For a home build/new design fuse both sides of the HV winding, regardless of if using a centre tapped winding with 2 diodes or a non centre tapped winding with bridge rectifier. I did'nt check but I would guess that is what Merlin would suggest, its certainly what Kevin O'Connor recommends
and FWIW its what Gingertube does and recommends.
That destroyed HiWatt I referenced above made me a convert.
Cheers,
Ian
The only time an HT fuse would seem okay would be in a cathode biased amp, where there is no fixed bias supply, of course. Kevin O'Connor does use that on the AC30 in TUT3... not sure really why, however when he could have just done his usual recommended approach.
I've said this before (and learned the hard way) that if you must go the Fender route use small wattage screen resistors like they did, or your PT may go before the screen resistor does. They are pretty much screen fuses for screen shorts, which seem like a common tube failure in my experience.
Power tube cathode fuses are a nice addition too, opposed to an OT center-tap fuse. Just remember to put a resistor across them so the HT is not floating if they blow.
Be careful with a cathode biased amp.
If it includes a standby switch where you can turn on the HT with hot cathodes on the output tubes then there is NO BIAS on the tubes untill the cathode bypass caps charge up and during this time you can get huge peak currents through the output tubes. I had to delete the standby switch in a carthode biased amp to stop it blowing HT fuses.
Rewired it as an output tube grid mute switch instead.
Cheers,
Ian
gingertube wrote:For a home build/new design fuse both sides of the HV winding, regardless of if using a centre tapped winding with 2 diodes or a non centre tapped winding with bridge rectifier.
For a FWB with no CT, fusing one side would sufficient, no?
gingertube wrote:Be careful with a cathode biased amp.
If it includes a standby switch where you can turn on the HT with hot cathodes on the output tubes then there is NO BIAS on the tubes untill the cathode bypass caps charge up and during this time you can get huge peak currents through the output tubes. I had to delete the standby switch in a carthode biased amp to stop it blowing HT fuses.
Rewired it as an output tube grid mute switch instead.
Cheers,
Ian
Ian, that's interesting. What do you mean by "hot cathodes?" Opposed to cold cathodes?
I believe Ian is referring to the usual setup with a power and a standby switch, where the power switch energizes the filaments and the standby turns on the HV. If the amp is cathode biased, and the cathode resistor is bypassed, the bypass cap will delay the rise in cathode voltage. With the cathodes already in a hot state, there will be a large current surge.
Nothing wrong with having a mains fuse, and you should absolutely include one. The issue is it is usually somewhat overrated and/or a time delay type. This is so it can survive the initial startup surge, and then during use it may not open fast enough to prevent damage on the secondary side. Some new amps have a NTC thermistor in series with the AC line to limit the startup surge, which allows a lower rating for the mains fuse. Still, fast-blow fuses in strategic locations can help reduce collateral damage in a failure event.
Martin, thanks for the previous diagram.
I agree that it looks like it should be a problem.
Unfortunately I'm still having trouble working out exactly which bias supply components would be overstressed by an open CT, and why.
Can someone assist with the sequence, eg starting with both PT B+ winding legs at a notional zero crossing point?
Thanks!
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With the CT disconnected from ground, the bias supply circuit will provide an alternate path with ~30k ohms between ground and the bias tap on the secondary. That point on the secondary will be pushed way below ground, putting a very large voltage across the bias supply caps.
