ZackPlonk,
Take a 300B with 70V grid bias.
The driver needs to swing +/- 70V (140V peak to peak) and do that with low distortion.
Your mileage may very, the bias may only be 60V (120V peak to peak), or it may be 90V (180V peak to peak).
Take a KT66 with 35V grid bias.
The driver needs to swing +/- 35V (70V peak to peak) and do that with low distortion.
Your mileage may very, the bias may only be 25V (50V peak to peak), or it may be 45V (90V peak to peak).
Take an EL84, with 7 to 12V grid bias.
Easy to drive.
Both the driver gain, and the driver linear output (distortion free voltage swing) are very different, according to the output tube type and surrounding circuits.
Take a 300B with 70V grid bias.
The driver needs to swing +/- 70V (140V peak to peak) and do that with low distortion.
Your mileage may very, the bias may only be 60V (120V peak to peak), or it may be 90V (180V peak to peak).
Take a KT66 with 35V grid bias.
The driver needs to swing +/- 35V (70V peak to peak) and do that with low distortion.
Your mileage may very, the bias may only be 25V (50V peak to peak), or it may be 45V (90V peak to peak).
Take an EL84, with 7 to 12V grid bias.
Easy to drive.
Both the driver gain, and the driver linear output (distortion free voltage swing) are very different, according to the output tube type and surrounding circuits.
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to Zac
The perfect drive for 300B is hard to develop.
I have mentioned the AN kit One as one of the best circuit for this tube due the configuration of the gain stage but also for dirver stage.
The 5687 coinfigured in that way it can deliver a high swing, good distortion with a little Zout
In my opinion if you have decided to build a good one, this circuit is simple and efficent but you have to put the hand on your pocket to pay money for a very good OT,
No way
Walter
The perfect drive for 300B is hard to develop.
I have mentioned the AN kit One as one of the best circuit for this tube due the configuration of the gain stage but also for dirver stage.
The 5687 coinfigured in that way it can deliver a high swing, good distortion with a little Zout
In my opinion if you have decided to build a good one, this circuit is simple and efficent but you have to put the hand on your pocket to pay money for a very good OT,
No way
Walter
Yes, size matters when it comes to tubes. I am not talking about Freudian factors, but rather generous and spaced structures within the tube that result in a capability with high linearity. Although the use of beefy tubes as drivers has a long legacy prior to World War II, Sakuma's "same tube for driver and power" scored the points excellently. No tube is too big or too strong to be used as a driver, and the driver should better be similar in size or a tad smaller than the output tube. The mighty 845 is not powered by a 9-pin light bulb. No, it doesn't have to be the exact same tube for driver and output (Sakuma wasn't and is not overly dogmatic either), but it's a good starting point for a design exploration.
Thanks!
Waltube, I’m definitely looking at the AN design, thanks for pointing that out.
With what I have read so far, a DC coupled design would be an awesome challenge…
About the grid resistor, what is the reason for different types of tubes stating a max value there? What happens if that value is exceeded?
With 200k and 140Vpp swing the driver needs to deliver 0,7 mA, right? That can’t be so bad…
Waltube, I’m definitely looking at the AN design, thanks for pointing that out.
With what I have read so far, a DC coupled design would be an awesome challenge…
About the grid resistor, what is the reason for different types of tubes stating a max value there? What happens if that value is exceeded?
With 200k and 140Vpp swing the driver needs to deliver 0,7 mA, right? That can’t be so bad…
ZackPlonk,
The reason for the maximum control grid resistor specification is to keep the tube from going into thermal run away.
The maximum control grid resistor specification is usually lower resistance for fixed bias/fixed adjustable bias, versus the higher maximum control grid resistor specification for self bias.
Thermal Run Away . . . the tube starts to draw more current, gets hotter, then draws even more current, gets even hotter, draws more current . . . and keeps going until the tube is destroyed, some amplifier parts are destroyed, or all are destroyed.
Exceed the maximum control grid resistance at your own risk.
I did that one time with a 6550 (I will never do that again).
