the facts about 6x5g,gt

[ALBATROS1234]

one of my guitar amp builds has pt from a revere t-100 r2r. it was designed with a 6x5 because the pt had only ht and one 2.5a 6.3v sec. the heater of this rectifier is parallel to the heaters of a 6SL7 and 6L6 se. the plate voltage on the 6L6 is around 370vdc. so the 6x5 was being taxed a bit i think, epecially with having read there have been problems with 6x5 types(especially G type st shaped bottle) cathode coating break down and arc to the heater in a progressive way so that it is not really detectable until it shorts and takes out your pt ht winding. so i upgraded to 6ax5gt a while back but have also read that these tubes "may" be succeptable to similar failures? the 6ax5gt draws 1.2aheater current a bit more than the 6x5gt heater . it can also handle higher voltage of 375 at 125ma. i just have read on different forums differing views so would like to ask of past experiences with these tubes.
 is the 6x5gt really that bad or is it just the "G" types?

 is there really a difference between X or + shaped plate types being more reliable?

 am i fooling myself having a 6ax5gt in it or is as likely to fail?

beside 6ca4 what are the better 6.3volt rectifier types?

the amp sounds pretty sweet so there has to be a solution.

[sluckey]

the amp sounds pretty sweet so there has to be a solution.

Wish we all had that problem. 

You may be on the path to silicon.

[ALBATROS1234]

this is my only tube recti amp. was hoping to keep it that way. i have individual diodes and bridges. so i guess this is your way of saying if its not a directly heated rectifier tube the its not worth the trouble? maybe i can get a separate 5v supply and change to a 5y3, i got a couple. so even a 6ca4 is no good and could short out?

[sluckey]

so i guess this is your way of saying if its not a directly heated rectifier tube the its not worth the trouble?

Whoa. I didn't say anything like that. I don't think you have anything to worry about. I was just funnin' with you, mostly about the sentence I quoted. 

[kagliostro]

if you are in doubt, why you don't add fuses ?


Franco

[tubeswell]

The rated h-k max is 450V on this data sheet https://frank.pocnet.net/sheets/049/6/6X5.pdf


While the 6X5 datasheet says 450V max h-k, it also says this rectifier was designed for use in cars. The voltages in a mains-powered tube amp are a different kettle of fish.


The reason we like to have a separate rectifier filament winding on the PT, is that this winding usually sits at somewhere around the B+ voltage (especially if your tube rectifier has a directly-heated rectifier cathode).


With a directly-heated cathode, if you use the same filament winding as the rest of your heaters, then the heater voltage on all your other tubes is going to be at that same B+ reference point, and the h-k insulation on those other tubes won't last long.

Conversely, with an indirectly-heated rectifier on a shared filament winding, the h-k differential on the rectifier tube will be high if your heater winding is referenced close to ground potential - for the sake of protecting the h-k insulation on all the other tubes. Even though the h-k max is supposed to be 450V, i doubt the tube will be very reliable if the h-k voltage is anywhere near that.

[terminalgs]

regarding the 6X5 and the 6X4. I have several PTs with no 5v windings and recently contemplated builds using the 6x4 and 6x5 tubes.  it sounds like you probably read the same references to old ham radio guys warnings about these tubes (if not, google  "6X5 tube shorted").


I finally decided there would only be two outcomes in my case:  (1)  the tube would short and take the PT with it or (2)  I would always worry that the tube might short any moment and take the PT with it.   So I decided to not use them in new builds, and for my old amps that have them, I'll rewire the socket to use unused pins and put 1N4007's in series between PT winding ends and plates.


as for fuse protection:  everything I read said a fuse maybe or may not help.  the fuse on the main side would act too slow, and fuses on the secondary side are tedious to size, and if you are putting something in series, you might has well make it easier and put a SS diode in there.


I think the EZ80 falls into the same boat.  maybe the EZ81?  I never saw it mentioned in those shorted rectifier threads.  maybe its ok?


Also, since you have a SE amp,  I don't know if the tube vs SS in the rectifier department will be noticeable to your ear.

[jjasilli]

Ditto to sluckey Reply #1 & terminalags Reply #6, last sentence.


