Broskie P-P Bridged Output Stage - diyAudio
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Old 9th April 2012, 01:26 AM   #1
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Default Broskie P-P Bridged Output Stage

Has anyone built John Broskie's H-Bridged output stage shown in his Tubecad blog number 192? (See the link below.) I've got it breadboarded, but getting enough balance in all those triodes to limit dc offset is challenging. Would be interesting to hear thoughts on possible routes to achieving balance. Thanks much.

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Old 9th April 2012, 03:32 AM   #2
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There are several diagrams on that page. I built the one with 4 output tubes all driven by a driver transformer with 4 secondaries. I did it because I used to make some extremely high powered solid state amps that way back in high school. They used the same circuit to make 300 to 1200 watts, which was stupid power in 1970. Back then I used to wind my own driver transformers or use a pair of Heathkit transformers with their primaries in parallel.

Triode electronics has a tube type "universal driver transformer" with a center tapped primary and 4 independent secondaries. I bought a pair of the early ones and tried the BTL circuit with several different tubes. The results were the same. the driver transformer starts rolling off at 10KHz and is 6db down at 20 KHz. The low end isn't great either.

They now have a "version 2" of the driver transformer, but I don't know if I will try it or not.
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
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Old 12th April 2012, 12:14 AM   #3
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While trying to figure out how Broskie's double bootstrap worked in that blog #192, I realized that there is another equivalent way to do the same using the more usual differential pentode driver stage driving a totem pole output stage. Usually the bootstrap from the output midpoint goes to the top of just one or the other of the diffl. driver plate resistors (per side, if talking an H bridge config, and a bootstrap cap is usually there too).

In one case the boot boosts the drive for the upper totem pole device, or in the other case it reduces the drive for the lower totem pole device. But it is possible to do both bootstraps at once like Broskie effectively did by reducing the bootstrap signal by 1/2. Connect both diffl. driver plate load resistors together to an R going to B+ and another equal R going to the output. The drivers sum to a constant current, so they only cause DC loading on the R/R divider. The two equal R's then divide the output AC level in half for the equal 1/2 bootstrapping to each side. I've never seen that done before. (one might have to play with the driver stage DC plate V or B+, maybe a bootstrap cap)

This approach eliminates two floating (filament issue) tube elements from the driver stage versus the Broskie version.
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Last edited by smoking-amp; 12th April 2012 at 12:24 AM.
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