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 Post subject: 100 watt PP 300B amplifier
PostPosted: Jan Mon 27, 2020 5:21 am 
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Not sure I see how this is possible?
The OPT was a FREED 18777. 1650 ohms on the Primary
https://i.pinimg.com/originals/8b/1e/00/8b1e00728ef12eb4be4d8b8aa7b075d2.jpg

How does a quad of 300B's develop a 100 watts ???
I would have thought 45 maybe 47 would be the max.

Image


Full article is here
https://www.americanradiohistory.com/Ar ... 955-04.pdf


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Jan Mon 27, 2020 5:30 am 
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It doesn't say RMS watts, so maybe they mean IHF.


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Jan Mon 27, 2020 6:56 am 
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Never produce anything near 100 watts RMS.


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Jan Mon 27, 2020 7:11 am 
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jimbenedict wrote:
Never produce anything near 100 watts RMS.


Sure it could. But not for long.

Brett


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Jan Mon 27, 2020 7:11 pm 
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archeus68 wrote:
How does a quad of 300B's develop a 100 watts ???


Class AB2 per article. All published specifications for the 300B are for Class A only.

Norman

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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Jan Mon 27, 2020 7:38 pm 
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https://frank.pocnet.net/sheets/084/3/300B.pdf

Most likely 40 to 50 watts with 40 being more realistic .

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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Jan Fri 31, 2020 9:11 am 
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Location: New York City, New York
This design has been discussed at length on other forums- it is certainly capable in class AB2 of putting out 100W.

Electra-Print also has a published design (https://www.electra-print.com/pushpull_a2.php) of a 300B push pull amp that outputs 50W.


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Jan Fri 31, 2020 12:27 pm 
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"8417"
User name checks out. i had a old Bogen PA that had a pair of them for the outputs, transformers were huge.


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Feb Tue 04, 2020 1:44 am 
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If you consider that the output tubes are driven into positive grid area (which the CF driver is very likely capable of) the amp works in class AB2 and could deliver his stated power of 100 watts.


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Feb Tue 04, 2020 3:40 am 
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That's quite a circuit! I wonder if anyone has built one of those; the price of the transformers would be substantial. It certainly would be a beast.

The values of the grid resistors on the 12BH7 are not given.

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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Feb Tue 04, 2020 4:02 am 
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No fiddling with bias adjustments.

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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Apr Mon 13, 2020 9:38 pm 
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What struck me was the 1650 ohm Primary of the OPT.
A quad of 300Bs usually runs 2500 Ohms @ 40 watts.

So yeah 1650 is considerably lower so it'll definitely draw more power from the quad.
Didn't know you could go that low.


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Apr Tue 14, 2020 6:13 am 
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It is an impressive design. However, anyone who does not already have the major parts would spend far less buying a new amplifier.

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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Apr Tue 14, 2020 9:50 pm 
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The catch is, that is a waste of expensive tubes as all the magical attributes to 300B amps are using them in the class
A configuration instead of a variety of B class. Might as well run pentodes in AB2.

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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: Apr Wed 15, 2020 3:58 am 
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Location: Boston, MA USA
300B was nothing special in 1955 — just a garden-variety triode. Not expensive at that time, either.

-David


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: May Thu 14, 2020 11:21 pm 
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This amplifier schematic, along with a series of others all loosely based on the Williamson, was drawn up by M.V. Kiebert of Mallory. It will in fact produce the 100 watts RMS claimed. In fact, in his paper explaining the series of amplifiers developed, he notes that this amplifier was originally designed for disc cutting service at the BBC. Suspiciously though, while he gives IM and harmonic distortion data for all the other amps in that paper, he leaves this one off. I assume it doesn't meet his usual 1% IM, 0.1% harmonic standard.

His "kicker" circuit used in this amplifier and others, is actually a pretty slick design, and it simulates well. If you have a CF and don't tightly regulate B+ to it, the distortion rises precipitously once you swing positive and the outputs start drawing grid current. By holding the B+ on the CF extremely steady, most of the issues in drawing grid current from the cathode follower drivers are solved.

Of course his higher power designs are rendered pointless if you have efficient drivers, and the more efficient the driver, the lower the THD from the speaker system (as a general rule, that's not always true). It would have been more sensible back in the day to buy a Jensen Imperial, a Klipschorn, or even some no-name coax in a Karlson Coupler, and drive it until your ears bled with a pair of 6V6s, or if you must, a pair of triode strapped war surplus 807s in a bog-standard Williamson.

Edit: I just noticed he mentions the BBC here as well. If I can find his original paper, I'll link to it here.


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 Post subject: Re: 100 watt PP 300B amplifier
PostPosted: May Thu 14, 2020 11:31 pm 
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Location: Metamora MI, 48455
Here's a link to his paper from 1954, which is the first reference I've been able to find to this amplifier, and his entire family of higher power AB2 Williamson derivatives. It's interesting to note that this wasn't even that novel of an idea; Avery Fisher had been manufacturing the 50A which squeezes ~40 clean watts from a pair of triode strapped 1614s with a similar topology; Kieberts 45 watter is markedly similar. The biggest difference is that Fisher went with transformer coupling between the CF driver and the output grids, Kiebert direct coupled it.

It's an interesting read in any case, but the real jewel of this paper is the 10 watt lab amplifier in figure 3. To date it's one of the cleanest designs I've simulated in LTSpice, and I have little doubt it would perform well if/when I actually get around to building it. It's a Williamson variant like the others, but with some tweaking of component values that never made it into the other amps for some reason...


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