I know that its strongly recommended to respect the 300v screen voltage limit on the 807/1625.
I created a Ltspice model of the RH807 amp I am building and it simulates a screen waveform in the snip attached.
So I assume (asking for corrections in my assumptions):
1. The brief excursions above 300v is not a problem because the majority of the waveform is well under 300V?
2. The 300V rating in this design is to be respected at the DC operating point?
3. There is a big compromise is dropping the screen voltage from the B+ using a resistor because the screen is operating well under 300V when real music is playing?
I created a Ltspice model of the RH807 amp I am building and it simulates a screen waveform in the snip attached.
So I assume (asking for corrections in my assumptions):
1. The brief excursions above 300v is not a problem because the majority of the waveform is well under 300V?
2. The 300V rating in this design is to be respected at the DC operating point?
3. There is a big compromise is dropping the screen voltage from the B+ using a resistor because the screen is operating well under 300V when real music is playing?
Attachments
Thank you for that link, very interesting. But paragraph 6 in that article doesn't answer all (any) of my questions in the opening post, at least not in an non unsubtle way.
The 807 RH amplifier has the 807 in Beam Power mode, Right?
(for some, they call it Beam Tetrode mode).
If you use a series resistor in the screen of a Beam Power tube, the screen voltage will vary. That is because as the control grid voltage varies, and as the plate voltage varies, the screen current varies. Varying current across a series screen resistor causes screen voltage to vary.
Nothing surprising here.
If you Regulate the 807 screen (a couple of OD3 tubes in series, for example) the screen will be at 300V, and only vary by up to 8V (4V per OD3) (less varying if you keep the plate from going too low of a voltage during large signal swings, the screen current changes a lot when the plate voltage gets near 100V, and even more with the plate at 50V).
And if you go to the trouble to regulate the screen voltage, I recommend using a 100 Ohm screen resistor, no more, no less.
An 807 in triode mode rates the maximum screen voltage at 400V.
When the control grid goes more negative, then the plate voltage goes more positive, and the screen voltage also goes more positive. But those larger plate voltages and screen voltages are at lower currents, because the control grid voltage is more negative (dictating those lower currents at higher voltages).
An 807 in Ultra Linear mode, with a 40% UL tap, will work with about 340V on the screen.
When the control grid goes more negative, and the plate voltage goes more positive, the screen voltage also goes more positive, but those larger plate voltages and screen voltages are at lower currents, because the control grid voltage is more negative (dictating lower currents).
I hope that explains the ins and outs of using the 807 in Audio applications, especially in single ended, which has to be class A. Any other Class than Class A in single ended sounds terrible, because it is clipping.
(for some, they call it Beam Tetrode mode).
If you use a series resistor in the screen of a Beam Power tube, the screen voltage will vary. That is because as the control grid voltage varies, and as the plate voltage varies, the screen current varies. Varying current across a series screen resistor causes screen voltage to vary.
Nothing surprising here.
If you Regulate the 807 screen (a couple of OD3 tubes in series, for example) the screen will be at 300V, and only vary by up to 8V (4V per OD3) (less varying if you keep the plate from going too low of a voltage during large signal swings, the screen current changes a lot when the plate voltage gets near 100V, and even more with the plate at 50V).
And if you go to the trouble to regulate the screen voltage, I recommend using a 100 Ohm screen resistor, no more, no less.
An 807 in triode mode rates the maximum screen voltage at 400V.
When the control grid goes more negative, then the plate voltage goes more positive, and the screen voltage also goes more positive. But those larger plate voltages and screen voltages are at lower currents, because the control grid voltage is more negative (dictating those lower currents at higher voltages).
An 807 in Ultra Linear mode, with a 40% UL tap, will work with about 340V on the screen.
When the control grid goes more negative, and the plate voltage goes more positive, the screen voltage also goes more positive, but those larger plate voltages and screen voltages are at lower currents, because the control grid voltage is more negative (dictating lower currents).
I hope that explains the ins and outs of using the 807 in Audio applications, especially in single ended, which has to be class A. Any other Class than Class A in single ended sounds terrible, because it is clipping.
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I'm not familiar with the RH807, or SE amps in general, but the Wayback machine at least showed a schematic. My understanding is the screen's 300 volt rating is for a pentode connection. When connected as a triode, or UL, 425 volts is quite common. I recently found a Patrick Turner old RAT post where he said 450 was permissible. I'd guess you're safe.
807 tubes are not Pentodes.
There is no Suppressor Grid (G3) in an 807.
807 tubes, similar to 6L6 tubes, have Beam Formers.
They are called Beam Power tubes, or Beam Tetrode tubes.
Pick one.
Screen ratings are 300V and 400v for Beam Power mode, and Triode mode respectively.
Take a look at the curves that show both plate current and screen current for Beam Power mode.
You will see the enormous screen current when the plate goes to 100V or even worse at plate voltage of 50V.
Do like a Guitar that is played continuously into heavy clipping, and the screen current will be at maximum almost 50% of the time.
Or we can do a clipping test with a sine wave, and get a phone call, and forget about the clipping amplifier.
The 807 is well known for its robust character.
Go beyond the ratings at your own risk.
At some point, you will transition from Robust, to Bust-ed.
Your Mileage May Vary.
There is no Suppressor Grid (G3) in an 807.
807 tubes, similar to 6L6 tubes, have Beam Formers.
They are called Beam Power tubes, or Beam Tetrode tubes.
Pick one.
Screen ratings are 300V and 400v for Beam Power mode, and Triode mode respectively.
Take a look at the curves that show both plate current and screen current for Beam Power mode.
You will see the enormous screen current when the plate goes to 100V or even worse at plate voltage of 50V.
Do like a Guitar that is played continuously into heavy clipping, and the screen current will be at maximum almost 50% of the time.
Or we can do a clipping test with a sine wave, and get a phone call, and forget about the clipping amplifier.
The 807 is well known for its robust character.
Go beyond the ratings at your own risk.
At some point, you will transition from Robust, to Bust-ed.
Your Mileage May Vary.
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Newbies might as well know the correct nomenclature.
A KT77 and an EL34 are similar in specifications.
But tube rollers may wonder if, or if not, and why, or why not, they sound different in the same amplifier.
Beam Power, versus Pentode.
As for nomenclature, a common cathode phase splitter with either LTP or current sink have been referred to as a grounded grid amplifier.
Every General Class Amateur Radio operator knows that is untrue.
A KT77 and an EL34 are similar in specifications.
But tube rollers may wonder if, or if not, and why, or why not, they sound different in the same amplifier.
Beam Power, versus Pentode.
As for nomenclature, a common cathode phase splitter with either LTP or current sink have been referred to as a grounded grid amplifier.
Every General Class Amateur Radio operator knows that is untrue.
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Thank you. Yes I wasn't surprised but I did the spice model to find how much the screen voltage varied in this arrangement.
So my question, is this a good thing as compared to a regulated screen? I suspect the answer will be "it depends". What benefits might I see if I regulated the screen voltage in the RH807 all other things being equal?
True.
However, there was reference to the mode of operation not to the internal structure.
The mode of operation, circuit, connection (whatever you call it) is exactly the same whether you use beam tetrodes or real pentodes.
Besides, when you reject the term 'pentode mode' for a beam tetrode, you certainly can't use 'triode mode'! 😀
Parafeed813,
Agreed.
I get lazy too.
Triode wired mode (but does not tell us if it is a 4-65A tetrode, or a Beam Power, or a Pentode). So, more correctly . . .
Triode wired Beam Power tube,
And Triode wired Pentode.
Or Triode wired Tetrode (as I did once).
Agreed.
I get lazy too.
Triode wired mode (but does not tell us if it is a 4-65A tetrode, or a Beam Power, or a Pentode). So, more correctly . . .
Triode wired Beam Power tube,
And Triode wired Pentode.
Or Triode wired Tetrode (as I did once).
jderimig,
Just what was the series screen resistor value?
10k?
Or what?
A regulated screen gives more transconductance than you get with a large series resistor to the screen.
And more transconductance into a given load impedance, gives more open loop gain.
And more open loop gain, combined with a fixed Schade negative feedback resistor, gives a higher open loop to closed loop number (effectively more negative feedback).
That could be lower distortion.
That could also be less stable.
Generally, a Beam Power tube, and a Pentode tube are more linear with a regulated screen.
The stability depends on the circuit values, output transformer, and the complex speaker load reflecting back to the primary, where the Schade feedback is taken from. It is not just about speaker impedance, it is about those frequencies where the speaker has capacitive or inductive reactances.
Just what was the series screen resistor value?
10k?
Or what?
A regulated screen gives more transconductance than you get with a large series resistor to the screen.
And more transconductance into a given load impedance, gives more open loop gain.
And more open loop gain, combined with a fixed Schade negative feedback resistor, gives a higher open loop to closed loop number (effectively more negative feedback).
That could be lower distortion.
That could also be less stable.
Generally, a Beam Power tube, and a Pentode tube are more linear with a regulated screen.
The stability depends on the circuit values, output transformer, and the complex speaker load reflecting back to the primary, where the Schade feedback is taken from. It is not just about speaker impedance, it is about those frequencies where the speaker has capacitive or inductive reactances.
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Just as I suspected. 10K screen resistor.
Take a look at the Parallel 807 amplifier by Sarris.
i can not copy the link, sorry. It claims that it is not secure, my system will not permit copying the link.
10k screen resistors.
Schade Feedback.
Deja Vu all over again.
Take a look at the Parallel 807 amplifier by Sarris.
i can not copy the link, sorry. It claims that it is not secure, my system will not permit copying the link.
10k screen resistors.
Schade Feedback.
Deja Vu all over again.
Your right about the website a special app I have completely blocked it--nothing to do with HTTP etc , but the last time I mentioned this here I was "shot down in flames " but I was only trying to keep people safe .
So my personal advice only ---don't try to reach it but I am sure many will ignore that.
So my personal advice only ---don't try to reach it but I am sure many will ignore that.