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 Post subject: Wow what an amp - interesting
PostPosted: Jul Fri 06, 2018 9:37 pm 
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My neighbor, a young fellow asked me if I could help him locate a new tube for his parents' Motorola console. They live in the DR and sent him a photo of the tube ... but it had no clear id numbers on the glass in the picture so I asked him to have them also send photos of the raadio and the model number.

When I looked up the Motorola model number SK79W, HS968, HS969
https://www.radiomuseum.org/r/motorola_ ... 9hs96.html

I could easily see the rectifier was the tube he was seeking. A 3DG4

That was a surprise to me too as I'd never heard of it.
But holy cow it takes nearly 4 amps on the filament !!

But as I looked at the whole schematic I was blown away by the tube count of 21.

It uses six 12ax7s and six 6V6s.
Three output stages include a middle BASS amp.

Very interesting...
Seems Motorola went "all out on this design"

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Fri 06, 2018 9:52 pm 
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Motorola made a bunch of very nice stereos in that era. Many in furniture grade cabinets. When they went solid state their "Stereos on the Lid" also had great tuners.

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 07, 2018 4:26 pm 
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The 3DG4 was used in the first Zenith color TV chassis, the 29JC20, and it may have been used in one of their B&W sets in the 1961-62 era as well. They should still be available; I might even have one in the shed.

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 07, 2018 9:33 pm 
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Tim Tress wrote:
The 3DG4 was used in the first Zenith color TV chassis, the 29JC20, and it may have been used in one of their B&W sets in the 1961-62 era as well. They should still be available; I might even have one in the shed.

Thanks Tim :
We found them on ebay cheap... 3 or 4 bucks. So I think he already ordered one.
Appreciate it though very much.

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Mon 16, 2018 1:36 am 
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Times sure have changed.

A 6v6 pair puts out about 20 watts max. In this case they have another pair for the main amp, so they are using the same power rating for both amps.

Recently I put together a modern system for modern music. It's an active system, which uses a DSP box, splitting the signal up for each driver, which then requires it's own amp. The tweeter horn has about 20 watts. The 6" mid uses about 60 watts, the 15" mid-bass about 150 watts, and for the bass I'm using two 18" subwoofers using about 300 watts each, so 600 watts total needed for the bass, though my bass amp puts out 450 per speaker. I think the rough rule is for half the frequency you need four times the power to reproduce it faithfully, all other things being equal.

That Motorola system was no doubt pretty good for the day, but honestly it took some trial and error for me to put together the system above - anything less ran out of gas pretty quick as soon as I put on some dubstep or metal, and even lounge tracks can have a lot of bass going on.

If you can't feel the bass in this, it's because your system just isn't up to the task - if your system is flat to 25 hertz, you will feel it:

https://www.youtube.com/watch?v=fASdRb5iUTY

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Thu 26, 2018 6:30 pm 
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That rectifier sounds like an industrial type. I have a pair of 3 something or other in my tube collection. They came from the maintenance department at the factory where I worked.
RW

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Thu 26, 2018 6:47 pm 
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The 3DG4 was used in some of Zenith's b&w tv sets as well.


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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Fri 27, 2018 4:40 pm 
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3DG4 was only in use for a short time as OEM, because that was the era when manufacturers were starting to use silicon rectifiers instead of vacuum tube rectifiers. It has very different specs than anything else, and required a power transformer with a special filament winding for it.

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Fri 27, 2018 8:21 pm 
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Mr. Detrola wrote:
3DG4 was only in use for a short time as OEM, because that was the era when manufacturers were starting to use silicon rectifiers instead of vacuum tube rectifiers. It has very different specs than anything else, and required a power transformer with a special filament winding for it.

What has to be particularly special about the 3v winding?
I agree that as far as filament windings go, a 3v winding is certainly unusual since 5v was typical. But is there anything else "special" about it other than the 3v?
And, btw, why in the world did they "need" to use or choose to use 3v vs 5v?

Makes me think about why did portable radios choose 1v tubes too?

