ANTIQUE-RADIO-LAB

Antique Radio Forum for Collectors antique-radio-lab.forumotion.com
 
HomeHome  RegisterRegister  FAQFAQ  SearchSearch  Log inLog in  

Share | 
 

 *SOLVED*Tube substitution

View previous topic View next topic Go down 
AuthorMessage
copain



Join date : 2011-12-28

PostSubject: *SOLVED*Tube substitution   2012-10-03, 3:10 pm

In a previous post I mention the Airline 64WG-2700A I've been working on. When I had finished cleaning 65 years of dirt, grime, and dust from the chassis and big cabinet, recapping, and installing a new line cord and plug, I turned on the unit. I had that pleasant rush of success as reception started to come in. However, when I advanced the volume control a small amount, some unpleasant audio distortion appeared. Substituting tubes did not end it, nor did voltage readings reveal anything that would cause the distortion. Checking in Marcus and Levy's Elements of Radio Servicing, I learn that almost all audio distortion is caused by tubes. But I had already checked the tubes to no avail. Then, maybe from desperation, I removed the output tube, a 6V6GT, and substituted a 6F6. Guess what? The distortion was gone and the unit played louder! The unit ran about two hours and no problems showed up. Now, I know that 6F6 is an ordinary power pentode and that the 6V6 is a beam power amplifier and that the pin-out is the same for each one. But why would the 6F6 yield better performance? I left it in and told the owner his radio was ready. He picked it up today and was highly pleased with its restored appearance and performance.
Back to top Go down
Cliff Jones
Site Administrator
Site Administrator
avatar

Join date : 2010-11-22

PostSubject: Re: *SOLVED*Tube substitution   2012-10-04, 11:59 am

The 6V6 Draws more current for the filament and grid and plate, so this means that you have a circuit that either is failing to meet the load or bad resistors or caps that are out of tolerance. I would suspect both due to the age. Resistors climb in value with age and limit current more than they should, or caps that are starting to short or leaking. To me the symptoms show the need for further checking. Even the grid voltages are different. Look at a book on tubes and compare the parameters against each other they are significantly different even though the pinout is the same one and also because of design difference one can have a larger voltage swing and since the voltage is restricted by out of tolerance components, distortion is more apparent. Start by looking at the actual measured component values of each.

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
Back to top Go down
copain



Join date : 2011-12-28

PostSubject: Re: *SOLVED*Tube substitution   2012-10-04, 1:19 pm

I guess I thought you would assume that I had routinely changed all the capacitors plus three or four out-of-range resistors. Also, in the original post I clearly stated that voltage readings revealed nothing that would be causing the distortion. I also stated that the unit ran two hours after the tube substitution with no problems showing up. Your reply seems to indicate that I had not done these perfunctory tasks, although maybe I'm misinterpreting.
Back to top Go down
Cliff Jones
Site Administrator
Site Administrator
avatar

Join date : 2010-11-22

PostSubject: Re: *SOLVED*Tube substitution   2012-10-04, 4:42 pm

When you said re-capping I assumed you meant just the power supply caps, (as that is what most people do first off) since all caps were not mentioned, I know, I know, getting a little technical, but I cannot read between the lines and have to make an assumption here and there till clarification. Now you state after it was up and running then it changed, because of that statement I was assuming you didn't change all the dubious parts. All electrolytics and all paper caps defective or not.
Did you check for hairline cracks in resistors? corrosion in tube sockets? Dirty tube pins?

There are a couple of items that should be addressed, Not all replaced components are considered as working as intended. Many new parts fail by breaking down after replacement, so, even though you say it worked, is no reason to assume that's still not a problem area. Always consider new replacements, will and do fail often. So back to the drawing board. Testing of tubes is a weak link also, as they may check out fine on a tube tester, but fail under actual voltages and loads, A tube tester cannot provide a load to the plate or operating grid voltages the same way, as when in the actual circuit. They however can simulate some parameters such as a 60 cycle or 400 cycle signal to the grid, not to say there aren't tube testers that can accomplish that.

Cold solder joints fractured solder joints and connections are always a possibility.

