So here's an interesting old circuit Page 52.
https://www.americanradiohistory.com/Archive-Radio-News/Radio-News-Specials/Radio-&-Television-Hi-Fi-Annual-1956.pdf
Why would you put OB2 volt regulators on the grids of the 300Bs ?
Never seen that before. Seems out of place. The article mentions they're for protection.
https://www.americanradiohistory.com/Archive-Radio-News/Radio-News-Specials/Radio-&-Television-Hi-Fi-Annual-1956.pdf
Why would you put OB2 volt regulators on the grids of the 300Bs ?
Never seen that before. Seems out of place. The article mentions they're for protection.
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Neither of the links work for me classroom they’re talking about regenerative receivers which use positive feedback. This does the opposite of negative feedback is that it narrows the bandwidth as well as increasing gain which both are benefit to our old radio designs
The 300B tubes warm up instantly, ready to conduct lots of current.
The Driver tubes warm up slowly (no plate current), therefore driver tube plate load, RL and the coupling cap puts a large portion of the driver B+ directly to the grid of the 300B.
Want to chance that anybody?
The Driver tubes warm up slowly (no plate current), therefore driver tube plate load, RL and the coupling cap puts a large portion of the driver B+ directly to the grid of the 300B.
Want to chance that anybody?
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As stated there will be B+ voltage on the grids of the 300B's at turn on which will slowly ramp downward as the coupling caps charge. The regulator tubes will clamp this to a less damaging voltage.
I have seen the same thing done in other amp designs, sometimes with a silicon diode. Silicon diodes are Voltage Varying Capacitances under reverse bias, and could cause distortion or phase shift issues.
I use an ordinary neon bulb in some of my amps because it is a relatively benign 1 pF cap until it ionizes at 75 to 90 volts. As soon as all of the heaters are hot the DC voltages stabilize, and it's a 1 pF cap again.
I have seen the same thing done in other amp designs, sometimes with a silicon diode. Silicon diodes are Voltage Varying Capacitances under reverse bias, and could cause distortion or phase shift issues.
I use an ordinary neon bulb in some of my amps because it is a relatively benign 1 pF cap until it ionizes at 75 to 90 volts. As soon as all of the heaters are hot the DC voltages stabilize, and it's a 1 pF cap again.
Really? A cathode biased/cap coupled 300B amp with a 100K grid leak needs this sort of protection? If the 300B attempts to draw a lot of current, its cathode bias resistor is very much in the way of doing that. Will that tiny coupling cap really take that long to charge? How about that tube rectifier that will also have to heat up and start conducting? Saying stuff like this will lead people to ask every manufacturer of every 300B amp why they don't have a cold cathode regulator tube from grid to ground...
From the article:
" If the output leads of the amplifier are shorted together the negative feedback is removed and the oscillation of the drivers then tends to drive the grids of the output tubes well into the positive region. 300B's are not intended for operation in this region and will quickly be destroyed by this eventuality. The same type of destruction will occur if the output is efectively short-circuited such as by severely overdriving the speaker. A number of protective devices have been employed to prevent this type of destruction. Not only the plate supply but also the speaker line is fused and OB2 voltage-regulator tubes are placed across the 300B grids. Of these three precautions the OB2's are the most efective. "
From the article:
" If the output leads of the amplifier are shorted together the negative feedback is removed and the oscillation of the drivers then tends to drive the grids of the output tubes well into the positive region. 300B's are not intended for operation in this region and will quickly be destroyed by this eventuality. The same type of destruction will occur if the output is efectively short-circuited such as by severely overdriving the speaker. A number of protective devices have been employed to prevent this type of destruction. Not only the plate supply but also the speaker line is fused and OB2 voltage-regulator tubes are placed across the 300B grids. Of these three precautions the OB2's are the most efective. "
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Maybe, maybe not. It's up to you, but I'll stick a 50 cent neon bulb there in my amps. This design uses 300B's which can conduct a couple seconds after power on. The 5R4 DH rectifier is almost as fast, while the IDH 6C5's are far slower. The 300B's could see a brief surge of plate current at power on. They are expensive, an NE-2 is cheap, so I would use one.
I have not built this amp, and I doubt that I ever will, but some experiments with a similar circuit that I found somewhere on the web several years ago that used 6550's I think, led me to abandon the concept. This design really doesn't like being overdriven by an idiot with guitar preamp....one of the test criteria for my amp designs. Yes, my 300B amp will eat my guitar playing.