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PT with bias winding instead of tap

1/16/2006 11:19 PM
Chris333 PT with bias winding instead of tap
Got a PT with a bias winding instead of a tap, and I'm not sure how to use it. One Marshall schematic shows it hooked up like a regular half-wave rectifier w/ a backwards diode and cap (and the other leg grounded). Another shows backwards diodes on each leg joined together, kinda like a full-wave CT rectifier, but no CT.  
And that got me wondering (here's my stupid question): could you use a full-wave bridge rectifier with the diodes backwards?
1/17/2006 3:32 AM
Chris/CMW amps

Hi Chris  
You can use both ways but the "simple one" still does work greatly after being used for at least 35 years.  
In a few words: just use the Marshall style one with 1 diode, 2 e-caps, some resistors and the trimp-pot. If the grounding is good you shouldn't get any hum-related-problems.  
Hope this helps,  
Ps Yep, it's certainly (and also) possible to use a bridge rectifier by just reversing the two dc-outputs.
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1/17/2006 12:37 PM

ChrisW has got probably the best approach, but I'll go into more detail in case it helps anyone.  
Basically an isolated, dedicated bias winding is just like having a separate low-secondary-voltage transformer available to base a bias power supply on.  
You could think of the main B+ supply as a unit and the bias supply as a separate unit, for instance if you had a cathode biased amp working well with only the main B+ supply (and heater supply of course), if you then wanted to change to fixed bias one of the key steps is to add just about any power supply that will reliably give you the exact desired negative voltage you need, or like most Ampagers prefer, an adjustable negative voltage over an appropriate range.  
This bias supply could be any of a number of stand-alone low-noise low-current DC supplies (or just plain batteries like the old radio days) or of course you build whatever topology supply you want from an appropriate transformer or winding. Half-wave, full-wave, bridge, whatever. Once you have a working source of the absolute DC voltage you need, and when that source still has no connection (through the PT winding, ground or otherwise) in common with the rest of the amp and its other power supply(s), it is still a floating power supply at this point, so you finally connect it to the amp.  
You do not even need a *negative* power supply, since this is the time you choose which terminal of an isolated bias supply you are going to connect to your amp's ground, and the other terminal goes to the bias-splitting resistors. Since you want a negative voltage to appear at the grids, you simply connect the positive terminal of the isolated bias supply to ground and the negative terminal to the point you want the negative voltage to enter the circuit from.  
the bias supply is only negative in relation to the rest of the amp, when you have an isolated separate winding or transformer to start with, you can turn it to DC any way you like, for instance using examples of common *positive* supplies, then just make sure you hook it up to your audio circuit with the positive terminal grounded and the remaining terminal will then be a source of negative voltage for biasing.  
1/19/2006 10:28 AM
Chris/CMW amps

Hi Mike  
Wish my English was that good :)  
1/19/2006 11:07 PM
Chris and Mike--thanks for the responses--the bias supply seems to be working and, finally, I understand why.
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