Silicon Chip mag "Currawong" amp

[Liamc]

G1 should be connected to ground through a resistor. These are shown as 1 Meg in the layout but as 6A3Summer says in post #68, they should be no more than 500k. Try replacing them with 470k and make sure you measure 480k (470k + 10k grid stopper) between G1 (pin 5) and ground.

Would you recommend changing out the 1meg anyway now all is working?

 

[tikiroo]

If it works OK you could leave them in. Gingertube in Post#13 notes that they get away with it because the output tubes are not working very hard. But you do increase the risk of the current running away if a slightly dodgy tube is installed. Lowering to 470k increases the load for the phase splitter which might increase distortion, also changes low frequency cutoff. But that phase splitter is a pretty poor design to begin with so you might not notice

 

[Liamc]

Might give it a go, I’ve got a set of kt66’s that I want to give a go in the amp. So when I decide to run them I’ll swap out to the 470k resistors.

Cheers man!

 

[wg_ski]

Never mind guys, found the issue…. Sometimes it pays to triple check your work 🤦

Forgot to add these links in, they were never mentioned in the drawings but when I was tracing it out I found them 😂

Thank you all for your help!

That would make me want to slap a PCB designer. I see no reason a jumper needs to be used - that ground connection should have and could have been made directly.


I bet you’re not the first person to do this.

 

[6A3sUMMER]

Depending on the rest of the PCB, jumpers might allow you to EASILY use either fixed bias, fixed adjustable bias, or self bias.
Perhaps the G1 jumper allowed the use of an external fixed bias or external fixed adjustable bias circuit.
Nice added features do trip up people.

 

[Liamc]

That would make me want to slap a PCB designer. I see no reason a jumper needs to be used - that ground connection should have and could have been made directly.


I bet you’re not the first person to do this.

Yeah, I had a look online at other people finished kits and no one else had it in 😂 it’s not mentioned at all in the booklet it comes with. But I have confirmed with silicone chip that it’s meant to be installed 😂 ah well live and learn