Do low voltage, high current batteries last longer or weigh less than a higher voltage lower current one?

There must have been a good logical reason for all this, but as I think back to early table-top sets they used 2v tubes too.... and then went to 6v and then 12v... but not aware of why exactly?
Inquiring minds need to know?

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 12:18 am 
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Pbpix wrote:
....Makes me think about why did portable radios choose 1v tubes too?

Do low voltage, high current batteries last longer or weigh less than a higher voltage lower current one?

There must have been a good logical reason for all this, but as I think back to early table-top sets they used 2v tubes too.... and then went to 6v and then 12v... but not aware of why exactly?
Inquiring minds need to know?
1V (actually 1.4V) and 2V tubes are for native operation on batteries. Dry cell for the 1V (1.4V) types and lead storage cell for the 2V types.


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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 12:50 am 
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IIRC 6.3V tubes were introduced for auto radios, so the A supply could come from the vehicle battery while a small dynamotor supplied B+. Then 12.6V tubes for vehicles with 12V batteries.

-David


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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 1:50 am 
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Both last two answers are very good... thanks.. yes.

But why 3v on the 3DG4? ... and btw... why 5v?

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 6:17 am 
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If you examine the internal construction of a 3DG4 you will find it very different from a 5U4 or 5Y3. it looks more like a 5AR4. But it isn't indirectly heated. The tubes that form the cathodes are actually the filaments. Because of the large diameter of these filaments, they require higher current at lower voltage. The object was probably to get the absolute minimum voltage drop between cathode and plate to help stave off the conversion from tube to silicon rectifiers. The tube manufacturers did some mighty clever things in their last days to try and hold onto sales.

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 7:18 am 
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I too, kinda thought maybe they were trying to replicate the voltage drop of silicon.... so that's a good thought I guess.

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 9:50 am 
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Interesting historical anecdotes...suddenly I have an image of someone developing a tube to emulate the characteristics of a MOSFET......

As for rectifiers, it's pretty hard to imagine how tubes could ever have competed with a Ge or Si diode. Once SS reached even moderate reliability, I would have thought that tube rectifiers would have disappeared very quickly.........except of course where their sonic qualities were needed....;)

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 10:46 pm 
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pixellany wrote:
Interesting historical anecdotes...suddenly I have an image of someone developing a tube to emulate the characteristics of a MOSFET......

As for rectifiers, it's pretty hard to imagine how tubes could ever have competed with a Ge or Si diode. Once SS reached even moderate reliability, I would have thought that tube rectifiers would have disappeared very quickly.........except of course where their sonic qualities were needed....;)

of course... :lol: :lol:

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sat 28, 2018 11:57 pm 
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pixellany wrote:
Interesting historical anecdotes...suddenly I have an image of someone developing a tube to emulate the characteristics of a MOSFET......

As for rectifiers, it's pretty hard to imagine how tubes could ever have competed with a Ge or Si diode. Once SS reached even moderate reliability, I would have thought that tube rectifiers would have disappeared very quickly.........except of course where their sonic qualities were needed....;)

I think the big issue was cost. It took several silicon or germanium diodes to equal the performance of a powerful tube, particularly the voltage ratings. The diodes were relatively expensive at that time, so tube manufacturers were bent on improving performance and reducing cost.

The same philosophy applied to transistors, giving us Nuvistors and Compactrons.

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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Sun 29, 2018 7:05 pm 
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If you look you can see that fet and pentode characteristics as similar but the tube has a higher plate resistance. The pentode offers more flexibility as you can change the other grid voltages to change the behavior and a standard fet is a triode that thinks it is a pentode with no control of the other elements. High voltage fets are not very forgiving while a short overload on the pentode it usually survives. I prefer the fets for low voltages and tubes for higher voltages.


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 Post subject: Re: Wow what an amp - interesting
PostPosted: Jul Mon 30, 2018 3:36 am 
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I think they've licked the FET voltage problem. IgFETs are used in synchronous rectifiers and inverters for 2MV ultra-high-voltage DC transmission lines.

-David


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