Hi all.
Can someone please enlighten me and tell me what this 25K resistor in between the screen and the cathode of the output tubes does in this schematic?
Is it also part of biasing up the filament string?
Thanks in advance!
Explain schematic.
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Stevem
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Explain schematic.
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When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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sluckey
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Re: Explain schematic.
The 25K, 13K, and 150Ω resistors make a voltage divider on the B+ string. When in "STBY" mode the BIG voltage dropped across the 13K + 150Ω is applied to the output tubes cathodes which will cut off the output tubes. When in "ON" mode the 13K is shorted and the voltage divider now consists of only the 25K and 150Ω resistors. This puts a much smaller fixed voltage on the cathodes, allowing the output tubes to operate normally. So, you actually have fixed bias AND cathode bias on the tubes. And the filament string is elevated by the voltage across the 150Ω resistor.
https://sluckeyamps.com/index.htm
Re: Explain schematic.
I recall RDH4 shows that sort of arrangement (without the standby) as an example of fixed bias. I suppose the ratio of cathode current to straight HT current flowing in the cathode resistor determines how 'fixed' or 'mixed' the bias is.
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Stevem
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Re: Explain schematic.
Thanks much for the explanation folks!
I don’t know why my mind could not accept that it could be some of both forms of cathode bias!
As always it seems that back in 54 when this amp was made Gibson engineers could not resist playing around.
I don’t know why my mind could not accept that it could be some of both forms of cathode bias!
As always it seems that back in 54 when this amp was made Gibson engineers could not resist playing around.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Explain schematic.
I think in ON mode the effect of the 25k on cathode voltage is rather insignificant (<2V).
The other effect of the 25k is lowering the screen voltage by increasing the voltage drop across the 4k dropper by maybe 40V.
The other effect of the 25k is lowering the screen voltage by increasing the voltage drop across the 4k dropper by maybe 40V.