Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Jun Wed 28, 2017 1:18 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 34 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Cap in output transformer
PostPosted: Jan Sun 08, 2017 9:42 pm 
Member

Joined: May Wed 31, 2006 6:41 am
Posts: 191
I found the other 6v6

I tested it on my tube tester and it is low on emissions.
Also i tested the tube that was installed and it is also low, but also when i move it the needle on the tester starts to jump.

I put the other tube and the radio works.
I have a biasrite, so i measured bias.
Voltage 232 volts
Current: 21.5 mA

This voltage is a little high. According to service manual the plate to ground voltage should be 220V and the cathode voltage should be 12V. so the voltage on the biasrite should be 208V

Didn't measured the actual cathode voltage, because i have to get some alligator clips ( i had some, but i don't know where they are.). I don't like introducing my hands on a plugged radio :P.

http://www.arimi.it/wp-content/Geloso/Bo39.pdf Here is the service manual

I simulated the amplifier part on ltspice (i put a 255V DC supply instead of the rectifier and transformer to make it easier).
according to service manual 255V is the voltage that should be on the second filter.

In the simulation always i get around 12V on the cathode of the 6v6, but to get 220V on the plate of the 6v6 i had to put a resistance of 790 ohms on the primary of the output transformer. The dc resistance of the actual trasformer installed is of 300 ohms. On the simulation the current is around 48mA.


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Sun 08, 2017 11:56 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 8616
Location: Victoria, Australia
The accepted variance in voltage is +/- 20%. The voltage across the 6V6 cathode resistor indicates its current the 25 Ohm resistor on the CT would have 1.63V across it if it is drawing its maximum (that resistor will be added to the cathode of the 6V6). It is not clear as to that being the limit of the transformer or what the set draws. However is is a guide to overload. Replacing parts especially filter caps & leaking paper caps can see a slight rise in voltage mainly due to the electrolytics not drawing as much current (10mA is quoted as the limit, on one tester I have here for the old ones. That is significant and would have been allowed for in design).

220V looks like the same as most of Europe. Most of those tubes should take 250V DC 6V6 -12.5V grid bias @250V plate. if its within the 20% I would not worry (and you still have a primary tap that will lower it anyway: viz 280)

Low emission is not always a guide to a tube not working. I have brand new 6V6's that test 65% and work just as well as some from the forties that refuse to die.

Long probes with very little metal at the tip and a lip to stop fingers slipping down are best. For monitoring I use IC clip leads.

I am not a fan of the 0.01cap from utility supply (mains) to chassis. It should be a line cap (X, X/Y) and on Neutral. It is there to provide an RF path to ground, as Neutral in most cases is connected to ground. Unfortunately with the NEMA plug (or incorrect wiring) it can be on Active which can produce undesirable consequences if it lets go. Here, we tend to retrofit cables with an earth lead to the transformer sets.

Marc


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Wed 11, 2017 11:24 pm 
Member

Joined: May Wed 31, 2006 6:41 am
Posts: 191
I already put a safety cap.
I also thought of retrofiting a cable with earth lead, but i didn't yet.
Also i plan to put a fuse.

Today the new tube arrived. I installed it and i noticed that i have to wait a lot less to listen. Also i think it's sounding better, but that may be psychological. i only listened the radio for a few minutes.

Now i will measure voltages again.


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 12:03 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 30083
Location: Maryland 20709, USA
Radio Fixer wrote:
A good thing is the cap is across the output transformer and not from the anode to B- (ground here) and so would have virtually the full B+ across it. All it has is the drop across the transformer primary, say 50V and the AC component so is having an easy life.

Don't know where Chaz is finding these high voltage capacitor destroying spikes from?
Would have to be massive to do that surely?

Chas is absolutely correct that you can have very high voltage transients across that cap.
This is true regardless of whether it's directly across the primary or from plate to ground.

If the tube's plate current gets completely cut off, as with a static burst, the voltage at the plate is generated by the inductive kick from the transformer, which can rise very high.

The voltage rating of capacitors at the output tube plate are usually 1000 volts or more for that reason.

- Leigh

_________________
73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 10:20 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2219
Location: England
Never fitted more than 630V plastic film myself Leigh and never had one fail. That's about what everyone uses over here.

Good point about inductive kick back.

Gary


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 11:10 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 30083
Location: Maryland 20709, USA
Radio Fixer wrote:
Never fitted more than 630V plastic film myself Leigh and never had one fail.
Meaningless response.
Unless you've experienced a statistically-significant number of lightning events...
which would be in the millions.

Engineering "best practice" is not based on what folks have observed.

Sorry, but there's no way you can justify using a cap with a lower voltage than the original part.
High-voltage caps cost more than lower-voltage caps. The manufacturer used them for a reason.

- Leigh

_________________
73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Last edited by Leigh on Jan Thu 12, 2017 11:25 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 11:24 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2219
Location: England
Well that's true. How many of the original wax paper do you find blown? How did any survive? Leaky maybe ...

What did the makers fit? Have to look in some service manuals. I wouldn't have fitted a lesser voltage than the original part I think ... need to check

Gary


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 11:28 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 30083
Location: Maryland 20709, USA
Radio Fixer wrote:
What did the makers fit? Have to look in some service manuals. I wouldn't have fitted a lesser voltage than the original part I think ... need to check
Gary,

This thread is about one specific capacitor at one specific location in the radio, not about caps in general.

