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 Post subject: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 2:52 am 
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I keep running across something I don't understand.
If you look at the output circuit of a Capehart 115P2-BQ see here http://bit.ly/2NbiUAT You'll notice there is a driver transformer T3 driving the grids of four 2A3 output tubes. Now if you look closely one of the output tubes in each section has a throttle resistor. see R117 & R118 both are 510 ohms.
So looks like two of the tubes would see the full signal from the driver transformer and the other two would see a slightly lower signal. This same design pops up in UTC 45/2A3 amplifiers as well, see here for example http://bit.ly/2NcEbKz looking at R7 & R8 valued at 2500 ohms each, and in another UTC circuit they come in at 2 meg ohms.
Now I haven't heard a real good explanation why you would need or want to throttle one of the output tubes in each section? One person did tell me that it wasn't uncommon for engineers to put in design flaws to trip up those who would copy their work. That may or may not be true. But the fact that I've seen this identical circuit from different manufactures makes me think there is a legitimate purpose for their existence.
Thoughts ideas?


Last edited by archeus68 on Jul Fri 13, 2018 4:11 am, edited 1 time in total.

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 4:00 am 
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It would help if I could see the schematics but the links you posted don't allow it.


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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 4:12 am 
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Audio power tubes in push-pull parallel will sometimes oscillate automatically. Slightly different values, either in grid or plate circuits will prevent this.

DG

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 4:22 am 
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Flipperhome wrote:
It would help if I could see the schematics but the links you posted don't allow it.

The problem most likely is on your end. radiomuseum.org and pintrist are very common and readily used websites on the internet. And the links work fine for me. But yes they do require membership. These are just the easiest way I have to post images.
There is a world of wealth here as far as being able to reference image data. I highly recommend investigating in using them.


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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 4:30 am 
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W7TFO wrote:
Audio power tubes in push-pull parallel will sometimes oscillate automatically. Slightly different values, either in grid or plate circuits will prevent this.

DG

Preventing Parasitic oscillations does seem like the most realistic answer. Thanks for your reply.


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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 5:09 am 
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archeus68 wrote:
Flipperhome wrote:
It would help if I could see the schematics but the links you posted don't allow it.

The problem most likely is on your end. radiomuseum.org and pintrist are very common and readily used websites on the internet. And the links work fine for me. But yes they do require membership. These are just the easiest way I have to post images.
There is a world of wealth here as far as being able to reference image data. I highly recommend investigating in using them.
Pardon me but the "problem" is they require membership.


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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 6:27 am 
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"throttle resistor" Interesting term.

Dave


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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Fri 13, 2018 6:45 pm 
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As a member of radiomuseum, every time you download
a schematic, you use up a portion of your membership allotment.

You build up your allotment by uploading things they want, or
by writing a review.

When someone has a question on this forum and references
radiomuseum material, why should responders use up their
allotments to reply ?

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sat 14, 2018 4:23 pm 
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These are known as "grid stoppers" and yes, they are to prevent parasitic oscillations. Resistors and/or inductors are almost always needed with parallel power tubes.

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sun 15, 2018 1:06 am 
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Image


A 1k ohm 1 watt resistor in series with each 2A3/45 grid would be the modern standard .

A small value inductor in series with each plate would be even better , seven turns of a number
16 solid wire loosely wound around a 3/8 inch dowel rod about 2 inches long and slid of the dowel
will keep them from oscillating ,

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sun 15, 2018 1:15 am 
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Flipperhome wrote:
archeus68 wrote:
Flipperhome wrote:
It would help if I could see the schematics but the links you posted don't allow it.

The problem most likely is on your end. radiomuseum.org and pintrist are very common and readily used websites on the internet. And the links work fine for me. But yes they do require membership. These are just the easiest way I have to post images.
There is a world of wealth here as far as being able to reference image data. I highly recommend investigating in using them.
Pardon me but the "problem" is they require membership.

