Y=+1 so Y-X= - 12v so in fact we have our correct negative bias on the grid. Looking over the thread the problem you have might not be too bad. Reading between the Grid and Chassis doesn't necessarily give you a reading at zero, tiny leakages can give readings. Hope this helps

[Murphyv310]

Hi.
In addition. We actually don't want the voltage between grid 1 and cathode to be "too" negative as this will eventually turn the valve off and when you feed in an audio signal only the positive half cycles of the waveform will be amplified, resulting in severe distortion. Class A amplifiers need sufficient bias so it won't clip with the full expected drive signal. There is also a bit of headroom added to the bias.
Cathode resistors often drop in value as they age unlike high values that go higher, its a good idea to replace the cathode resistor if its dropped even by a few percent as its enough to increase cathode current, shortening the valve life.

[Danndad]

I have actually put this back together and have been using it, but since Trevor has posted again I have taken the back cover off and can get in there with test hooks to do more measuring.
I am using the trader sheet 1395. The cathode resistor is a 470r which was replaced. The cathode voltage for the ul84 is 14.5v according to the trader sheet, using the dmm with the neg lead of meter to chassis I have a reading of +13.45v. on the cathode of the ul84.
Trevor has suggested that I connect my pos meter lead to the grid and the neg lead to the cathode of the ul84, instead of chassis , the resulting voltage is the same but negative -13.45v.
Forgive me for not understanding, but what does this test prove?

[Mike Watterson]

That cathode voltage seems OK.