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 6CA4 Rectifier - destruction and substitute?
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 Return to top of page · Post #: 1 · Written at 8:28:56 PM on 22 May 2014.
Maven's Gravatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

I thought the 6CA4 in the Pye R17-1A I am working on looked sooty, but after checking the chassis, transformer voltages and socket for continuities and shorts, and inspecting for any dirt or loose connections, I thought I would risk firing it up.

The filament immediately burned brightly for a few seconds, sparked, and then what looked like a plate halfway up the valve glowed hotter and hotter to almost white heat until I reached the off-switch. During this period there was some buzz in the speakers.

Switching on again, the plate maintains a glow, about what you would expert from a heater filament. All inputs test OK but B+ is zero.

Q1. Does this suggest a problem down the B+ circuit that needs finding before trying another valve? I know there is no short to ground.

Q2. While waiting to get a replacement 6ac4, can I substitute another (compatible) valve type to get enough B+ to test out voltages to the rest of the circuit?

Q3. I have a spare 6V4 that I read somewhere is pin-compatible but lower power - can I safely try that in the socket?

Q4. I have several 6X5GTs. Is it safe to patch one of these up to the 6CA4 socket to give me a temporary B+? Voltage specs seem to be within range. I'm aware I would be patching 240vac and 250vdc as well as heater circuit.

Q5. This might be heresy, but how safe would it be to put in (a) a diode bridge rectifier or (b)a single diode for half-wave, with a resistor in series?

Maven


 
 Return to top of page · Post #: 2 · Written at 12:00:11 AM on 23 May 2014.
Viccadillac's Gravatar
 Location: Perth, WA
 Member since 7 May 2012
 Member #: 1140
 Postcount: 157

Hi Maven,

I presume you replaced all the capacitors before you switched it on,

The 6AC4 is shorted and I have had a few do what yours did even after doing the capacitors. Don't keep switching it on or the mains transformer will burn up.

Wait for the new tube arrive even though you can muck around and replace it with modifications. Several US suppliers and Evatco have them.

Cheers
Vic


 
 Return to top of page · Post #: 3 · Written at 12:06:53 AM on 23 May 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6678

If the rectifier is running seriously red in the face, then the usual suspects would have to be the filter caps providing short circuits, or near to shorts, to the rectifier.

Before even powering it up, I would do a re-cap as a priority on any unit as old as that one!!

According to my data, 6V4 has same pin config and similar characteristics to 6CA4.


 
 Return to top of page · Post #: 4 · Written at 8:44:20 AM on 23 May 2014.
Maven's Gravatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

[sigh!] I keep hoping I have picked up an item where it is not necessary to replace every cap in sight, just in case.

I usually end up paying more for the set of caps than the final value of the object I'm working on - since I only do rescue jobs on derelicts that nobody wants for aesthetic reasons. Looks like a recap is the only option, again.

Maven


 
 Return to top of page · Post #: 5 · Written at 1:05:05 PM on 23 May 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6678

Yes, re-capping can be expensive, but it's really a given as regards the electrolytics and probably the wax/papers too (where some are notoriously worse than others).

However, what is even more expensive is allowing a bad cap to result in a burned out transformer.

So, when deciding to bring one home, I think one should look at any unrestored set as a patient in need of some basic surgery, and factor that in to the amount you are willing to pay for it, and/or the amount of time you are willing to spend on it, to make it operational and safe (especially as to fire risk).

In summary, when looking at any derelict set under consideration for adoption, I think you should say to yourself, "Hello my little re-capping opportunity".


 
 Return to top of page · Post #: 6 · Written at 2:34:38 PM on 23 May 2014.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 7290

I've had a few condensers go on me and they either pop very loudly, scaring everyone in a two km radius, or damage other components or both.

I just replace all electrolytics, paper and the black ones as a matter of course now. So that no new materials are wasted I will remove all valves, clean the chassis, check for mains faults and then test the transformer first.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 7 · Written at 4:03:54 PM on 23 May 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

Which "black" ones. AWA used round black ones with a brittle casing, they often snapped off at the ends.

