Fuse on HV CT to Ground, Yea or Nay?

[NickC]

Building an AB763 Blackface Deluxe Reverb. Nearing the first power up (w/ lightbulb limiter, slow ramp up on variac, etc) and measurement phase.

I added a 500mA fuse inline with the HV center tap to ground, thinking it would be a good safety protocol (put in place of the unwanted ground switch). Borrowed the idea from a Marshall circuit.

Is this a good idea, or should I undo it? Thanks for any advice and opinions, much appreciated!

[martin manning]

Read up here: http://www.valvewizard.co.uk/fuses.html

Add to that the caution that a CT fuse in a FW rectifier will not protect the PT from a failure where one rectifier diode shorts. Half-wave rectified current will flow through the secondary, so the mains fuse will have to cover that.

[NickC]

Read up here: http://www.valvewizard.co.uk/fuses.html

Add to that the caution that a CT fuse in a FW rectifier will not protect the PT from a failure where one rectifier diode shorts. Half-wave rectified current will flow through the secondary, so the mains fuse will have to cover that.

Thanks Martin. I read that excellent article during the planning phase. I understand the fuse in the HV CT to ground line isn't optimal. Also understand that fusing both sides of the HV secondary BEFORE the rectifier (in this case GZ34) is recommended. Sadly, it would be a tight squeeze to fit two internal fuse holders to accommodate that at this point.

I figured using the empty hole where the ground switch ordinarily goes would better serve as a chassis mounted (external access) fuse holder for the HV CT. I understand it's not optimal. But it seems better than not fusing the HV CT at all?

[martin manning]

The other risk is that if the CT fuse opens up the bias circuit wants to be your new CT, and damage to the bias supply components will result. Better than no fuse? I dunno, maybe not.

[NickC]

The other risk is that if the CT fuse opens up the bias circuit wants to be your new CT, and damage to the bias supply components will result. Better than no fuse? I dunno, maybe not.

Thanks again Martin! Excellent point. I'm going to undo it.

[Phil_S]

Nick -- This is just an example of what I have in mind and they don't make it easy to select the right type: http://www.littelfuse.com/~/media/elect ... asheet.pdf
Are you saying you don't have room inside to interrupt the high voltage secondary between the PT and the rectifier to install a pair of these? It could even go on the side wall.

[pdf64]

The other risk is that if the CT fuse opens up the bias circuit wants to be your new CT, and damage to the bias supply components will result

I'm having trouble working out what exactly would happen in this case.
Assuming that the fuse blew due to a manufacturing fault or whatever, ie there's nothing actually wrong with the amp, what happens next?

[NickC]

Nick -- This is just an example of what I have in mind and they don't make it easy to select the right type: http://www.littelfuse.com/~/media/elect ... asheet.pdf
Are you saying you don't have room inside to interrupt the high voltage secondary between the PT and the rectifier to install a pair of these? It could even go on the side wall.

That looks good! I have some internal, chassis mount holders but they're larger than that. I could probably cram a couple of the type you posted in the crowded chassis I have. Food for thought. Thanks Phil!

[gingertube]

In this amp:
http://bmamps.com/Schematics/hiwatt/hiwatt_sa412.pdf
The rectifier diodes went short and it took out the power tranny. I had to get the power tranny stripped and rewound by HD Transformers here in Adelaide to repair it. This was actually cheaper than a replacement power tranny from the UK.

The mains fuse should have gone but someone had put a STUPID high value fuse in place of the original and so "the power tranny blew to protect the mains fuse".

The HT fuse in the CT is something I never use in amp builds.

Cheers,
Ian

[martin manning]

The other risk is that if the CT fuse opens up the bias circuit wants to be your new CT, and damage to the bias supply components will result

I'm having trouble working out what exactly would happen in this case.
Assuming that the fuse blew due to a manufacturing fault or whatever, ie there's nothing actually wrong with the amp, what happens next?

See below.

[Stevem]

Just make it easy on yourself and install a inline (series) 160 ma fast blow fuse in the OT center tap feed and call it a day!

[martin manning]

Just make it easy on yourself and install a inline (series) 160 ma fast blow fuse in the OT center tap feed and call it a day!

That leaves a lot of the HT circuit unprotected. If you want to fuse the HT find room for two, one on each leg of the PT secondary, call it a night, and sleep well.

[Stevem]

You are right, but percentage wise it's the output tube (s) going south that makes for a issue to the tune if 95%!

[martin manning]

You are right, but percentage wise it's the output tube (s) going south that makes for a issue to the tune if 95%!

Got any data to back that figure up? ;^) I would like to protect against rectifier shorts too. Furthermore, in power tube failures it is often the screen that fails. Fusing the anode circuit at the OT CT will not help in that scenario.

[VacuumVoodoo]

Don't forget the rectifier ripple smoothing electrolytic caps. They can fail open or short circuit.