Although my comment about cap voltages applies to all.

- Leigh

_________________
73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 1:09 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2219
Location: England
Alas, I didn't make myself clear ! Sorry!

I was referring to that particular cap that started the Thread. I have found blown original caps on the primary and secondary of the mains transformer but not on an output transformer. Yes! statistically that proves nothing but its a lot of radios ... not millions though.

I accept the reason to fit a high voltage quality cap, inductive kick back in this position is valid. Something else learned.

Have to look through some service manuals and see if they give the voltage for the cap across the output transformer.

Right not definitive I know: Zenith no cap > 600V, AK 500V, Defiant (the cap we are talking about) 460V, Murphy (in a top cut cct to ground) 1000V.

Personally I feel happy to leave my 630V plastic film types in. I wouldn't be concerned to leave the 1000V metalised film type that the OP fitted either, its a branded maker Panasonic and should have quality control even if made in China.

thanks Gary


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 3:48 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 10805
Location: S. Dartmouth MA 02748-1225 USA
See: https://en.wikipedia.org/wiki/Film_capa ... ge_ratings

Chas

_________________
There Are None So Blind As Those Who Will Not See: John Heywood, 1546


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 6:51 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2219
Location: England
That's quite a read Chas. Will have a go at it in bed :D

I buy these: http://www.farnell.com/datasheets/20506 ... 1460531717

as a one voltage fits all, in other words just buy the one voltage (630V) for everything (excepting electros and X / Y of course). All the folks I know do but they may be a different make.

"There Are None So Blind As Those Who Will Not See: John Heywood, 1546"

Hope this wasn't intended for me; trying to get with you :D


Gary


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Thu 12, 2017 8:49 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 10805
Location: S. Dartmouth MA 02748-1225 USA
Quote:
Hope this wasn't intended for me; trying to get with you
Not really, I wanted to find out if there were some other "investigations" on metalized caps. I was quite surprised at finding a WIKI. So I thought I would share it. The data seems valid, to my surprise there ARE metalized for pulse applications. The internal construction is different. I also see that foil/flim if misapplied, say for an across the line cap is a poor choice as some, again its the internal construction, will fail short.

I note that a cap application chart in the WIKI looks familiar. Seems a member chose to post the chart in another thread...

Chas

_________________
There Are None So Blind As Those Who Will Not See: John Heywood, 1546


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Sat 14, 2017 11:03 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2219
Location: England
This thread turned out to be interesting and informative. I now know something about the poly caps that I used to just use, a great piece of work that article on Wiki.

It seems the metalized film is almost universal with polyester dielectric PET. I could find no axial film / film types from one of the UKs biggest suppliers who stock millions of items (4917 film caps). Other metalized film types are available using polypropylene PP dielectric but in a limited range of values with no obvious advantage. Of course they are more expensive.

Can buy Vishay metalized film as I do at 630V which are rated for 1.6 times that for 1 minute so I kV (actually they will stand it for as long as I want to leave my battery tester on, of course that proves nothing). They are sold in quantity to BVWS members about 1300 as, to quote: "... to replace all old paper types in vintage equipment".

1000V Vishay are available but at twice the price (£.948 / £2.33) so I doubt anyone uses them. Anyway the 630V ones do the job and in virtually all cases are better rated than all original parts in old radios.

Can also buy much the same spec 630V cap from Cornell Dubilier but its imported from the US so a £15.95 hit for shipping. Also the caps cost £2.71 each

These prices are for .1 mf (used as an example) and drop to about 60% for quantity.

On Brit Forums it is not unusual to read about replacement blown X or Y caps but I can never remember reading about replacement blown caps on output transformers. Whilst excepting that the output valve could 'hit the rails' on noise transients and inductive kick happen, IMHO it would need all other stages of the radio not to limit and the volume control to be turned to maximum. Unlikely for most radios and most people operating old receivers. Don't leave them on and unattended if grand kids are around :D .

So in conclusion I'm happy with the capacitor type and rating I'm using. Of course any other restorer is free to use higher voltage, spec and priced items if they wish.

Gary


Top
 Profile  
 
 Post subject: Re: Cap in output transformer
PostPosted: Jan Sat 14, 2017 3:24 pm 
Member

Joined: May Wed 31, 2006 6:41 am
Posts: 191
Indeed this is an interesting therad.
When you have so many options sometimes is difficult to decide wich is better.

I know where to use safety caps. Also i know where don't use ceramic caps. but, for example i didn't knew about the pulse rating of the caps.

I have the service manual for this radio and it calls for a 2000pF 1500V paper cap. I think this value is here for a reason.

Finally i found 2000pF 2000V polyester. ( i found them at home, i didn't remembered tha i buyed some years ago). I used this and seems to be working good.

The radio is working, but i found another problem.
I will make another thread for this.


Top
 Profile  
 
Post New Topic Post Reply  [ 34 posts ]  Moderators: Marcc, Norm Leal Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: CLC and 14 guests



Search for:
Jump to:  




















Privacy Policy :: Powered by phpBB