I have to ask about this diode deal on the rect HT leads in case a rect shorts

[catnine]

. I understand how if a rect tube shorts out then you have ac to the filter caps and they can blow and then blow the PT because the diodes will not allow the voltage to flow back to the PT HT windings . What I don't understand is how the diode can keep the amp working and for two reasons that have yet to be explained. If the filter caps blow you are done anyway , and lets say the caps don't blow how does the HT voltage reach the heaters or rather find a path to the B+ ?

 The rect tube changes a/c to d/c now wouldn't it sort of depend on what shorted in the rect tube to begin with?

 What am I missing here? I have had a rect tube long ago short in a sf champ and the main fuse blew . I put in another fuse and powered up and the fuse blew again. I went to a local dealer who sold amps and guitar and all the varied stuff and he told me I needed a new 5Y3 . I put it in with a new fuse amp worked.

 So just what part of a rect tube shorts in the first place . Fender never put diodes in the HT leads and if not why not . I have not read about adding diodes even on re -issue models of the old point to point fender amps that still use rect tubes. I am certain fender knew about tubes shorting out and that they would not risk replacing PT's for the cost of a few diodes.

[HotBluePlates]

So just what part of a rect tube shorts in the first place.

You should have started with this question.

A rectifier tube is a diode, meaning it has only 2 elements (yes, an indirectly heated tube has an extra element, but ignore that for now). Those are the anode and cathode. In a rectifier tube, current can only flow when the anode is more positive than the cathode. Therefore, current flows in only 1 direction, and the output is always positive (in the case of your B+ circuit). The rectifier acts as a one-way valve.

Electrolytic filter caps have a polarity. The dielectric insulation between the 2 plates of the cap is formed by a thin layer of aluminum oxide that is formed by applying an electric charge in the correct polarity. If you reverse the polarity of the charge, that insulating layer vanishes, and the cap acts as a short-circuit.

Well, there's only 2 parts in your rectifier tube that are there to short together. Somehow, a sneak-path for current forms between the anode and cathode, other than the empty space between them. Now the rectifier is no longer a one-way valve, but current can flow in both directions with equal ease. Once that happens, the output is no longer positive-only; it outputs essentially the same a.c. as is coming into the rectifier. On the negative half-cycles, the electrolytic caps' insulation layer vanishes and BAM... your fuse pops due to excessive current draw.

You pop in another fuse, and get another blown fuse. You change the rectifier and try another fuse, and everything's happy again.

... lets say the caps don't blow ...

I think I've demonstrated that isn't likely with electrolytic filter caps. It's possible, and it seems like it may have happened in your case. In any event, the momentary vanishing dielectric is the means for massive current flow when the rectifier shorts.

It might be possible if you use those enormous plastic film filter caps (such as Solen), assuming the excessive current doesn't damage the cap. It very well might pop the main fuse before damaging the cap. Or maybe the cap dies too.

The rect tube changes a/c to d/c ...

It is important (to make sense of future symptoms) to know the rectifier does not change a.c. to d.c. (at least, not by itself).

It takes an incoming a.c., which has positive and negative pulses, and converts that into a series of pulses in one direction only. Whether they are positive or negative depends on the direction of the rectifier.

The cap and load resistance attached to the output of the rectifier is what turns the pulses into something resembling d.c. The cap charges to the peak value of the incoming pulses, and the load resistance (if high) causes the cap to discharge slowly enough that the average voltage remains somewhat close to the peak value of the incoming pulse.

If the cap value is small, or if the load resistance is low (drawing a lot of current from the cap), then the cap does a poor job of maintaining its voltage level, and the output of the rectifier is mostly big pulses.

[jeff]

Another thing, In your case you had a bad 5Y3, but that doesn't neccessarily mean yours shorted. A rect. tube can go bad and not short, but open. I tested an old rect tube I got out of a old TV set on a tube tester. It tested negative for short but one of the plates was not conducting.

