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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Mon 23, 2017 1:15 am 
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The IF bandwidth is not a problem.

I recommend taking the signal at the 15K to 180 pF junction, and
reducing that cap's value by quite a bit more than the capacitance
of the cable going to the decoder.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 28, 2017 2:25 pm 
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dtvmcdonald wrote:
The IF bandwidth is not a problem.

No, it is not. I just tested the little tuner and it can pass signals well into the 100's of khz.
So, I'm stoked! Lets see how far I get this weekend.

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 28, 2017 3:12 pm 
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Fun project.

Certainly it is clear to the O.P. that program source can be most any stereo source, mp3, computer streaming, Digital TV, records, tapes, even the Indy 500, Roller Derby?, For example the Ball Game on the big screen the audio coming from the Zenith & Sparton :?

The system from the mid-50's was Binaural, I had a couple of binaural receivers, a Bell (TRW) and a Horizon Criterion...

Chas


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sun 29, 2017 1:39 pm 
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Chas wrote:
Certainly it is clear to the O.P. that program source can be most any stereo source, mp3, computer streaming, Digital TV, records, tapes, even the Indy 500, Roller Derby?, For example the Ball Game on the big screen the audio coming from the Zenith & Sparton :?

Oh yes, I get that, and will be adding inputs for that purpose.
The MPX unit did not work as of when I quit yesterday afternoon, but when I got up this morning I replaced a couple of small value electrolytics and got the MPX working.
One cap was a bypass and the other was a coupling. There is some "tweaking" on both the tuner and the MPX still to be done, but it works, and it is stereo.
Now I can proceed with the chassis as time allows.

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Wed 01, 2017 2:03 am 
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Okay,I'm having fun. the wires are running across the floor to the theremin amp on one side and the six tube zenith on the other.
Steroe..LOL fun
The tuner and MPX are on the kitchen counter,with the wires hanging by threads.
I need the transmitters and the chassis.... to much work so little time.

This a fun project and I think if you like it you should try it.
Anybody can make this stuff with IC's and transistors, I like convincing tubes to do what I want.
Power supplies are still cut and fit for me. And I can hear them sometimes. So I fix it.

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Tue 07, 2017 12:12 pm 
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The chassis metal is brass, recycled from old door kick plates.
After cutting and bending, I attempted to strip the clear coat off the brass.
One kick plate stripped fairly well with methylene chloride, but the other kick plate had a tenacious coating on it.
I tried chemicals, heat gun, and mapp torch. Nothing touched it. The torch just made it stick more.
Out of viable options, I experimented with electrolysis. That worked.
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Here is the rig
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The brass with out the coating only needed a few seconds to respond to cleaning
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A wood frame for the brass chassis was cobbled up
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The transformers that I scrounged up to use were ugly and mismatched, so some time was spent switching end bells.
The closest to spec power transformer needed had no filament windings, and output volts too high.
The chosen filament transformer was also over volt by a volt or two.
A transistor center tapped power transformer was salvaged to buck both the power and filament transformers each by an appropriate factor.
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This is about what I have in mind
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rear_view.jpg
rear_view.jpg [ 69.34 KiB | Viewed 423 times ]

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 7:55 pm 
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I have been working out what I need for filament power and I noticed something that is sort of counter-intuitive, at least to me.
The 6888 tube filament draws 800ma @ 6.3v.
I wired the tube filaments in series to utilize a 12v transformer.
So the tube filaments in series on 12 volts, the current is still 800ma.
This should not surprise me should it?
Is this a classic example of current and voltage are inversely proportional?

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 8:25 pm 
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kc5gym wrote:
I have been working out what I need for filament power and I noticed something that is sort of counter-intuitive, at least to me.
The 6888 tube filament draws 800ma @ 6.3v.
I wired the tube filaments in series to utilize a 12v transformer.
So the tube filaments in series on 12 volts, the current is still 800ma.
This should not surprise me should it?
Is this a classic example of current and voltage are inversely proportional?
It's closer to an example of Kirchhoff's Law. With equal value resistors in series the current through one has to be equal to the current through the other.


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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 8:44 pm 
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So for a somewhat silly example, if twenty 6888 filaments were wired in series across the 120v line the current will be 800ma?

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 9:03 pm 
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kc5gym wrote:
So for a somewhat silly example, if twenty 6888 filaments were wired in series across the 120v line the current will be 800ma?