They also used a black encapsulated Mica, and an open construction wafer mica the same. I put a bandmaster D184E back into service with every original one of those open laminated Mica's recently. .500V they did not fail. Never seen a black one of those Mica's fail.


 
 Return to top of page · Post #: 8 · Written at 4:20:35 PM on 23 May 2014.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 7290

The axial jobs, about the size of an underweight C cell. They expand and crack like a WACA test wicket.

I have to say that I have had very few problems with mica condensers. I may have replaced two or three in my 26 years of collecting. One was due to the porcelain being cracked and putting things out of alignment.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 9 · Written at 5:02:43 PM on 23 May 2014.
DJ Oz's avatar
 Location: Central Coast, NSW
 Member since 18 April 2014
 Member #: 1554
 Postcount: 215

Well thats kinda sad Maven Sad (and understand your point of view)

But as you know I am a noob in this.

Still one thing I do know from electronics is all electrolytics are finite in life so it usually better to cut to the chase and get rid of them to rule out further damage...filter caps are work horses and do take a beating all of their lifes...so even if working its pretty much for how much longer and really after so many years they wouldn't be in an optimal state anyway for the job...so you'd have somewhat degraded performance to start with.

On the Valve Diode questions I'll leave that to the others that do know but will say what your suggested is possible.

But it really comes down to how the circuit operation will be affected by the psu circuit mod in question or more so how well the psu and circuit will behave together for the different configurations.

From what I know of you here I think you understand what I mean (and probably why your asking the questions I cant directly answer...sadly)

Seem a lot just go "stuff the tube rec" and whack Diodes in but doing so can possibly not be such a great Idea without pandering to the whims of the circuit demands.

I've been reading a lot of posts on the 6X5 and think that would probably be a risky idea due to how the circuit might behave if you push those things too hard...they can and it seems do fail with higher risks of very bad results happening.

Quite controversial that tube but well some say the GT version and operating it within it rights parameters is safe enough. So I am erring on the side of caution in an unknown medium (your set and it as a mod) in my comments.

So id be wary of using them as a test bed for the PSU.

I'd be incline if you unsure to use diodes but design the circuit to mimic the behaviour of the 6CA4 this seem to be the done thing if using SS Diodes as replacements in radio Sets anyway (diodes are cheap but it would have to be done right I guess first time out if your having issues)

Still, if you got one and are prepared to risk the tube GTC answer is probably the way to go.

Heres a pdf on the 6CA4:-
http://frank.pocnet.net/sheets/093/6/6CA4.pdf.

6V4 (or it seems EZ80):-
http://www.r-type.org/pdfs/6v4.pdf.

Bit fuller but in French I think:-
http://frank.pocnet.net/sheets/020/e/EZ80.pdf.

No idea on the actual info thought, how accurate or correct.

Edit on the 6V4 being not as powerful.....I may get this totally wrong but I'll say it anyway as I think that was an important question.

Provided the the Circuit demands dont exceed the Safe operating specifications of the 6V4 it should be OK to use it...its really how much the circuit requires Current wise to operate.... thought yes there are other factors to consider with that ...voltage, plate dissipation and surge currents too I suppose but like I said NOOB here.

So how these play out I have no idea really with what your working on.

GTC Seems to think you'll be ok so I'll bow to his vast experience here.

Just want to emphasis the point that using something of less then designed for you really need to be sure your safe to do so thats all.

Personally I like the idea of fuses (even as temporary preventative) at lest on the mains side.... in the hope transformer wont blow to protect the fuse..the transformer is as has been said above your most "valuable" and hard to replace item...thought I guess its fused anyway.

Hope my NOOB comments are of some use to you.

But yeah it seems preemptive "Recapping" is the price of doing business with Old Vavle Tech...so you'll just have to get use to the idea of it being a first step (most of the time)

HTH Cheers Smile

And remember Noob Here so I am quite liable to get it wrong.


 
 Return to top of page · Post #: 10 · Written at 5:59:53 PM on 23 May 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6678

GTC Seems to think you'll be ok so I'll bow to his vast experience here

I wouldn't refer to my experience as vast by any means. I'm just a hobbyist. There are members on here who either did this for a living, or still do. (Marcc seems to be doing a set a week, if not more.)