They allow non members to request the print by email... a quick little step

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sun 15, 2018 3:09 am 
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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sun 15, 2018 3:20 am 
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I really like amplifiers designed by a transformer company. :mrgreen:

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sun 15, 2018 6:45 am 
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archeus68 wrote:
I keep running across something I don't understand.
If you look at the output circuit of a Capehart 115P2-BQ see here http://bit.ly/2NbiUAT You'll notice there is a driver transformer T3 driving the grids of four 2A3 output tubes. Now if you look closely one of the output tubes in each section has a throttle resistor. see R117 & R118 both are 510 ohms.
So looks like two of the tubes would see the full signal from the driver transformer and the other two would see a slightly lower signal. This same design pops up in UTC 45/2A3 amplifiers as well, see here for example http://bit.ly/2NcEbKz looking at R7 & R8 valued at 2500 ohms each, and in another UTC circuit they come in at 2 meg ohms.
Now I haven't heard a real good explanation why you would need or want to throttle one of the output tubes in each section? One person did tell me that it wasn't uncommon for engineers to put in design flaws to trip up those who would copy their work. That may or may not be true. But the fact that I've seen this identical circuit from different manufactures makes me think there is a legitimate purpose for their existence.
Thoughts ideas?


The 2M is and old way of saying 2 K ohms .

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sun 15, 2018 6:56 am 
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Yep, the upper case M is the same as the Roman numeral for 1000. The lower case m stands for meg.

Occasionally this: ω will show up in a schematic, it is the lower case of Ω.

Dennis

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Sun 15, 2018 7:56 am 
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Cant see ALL schematics. however the Pinterest link leads to some
ripe language graphic not allowed here.

Have a feeling that transformer characteristics (or lack of)
Radiotron Designer's, p580-581, composite curves , type 45 and 2A3,
need tweaking.

With the parallel driver tubes, in class AB1,
bias on a 2A3 is -62 volts. At part of the cycle
the peak signal wont (shouldn't) exceed the bias.

Were the grids in parallel, one tube might start
causing plate current to change, before
the other, because no two tubes are exactly
similar. This on a overload trying to drive
the stage AB2.

Attachment:
Snivets.jpg
Snivets.jpg [ 25.2 KiB | Viewed 1623 times ]

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Steve Dow
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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Tue 17, 2018 4:03 am 
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With all those transformers, the big old amplifiers were HEAVY! Mark, what amplifier is the second schematic of? It looks to be in the 250-300 watt rating.

RCA liked to put stopper resistors in series with each plate, in their PPP amplifiers; typically 47 ohms at 1 watt each. They also served as fuses, protecting the output transformer if one of the tubes f!ashed over.

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 Post subject: Re: PP Throttle Resistors.
PostPosted: Jul Wed 18, 2018 9:29 pm 
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Tim Tress wrote:
With all those transformers, the big old amplifiers were HEAVY! Mark, what amplifier is the second schematic of? It looks to be in the 250-300 watt rating.

RCA liked to put stopper resistors in series with each plate, in their PPP amplifiers; typically 47 ohms at 1 watt each. They also served as fuses, protecting the output transformer if one of the tubes f!ashed over.



http://www.bunkerofdoom.com/xfm/UTC_1936/UTC1936.pdf page 6


It is a 500 watt modulator .


http://photobucket.com/gallery/user/bat ... nBn/?ref=1
http://photobucket.com/gallery/user/bat ... nBn/?ref=1
http://photobucket.com/gallery/user/bat ... nBn/?ref=1

http://photobucket.com/gallery/user/bat ... nBn/?ref=1
http://photobucket.com/gallery/user/bat ... w==/?ref=1

I have an affinity for utc transfomers , Arch-68 bought my pair LS-58 frome me is why he is curious about the the grid stoppers and PPP amps

The LS-57 amp was built with two LS-57 and two UTC chokes that i got from Alan Douglas .

Modern amps only need an input transfomer or an interstage transfomer with all the gain with a preamp or even CD player has
I have atarte a PP 300B amp with LS-55's but one of them is bad and have had to put the project on hold do to that and health problems .

Mark

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