Restoring the VE 9-55 and VE 9-25 Amplifiers

[ChuckA]

Wayne, If you go the F1/F3 route, it would be better to put F3 in line with the plate circuit, between the output transformer and the power transformer. That way, if there is an overload, all DC power will be disconnected from the amplifier.
Keep in mind that the voltage along that circuit will be about 400 VDC above chassis ground, so isolate the fuse holder well and try to find a fuse that is rated above 400 V.
Cheers, Russie

First of all the voltage at the output of the filter network is a little over 500V. The maximum current that can be drawn from a pair of 81 tubes is about 180mA. So the with a little fudge factor added use a 250mA fuse or give it a little more range use a 500mA fuse rated for 600V. This size fuse is about ½"Dia x 1 ½"Long and cost is between $20 & $30 each. Another $10 to $20 for a fuse block or fuse holder
Bussman #FNQ-R-½ SB
Bussman #KTK-R-½ FB
Bussman #FNQ-R-¼ FB

Put it in the return line and you only need a 150V rated fuse at the same current. $2 for a regular 3AG fuse at list prices and a $3 fuse holder.

At the rate this discussion is going you might as well put a fuse in every node that has voltage on it to protect every component, I count 10 needed to protect everything.

Chuck

[Victrola-Monkey]

Ok. I wish I had the knowledge to keep up with you guys. I take it that I had F3 on the wrong center tap. I think I moved it in the schematic below to where Chuck initially suggested and Russie told me tonight. Is this correct? But, it sounds that would require a whopper fuse. Therefore Chuck suggests moving it to the return line. I’m not sure where that is. Is it where I had F3 in the previous schematic? Oh, maybe the other line on the plug, is that what you are referring to, Chuck?

[startgroove]

Gosh Chuck, I don't mean to sound argumentative. There are many ways to accomplish the same task, just ask a Cat! I think the goal is to implement a way which will protect against the most likely failures first, without sacrificing safety. In my opinion, based on experience, the first failure would be the #10 tube, followed by the output transformer. I believe those are two most likely failures, and that is partly why I recommended the F2 fuse placement.

I agree that F3 would be a good position for the fuse as long as the safety concerns are addressed. You may have a different opinions on what I suggest, and that would be just as valuable as my opinions, since all points should be considered. In the end, a compromise may be the final solution.

In checking the ratings for the #10 tube, the maximum plate voltage is 425. I like to keep the plate voltage below that at a maximum of around 400, so that's where that number came from. If the plate voltage is above 400 VDC, something should be done to bring it down. That would be yet another discussion.

BTW, I have first hand experience with a #10 tube that was run at higher than its maximum plate rating. It turned into a glowing light blue light bulb!

Wayne, in my opinion you are on the right track. This latest diagram where you have relocated F3 will disconnect all high voltage from the amplifier, if the fuse opens. It will also protect all parts of the amplifier except the power transformer and anything before the power transformer. The trick is find the voltage tolerant parts.

Just checked and found a 600v rated fuse holder on Ebay, 3 for $2.99, https://www.ebay.com/itm/3x-Inline-Roun ... 3746330703 This is the critical high voltage item. Any AGC style fuse can be used in it.

Cheers, Russie

[Victrola-Monkey]

Thanks Russie,

I went ahead and bought them.

[Victrola-Monkey]

Also, I would like to thank Chuck for your fuse suggestions for the F3 fuse. However, the size (.5" x 1.5") and price ($30) certainly was a shocker. When Russie suggested the smaller size (.25" x 1.25") 600V rated fuse holder on eBay for only $2.99 for three, I was hoping to find the smaller fuse to go with it, though I am unaware of the benefits of a large physical sized fuse. I think I found one that may work for only $5.16 each. Will this work?:

500mA 500V AC DC Fuse Cartridge, ¼" x 1-¼"

https://www.digikey.com/product-detail/ ... =291994550

Thanks to both Russie and Chuck for helping.

[PeterF]

No, Wayne, Chuck cited the big fuses as what NOT to do.

You only need a 150V rated fuse holder and a regular 3AG fuse at either ¼ or ½ amp, slow blow. You put it in the high voltage return line (center tap of the HV winding). You already had that understanding earlier in the discussion; it’s marked as F3 in your first annotated version of the schematic.

This is what I’ll be installing.

I’m also considering getting rid of the original power xfmr and ballast. Those big hot ballast tubes (so hot they require chimneys) are nasty and scary, and probably more dangerous than the high voltage, when you think about it.

[startgroove]

Wayne,
Yes, I believe you will be fine with that fuse. The most important part is the fuse holder, it's 600 volt rating is important to prevent the possibility of arcing to the chassis on humid days, or under surge conditions, and to minimize the possibility of an electrical breakdown of its housing.
I think that combination will work fine if you choose to use the fuse position which disconnects all DC power from the circuit when there is an overload. If you choose the other position, then what Chuck (and Peter) say about the fuse rating is sufficient.
Be sure to check the plate voltage on the #10 tube.
Cheers, Russie

[Victrola-Monkey]

Thank you Peter and Russie, I now understand.

Btw, the cap cans have their primer coat (sitting on large washers) and now I gotta wait 48 hours before applying the top black coat.

[TonyJ]

Ok. I wish I had the knowledge to keep up with you guys. I take it that I had F3 on the wrong center tap. I think I moved it in the schematic below to where Chuck initially suggested and Russie told me tonight. Is this correct? But, it sounds that would require a whopper fuse. Therefore Chuck suggests moving it to the return line. I’m not sure where that is. Is it where I had F3 in the previous schematic? Oh, maybe the other line on the plug, is that what you are referring to, Chuck?

Just a suggestion on the placement of F1 as well - if you are replacing the power cord (or the existing cord is polarized), I would place F1 inline with the "hot" feed and make that be the "right hand" connection from the power input on your schematic . and place the "neutral" return on the "left hand" side (going to the 2mfd cap). That way if the fuse does blow, you won't potentially still have 120vac "hot" in there, and if there was ever an issue with the 2mfd cap, you wouldn't have a "hot" chassis. Again, this is all based on if the cord is or going to be polarized (and the wiring to the particular outlet being used is correct).

[Victrola-Monkey]

Thank you TonyJ. I will incorporate your advise and have included it in the two schematics below that give the two different choices of the type and location for F3, as previously discussed. The new power cord for the VE 9-55 is set up to be polarity sensitive but as you know, the male prongs on the back of the VE 9-55 are not. I will either need to replace those prongs on the back of the VE 9-55 with polarity sensitive ones or just indicate with writing on which way the power cord should be plugged in to it.