Did some more research and now I'm sure the problem is with V5.
With any tube in V5 and no tube in V4, I get no sound.

Seeing as how the bias current gets to the tube through that 1 ohm resistor and its open, the current's not getting there, even though I get a reading.

So, I'm wondering - can I bypass that 1 ohm resistor and see if that fixes the problem ?
V5 should then get sound even if V4 is pulled out ?

What I don't understand is - if that resistor is creating an open circuit, where are the mV on pins 1 & 8 coming from ?
Why don't pins 1 & 8 read zero mV ?

THanks Gregg,
I (sort of) understand what the 1 ohm resistor is for - converting mA to mV so you can easily measure the bias.
What I'm wondering is if I can bypass that resistor with a piece of straight wire to see if that gets V5 working again.
I know I can't measure that side in that case.

I want to see if the problem is just the resistor, or there's something else going on upstream from there.

Pins 8 & 1 are connected together and are getting voltage - the same reading as at the bias lug.  around 35 mV.
And it changes as I adjust the bias pot. I don't understand how that's possible if the resistor is not passing any current.

Or is it just that pins 1 & 8 are not grounded ? Could that be why V5 is not firing up ?

I'm getting 469 V on pin 3. Master Volume is turned off.

Unplugging the C-lator with the amp on is ok if the volume is low. Its never been a problem - I don't think connecting/disconnecting it can harm the amp.  Maybe blow a speaker if the volume is up.

Yup, you can run the cathode straight to ground, just can't test the bias on that one without some different equipment, but you can bypass it.

Gregg

Bypassed the 1 ohm resistor and all is well.  Going to get a replacement now.
Thanks Gregg.  You saved me a small bundle.
Cheers,
Jon

Hopefully, it won't happen again.  Its unusual for a resistor to just quit with no sign of burn, isn't it ?

Anyway, another question - really for Pickmaster, but since we're already talking and you may know the answer....

While at the electronics store I bought some extra 100K and 68K resistors to try Pickmaster's slope resistor mod.
I couldn't find any brown ones like the one that's in there, so I just got plain carbon fibre, 2 Watters.
I believe the slope resistor is 3W.  I don't know if the brown ones are carbon fibre or what.

Can I put that 2 Watt resistor in there ?

I wonder if something was wonky from day 1.   After replacing the 1 ohm resistor and putting back the pair of JJ's I've been using for the last year, the bias measurement was a lot closer from tube to tube.

Anyway, I did the slope resistor mod - with a temporary internal switch from 100K to 68K.  Been playing with the 68K and loving it!  Still have to see how it sounds at gig level, but sounding rather stellar at home.
I'm also running the bias a bit cooler than before.  Found a bias calculator online - and now that I know the voltage on the pins, I can bias more accurately.  Apparently, the higher the voltage, the lower the bias setting to get the same dissipation.

The amp is starting to sound like what I bought it for.   Can't wait to gig with it this weekend.

Thanks again to  plasticvonaband, Pickmaster and others who've been so generous with their knowledge and skill.