Half wave to full wave question

[Colas LeGrippa]

Bonjour! Got a TEISCO checkmate 20 on my bench (that doesn't correspond totally to the schem found on the net). The amp starts hummming when the volume pot is at middle. The power supply is halfwave rectified and I'd like to convert it to a full wave ( for more quiet operation).  This isn't a big deal though, but I just want to make sure I ain't missing something I wouldn't have seen at first glance.The high voltage taps have no center tap. Two diodes are tied to one HV red lead the usual way. The other red lead from the HV secondary goes to a HUGE 220 OHM resistor ( 20-50W approx. ) and the return from this resistor to the first filter cap positive side.  What is this resistor for ? Does it act like a choke? I would disconnect it completely and fit in two more diodes for full wave recto. But, why then has the manufacturer installed a component worth tens of diodes instead of simply install a full bridge rectifier ? Other question: is that possible that a half wave rectified amplifier be humfree, as a full wave one ?

Colas LeGrippa[/color]

[jjasilli]

Here's the official schematic:  http://www.preoccupationrecords.com/teisco/ckmt20_sch.gif  This is a voltage doubler circuit with full-wave rectification.  The rectifier diodes are not in series -- the "usual" way.  Instead one is in series with the HT lead; the other diode is shunted to ground.  The other HT lead goes to the junction of the 2 first stage filter caps.

I think you need to post a schematic of the circuit in the amp on your bench.

[Colas LeGrippa]

Hi,

I have already this schematic.
The usual way of hooking diodes in case of a non centered tap PT , I meant .
Thanks
Colas

PS: I'll try to draw what I've got on the bench and post it here.

[Willabe]

If the amp you have on your bench is wired as a voltage doubler and you want to change it to a FWB it will drop the B+ to about half of what it is now.

Wont work.     


                    Brad           

[Colas LeGrippa]

That's for sure.

[Willabe]

You could wire it as a FW doubler, but I think that will cut the B+ current in half, but maybe not because it's already a doubler?


               Brad        

[Colas LeGrippa]

I am not gonna fool around with something I'm not familiar with. I replaced the 2 filter caps out of capacitance and the hum diminished to an acceptable level.
I still don't know what is the big 220 / 63 resistor for
Regards
Colas

[jjasilli]

I'm guessing that you still have a full-wave voltage doubler circuit.  But someone installed that big dropping resistor in series with the B+ lead which is connecting to the junction of 2 filter caps (the caps are in series).  The resistor is affecting only one phase of the ripple waveform, throwing the circuit out of balance, and causing hum.  In other words, I think 1/2 the R value should have been used on ea leg of the B+ lead wires.  Then ea phase of the 60 cycle ripple (summing to 120 cycle) will be in balance.  The purpose of the Resistor would be to drop B+ voltage, and/or to simulate the sag of tube rectification.  Right idea, bad implementation.

If my guess is correct then:

* one of your red HT leads goes to the junction of 2 diodes; then to the top (+)side of the top filter cap.
* the other red HT lead goes to the Resistor, then to the junction of 2 filter caps; i.e., the junction of: the (-)bottom of the top filter cap, With the (+)top of the lower filter cap (the caps are in series, as stated above).

[tubeswell]

Whats the VAC on the HT secondary?

[Willabe]

I ment to say to re-wire it as a FWFB doubler, total of 4 diodes, not 2.

Sorry       

[Colas LeGrippa]

I will post a drawing for simpler discussion.

Thanks

Colas