If it shorts, it passes AC, but if it opens it passes nothing. Maybe the heater just blew in yours, although you were blowing fuses???? Maybe the heater shorted the 5V tap???

[HotBluePlates]

I have had a 6L6 which shorted plate to heater (from pin 3 to pin 2), and that popped the fuse.

The key is that a 5Y3 is a directly-heated rectifier... that means the heater and cathode are one element internally. So the plate can't short to the heater independently of the cathode.

The heater could be looked at as the low-impedance path that caused the fuse to blow. Or you could say the rectifier now passes current in 2 directions instead of one. Either has as good a chance of being the culprit.

[jeff]

So do you put the diodes between the AC and the tube's plates? Seems like cheap insurance and can be mounted right on the tube socket. Is a single 1N4007 for each plate good enough?

 I mizwel do it.

If your rect tube does short the voltage will go up because there's no rect drop and now you have SS rectification? So I guess you should also have caps rated for that voltage since were trying to keep the caps from failing if the rect tube fails.

[catnine]

So just what part of a rect tube shorts in the first place.

You should have started with this question.

A rectifier tube is a diode, meaning it has only 2 elements (yes, an indirectly heated tube has an extra element, but ignore that for now). Those are the anode and cathode. In a rectifier tube, current can only flow when the anode is more positive than the cathode. Therefore, current flows in only 1 direction, and the output is always positive (in the case of your B+ circuit). The rectifier acts as a one-way valve.

Electrolytic filter caps have a polarity. The dielectric insulation between the 2 plates of the cap is formed by a thin layer of aluminum oxide that is formed by applying an electric charge in the correct polarity. If you reverse the polarity of the charge, that insulating layer vanishes, and the cap acts as a short-circuit.

Well, there's only 2 parts in your rectifier tube that are there to short together. Somehow, a sneak-path for current forms between the anode and cathode, other than the empty space between them. Now the rectifier is no longer a one-way valve, but current can flow in both directions with equal ease. Once that happens, the output is no longer positive-only; it outputs essentially the same a.c. as is coming into the rectifier. On the negative half-cycles, the electrolytic caps' insulation layer vanishes and BAM... your fuse pops due to excessive current draw.

You pop in another fuse, and get another blown fuse. You change the rectifier and try another fuse, and everything's happy again.

... lets say the caps don't blow ...

I think I've demonstrated that isn't likely with electrolytic filter caps. It's possible, and it seems like it may have happened in your case. In any event, the momentary vanishing dielectric is the means for massive current flow when the rectifier shorts.

It might be possible if you use those enormous plastic film filter caps (such as Solen), assuming the excessive current doesn't damage the cap. It very well might pop the main fuse before damaging the cap. Or maybe the cap dies too.

The rect tube changes a/c to d/c ...

It is important (to make sense of future symptoms) to know the rectifier does not change a.c. to d.c. (at least, not by itself).

It takes an incoming a.c., which has positive and negative pulses, and converts that into a series of pulses in one direction only. Whether they are positive or negative depends on the direction of the rectifier.

The cap and load resistance attached to the output of the rectifier is what turns the pulses into something resembling d.c. The cap charges to the peak value of the incoming pulses, and the load resistance (if high) causes the cap to discharge slowly enough that the average voltage remains somewhat close to the peak value of the incoming pulse.

If the cap value is small, or if the load resistance is low (drawing a lot of current from the cap), then the cap does a poor job of maintaining its voltage level, and the output of the rectifier is mostly big pulses.