 
 Return to top of page · Post #: 11 · Written at 6:19:53 PM on 23 May 2014.
DJ Oz's avatar
 Location: Central Coast, NSW
 Member since 18 April 2014
 Member #: 1554
 Postcount: 215

Well from reading some of your posts GTC seems you know a fair amount and have a fair bit of the really needed experience there, which I dont have at all really...not with valves lest wise.

Yes Marc is very knowledgeable I've been tripping over his posts in many a forums thread LOL.

I dont know how he finds the time...but his input is welcome.

Anyway hope some of what I said is useful to Maven.
Thanks GTC.


 
 Return to top of page · Post #: 12 · Written at 6:37:27 PM on 23 May 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6678

Well from reading some of your posts GTC seems you know a fair amount and have a fair bit of the really needed experience there,

I was heavily into electronics as a kid, and that meant valves. I spent some time in telecommunications after leaving school, but lost interest after a few years and got right out of the electronics field. I took up valve equipment restoration for my own amusement about 6 years ago.

I soon realised that I had forgotten a lot of valve circuit theory and had to hit the text books again. In reading those, and as things came back to me, it struck me that as a kid I knew a heck of a lot about this stuff. Amazing how it fades when you don't use it on a daily basis, but it can be recalled when you hit the books again.

A funny thought came to me as I wrote this. Back in high school for one year I sat beside a guy who was also into electronics. He was dux of the school and when asked what he wanted for his prize he nominated the book "Scroggie's Foundations of Wireless and Electronics", which was an expensive hardcover tome back in the day.

He didn't want to read it immediately, preferring to concentrate on his studies (he became a doctor), so I read it from cover to cover during school hours whenever I could get away with it.

I was reading my own copy of that book a few years ago and schooldays memories came flooding back.


 
 Return to top of page · Post #: 13 · Written at 9:13:12 PM on 23 May 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

Australian Mica's rarely fail, some of the ones in the really early sets, were unpredictable but the ones in that D184E era survived 500V and as said, those are not a worry, the SM's in the Bakelite pack will fail at times.

Normally it is the ones on HV DC that are the common fails, but not exclusively. I will leak test them if I have an end out of circuit.

If you get a US set very different story. There is a brand called "Micamold" caps are not all Mica. Anything with that brand should be treated with contempt & replaced.

Marc


 
 Return to top of page · Post #: 14 · Written at 9:54:33 PM on 23 May 2014.
Maven's Gravatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

No advice is wasted - thanks all.

This is the most "modern" set I have worked on (1960). It's the last Pye portable in their "briefcase" range to use valves. They were selling transistors by the end of that year.

I'm biting the bullet and ordered replacements for all electros and some greasy waxed paper caps I found lurking around the transformer. I had thought those were obsolete by 1960.

Also ordered EZ81 NOS tube -it will take a while by post.

As to cost - finding the advertisements in Trove cheered me up as I found that this model was selling for 59guineas in 1960 - so I'll still be marginally ahead even ignoring inflation. I've sent the ad to Brad for attachment.

There was a cheaper model Stereo 22 that was player only, the "Major" model added the radio for ten pounds extra.

Maven

Pye Stereo 22 Tablegram


 
 Return to top of page · Post #: 15 · Written at 10:07:43 AM on 24 May 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

I do not mess about with sets, and only concentrate on the electrics, albeit I will do the occasional cabinet minor repair. It actually does not translate to one a week, but it has happened.

I did cop over 200 in one two year period, around ten years ago. I was being a carer, so was chained to the house.

Having common parts on hand and no blown transformers, complications (hackers), or valves, does mean that you should have a 5 valve set chassis refitted, re-calibrated & burn tested within 3 days. Considerably longer, of course If the chassis needs work, or you have to wait, find, or make parts.

My main issue is that my back is rapidly running out of disks, several are no longer there. Not helped by the Motorcycle & visually impaired people who should not be on the road: Knees have taken a hammering.

So one works in cycles. When the back, or knees have had enough, one sits, that does not last long as the back starts complaining. Sitting idle achieves naught, and so one might as well do something productive in that period.

That is the main reason for appearing to have time.


 
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