The simple solution, pug & play, is a Weber Coppercap:  http://thirdbayradios.com/6X5_Tube_Replacement.html


Or cheaper: bypass the tube rectifier with SS diodes and a dropping R.


Or live life on the edge and keep the 6X5.

[ALBATROS1234]

thank you gentlemen for the replies. i no longer have the 6x5gt in use. i substituted the 6ax5gt which from my casual reading most seem to think the 6ax5gt to be fine, a smaller percentage of guys say it is or could be succeptable. i do have a 6ca4/ez81 as well as diodes.so...

1). since some say the 6ax5gt could perhaps have similar failings i guys the intelligent thing would be to omit this tube , although i had hoped a few guys would chime in and tell me this tube would be fine.

2). i have read about fusing the heater winding connections with a "47" or similar bulbs to act as fuses. not sure why this would be better than a regular fuse unless its the "slow blow" aspect that causes the problem?

3). are there any other opinions on the ez81/6ca4 tube dependability and/or potential to short like the 6x5/6x4?

4). if i do go with "ss" , i have never done 2 individual diodes ,but i also have some hearty bridge units. i want to keep voltage from climbing higher so iirc 2 individual diodes with a series dropping resistor will be the method that will produce least voltage climb.

5). in my situation what would be an approximate size for the dropping resistor for a ss rectifier.

6). last option could be a complete redesign of the ps . if so i would probably add a larger resistor on the 3rd node as well to drop preamp tube voltages in order to get a bit more overdrive from the 6sl7 at lower volume. in my limited experience ,contrary to what i thought would be the case, i find lower preamp tube voltages seem to provide earlier breakup.

7) are there any advantages to using 2 diodes in conjunction with a tube rectifier to simulate a bridge situation?

thanks for all the help.

[jjasilli]

Bridge vs. dual diodes rectification has different results for resulting voltage & current.  Google:  hammond transformer selection guide; scroll to:  PDFDesign Guide for Rectifier Use


download the pdf document.  That addresses your questions.  Pick a solution that comes near your desired B+ voltage at idle.  Use a power resistor to hit your target voltage.  I use an inductive power resistor to simulate the reactance of a tube rectifier.  Calculate the R value for the voltage drop using Ohm's Law, using the current drawn b y your tubes.

[sluckey]

You are way overthinking this. Just solder a couple diodes on the tube socket. Remove the 6.3VAC wiring from the socket so no one can ever operate a 6X5. Otherwise, read on...

2). i have read about fusing the heater winding connections with a "47" or similar bulbs to act as fuses. not sure why this would be better than a regular fuse unless its the "slow blow" aspect that causes the problem?

That makes no sense to me.

3). are there any other opinions on the ez81/6ca4 tube dependability and/or potential to short like the 6x5/6x4?

I have a high opinion of that tube. I have several amps that use this tube. Hammond used it in thousands of their small amps. So did Marshall. It's a proven winner. I suggest you use this if you don't mind changing the socket to a 9-pin noval.

5). in my situation what would be an approximate size for the dropping resistor for a ss rectifier.

You may not even need one. But if you still think you do, experiment with 100Ω to 500Ω. And use 10 watts.

7) are there any advantages to using 2 diodes in conjunction with a tube rectifier to simulate a bridge situation?

This is not an option for this amp. You cannot just replace a conventional full wave rectifier with a bridge. Your B+ would double. Might blow up caps and other goodies.

[ALBATROS1234]

thanks guys. i do have an overthinking problem . probably going to just switch to noval socket and do the ez81. just wish there was a way to ease my mind over the 6ax5 upgrade i did a few months back as it seem the ratings are more than sufficient  and it seems the 6x5 issues result from this wimpy rectifier been overtaxed in a higher voltage/amperage situation as opposed to the 6x5 in a car radio or test equipment where the ht is below 300vdc sometimes closer to 200vdc. it did not make sense using a 6x5 with a 6L6 se at 370vdc. it was sure to fail me. the 6ax5gt gave me solice temporarily due to peak plate of 375 max, h to k of 450v max, and being able to supply a bit more current than a 5y3, the 6x5 is close to half a 5y3 current supply.

[DummyLoad]

6CA4 / EZ81 << ---- +1
--pete