Ok I unstand that. This was an old Sf champ , the build I have now is not shorting out the rect tube. I have now is not shorting out and it draws little current so then the cap is not to small being 47UF yet if the current draw is small then is it safe to assume the most of the current is from the large 47uf 1st cap and if that is the case then would  a smaller reservoir cap be a better choice? I am really tryong to understand this . there are two other filter caps yet it is the 5y3 and the reservoir cap that I am concerned about , so since the res cap is 47 uf this is what the 5y3 is charging up and if the amp with one 12ax7 and a 6V6 plus the heaters are not drawing much , the PT gets only warm . Yet if the 1st cap after the 5Y3 is doing amost of the the work  then is it safe to say that the rect is putting out big pulses rahter than maintaining a balance where the res cap does the job it's designed to do? What I am trying to say is if the res cap is charged yet it is the main source that the rect is charging and the other two filter caps being 22uf are part of the filter and are doing their job seems to me that the rect is on a continuois duty of charging the 47uf 1st cap as the overall current draw is low is it possible that this constant charging of the 47uf res cap cap can cause my low 3 volt voltage drop across the 1st node of 1k ohms.

[catnine]

trying to quote just makes it impossible because the area I have left to type keeps making me go back I have no idea why.

 Let me just say this and hope I get the answer I am looking for.

 HBP you have the most in depth answers I have seen . Here is what has happened .

 When I first built this home breww AA764 champ I got the filter caps based on what were in my great sounding 73 SF champ without  trem. There were F&T cpas a 47uf res and a 22 uf and anothe 22uf after that .

 Then after reading about the stress a 47 uf res cap can place on a 5Y3 I freaked out and got a 30 uf F&T cap for the res cap . At that point is when the icepick highs came into play , before that the amp sounded great.  Now I have no way of measuring a caps value , I have no meter to test caps values so for all I know the original 47 uf cap could ahve been much higher or lower in value and then the 30 uf could have been the same higher or lower. , I just don't know.

 What I do know is I built the eyelet board out of the chassis and used the 47uf cap to go to the eyelet for the B+ so in order to remove it I had to clip the leads and they were to short to use in the same place again so i got a new 47 uf cap and agian have no idea what the actual value of the new stamped value 47 uf cap really is but after pulling out the old 47uf and then putting in the new 30 uf and then the icepick highs came into play . That is all I did . then when I installed the new F&T 47 uf cap the icepick highs remained , again I have no idea of the actual value of the new 47 uf cap .

 I had no hum with any of the caps at all. Bottom line since I really do not fully understand the ripple affect and can't seem to measure it then what I am left with is this . I can install a 22uf F&T cap as the res cap off the 5y3 a cap that I got at the same time as the other two 22 uf F&T caps and still have no idea what it's actual value is .

 My point is this . If the 47uf give or take is working the 5y3 just to keep it charged through duty cycles  and the amp is drawing little current then it seems the 5y3 is just working to keep that cap charged until the low current draw of the amp draws it down yet it will keep this cycle . so what is the point of the 47 uf cap and what affect does it have when it is in a constant long charge mode if that is the proper term.  Another thing I do know is when I had the 73 SF champ I know I needed a new cap can and it was original and was 40/20/20 uf and I know the new cap cna I got was measured to be 40/20/20 uf because the fellow i got it from checked everything and even if you got tubes from him he would burn them in for a matched pair and would ship the parts with the exact measurement of each part written down so you knew what you had to begin with.
 I can only assume the fisrt F&T 47uf cap was at the least 47 uf or above because the amp did not have the highs , I cannot say with the 30uf or the new 47uf but they must be lower who knows it's all gues work on my part. . Yet if like I said the 5Y3 is doing most of it's work just to keep the res cap charged for what it will take then perhaps I would be better off with the 22UF from the same batch . I personally really don't know , right now with the 47uf cap as a res cap i see no issues of hum nore did I with the 30 uf . Perhaps the first 47uf cap i had was lower than the 30 or second 47 uf in the amp now and that is why I have the highs I did not have before . I just want the amp to sound and work as it should . I wish I had left it alone when I first built it because it sounded great then. , to damn late now.

[sluckey]

trying to quote just makes it impossible because the area I have left to type keeps making me go back I have no idea why.

Sounds like an IE-8 issue. Use "Compatability View" mode to correct. The compatability button is just to the right of the address bar.