One big culprit is heat, and after you said for two ours it played OK then went into distortion, can happen as everything heats up, even solder connections that look good, Transformers heat-up and if to close to upper design limits can breakdown and cause internal windings to short, capacitors and resistors can change values even when new if they are cheap. Check the input voltage especially when working with older radios, Higher line voltages can effect radios, there are just so many variables its hard to pinpoint in a forum easily. Be open to speculation as I cannot know exactly what you did and did not accomplished using generalities, even though you think you were clear in your explanation.

I am used to using details in my previous work, and had to follow precise written instructions, then we had to explain in written detail even which parts were tested and to put down the measured values,(voltages, currents, signal strengths, dBMs, calibration tolerances, operational parameters and test equipment used and required and their calibration dates and expiration's, all in triplet and even more copies to make a paper trail.) before and after changes.

I always made suspect and resolution notes for my own future reference because It helped to see if there was a pattern developing and common errors, no stone was left unturned Suspect . Physically being there, One on one is always easier, However explaining by forum isn't always the most efficient way to go, but since you haven't found the reason for the conundrum you are facing, one would have to start to make assumptions based on our own personal experiences, and apply the same methods by asking and double checking.

In closing just remember we are here to help and some times its a slow diagnostic process, and other times it can be quick. study

Did you do a comparison of voltage checks before the problem and after the problem arose?
I would, by reinstalling the correct tube and see if problem is evident to start with or if it again goes a certain amount of time before developing symptoms. If it is already present when using the old tube then I would retest the tube. Just to make sure that the tube wasn't damaged. More detective work ahead.

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
Back to top Go down
FrankB
Moderator
Moderator
avatar

Join date : 2010-11-22

PostSubject: Re: *SOLVED*Tube substitution   2012-10-06, 8:11 pm

More than once i have been stung by a tube that tested fine in a checker but that failed in the set under actual operating conditions.
I also like to tap on the tube while in the tester and radio. That tapping can reveal problems that a non tapping test wont show.
Ie intermittently shorting elements or - most often- microphonics.
Back to top Go down
copain



Join date : 2011-12-28

PostSubject: Re: *SOLVED*Tube substitution   2012-11-15, 4:07 pm

You said: "One big culprit is heat, and after you said for two ours it played OK then went into distortion. . . ."

I did not say that. Please return to my original post and re-read it carefully.

You might try printing out a post, especially a longish one, that you intend to reply to for quick reference as to what was actually stated, rather than attempting to remember and then posting an erroneous response.


Back to top Go down
Cliff Jones
Site Administrator
Site Administrator
avatar

Join date : 2010-11-22

PostSubject: Re: *SOLVED*Tube substitution   2012-11-15, 9:49 pm

I stand corrected.


You have solved your issue by substitution, but the underlying cause needs to be investigated as to why the difference from the original design.

A tube can cause distortion because it is operating outside of it's characteristic curve. One condition would be, that the negative grid voltage has changed to the point where the plate current can no longer operate in the linear portion of the operating curve for that tube.
Since you replaced the tube with an exact replacement and the distortion was still present then that would verify that the previous tube used was not at fault. But this would be a clue that a voltage was off. (Yes I did note your comment of voltage checks revealed nothing)

Either the B+ or the grid voltage. A very minor change in the grid voltage would change the signal swing scale into the non-linear portion of the curve and would result in plate current change and distortion. So a needed change would be to reduce the amplitude of the alternating voltage applied to the grid. (This is a clue to research to completion to see what has changed and maybe the best solution ). Or to change operating characteristics by using a different type tube which you previously stated that will accommodate a larger signal input, . That does solve your problem, (However, still I think may just mask the underlying cause.)
I will mark this as solved.

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
Back to top Go down
 
*SOLVED*Tube substitution
View previous topic View next topic Back to top 
Page 1 of 1
 Similar topics
-
» Young Woman Loses Breathing Tube In Dream...help please
» GCL & COMPANY LAW CASES SOLVED!!!!!!!
» Abadi of the Organization of Islamic Cooperation: conflicts are not solved by war
» Substitution of the 10% Retention Money for Infrastructure
» 99 ballons..and a dear friend (requesting prayer tor their family)

Permissions in this forum:You cannot reply to topics in this forum
ANTIQUE-RADIO-LAB :: Radio Repair Bench-
Jump to: