Mojotone Custom 100 (Hiwatt 103 copy) Build Question

[ThreeChordWonder]

BTW my paternal grandfather was the BBC's Chief Engineer when he retired, still in the tube era, in 1968. Some things just rub off on you, like not relying on wiggling things to make them work.

[martin manning]

Some things just rub off on you, like not relying on wiggling things to make them work.

Yea, but you might locate the source of the problem that way.
With the PI plate resistors at 82k and 91k, and a balanced PI tube, the outputs should balance nicely.
Here are some better DC voltages for you:

[sluckey]

What voltage do you measure on pin 2 and pin 7 of V4?

[maxkracht]

BTW my paternal grandfather was the BBC's Chief Engineer when he retired, still in the tube era, in 1968. Some things just rub off on you, like not relying on wiggling things to make them work.

It's not to make it work, it's to find the source of the problem. Wiggling and poking things with a stick are very useful troubleshooting tools. You can have something that looks connected, but isn't fully connected, or something that tests fine with your meter but disconnects with heat or vibration. Poking or wiggling can be the best way to point you toward a problem so you can fix it properly.

[ThreeChordWonder]

What voltage do you measure on pin 2 and pin 7 of V4?

P1 = 164.4 V
P2 = 3 mV
P3 = 87.5 V
P4 = 6.6 VAC
P5 = 6.6 VAC
P6 = 455.6 V
P7 = 3 mV
P8 = 87.5 V
P9 = 6.6 VAC

That's without power tubes fitted.

[ThreeChordWonder]

BTW my paternal grandfather was the BBC's Chief Engineer when he retired, still in the tube era, in 1968. Some things just rub off on you, like not relying on wiggling things to make them work.

It's not to make it work, it's to find the source of the problem. Wiggling and poking things with a stick are very useful troubleshooting tools. You can have something that looks connected, but isn't fully connected, or something that tests fine with your meter but disconnects with heat or vibration. Poking or wiggling can be the best way to point you toward a problem so you can fix it properly.

Offshore, we used to prod Norwegians with a stick. To see if they were dead or just asleep

[cdemike]

Yeah not trying to offend you or make assumptions about your credentials or ability. Just passing on advice I've gotten to help diagnose (not fix) similar issues in the past, as Max and Martin have pointed out. Very cool about your granddad -- I'll bet there were some great stories about his work. You're right that it's generally not a good idea to put multimeter probes into the tube sockets since it'll loosen them up too much, but you may want to try using clip multimeter probes attached to wire running into the sockets. No risk of stretching out the socket grips that way.

+1 on Sluckey's suggestion to check bias voltage at the PI. I'd still recommend checking the voltages at the socket with V4 removed: if you suspect the socket may be bad, it's a lot less work to rule out the socket with the wire method above in advance of replacing it than to swap out a potentially good socket.

What voltage do you measure on pin 2 and pin 7 of V4?

P1 = 164.4 V
P2 = 3 mV
P3 = 87.5 V
P4 = 6.6 VAC
P5 = 6.6 VAC
P6 = 455.6 V
P7 = 3 mV
P8 = 87.5 V
P9 = 6.6 VAC

That's without power tubes fitted.

These measurements show you're not getting bias voltage on the grids of V4. You should be getting around 80V on pins 2 and 7. What does V3 pin 3 read?

Edited for clarity.

[sluckey]

Your pin 2 and pin 7 voltages should be many, many volts, certainly not 3mV! Look for a bad connection between V3 pin 3 and V4 pin 2.

[sluckey]

Post a closeup hi-rez pic that clearly shows V3 and V4 sockets.

[martin manning]

And what are the voltages on V3 pins 1, 2, and 3?

[ThreeChordWonder]

Corrections:

P1 = 276.0 V
P2 = 82.9 V
P3 = 86.4 V
P4 = 6.6 VAC
P5 = 6.6 VAC
P6 = 277.2 V
P7 = 75.6 V
P8 = 86.8 V
P9 = 6.6 VAC

I think I've fixed it!

[ThreeChordWonder]

Turned out I had a bad 1 meg resistor I hadn't checked before. I swapped that out and Hey Presto! everything was on the money. Voltages are down a bit because our mains voltage is too, I expect - 100+ deg. F. outside, Sunday afternoon, everyone at home, with their ACs going full tilt.

Thanks for all your help guys!

[ThreeChordWonder]

The problem came back so I went ahead and fitted a new socket, which turned out to be much "tighter" than the old one.

I found a similar issue on one of the EL34 sockets so now I've ordered a replacement one of those too.

Thanks everybody.

[ThreeChordWonder]

Finished!