Individual Tube Fuses - Easy Swap Out Method - Auto-Bias Board Users

I received the fuses from Roy yesterday, they are ¼ Amp with long leads. Instructions to solder them between Pin 8 > 1.

A pal of mine clued me in to a cool way to install with a fuse holder to use with these same type of fuses.
When they blow, ya just replace the fuse / tube. Perhaps diagnose your problem.

The fuse holder is this little guy:

Tube Fuse Holder

Basically, they are a small 3 pin break away pin socket with the middle pin removed.
I was thinking, heck... I could have made that!

40 PIN Machined Break Away Female Header Connector

My pal suggested these fuses:

315 mA fuses for a KT88 tube:
315mA KT88 Fuses

These fit right in without trimming the leads. Use these or the fuses Roy sent along, just give them a little trim. I think my pal is being more conservative, Roy’s mA rating being less will blow faster, perhaps safer.

Soldering instructions for me, the hobbyist, not a vendor

Quote:
If you twist the lugs on the octal socket ever so slightly you can get them in.  Put a pliers under them to hold them up at an angle.  Tack one pin with solder.  Then make sure the other is lined up and solder it.  Then put more solder on the first pin.  Do not overheat.   Solder quickly.  Buy a couple of spares in case you screw up the first one.   Once installed just plug the fuse in and you are done.

If a tube blows it will just blow the fuse.  You can tell by simply measuring the resistance between the pin 1 and 8 socket holes from above with the amp off and tube removed.   It will read 1 to 1.3 ohms when fuse is fine.   Infinite of course if you blew the fuse. Then simply pull the bottom of the amp, plug in new fuse, install new KT88 and the auto bias will take care of it…
End Quote

Hope this helps, makes swapping fuses a bit easier.

Aguaazul

A fuse will take some time to break. A 1/4w resistor of film type will blow much faster.

For fusing the cathodes you need a very specific, high-breaking, ceramic fuse like https://www.digikey.com/products/en?keywords=%20F4603-ND

Ordinary fuses won't do. And if you have to go under the hood to replace the fuse then you might as well not bother since you would do the same for the resistor. It's really more of a convenience to use them. I use panel mount fuse holders so replacing the fuse is quick and easy. It also beats the smoke and smell of a fried resistor.

these are fast-blow fuses (250ma, and 0.25 ohm) I'm supplying, so they should blow out about as fast as a 1/4w resistor

corndog71 wrote:For fusing the cathodes you need a very specific, high-breaking, ceramic fuse like https://www.digikey.com/products/en?keywords=%20F4603-ND

Ordinary fuses won't do.  And if you have to go under the hood to replace the fuse then you might as well not bother since you would do the same for the resistor.  It's really more of a convenience to use them.  I use panel mount fuse holders so replacing the fuse is quick and easy.  It also beats the smoke and smell of a fried resistor.

WOW!!!!!

210% of rating for 30 minutes?

Really?

http://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_215_datasheet.pdf.pdf

yeah WOW, so looks like with those fuses, if you really want a 1 amp rating, better get 1/4 amp fuses to blow in 1 to 10 seconds ! I'll stick with fast-blow fuses!
Although the MDL spec fuses I usually use spec a 5sec blow at 200% of rating.

The fuses I use are what Roger Modjeski has been using in his amps for 20 years. He wrote a lot about fuses in the Music Reference forum on audiocircle.com. (It’s closed and archived but you can still read it). He was adamant about using the proper fuse for the job.

corndog71 wrote:The fuses I use are what Roger Modjeski has been using in his amps for 20 years.  He wrote a lot about fuses in the Music Reference forum on audiocircle.com.  (It’s closed and archived but you can still read it). He was adamant about using the proper fuse for the job.

FYI - the data sheet for those shows a 210% blow time of 1/2 hour as well. Even "fast-blow" (as opposed to "normal-blow" or "slow-blow" fuses) are relatively slow acting devices. Since the fuses blow because of excess heat through the element there will always be some of what some call "thermal capacitance" affecting fuse performance. Heating the of element would occur more quickly if some of the element heat wasn't dissipated through the clips or leads as well as radiating a bit through the air/glass. So for the fuse to blow it has to have excess current which in turn heats the element enough to overcome the "thermal capacitance" and melt the element. Less excess current means less heat and therefore longer blow times.

Obviously the higher the overload the more heat and the faster blow time.

To shut the circuit down faster than a fast blow fuse is going to require much more than a simple fuse. Roy's choice of fuses is about as good as you can get without creating either a lot of expense and added complexity and/or situations where frequent undesired fuse blowing can occur.

The goal is to get the rating of the fuse as close to the operating current of the device as possible. However, issues with this might include nuisance blowing if that rating is too close and/or if the devices has a high inrush at start.

Ways around this include:

a) Dual-Element/Time-Delay fuses that are designed to handle inrush, but then revert to standard or quick-blow once the inrush is complete. These fuses are available in fractional amperage down to 1/10A but are quite expensive as compared to other fuses. Interesting to note that such fuses were OEM in most Dynaco products back in the day.

b) Slow-Blow (Devils' Spawn) fuses that are designed to absorb specific levels of overload for specific periods of time on the theory that such overloads are unlikely to start a fire, but damage to the equipment is not a concern. 210% of load for 1/2 hour is an excellent example of such a device.

c) Eutectic-Element fuses (often designated as "Type R"), designed such that when the element is overloaded and reaches failure temperature, it goes liquid all-at-once without any sagging or stretching. These are also available in fractional amperage but are expensive and hard to find in AGC sizes. Dual-Element fuses are also available with eutectic elements, its only money. But would represent the best-available conventional technology to protect our equipment - in addition to our real-estate.

