Fixed bias using PT with no CT ?

[2deaf]

Better attachment

[HotBluePlates]

...  One of the gurus will correct me if I am wrong ...

Standard Response

[2deaf]

Attached is the portion of the Ampeg power supply that generates the bias voltage.

[2deaf]

In reference to the above Ampeg Bias 3 attachment, I'm still not seeing an AC signal path.

to make it full wave, you need another diode, where the HT signal is the other leg on the transformer.

I'd like to see that one.

"The ground on the bridge provides an artificial centertap for the PT.  So the circuit generates a signal that is half wave.

How would the arbitrary assignment of the ground symbol generate a half wave?

With all the filtering on the B rail, the ground is mostly DC.

Relative to what?  To ground?

the full wave bridge, and the bias diode are upstream of any capacitors used to convert an AC signal to rippled DC.

The capacitors convert an AC signal to DC?  Mine blow up when I try that. 


Like I said at the start, I don't see an AC signal path.  I do see a path for a nasty-ass rippled DC signal.

[AZJimC]

 A single diode fed an AC signal, in this case the 60hz from the transformer winding, (which comes through the .047 mfd cap, used as a coupling cap), produces a half wave rippled DC, which then charges a 22mfd, used as a filter cap.

That 22mfd value is enough to smooth a half wave DC since the current draw is very minimal. The diode facing backwards,  allows the positive side of the AC to be cut off, relative to ground, and thus produces a DC voltage negative relative to ground. Notice the 22mfd filter cap is installed with it's positive leg to ground, so it can be negative of ground at the other end, where the diode feeds it only the negative side of the AC ripple.

[2deaf]

A single diode fed an AC signal, in this case the 60hz from the transformer winding, (which comes through the .047 mfd cap, used as a coupling cap),

Yeah, I already know how it works.  I was just trying to bait some people into saying something since there has been a lot of misinformation in this thread.  Your words can be interpreted to be correct since the bias diode is fed an AC signal that was ultimately derived from the 60 Hz AC transformer winding.  However, the 60 Hz secondary signal has been converted to half wave DC by the B+ diode.  Now the fluctuating DC is converted to AC relative to the chosen ground point by the .047uf coupling cap and the 56K resistor.  The cap. and the resistor are not used as a voltage divider but instead are used in the same way that a triode stage uses them when capacitor coupled.  The table I presented above shows the pair acting exactly as expected for cap. coupling.

I have attached my final drawing for this thread.  I have redrawn the pertinent circuit to look like a cap. coupled triode stage in an effort to illuminate how it works.

[HotBluePlates]

Like I said at the start, I don't see an AC signal path.

Right there isn't a signal path.

As you already know, the cap just couples a.c. and is an impedance at 60Hz to work against the resistor to ground ahead of the rectifier to divide the raw a.c. input.

... there has been a lot of misinformation in this thread. ...

The wasted time I've spent in the past to correct misinformation, when all that was wanted was argument, is why I've resorted to the "Standard Response" I posted earlier. That, and simply ignoring the posts unless someone different asks a question.

[sluckey]

Hopefully this pdf will help people understand this circuit without needing a fancy calculator and reference books from the local university library. This is a real world circuit with real world test equipment readings. I can't think of anything else to say about this circuit.

[2deaf]

Hopefully this pdf will help people understand this circuit without needing a fancy calculator and reference books from the local university library. This is a real world circuit with real world test equipment readings. I can't think of anything else to say about this circuit.

Use this explanation with my Ampeg Bias 3B re-draw of the circuit above and it should be even more clear. 

[sluckey]

Be my guest. I'm done.   

[dude]

Glad you guys got that worked out

All I know at this point is it works "well", the amp plays well.   PS: the 100K R to the NFB on the Presence Pot works much better too. Thanks Sluckey. I did use Marks Huss's schematic and crossed it with an 18 watt lite with 6V6's but missed that NFB R valve. The first amp I build without a layout of someone else-es.

HBP, helped me change the original Crate VC20 many ways to better the tone, CF, changing many R's and caps but suggested to gut the thing and build something useful. I took his advice and this amp is one of my favorites now. I was going to put in a different PT with a CT (it wouldn't fix) but Sluckey, fixed that idea for me too, I read all over the place and found no way to do it without a CT on a FWB.

A world of knowledge is here, hope I didn't piss anyone off with my novice questions, especially HBP as I always look forward to his replys too, thanks HBP.

Sincerely thanks again,
al

[HotBluePlates]

No problem, Al. That's what this place is for.