Power Entry module (Twin fused?)

[Champ81]

Hi all,

I was looking for a power entry module from digikey. There are fused PEMs which I was looking for to fit the chassis cutout.  There are ones with one fuse in the drawer. Now I see ones with "twin fuses" so the drawer in fact needs two. Any benefit or less benefit to using two vs one anyone know of?  There is definitely a reason to incorporate twin fuses of course which is why they manufacture them. But not sure why.

My thoughts process was that if there was a power surge any one of the terminals other than ground would blow one or both fuses. I heard they use these on medical equipment. But just curious if these would either be redundant, have more safety or less safety.

Here is one I see on digikey.

https://www.digikey.ca/en/products/detai...100/641415

Thanks.

[K O'Connor]

Hi Guys

I do not see the point to having a fuse drawer as part of the IEC socket. And really, there should not be an external fuse position anyway.

If a fuse blows in your amp, there is likely something that needs attention and some investigation. Field replacements often cause more damage to the amp since it is inevitably a wrong and higher value that is subbed in.

When the fuse values and breaking characteristics are chosen correctly, there should not be nuisance failures.

Buy the $2-3 entry socket and fuse internally as TUT3 illustrates.

Have fun

[makinrose]

I agree with Kevin here.  I'm slowly redesigning all the amps I build to only have internal fuse holders because I run into exactly what he is describing.  It seems like many players do not understand what the function of a fuse is and that they blow to protect the amp so they will install higher value fuses or a piece of foil around a match stick thinking that the fuse itself is the problem. Many a transformer has been killed that way!   I like to use a through hole holder that mounts with a screw or rivet to the chassis rather than mounting them on the board but that is just a personal preference.  If it's your personal amp and you know better than I guess it won't matter as much.

[Champ81]

Thanks for the replies. Yeah most amp kits come with a mains fuse. I figured it was there in case of power surges? I 100% agree that the fuse is there to protect the circuit inside the amp of course.  I am starting again to build amps so I'm getting some kits from I guess the old Marshall way of doing things.  Unfortunately the chassis has a slot cutout that only fits a fused PEM so im just getting them to fit. Like Kevin mentioned in his books I also am eliminating the standby switch from all these and have been doing so. Also of course the galactic grounding.

[makinrose]

Mains fuse protects the power transformer against high current draw. Unfortunately, a single fuse is pretty poor protection---sometimes by the time it blows parts of the amp can be damaged. That's why more modern amps have additional fusing so that those fuses can protect individual windings on the transformers and better protect the amp. If you are building kits, you'd do well to add some more fusing....kit companies often copy old amps warts and all. Heck I've seen quite a few kits that don't even update amps to have a bias pot.

[Champ81]

I've recently finished building a 20W jcm800 style build with two el84's. I've been contemplating on adding seperate bias pots via Kevin's bias kit. However is it worth biasing these tubes? Most el84s I've seen are cathode biased.

[K O'Connor]

Hi Guys

The old 20W amps were fixed-biased. Definitely use two bias pots for your 2x EL84 amp, then you can set how hot they run and what tone they produce (within the limits of the tube for both). Apart from being able to get more life out of the tubes and potentially eliminate the mud that comes with cathode-bias heat, the output stage can be hum-balanced, which is different than DC balance.

In fixed-bias push-pull there should always be separate bias pots for the push and pull sides. This is generally better and easier to configure than using a bias + balance pot, as TUTs show. I have to admit that early on I was enamoured with the latter but found later that the separate bias pots is a lot easier to deal with in every way.

Have fun

[makinrose]

Hi Guys

The old 20W amps were fixed-biased. Definitely use two bias pots for your 2x EL84 amp, then you can set how hot they run and what tone they produce (within the limits of the tube for both). Apart from being able to get more life out of the tubes and potentially eliminate the mud that comes with cathode-bias heat, the output stage can be hum-balanced, which is different than DC balance.

In fixed-bias push-pull there should always be separate bias pots for the push and pull sides. This is generally better and easier to configure than using a bias + balance pot, as TUTs show. I have to admit that early on I was enamoured with the latter but found later that the separate bias pots is a lot easier to deal with in every way.

Have fun

I'm don't think any of the Marshall El-84 based amps made during the Plexi period were fixed bias.  The 18W ones most kit builders are familiar with like the 1974X, 1953, and 1973 and the 20W Lead and Bass 2061X were cathode biased and like Vox's offering they were biased really hot with tiny or non-existent screen resistors.  Definitely amps that need some updating.  All of Marshall's bigger amp were fixed bias during the Plexi era and I think that later on the El-84 offerings became fixed bias.

I've had some success with moding them with ideas from TUT 3's AC30 chapter and cooling down the bias a bit---I think that fixed bias would be good thing to try to make them sound more Marshall and less muddy     .

[Champ81]

The kit i got is cathode biased. So i am really planning on making it fixed. So basically i would need to make the cathode resistor a variable resistor.  With test points tied to the cathode (pin 3) of the el84s?  Im still relatively a novice in this whole thing so the questions will be very basic.  While learning. In TUT 2 it explains it clearly but it looks like its for a fixed biased amp converting to exclusive pots for each tube. I can only for now learn through paint by numbers type of explanation. Sorry.





























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[K O'Connor]

Hi Guys

Champ81: Since you say you have TUT2, you need to read it and look at the fixed-to-cathode-bias switching. That lets you use either method. In fixed-bias, it is simple to be able to hum-balance the output stage; in cathode-bias it is a bit more expensive, although there are ways that are not expensive but require a more complex grid circuit.

Making Rk variable will not dial the amp to fixed-bias;rather, it will be dialed to a melt-down.

Makinrose: You are correct; I was mistaken
Dpn Rickles says, "You win a cookie".