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/product-datasheets-b/Bus_Ele_DS_1019_FRN-R_1_10-60A.pdf

These fuses are large, expensive and did I mention large and expensive.

I use standard AGC DE Fuses run as close to actual operating amperage as I can find - and I determine AOA via my AC Power-supply meters, not the nameplate rating.

Ok, so I'm not 100% sure these are the exact fuses Modjeski uses but he just recommended Littelfuse because of their reputation as a certified fuse maker and that the fuse should be high breaking and ceramic. For EL34 and KT88's he recommended 1/4A Slow blow.



Now you guys focused on the 210% rating which does say 30 minutes before blowing. However if the current exceeds 275% it can blow from .25 - 80 seconds. If it exceeds 400% it ranges from .05 - 5 seconds.

This has me wondering how much current can be expected from a tube that goes bad?

The one time I blew one of these fuses the tube was completely dead and it blew on startup.

Hello Guys,
                  I am new here to the forum. I just received my Bob Dyanco VTA-ST120 KIT ON 6/21/21 and waiting on a roll of Kester 44 Solder Wire, 63/37 to start building the kit. I have been reading a lot here on the forum. And would like to add some of the things you guys have posted that will safeguard the amp. This mod is just one of them. Would anyone be willing to tell me what size fuse and holder I would need to buy and where I should place it on which tubes if you don't mind? I really appreciate the help. I thought it would save me a lot of trouble if I add everything while building the amp. Instead of taking it back apart. You guys have done a great job of sharing info on the Dyanco amps on this forum for guys like me. And I thank you.

edman44 wrote:Hello Guys,
                  I am new here to the forum. I just received my Bob Dyanco VTA-ST120 KIT ON 6/21/21 and waiting on a roll of Kester 44 Solder Wire, 63/37 to start building the kit. I have been reading a lot here on the forum. And would like to add some of the things you guys have posted that will safeguard the amp. This mod is just one of them. Would anyone be willing to tell me what size fuse and holder I would need to buy and where I should place it on which tubes if you don't mind? I really appreciate the help. I thought it would save me a lot of trouble if I add everything while building the amp. Instead of taking it back apart. You guys have done a great job of sharing info on the Dyanco amps on this forum for guys like me. And I thank you.

You have individual tube fuses in the form of small 10k resistors to the cathode.
They will evaporate if a tubes should flash-over and save the transformer from potential damage.
A fuse will not break current equally fast.

Hello peterh,
So you are saying no need for me to add the little round mini fuses to the tubes?

edman44 wrote:Hello peterh,
                  So you are saying no need for me to add the little round mini fuses to the tubes?

Yes, that is what i say.
Just make sure the 10ohm resistors are of film type, 1w is ok, 2w is what is delivered.

A fuse in general might have problem breaking DC 400Volt as an arc will be created, special fuses ( filled with sand ) is ok. A fuse will also burn off slower then an exploding filmresistor.

Just so I understand correctly. I should not add anything to the tube sockets? Should I leave the 2-watt resistors and not chance them to 1-watt? I am going to start the build sometime tomorrow and go over everything twice maybe three times. Thank you for all your time and help peterh.

edman44 wrote:Just so I understand correctly. I should not add anything to the tube sockets? Should I leave the 2-watt resistors and not chance them to 1-watt? I am going to start the build sometime tomorrow and go over everything twice maybe three times.  Thank you for all your time and help peterh.

Use the cathode resistors bob supplied. They work fine ( if you need them).

Depending on tubes it's unusual that a tube flashes ( i have used my VTA-70 for > 5900h and is using my third set of power tubes without any flashover or redplating events. Thus my 2w resistors are still original.

I suppose the old saying is true. If it ain't broken don't fix it. Thank you for your help.

edman44 wrote:Hello Guys,
                  I am new here to the forum. I just received my Bob Dyanco VTA-ST120 KIT ON 6/21/21 and waiting on a roll of Kester 44 Solder Wire, 63/37 to start building the kit. I have been reading a lot here on the forum. And would like to add some of the things you guys have posted that will safeguard the amp. This mod is just one of them. Would anyone be willing to tell me what size fuse and holder I would need to buy and where I should place it on which tubes if you don't mind? I really appreciate the help. I thought it would save me a lot of trouble if I add everything while building the amp. Instead of taking it back apart. You guys have done a great job of sharing info on the Dyanco amps on this forum for guys like me. And I thank you.

Hi Edman44,
This topic / mod is for users of the Autobias board only.
It's stated in the subject: Individual Tube Fuses - Easy Swap Out Method - Auto-Bias Board Users

My final product:

https://servimg.com/view/19891933/9

From this thread:

Auto-Bias board on Roy's website?

If you are going to use the Auto-Bias Board, these are a good idea.

Cheers,

Aguaazul

Thanks.