Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on July 10, 2011, 09:07:53 pm
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What does V3a (Pins 1,2,3) Do?
(http://hiwatt.org/Schematics/DR_Pre4Input_v2.gif)
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Notice there's no self-bias resistor on the long-tail, as you'd see in a Fender amp.
V3a is a cathode follower, but it only serves to set a voltage for the grids of the long-tail. It's a method of fixed-bias.
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Ditto. I wondered this when I built it last week.
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sets Vg (bias) for the long tail pair. they could have just used 2 resistors.
--DL
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Tubegeek... That's funny.
You think this method is worth the extra tube?
It appears to me they could have eliminated an entire tube.
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Tubegeek... That's funny.
You think this method is worth the extra tube?
It appears to me they could have eliminated an entire tube.
I suspect it's supposed to stabilize that voltage better than if just two resistors. I can't claim to really understand it very well, though. I was looking at the Custom 20 schematic the other day and there is a ton of funky stuff in there.
Chris
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Sometime ago someone asked the same thing
is a recursive question
my opinion as Spacelabstudio told is for a better stabilized voltage, may be someone can try to use the two resistors instead of the tube and report results
Kagliostro
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Sometime ago someone asked the same thing
is a recursive question
my opinion as Spacelabstudio told is for a better stabilized voltage, may be someone can try to use the two resistors instead of the tube and report results
Kagliostro
I think it might be some kind of shunt stabilizer, although I just looked at Merlin's power supply book, and it isn't an exact match for any of the stabilizer circuits in his book, so I might be wrong. It's possible that it's in there to reduce B+ sag for V1 and V3 and to also cleverly provide the PI bias as a nice side effect. But that's a wild assed guess. Not really sure.
Chris
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Tubegeek... That's funny.
You think this method is worth the extra tube?
It appears to me they could have eliminated an entire tube.
Assuming the purpose of V3a is to provide fixed bias for the LTPI (and I've got no reason to doubt HBP), seems to me you could eliminate an entire 12AX7 provided you're willing to allow solid state demons into your circuit. Seriously, couldn't you use a MOSFET/Zener voltage dropping circuit off the B+ rail to get "fixed bias" at whatever voltage that LTPI likes? You'd need a power MOSFET, a couple of zeners, at least one electrolytic cap and a few resistors. Certainly cheaper than a 12AX7 with an unused triode. Without really understanding the details of a LTPI, I would think that fixed bias there would have an effect on the whole feel of the amp and would provide more clean headroom in the PI - allowing more emphasis on power tube distortion maybe?
HTH
Chip
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V3A is totally superfluous. It's just a cathode follower. The real fixed bias is provided with that 1M and 220K voltage divider. V3A just cathode couples it's grid voltage (times about 0.9) to the PI bias input. You could have done exactly the same with just the two resistors.
V3B is used as a gain stage. I suspect since there was a triode left over that the engineers just decided to do something silly with it just to be unique.
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It's in the Hiwatt Bull Dog amp also.
KOC's TUT 9, 3-4, says; it's a dedicated voltage reference for the Schmitt splitter. A tube-buffered proportional power supply with a single supply node. The voltage reference provides an easy way to adjust the bias condition for the Schmitt, without frequency dependence. Hifi Schmitts tend to be direcet-coupled to the preceding voltage amplifier, which then sets the bias conition for the Schmitt.
Brad :icon_biggrin: