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 Post subject: Non U Linear Beam Power
PostPosted: Aug Sat 04, 2018 4:29 am 
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I ran across this Toshiba 6GB8 amplifier. 6GB8 is an esoteric tube I know. For this discussion just assume it's an 8417.
I noticed the circuit is pretty close to the Dynaco MKIII. But unlike the Dynaco, Toshiba didn't have it set up for Ultra Linear operation.

They feed the screens with a 500 ohm series resistor. Not sure that is worth much and may just raise the impedance of the screen circuit which is not good. It also does not have fixed bias like the Mark III so in general, I would expect it to be not quite as good... but who knows??

Are there pros here?
Thoughts?


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6gb8amp - Copy.jpg
6gb8amp - Copy.jpg [ 62.15 KiB | Viewed 876 times ]
dynaco-Dynakit-Mark-III-Schematic.png
dynaco-Dynakit-Mark-III-Schematic.png [ 119.63 KiB | Viewed 876 times ]
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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Sat 04, 2018 5:39 am 
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Quote:
They feed the screens with a 500 ohm series resistor.

I don't see that on the schematic you posted. I do see a 1K resistor in series with each control grid. Either way, the purpose of those resistors is to suppress VHF parasitic oscillation.

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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Sat 04, 2018 6:30 am 
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It's a conventional Class AB tube amp design and cathode bias was used by the majority of audio amps, as was the 'lack' of Ultra-Linear (UL) operation. UL was more of a niche market, possibly due to it being an Acro patent that you had to license to use.

Cathode bias has a few advantages over fixed bias, beside being simpler and less expansive. It 'automatically' (self) adjusts and is less affect by line voltage variations. The advantage to fixed bias is it stays 'fixed' when operated in Class B but being 'fixed' means it has to be manually adjusted and is also susceptible to line voltage variations. Fixed bias was used when 'necessary', such as Class B amps and Class AB when you need to deliver sustained maximum power (like in a guitar amp). For 'music', cathode bias is sufficient because 'max power' need only be provided during transient peaks.

The 500 Ohm resistor doesn't raise the screen impedance. For one, while in Class A operation the screen current from the opposing Push Pull tubes is constant (one goes up while the other goes down) and for the brief periods of transients, when the amp enters Class B operation, it's bypassed. It's simply a dropper to provide the 'proper' screen voltage for the tubes, plus it adds some additional filtering since the screens are the more 'sensitive' thing (as opposed to plate voltage) for hum.


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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Sun 05, 2018 9:30 am 
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What is the DCR of the output transformer primary?

Can you post the rest of the data sheet for 6GB8 " ?

What is the Gm?


The 8417 is .023 A/v

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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Sun 05, 2018 11:21 am 
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radiotechnician wrote:
What is the DCR of the output transformer primary?

Can you post the rest of the data sheet for 6GB8 " ?

What is the Gm?


The 8417 is .023 A/v

I hope you can read Japanese.

http://ayumi.cava.jp/datasheets/6G-B8.pdf


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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Sun 05, 2018 7:57 pm 
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That Toshiba schematic is very similar to the dozens of 6L6 and 6V6 based amps designed in the 1940s and 1950s, with a "split-load" phase inverter. The other popular inverter design used a second tube as the inverter, fed with a low level signal from a voltage divider on the grid of the first output tube.

Another big difference from the Dynaco schematic is that the two sections of the 6AN8 are not directly coupled, so it's not really a Williamson design.

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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Mon 06, 2018 1:12 am 
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Flipperhome wrote:
radiotechnician wrote:
What is the DCR of the output transformer primary?

Can you post the rest of the data sheet for 6GB8 " ?

What is the Gm?


The 8417 is .023 A/v

I hope you can read Japanese.

http://ayumi.cava.jp/datasheets/6G-B8.pdf


If you look at the data sheet of an 8417 and compare it to the 6GB8 they are very similar.
So for all practical purposes lets just assume their the same. In this post I was mainly curious of what benefit this circuit had over a UL design.
And the schematic shows the Primary Impedance at 2.5 K ohms


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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Mon 06, 2018 2:55 am 
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I thought you had the amplifier there. That is why I wanted the actual DC resistance of the transformer
primary sections. Also the actual cathode and screen voltages could be measured.

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 Post subject: Re: Non U Linear Beam Power
PostPosted: Aug Mon 06, 2018 3:52 am 
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Tim Tress wrote:
That Toshiba schematic is very similar to the dozens of 6L6 and 6V6 based amps designed in the 1940s and 1950s, with a "split-load" phase inverter. The other popular inverter design used a second tube as the inverter, fed with a low level signal from a voltage divider on the grid of the first output tube.

Another big difference from the Dynaco schematic is that the two sections of the 6AN8 are not directly coupled, so it's not really a Williamson design.
The Dynaco isn't a "Williamson" design ether. It uses pentodes (preamp and output), lacks a 'driver' stage (thanks to the higher gain of pentodes), and uses UL (rather than a "Williamson" transformer).

This is the Williamson design. (While the outputs are technically a pentode they're strapped for operation as a triode.)

Attachment:
Williamson.jpg
Williamson.jpg [ 90.42 KiB | Viewed 745 times ]


Btw, what seems to be little known in history, possibly because most, if not all, "Williamson" designs didn't use it, is that Williamson offered the paraphrase splitter as a "good alternative" to the split load phase splitter.

Attachment:
Paraphase.jpg
Paraphase.jpg [ 64.46 KiB | Viewed 743 times ]


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