Yup...

Well except for fact 20 6888 equals 126v, so 120v applied to the string would be slightly less than 800ma...


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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 9:14 pm 
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One reason that I ask is that the MPX unit has a 450ma tube 6gh8a and a 300ma tube 12at7.
Realizing that I needed to make an adjustment for voltage drop, I treated the 450ma @ 6v as a "section" and the parallel 300ma filaments of the 12at7 as a 6v "section"
Solving for the parallel resistor I got 40 ohms which was correct in real life.

Now I am wondering if I just got lucky, or solved correctly.

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 9:22 pm 
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kc5gym wrote:
One reason that I ask is that the MPX unit has a 450ma tube 6gh8a and a 300ma tube 12at7.
Realizing that I needed to make an adjustment for voltage drop, I treated the 450ma @ 6v as a "section" and the parallel 300ma filaments of the 12at7 as a 6v "section"
Solving for the parallel resistor I got 40 ohms which was correct in real life.

Now I am wondering if I just got lucky, or solved correctly.
Just to be sure, they're not 6 V but 6.3 V. You've got an 'extra' 150 mA to take up in the 12AT7 "section" so 6.3 V / .15 A = 42 Ohm.


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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 9:46 pm 
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Okay, I was not factoring in the 0.3v, but I get it.
Sort of.
I used 40 ohm 2watt. The resistors that I was picking through are NOS Philco parts. They have all drifted up a lot despite being "new." I go by the measured value not the color code. Will these resistors continue to get worse?

One other thing arose. I planned to use 6sq7's to drive the 6888's just like the "Homebrew contest" transmitter.
Some of the 6sq7's seem to be out of spec on the filament current. One tube has 5.9v across its filament and the other has 6.4v . Should I worry about it and pick a better pair?

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Sun 19, 2017 9:53 pm 
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Okay, I was not factoring in the 0.3v, but I get it.
Sort of.
I used 40 ohm 2watt. The resistors that I was picking through are NOS Philco parts. They have all drifted up a lot despite being "new." I go by the measured value not the color code. Will these resistors continue to get worse?

One other thing arose. I planned to use 6sq7's to drive the 6888's just like the "Homebrew contest" transmitter.
Some of the 6sq7's seem to be out of spec on the filament current. One tube has 5.9v across its filament and the other has 6.4v . Should I worry about it and pick a better pair?

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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Mon 20, 2017 3:00 pm 
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Types like 6SQ7 were not designed for series operation though there were radios that used them as such... Within 10% is considered OK so either of your tubes would be OK...

I checked a group of 400ma 6K6 when I was restoring my Warco that used it in a 300ma string of tubes... Those ranged from approx 395ma to over 420ma... To create a fairly balanced string, i added a 36v@100ma #1822 dial light in parallel across the 300ma tubes... Prior it took a minute or more to warm up and 6K6 never had over 4.2v on heater...


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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Mon 20, 2017 8:25 pm 
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kc5gym wrote:
Okay, I was not factoring in the 0.3v, but I get it.
Sort of.
I used 40 ohm 2watt. The resistors that I was picking through are NOS Philco parts. They have all drifted up a lot despite being "new." I go by the measured value not the color code. Will these resistors continue to get worse?

One other thing arose. I planned to use 6sq7's to drive the 6888's just like the "Homebrew contest" transmitter.
Some of the 6sq7's seem to be out of spec on the filament current. One tube has 5.9v across its filament and the other has 6.4v . Should I worry about it and pick a better pair?
They changed once, they'll likely change again. I wouldn't trust them.

Every rating has tolerance and tube heaters are no exceptions. In fact, there are no exceptions at all, just missing information. 10% is considered 'normal' for that type of heater.


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 Post subject: Re: Stereo Transmitters
PostPosted: Nov Tue 21, 2017 11:57 am 
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Thanks for the insight, guys.
Perhaps I will find time over the Thanksgiving holidays to make some more progress.
I will post more pictures when that happens.

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 Post subject: Re: Stereo Transmitters
PostPosted: Dec Sun 17, 2017 5:31 pm 
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It"s coming along. Plenty of mistakes were made on this one, but I believe it will look presentable when completed.
Attachment:
top_side.jpg
top_side.jpg [ 56.79 KiB | Viewed 22 times ]

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