Tube radio repair

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Re: Tube radio repair

Postby Mike Cleaver » Tue Mar 15, 2011 12:58 am

If it's working fine now, don't mess with anything else.
The reason that local AMs don't sound as good as distant ones is that they likely are overloading the front end because of high power and very high audio levels and compression, for which old radios were not designed.
You could put a switch in one of the loop antenna leads to disconnect it for the 50 thousand watt locals and there'd probably still be enough signal for decent sound.
As to heat, yes, they do get hot.
A look inside any old tube radio console will show you heat marks on the top cover that makes it look as if there's been a fire inside.
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Re: Tube radio repair

Postby TRENT310 » Thu Mar 17, 2011 12:23 am

Since the glue that I used to stick the flat loop onto to the back panel finally dried, I have been testing the GE C409's performance compared to the Electrohome, my favourite modern digital PLL radio, and my first one, an old Sanyo SW-MW tape recorder. I have considered all to have good AM broadcast band performance, which is important to me.

After filter capacitor replacement, the receive audio is very clean and quiet. It does not generate much internal noise, which is kind of surprising given that it is directly line powered.
I guess that eliminates a power transformer giving off stray line frequency that would be picked up by the radio portion. So that gives it a nice low noise floor and it is nicer to listen to weak distant signals. I generally use 1570 CKMW as a good test because the frequency is generally pretty clear and available most evenings, so I can hear how well the radio picks out the weak field strength. I wonder if the fact that it had 6 tubes instead of the usual 5 improved sensitivity.
Of course the inherent disadvantage to mechanical tuning variable capacitors is that they are not as precise compared to modern digital tuning, but the C409 has the classic air-dielectric solid vane variable capacitor that doesn't get scratchy like with the more compact film-dielectric types. I was also surprised that I did not have to clean the volume potentiometer on the GE, usually that is the first component I have to take care of with old radios.

The knobs are very nicely laid out, good for 2-handed operation when scanning the band. And it has a conveniently placed external antenna connection, indirectly coupled to the internal loop, which the other 2 do not have (although portable vs. tabletop design differences).

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Re: Tube radio repair

Postby Neumann Sennheiser » Thu Mar 17, 2011 3:59 am

The G.E. on the right?...now that's what I'm talking about! (nice that it's sitting on a vintage desk as well). My mom had one from that era tuned to CFAC each morning when I came out to the kitchen for my Captain Crunch. Yours seems locked in to CJCA.
"You don't know man! I was in radio man! I've seen things you wouldn't believe!"
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Re: Tube radio repair

Postby TRENT310 » Thu Mar 17, 2011 11:12 am

Neumann Sennheiser wrote:The G.E. on the right?...now that's what I'm talking about! (nice that it's sitting on a vintage desk as well). My mom had one from that era tuned to CFAC each morning when I came out to the kitchen for my Captain Crunch. Yours seems locked in to CJCA.

Yeah, the GE is the one in the middle as shown, and I guess I should tweak the indicator needle for higher accuracy. In the picture I was listening to KOMO 1000 on all 3 radios as a comparison test. Speaking of CJCA, the GE is actually capable of picking out 910 CKDQ from 930 CJCA, which has a very high field strength in my house (since I live like directly in the middle of its pattern).
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Re: Tube radio repair

Postby groundwave » Fri Mar 25, 2011 10:25 pm

"How hot are the tubes supposed to run? The 35V voltage drop ones, 35Z5GT rectifier sure runs hot when at full line voltage as well as the 35L6GT AF final. That's normal though, right?"

It's absolutely normal. According to my dusty, but still intact, tube data book, the series-wired "string" of all the filaments in that set are running at 150 mA. This means the 12 volt filaments (12SA7, 12SK7, 12SQ7 etc) are dissipating 1.8 Watts each. The 35 volt filament of the 35L6 power output stage as well as the 35Z5 rectifier draws a somewhat greater 5.25 watts.
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Re: Tube radio repair

Postby Anotherwpgguy » Sat Mar 26, 2011 9:45 pm

Although the tubes in The All Amrican Five" tube radios have always run hot, these days they run even hotter because in the late 50's to mid 60's when those 5 tube radios were pretty much at the peak of their popularity, the powerline voltage was about 110 volts. Then it crept up to 117 volts, then 120, and now its not unusual to see it around 122. That additional power is dissapated as heat ... from a type of tube which already ran hot in the first place.

On a couple of my AC circuits in my ham radio ops centre, I use a Variac to hold the line voltage down to 110 to 114 for my vintage Collins, Racal, and Drake tube equipment.

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Re: Tube radio repair

Postby Montroze » Sat Aug 27, 2011 12:11 pm

Hello, I have this same GE 409 that ive had in the garage forever, worked untill this year, opened her up and the dial (tuning) string is broken, does anyone have or could point me to a diagram on how to set up the string. Many Thanks
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Re: Tube radio repair

Postby Mike Cleaver » Sat Aug 27, 2011 4:51 pm

Ask Trent310 if he can pull his out of the case and take a couple of pictures of the stringing of the dial cord.
When you replace it, you need a cloth cord, not nylon or anything slippery.
There should be a spring or two included in the setup to keep tension on the cord.
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Re: Tube radio repair

Postby TRENT310 » Wed Aug 31, 2011 11:37 pm

I'll look to see if I have any pictures of that when I had it taken apart. I'd like to avoid pulling it apart unnecessarily again because I'll probably break something else.
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Re: Tube radio repair

Postby Mike Cleaver » Thu Sep 01, 2011 11:25 am

Looking back at the original pictures posted in this thread, it appears the dial cord stringing is pretty basic.
Seems to be a simple loop from the dial to the tuning capacitor.
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Re: Tube radio repair

Postby Montroze » Wed Aug 21, 2013 2:53 pm

Hi, I have been searching all day for a pic on how to restring the tuner, any pictures/ links would be awesome. Also does a new string have to be of a certain kind? (I could not tell on your picture) Thanks
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