Of all the things I have seen about mods for this amp, the issue in this post is an important one for reliability and safety!
The design of the output tube bias boards leaves out an important component, a "fail safe" or "emergency pull down" resistor in case the wiper on the potentiometer goes open. This can easily happen if some dust or dirt ends up inside it, corrosion or it simply fails. Without this resistor, if the wiper goes open, the output tube red plates and all sorts of bad things happen. I've read several reports of this happening to this amp.
Attached are schematics showing as built, and then with a 100K "fail safe" resistor added. You can see more about this at the Valve Wizard page here: https://www.valvewizard.co.uk/bias.html
"R3 is an emergency pull-down resistor. This ensures that if the pot wiper fails to make good contact with the rack (because it is dirty or worn out, say) the grid leaks will effectively be connected directly to the raw bias supply through R3 instead, biasing the valves safely cold. "
If you do nothing else to this amp, I highly recommend adding a pair of these resistors to the two bias boards. I uploaded a video showing how to do this.
The design of the output tube bias boards leaves out an important component, a "fail safe" or "emergency pull down" resistor in case the wiper on the potentiometer goes open. This can easily happen if some dust or dirt ends up inside it, corrosion or it simply fails. Without this resistor, if the wiper goes open, the output tube red plates and all sorts of bad things happen. I've read several reports of this happening to this amp.
Attached are schematics showing as built, and then with a 100K "fail safe" resistor added. You can see more about this at the Valve Wizard page here: https://www.valvewizard.co.uk/bias.html
"R3 is an emergency pull-down resistor. This ensures that if the pot wiper fails to make good contact with the rack (because it is dirty or worn out, say) the grid leaks will effectively be connected directly to the raw bias supply through R3 instead, biasing the valves safely cold. "
If you do nothing else to this amp, I highly recommend adding a pair of these resistors to the two bias boards. I uploaded a video showing how to do this.
Attachments
Hi Stephe!
Thanks for all your videos, really appreciate them 😍.
I did this mod yesterday, first time soldering small things like this 😆 but it went ok. At least everything is still working well after the modification.
Here is a picture after the 1st board was done. (the 100k resistors were a bit of a pain 😱 and I had to slightly drill bigger holes for the Nichicon 100v 100uf caps... didn't find the AudioNote you used near my place)
I think you also did another mod on the R8 which reduces or almost eliminates the slight hum which can be heard when very, very close to the speakers, but I cannot find it.... will go through all the R8 mods videos again but if you can pinpoint me which mod it was, would be awesome!
Thanks again for your awesome content on youtube!
Thanks for all your videos, really appreciate them 😍.
I did this mod yesterday, first time soldering small things like this 😆 but it went ok. At least everything is still working well after the modification.
Here is a picture after the 1st board was done. (the 100k resistors were a bit of a pain 😱 and I had to slightly drill bigger holes for the Nichicon 100v 100uf caps... didn't find the AudioNote you used near my place)
I think you also did another mod on the R8 which reduces or almost eliminates the slight hum which can be heard when very, very close to the speakers, but I cannot find it.... will go through all the R8 mods videos again but if you can pinpoint me which mod it was, would be awesome!
Thanks again for your awesome content on youtube!