18Watt.com

One thing I'm still having issues with is that it will blow a fuse. Maybe 1 out of 100 times at startup. With the standby switch in standby position (no DC going to the filter capacitors) it will sometimes blow the 1A fuse.

A few days ago I went over everything with a fine-toothed comb looking for out of whack voltage readings, loose anything, etc. Couldn't find anything that didn't look normal. With my light bulb limiter it never blew a fuse. And 99% of the time it works fine without the light bulb limiter.

I suspect that it's the inrush current from the toroid power transformer.

Two questions...

I notice here on the modern schematics of the 18w it shows using a 2A fuse. Mojotone's instructions and kit use a 1A. Could that be partially the issue?

Also if I were to use a thermistor as Rob shows here (https://robrobinette.com/Generic_Tube_A ... nt_Limiter) I'm guessing that would prevent any issue from the toroid PT. Could I just put it between the LOAD terminal of the IEC plug and the fuse holder?

I think it's just the random current spike that needs to be dealt with.

In the USA it's 2A, in the UK it's 1A. If you're on a 120VAC source, you need to use 2A.

JMPGuitars wrote: ↑

Mon 06/13/22 5:54 am

In the USA it's 2A, in the UK it's 1A. If you're on a 120VAC source, you need to use 2A.

Thanks. I'm running on 120VAC. Hmm, I wonder why Mojotone is doing it like that? Just one more issue for builders of their stuff do deal with.

After I first noticed the startup issue I hooked up my multimeter and set it to measure the MAX amp load when I flipped the mains switch on. Most of the time it would be below 1A or just above but there were a couple of times (which didn't blow the fuse) which spiked around 5 - 7A.

Unit_1 wrote: ↑

Sun 06/12/22 5:28 pm

joulupukki wrote: ↑

Sun 06/12/22 4:40 pm

I guess for now they act as backups to the tube rectifier.

I used this as well as a capacitor. It's to prevent PT failure if the rectifier tube melts down.

Now that makes a lot more sense.

joulupukki wrote: ↑

Mon 06/13/22 7:17 am

After I first noticed the startup issue I hooked up my multimeter and set it to measure the MAX amp load when I flipped the mains switch on. Most of the time it would be below 1A or just above but there were a couple of times (which didn't blow the fuse) which spiked around 5 - 7A.

That's why slow blow fuses are used. Make sure you don't get fast blow 2a fuses or you still may have a startup issue!!!

Unit_1 wrote: ↑

Tue 06/14/22 1:37 pm

joulupukki wrote: ↑

Mon 06/13/22 7:17 am

After I first noticed the startup issue I hooked up my multimeter and set it to measure the MAX amp load when I flipped the mains switch on. Most of the time it would be below 1A or just above but there were a couple of times (which didn't blow the fuse) which spiked around 5 - 7A.

That's why slow blow fuses are used. Make sure you don't get fast blow 2a fuses or you still may have a startup issue!!!

They are definitely slow blow fuses. The issue is inrush current with the toroid power transformer. I'm working at getting a thermistor to try that out and my guess it it should work great.

joulupukki wrote: ↑

Sun 06/12/22 4:40 pm

zaphod_phil wrote: ↑

Sun 06/12/22 3:26 pm

joulupukki wrote: ↑

Mon 06/06/22 12:28 am

I’ll probably also use two 1N4007 diodes before it hits the plate pins on the 6CA4 rectifier as current limiters.

How would those 1N4007s help?

Not sure now actually. At some point I had read that these diodes could be used as current limiters. Not sure if that’s the case at all, but I guess for now they act as backups to the tube rectifier.

The two diodes protect the rectifier from excessive PIV. Many of the modern rectifier tubes don't seem to have as hard a vacuum as NOS, so this is a common failure mode.

Jack