Screen resistor wattage rating on JMI-era Vox AC15

[seven]

Hi all,

Working on wiring up my AC15 clone, have a question on the 100-ohm screen resistors.

Referencing the available schematics, I didn't see that these were anything other than 1/2 watt types. I used 1/2 watt here.

Looking at some other builds online, I see that peeps are using 3 watt resistors here... can't get immediate access to 3-watts anytime soon...

I haven't powered up the amp yet. Will I burn up those resistors on power up? Or otherwise damage the transformers or amp?

Cheers,
Chris

[Jazz P Bass]

The schematics that I have seen for the AC 15 indicate a 100 ohm 1/2 watt resistor for the screens.
Go for it.

[Mick Bailey]

I always prefer to run amps with a minimum wattage for screen resistors and use flameproof types. If an output tube develops a screen short the screen resistor can go open and give a little more protection. This is especially the case where no HT fuse is fitted.

[pdf64]

Definitely not a carbon type resistor - vintage correctness be damned
MO is preferred, fusible versions are available from Mouser if defined breaking characteristics is desired.
eg here’s a 1ohm 1/2W fusible, great for cathode current sensing https://www.mouser.co.uk/ProductDeta...jHVeUw6Q%3D%3D

[Jon Snell]

Always fit 100R 1/2W Metal Oxide because they are fusible and do not set fire to your PCB or other components if built in the traditional way on turret board when the EL84s get a problem.

[seven]

Hi all,

Thanks for your replies.

Unfortunately, I utilized 1/2W carbon films for these... but based on your feedback (which now makes sense to me), I am going with 1/2W metal oxides:

https://www.digikey.com/en/products/...J100R/10211258

I'm sticking with 1/2W and not going for anything rated higher because if there's a problem, I want these to blow and not other components getting damaged. That's if I am understanding things correctly. You guys will let me know, I'm sure, if I am misunderstanding this...

Thanks again all!
Chris

[pdf64]

A HT fuse and protection diodes for the rectifier are beneficial mods.
A light bulb limiter is extremely beneficial when initially applying power to a new build, or after significant repairs, power valve or rectifier replacement etc.

[seven]

Thanks for the reply pdf64. Any info/guidance on performing those mods?

Also, was reading elsewhere that I should up the screen resistors, to something like 470-2k ohms?

[pdf64]

Thanks for the reply pdf64. Any info/guidance on performing those mods?

Also, was reading elsewhere that I should up the screen resistors, to something like 470-2k ohms?

It would be best to provide a link to the exact schematic you're planning to use, so that we're all referring to the same thing.

[seven]

[seven]

OK, so as I await your advice, I did some research and found this from R.G. Keen:

https://www.premierguitar.com/articl...mplifier-mod-1 and https://www.premierguitar.com/articl...s-ndash-pt-2-1

So, I'm thinking for the B+ to fuse right after the EZ81 but before the first filter cap, like this:



and for the EZ81 plates, 1N4007s.

And then for the filaments, was thinking of 4A fuses like this:

https://www.littelfuse.com/~/media/e...asheet.pdf.pdf

or

https://www.littelfuse.com/~/media/e...asheet.pdf.pdf

What do you guys think?

[pdf64]

Does this make sense? Option A uses a single HT fuse, option B uses 2 HT fuses.
I think it’s better to fuse the AC side of the rectifier, in case there’s a cathode to heater short on the EZ81.
For a lowish voltage amp like an AC15, 1N4007 should have sufficient reverse voltage.
I like the Picofuses, thanks for introducing me to them. As they solder in place, the usual heater fuse issue of contact resistance with the fuseholder can’t arise

[Dave H]

A genuine "back of an envelope" design

[pdf64]

No fag packets available

[Jon Snell]

Drawing A is preferred because there is only one fuse and that protects both sides of the rectifier. If B was employed, there is a possibility of unbalanced loading and one fuse may fail leaving the other half working overtime or indeed neither may fail due to the chosen value and availability.