I found a paper copy of this (attached) schematic in my basement and it's simple enough to be interesting.
I am wondering if anyone recognizes it (Hiraga?). Reading about it will help keep me from misinterpreting too much.
I figured out some voltage drops and currents. I assume it's a small OTL power amp rather than a line stage. I wonder if the NFB (22k//3pF) also helps keep the DC nulled on the output, or is just classic AC NFB.
I had 4 pages stapled together, apparently from different sources so I can't assume too much. English 6C33C-B data sheet from user Cigna at Ohio.edu (1998), audiomatica.it curves for 6C33C-B, a Glass Audio spice model for same, and the Japanese schematic.
This topology doesn't provide me obvious 6C33C-B bias recognition, but if I imagine a load line based on V&I that produce typical 45W max.anode dissipation, I can see some sense. Still looks like a power amp after those thoughts, but is the bias stable? Hmmm...
If I knew/read more I might decide whether to keep the hard copy or bookmark it online.
Understanding it better might also help me recognize if the small NPN power transistors are the critical part of bias stability and the JFET's & current source PNP's are just preamp or interactive with the output bias too.
The 5687WA looks kind of like a DCCF, so there might be plenty of bias interaction.
OTL with multiple tubes always turns me off, but this appears relatively simple in comparison.
But if it's an unstable PA that produces about as much power as a line stage...maybe I'll spend my lunch money elsewhere. (The non-free part).
I have the tubes & power supply stuff, so it's briefly got my attention
Thanks
Murray
I am wondering if anyone recognizes it (Hiraga?). Reading about it will help keep me from misinterpreting too much.
I figured out some voltage drops and currents. I assume it's a small OTL power amp rather than a line stage. I wonder if the NFB (22k//3pF) also helps keep the DC nulled on the output, or is just classic AC NFB.
I had 4 pages stapled together, apparently from different sources so I can't assume too much. English 6C33C-B data sheet from user Cigna at Ohio.edu (1998), audiomatica.it curves for 6C33C-B, a Glass Audio spice model for same, and the Japanese schematic.
This topology doesn't provide me obvious 6C33C-B bias recognition, but if I imagine a load line based on V&I that produce typical 45W max.anode dissipation, I can see some sense. Still looks like a power amp after those thoughts, but is the bias stable? Hmmm...
If I knew/read more I might decide whether to keep the hard copy or bookmark it online.
Understanding it better might also help me recognize if the small NPN power transistors are the critical part of bias stability and the JFET's & current source PNP's are just preamp or interactive with the output bias too.
The 5687WA looks kind of like a DCCF, so there might be plenty of bias interaction.
OTL with multiple tubes always turns me off, but this appears relatively simple in comparison.
But if it's an unstable PA that produces about as much power as a line stage...maybe I'll spend my lunch money elsewhere. (The non-free part).
I have the tubes & power supply stuff, so it's briefly got my attention
Thanks
Murray
Attachments
I am not at all familiar with WCF or power follower configurations, so this may be (obviously, to someone) very different than my naive interpretation.
It's a power OTL amp by Shinichi Kamijo, you can read all about it here:
https://www-ne-jp.translate.goog/as...l=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
5687 is used to replace plate resistor load in differential circuit with so called Super Triode or constant current source cathode follower. Quite similar to Tim Mellow OTL which used plate resistor in the differential driver, maybe modified to use triode as load instead as well.
https://www-ne-jp.translate.goog/as...l=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
5687 is used to replace plate resistor load in differential circuit with so called Super Triode or constant current source cathode follower. Quite similar to Tim Mellow OTL which used plate resistor in the differential driver, maybe modified to use triode as load instead as well.
It's not a WCF, but a symmetric push-pull.
Just the single pair of output devices? The 6C33c is not rated above the zero-grid line, which lays near 75-80 Ohms. So diving a 8 Ohm load, it could swing almost 1/10th of the supply voltage, 150V, or 15V swing. Rounding to 14V this is just 10Vrms on the load, or 12 Watts. Not a pip-squeak, but no wall-shaker. I've had fun with amps smaller than this, but they were also cooler (seems to be 60W dissipation at idle). Per channel. Neglecting ~40W of heater.
Just the single pair of output devices? The 6C33c is not rated above the zero-grid line, which lays near 75-80 Ohms. So diving a 8 Ohm load, it could swing almost 1/10th of the supply voltage, 150V, or 15V swing. Rounding to 14V this is just 10Vrms on the load, or 12 Watts. Not a pip-squeak, but no wall-shaker. I've had fun with amps smaller than this, but they were also cooler (seems to be 60W dissipation at idle). Per channel. Neglecting ~40W of heater.
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Since the feedback is DC coupled, it does correct for DC output offset.
The 3pF is phase correction for stability.
Jan
The 3pF is phase correction for stability.
Jan
Unless I'm missing something, the sum of the current through the two 2.4K cathode resistors (plus 4 base currents)=the current through 510R and VR2.
The 2SC2752, 2SD668A, & 2SA1486 are just cascodes so you can ignore them to look at the bias. The current source at top left (via the FET differential pair) sets the voltage across the bottom left 4.7k resistors, which sets the base voltage at the second differential pair. Subtract 1 VBE and that voltage across the 510R and VR2 sets the current through the NPN differential pair, which then sets the current through the cathode resistors in the driver tubes.
The 2SC2752, 2SD668A, & 2SA1486 are just cascodes so you can ignore them to look at the bias. The current source at top left (via the FET differential pair) sets the voltage across the bottom left 4.7k resistors, which sets the base voltage at the second differential pair. Subtract 1 VBE and that voltage across the 510R and VR2 sets the current through the NPN differential pair, which then sets the current through the cathode resistors in the driver tubes.
Of course, it's gets far more complicated, as one reads the article...regulated power supplies, inrush current protection, timer(s), etc.
But still a very interesting build (not just a schematic).
The article explains 100% P-G FB reducing the ra of the 6C33C-B's and the various effects of the global NFB.
Max. power is claimed to be 45W, but not continuously, as 6C33C-B dissipation would be exceeded. Sounds like a warning to me.
There are more words than pictures, and pretty readable even thru Google Translate. Today, anyway. I wonder if Google Translate is like JPEG deconpression-you get different results each time.
This might never go beyond reading material for me.
But still a very interesting build (not just a schematic).
The article explains 100% P-G FB reducing the ra of the 6C33C-B's and the various effects of the global NFB.
Max. power is claimed to be 45W, but not continuously, as 6C33C-B dissipation would be exceeded. Sounds like a warning to me.
There are more words than pictures, and pretty readable even thru Google Translate. Today, anyway. I wonder if Google Translate is like JPEG deconpression-you get different results each time.
This might never go beyond reading material for me.