Your Mileage May Vary.
The reason for the maximum control grid resistor specification is to keep the tube from going into thermal run away.
The maximum control grid resistor specification is usually lower resistance for fixed bias/fixed adjustable bias, versus the higher maximum control grid resistor specification for self bias.
Thermal Run Away . . . the tube starts to draw more current, gets hotter, then draws even more current, gets even hotter, draws more current . . . and keeps going until the tube is destroyed, some amplifier parts are destroyed, or all are destroyed.
Exceed the maximum control grid resistance at your own risk.
I did that one time with a 6550 (I will never do that again).
Your Mileage May Vary.
A 6L6GC can do 100k the same way.
A 6550 can often replace TWO 6L6. 17W vs 9W SE, 100W vs 50W P-P.
So if you are not married to dinky tubes for dinky jobs, the 6550 is a fine choice, drive vs output.
The KT88 essentially does everything the 6550 does, and does some things better . . .
The KT88 can work with much higher control grid return resistance (versus the 6550).
Both of those tube types can be used near to, at, or beyond their specified limits.
According to how much versus those limits, what circuit parameters, and which tube manufacturer . . . Your Mileage May Vary.
I do use KT88 tubes.
The KT88 can work with much higher control grid return resistance (versus the 6550).
Both of those tube types can be used near to, at, or beyond their specified limits.
According to how much versus those limits, what circuit parameters, and which tube manufacturer . . . Your Mileage May Vary.
I do use KT88 tubes.
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Just to keep us all grounded, we need to remember that those grid resistance specs were written long ago by guys (and it was mostly all guys back then) who are mostly dead now, and applied to manufacturing processes that are all gone decades ago. Like excess noise specs, grid current specs are very specific to materials, processes and techniques. None of these things have survived, and the numbers have zero relevance any more.
We all want certainty, and numbers make things look more certain, but we need to be aware when the numbers are foo fighters.
All good fortune,
Chris
We all want certainty, and numbers make things look more certain, but we need to be aware when the numbers are foo fighters.
All good fortune,
Chris
I completed the tests for the new amplifier with tube 300b for which i chose a load line following data Ua=450v Ia=90mA -Ug=90v P=8w Zp=5200 ohm THD=2,9% .The driver is a mu folower type with 6E5P and 6P1P that can deliver 108v with THD <1% but for 300b max 60v is needed. The problem is that toget the 8w, the input voltage is 2,2v compared to 0,7v as would need a preamplifier with Au=3 or an input transformer, what could be the solution ?
And the later (1950s) Western Electric 300B data sheet reduced the maximum plate dissipation to 36 Watts. Surprise!
All things are subject to change.
Remember, Western Electric was the design and manufacturing arm of AT&T.
WE built amplifiers for the movie theaters, and telephones for business and home.
Those products were all rented/leased. They designed them to work forever; it was to expensive to fix or replace
(both price; but much more importantly in the eyes of the customer who never experienced an outage; that makes for happy and loyal customers).
Later, the Great Telephone Experiment (GTE) came along and tried to compete with AT&T.
(GTE failed to do a good job). Governments dictated areas that would be served by GTE, and could not be served by AT&T.
So much for excellent service for the customer; he/she no longer counted in the government's eyes, and ladies choices were limited to her obstetrician; she could not choose her telephone company.
Your mileage may vary.
All things are subject to change.
Remember, Western Electric was the design and manufacturing arm of AT&T.
WE built amplifiers for the movie theaters, and telephones for business and home.
Those products were all rented/leased. They designed them to work forever; it was to expensive to fix or replace
(both price; but much more importantly in the eyes of the customer who never experienced an outage; that makes for happy and loyal customers).
Later, the Great Telephone Experiment (GTE) came along and tried to compete with AT&T.
(GTE failed to do a good job). Governments dictated areas that would be served by GTE, and could not be served by AT&T.
So much for excellent service for the customer; he/she no longer counted in the government's eyes, and ladies choices were limited to her obstetrician; she could not choose her telephone company.
Your mileage may vary.
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