Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on February 13, 2016, 08:48:06 pm
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I was looking at the schematics for two distinctive amps. One is the Twin Reverb and the other is the Plexi 100 watt Super Lead amp. What I was looking for is the difference between the two power supplies. The power transformer high voltage secondary tap for the Twin Reverb is 340-0-340 and the Plexi is 178-0-178. The B+ for both amps is somewhat similar. The high voltage secondary center tap on the Twin Reverb is tied to ground while the center tap on the Plexi goes to the junction of two electrolytic caps in series which feed the output transformer primary winding center tap.
I watched the Uncle Doug video on youtube that explained the way the two high voltage secondary taps work. Essentially, one side goes positive and at the same time the other side goes negative and then true to a sine wave the positive side goes negative and the negative side goes positive. The wave goes forward to diodes. The positives go through the diodes and the negatives go to ground through the high voltage center tap in the case of the Twin Reverb and through the high voltage center tap to the junction of the two power capacitors in the case of the Plexi.
Am I correct in assuming the negatives that go to the capacitors for the Plexi are somehow converted to positive in order to achieve the voltage doubling effect? If so, how does that happen? If not, what am I not understanding?
Thanks
Mike
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With the Marshall, the PT secondary CT is biased up above ground to the lower filter caps B+dcv.
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With the Marshall, the PT secondary CT is biased up above ground to the lower filter caps B+dcv.
So that is how it doubles the voltage correct, from the negative voltage?
Thanks
Mike
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What - voltage?
The CT is not referenced to ground, it's referenced up/+ to the lower filter caps +dcv.
The CT is standing on that lower filter caps B+dcv.
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Google: "voltage doubler circuit"
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Google: "voltage doubler circuit"
Thanks for the suggestion. Will have to look at this in the morning. Not late night reading.
Thanks
Mike
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Don't look for voltage doubler. Just look for full wave bridge and conventional full wave rectifier. Then you will understand the differences.
Just forget that the plexi has the CT connected to the caps. All that connection does is force the voltage to split evenly across the two series filter caps. The plexi rectifier is a full wave bridge (FWB). The twin reverb (TR) is a standard conventional full wave rectifier.
The FWB uses the entire HV secondary all the time and produces twice the output voltage that the conventional rectifier produces. The conventional rectifier only uses half the HV secondary at a time.
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Don't look for voltage doubler. Just look for full wave bridge and conventional full wave rectifier. Then you will understand the differences.
Just forget that the plexi has the CT connected to the caps. All that connection does is force the voltage to split evenly across the two series filter caps. The plexi rectifier is a full wave bridge (FWB). The twin reverb (TR) is a standard conventional full wave rectifier.
The FWB uses the entire HV secondary all the time and produces twice the output voltage that the conventional rectifier produces. The conventional rectifier only uses half the HV secondary at a time.
Thank you for making it clearer. Looked up full wave conventional and full wave bridge rectifiers and it made sense. Not sure why Fender didn't want to use a FWB on the Twin Reverb except that the article I read suggested 100uF electrolytic caps before and after the choke (B+ and the screen supply caps) exactly as Marshall did whereas Fender used 70uF and 20uF filtration. Maybe Fender didn't want as much supply filtering as Marshall did. Wouldn't want 100uF cap on any rectifier tube that I am familiar with so it makes sense why you wouldn't use a FWB with a tube rectifier.
Thanks
Mike
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Twin Reverb never used a tube rectifier. It was always silicon. Filter caps are not a concern about whether you chose FWB or conventional type rectifier. They are a factor chosing between tube or silicon.
BTW, the reissue Twin Reverb does use a FWB rectifier.
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Navigate to NEETS (http://jacquesricher.com/NEETS/). Read Module 6. If any of it is unclear, you can reference back to Modules 1 & 2.
I was looking at the schematics for two distinctive amps. One is the Twin Reverb and the other is the Plexi 100 watt Super Lead amp. What I was looking for is the difference between the two power supplies. The power transformer high voltage secondary tap for the Twin Reverb is 340-0-340 and the Plexi is 178-0-178. The B+ for both amps is somewhat similar. The high voltage secondary center tap on the Twin Reverb is tied to ground while the center tap on the Plexi goes to the junction of two electrolytic caps in series which feed the output transformer primary winding center tap. ...
Full-wave rectifier (Twin) vs. Full-wave bridge rectifier (Marshall).
The Twin's full-wave rectifier is only using half of the PT's high voltage winding at any instant, because the center tap is tied to ground. It is rectifying 340vac into d.c.
The Marshall's full-wave bridge is using a different arrangement of diodes to use the entire winding, all the time. 178vac + 178vac = 356vac, or about the same as the Twin's 340vac from a half-winding. So the rectified output is roughly-same as the Twin's rectifier.
That the Marshall ties the center tap to the midpoint of series filter caps is only an interesting side-issue. Doing that helps force equal voltage across each cap, which is one of the functions the Twin's 220kΩ resistors across series filter caps.
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Thank you to everyone for helping me understand this matter. One of the reasons I was curious is I have an extra 173-0-173 PT to use in a project. Decided to use the Marshall power supply values and probably build another Steel String Singer clone Marshall style in a head cabinet. Really like the sound of those amps and can use a PT that is now on the shelf.
If I build another SSS will try to do a Visio schematic and layout. In my opinion this is one of the best, or maybe the best, clean amps ever made. Should have a schematic and layout for the amp in the Hoffman schematic library.
Thanks
Mike