So I've consulted with tim164 on the parts list and wanted to get anyone else's feedback on a parts list I assembled for the amp. For reference to Alex's schematic, I added some identifiers (e.g., R101, C101, etc.). I hope this was okay to do that. I am open to anyone's recommnedations on various parts. tim164 was very helpful in recommending the Transcendar SE OPTs and Edcor PT. I'm wondering about the power ratings of some of the resistors and whether I should stick with RN65's for the 1/2 watt resistors in the signal path and if I need Carbon composition or Carbon film resistors on the grid resistor positions.
View attachment RH 84 SE Schematic.pdf
View attachment RH 84 BOM.pdf
View attachment RH 84 SE Schematic.pdf
View attachment RH 84 BOM.pdf
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Mr Kitic, can I please have your email, to ask a few questions about this topology, without bothering anyone?
Thanks
Why not ask here and let others learn from the reply as well ? Anybody who might feel "bothered" by that can hit the X button in top right corner of the screen 😀
I would like to try to explore the possibilities of anode - to anode feedback, but
rather than implement your RH84 (I have tried RH84 about a year
ago), I would like to apply this promising topology to an amplifier
that I design myself, not necessarily using the 2-3 specific tubes you
used. I am thinking, for example, driving a KT77 with a E88CC, with a
voltage gain stage in front.
So, I would like to ask you, how do you define the operating points of
such a circuit? Which formulae do you use? Is it by trial and error?
(I can do that too, I have the euipment). And if you use a trial and
error approach, which is the criterion to judge the circuit as
succesful?
rather than implement your RH84 (I have tried RH84 about a year
ago), I would like to apply this promising topology to an amplifier
that I design myself, not necessarily using the 2-3 specific tubes you
used. I am thinking, for example, driving a KT77 with a E88CC, with a
voltage gain stage in front.
So, I would like to ask you, how do you define the operating points of
such a circuit? Which formulae do you use? Is it by trial and error?
(I can do that too, I have the euipment). And if you use a trial and
error approach, which is the criterion to judge the circuit as
succesful?
I don't think you will hear anything from Mr Kitic, if the "banned" under his username is correct.
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costis_n,
I would advise you to read this:
http://www.clarisonus.com/Archives/TubeTheory/Schade 1938 Beam Power Tubes.pdf
and this:
The Tube CAD Journal, partial feedback amplifier
before you embark on a design.
Ideally, the driver tube should possess both a high internal resistance and a high transconductance - however these attributes are often mutually exclusive. Pentodes are generally preferred for this reason. Out of the tubes I can think of, the 6AC7 probably possesses the best combination of these attributes. However, I have used 6AK5 to good effect.
A couple of general rules:
don't bypass the cathode resistor of the driver tube or use an LED in place of the resistor.
Initially aim for around 10% feedback. The feedback ratio is calculated by first determining the ratio of value of the following parallel resistances:
1. the input tube plate resistor, Rp
2. the internal resistance of the input tube Ri * its cathode resistor, Rk
3. the grid resistor of the output tube Rg
calculated in the usual way, ie Rtot = (1/Rp + 1/(Ri* Rk) + 1/Rg)
For the RH84
Rtot = (1/22k + 1/(12k*220) + 1/470k) = 10.42k
The actual Feedback ratio = Rtot / (Rfb + Rtot) = 10.42k/(100k + 10.42k)
= 0.094, or around 9%
Also try to run a reasonable amount of current in the driver tube (aim for >5mA)
I would advise you to read this:
http://www.clarisonus.com/Archives/TubeTheory/Schade 1938 Beam Power Tubes.pdf
and this:
The Tube CAD Journal, partial feedback amplifier
before you embark on a design.
Ideally, the driver tube should possess both a high internal resistance and a high transconductance - however these attributes are often mutually exclusive. Pentodes are generally preferred for this reason. Out of the tubes I can think of, the 6AC7 probably possesses the best combination of these attributes. However, I have used 6AK5 to good effect.
A couple of general rules:
don't bypass the cathode resistor of the driver tube or use an LED in place of the resistor.
Initially aim for around 10% feedback. The feedback ratio is calculated by first determining the ratio of value of the following parallel resistances:
1. the input tube plate resistor, Rp
2. the internal resistance of the input tube Ri * its cathode resistor, Rk
3. the grid resistor of the output tube Rg
calculated in the usual way, ie Rtot = (1/Rp + 1/(Ri* Rk) + 1/Rg)
For the RH84
Rtot = (1/22k + 1/(12k*220) + 1/470k) = 10.42k
The actual Feedback ratio = Rtot / (Rfb + Rtot) = 10.42k/(100k + 10.42k)
= 0.094, or around 9%
Also try to run a reasonable amount of current in the driver tube (aim for >5mA)
Correction:
Rtot = (1/Rp + 1/(Ri + (u+1) Rk) + 1/Rg)
= (1/22k +1/(12k + (61*250)) +1/470k) = 11.5k
Feedback ratio = 11.5/(11.5 + 100) = 10.3%
The moral: don't write equations with a hangover.
I wouldn't consider the E88CCto be an ideal candidate for a driver in this type of circuit.
If you contact Mr Kitic privately by email I think you will find him to be quite affable.
Rtot = (1/Rp + 1/(Ri + (u+1) Rk) + 1/Rg)
= (1/22k +1/(12k + (61*250)) +1/470k) = 11.5k
Feedback ratio = 11.5/(11.5 + 100) = 10.3%
The moral: don't write equations with a hangover.
I wouldn't consider the E88CCto be an ideal candidate for a driver in this type of circuit.
If you contact Mr Kitic privately by email I think you will find him to be quite affable.
Correction:
Rtot = (1/Rp + 1/(Ri + (u+1) Rk) + 1/Rg)
= (1/22k +1/(12k + (61*250)) +1/470k) = 11.5k
Feedback ratio = 11.5/(11.5 + 100) = 10.3%
The moral: don't write equations with a hangover.
I wouldn't consider the E88CCto be an ideal candidate for a driver in this type of circuit.
If you contact Mr Kitic privately by email I think you will find him to be quite affable.
I have (tried) to study these documents and they fly over my head. So thanks for the formulae! So I am going to use the EF80's from my dad's stash, and keep the E88CC for the 1st stage.
I have emailed mr. Kitic a couple of years ago with a similar question, and he was indeed affable, but he doesn't seem to read his mail address anymore 🙁 So thanks!
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