Help! Bias voltage from full wave bridge

[Chipotle]

Hello,

I am trying to derive a bias voltage from a full wave bridge a la the ampeg v4, http://www.schematicheaven.com/ampegamps/v4poweramp.pdf

The power transformer is around 300 volts and I'm getting 160 or so AC volts after the .047 cap. However, when I add the diode, I'm only getting around -8 volts dc! This is with no tubes loaded. I also don't have the 75k after the diode like in the ampeg schematic.

Any ideas?

[Mouthurst]

I have the similar 2 pwr tube version amp, V2-I recently re did the pwr xfrmr and it produced 408VAC for the HV.
I ended up getting roughly 596+ for the B+ and I think, -65 for the bias voltage.

Id say something is up with the xfrmr. Make sure the grounds are good and check it.

[Chipotle]

I have the similar 2 pwr tube version amp, V2-I recently re did the pwr xfrmr and it produced 408VAC for the HV.
I ended up getting roughly 596+ for the B+ and I think, -65 for the bias voltage.

Id say something is up with the xfrmr. Make sure the grounds are good and check it.

Hello,

Thanks for posting. It's actually a homemade amp, not a real Ampeg. I'm just trying to copy the bias design! So, it's supposed to be a 300 volt ac transformer. The bridge I'm using is a FRED bridge, and I've got the standby switch between the bridge and first filter.

The transformer is a Heyboer, the secondary is 300 volts with a 275 volt tap no center and it also has a 6.3 volt winding with a center tap which is connected to ground.

I do think it's ground related; I don't see how this bias setup can work right, since the other side of the transformer is connected to the bridge instead of ground. I've noticed that when I touch the chassis, the bias voltage shifts lower, too!

I'm thinking of trying this arrangement:

http://www.schematicheaven.com/voxam...0_poweramp.pdf

Any help or comments are much appreciated!

[R ski]

Low Bias

When I build amps, I noticed that the bias voltage can be offset by leaky coupling capacitors from the splitter.

Float the bias connection, if a radical increase happens. DC leakage
is countering the negative source.

[Chipotle]

Ok, I have noticed with this circuit, I have to measure the AC on one side of the cap before the bias diode in order to get a good reading. So, I think this cap needs to be charged up and measuring the AC is just enough current draw to get it charged. Then when I measure the DC voltage, it's slowly dropping.

I disconnected the bias caps, too, just to take the capacitors out of the equation (just pt, FRED bridge, .047 cap a la ampeg schematic, and 1n4007 bias diode) and it was the same results.

However, I swapped the .047 for .2 and the bias jumped up to -30 or so. Again, I had to measure the AC first in order to get the reading. So, I'm thinking of going with a 2 mf cap to try and get some decent voltage.

I suppose with some tubes drawing some current, this supply will stabilize..?

[Tom Phillips]

It sure sounds like the circuit path is not being completed. Like maybe the ground reference is missing after the cap. Try measuring the resistance between the cap and chassis ground. Measure it on the side connected to the diode with the amp turned off and all the cap voltages drained off. Does the reading make sense? (Value of the bias circuit divider resistor or less)
Tom

[Satamax]

Hello,

I am trying to derive a bias voltage from a full wave bridge a la the ampeg v4, http://www.schematicheaven.com/ampegamps/v4poweramp.pdf

The power transformer is around 300 volts and I'm getting 160 or so AC volts after the .047 cap. However, when I add the diode, I'm only getting around -8 volts dc! This is with no tubes loaded. I also don't have the 75k after the diode like in the ampeg schematic.

Any ideas?

It sure sounds like the circuit path is not being completed. Like maybe the ground reference is missing after the cap. Try measuring the resistance between the cap and chassis ground. Measure it on the side connected to the diode with the amp turned off and all the cap voltages drained off. Does the reading make sense? (Value of the bias circuit divider resistor or less)
Tom

Doesn't that sound similar? I think two different ground potential before and after the diode are nescessary, doing two different voltage drops, the difference between the two should create the "bias" voltage. Well, i'm not very clear, but it's real late where i am!

Bye.

Max.

[Chipotle]

Yes, I am definitely ground confused! Anyway, I stuck a 42k resistor to ground where the 56k is in the schematic, after the cap, before the diode. I had this in place before, but it had dropped the -8 volts to around -4. I stuck it back in, same result. However, when I flipped the standby to on, I got a steady -200 volts! I will probably add another after the diode simply to drop voltage.

Anyway, the standby is between the output of the bridge and the first filter cap for the B+. I don't know why the standby switch is having such a profound effect on the bias supply! It probably doesn't matter, though.

It looks like I'm getting far closer on the bias supply! Thanks for the help!

[Tom Phillips]

It appears that you have the standby switch in a position that disconnects the HV rectifier bridge from system ground. That will turn off the bias supply. It’s best to use a circuit that leaves the bias supply ON when the amp is in standby mode. One way to do this is to move the SB switch to the positive side of the supply rather than the negative side of the HV supply as shown in the first schematic link you posted.

After you get that fixed I suggest that you switch back to the original resistor and cap values in the bias supply. That will be your stating point and you will then need to adjust the resistor values to get the exact bias voltage your amp requires. If you search around you will also find capacitive bias tap circuits that include a bias adjust pot. It would be a good idea to incorporate the adjust pot into your amp. I’ll post a link if I find a suitable circuit.

[Chipotle]

Thanks, Tom. As it turns out, I have the standby switch on the positive side of the bridge already, not like the ampeg schematic. The ground of the bridge is hardwired. When the standby is off, the -200 volts begins to dissipate. When I flip the standby on, connecting the bridge to the rest of the filter caps, I get the steady -200 back!

Any ideas?

Here's an adjustable capacitor bias...

http://www.triodeel.com/dynawil1.gif

Interesting circuit. Thanks for the help, too!

[Tom Phillips]

Thanks, Tom. As it turns out, I have the standby switch on the positive side of the bridge already, not like the ampeg schematic. The ground of the bridge is hardwired. When the standby is off, the -200 volts begins to dissipate. When I flip the standby on, connecting the bridge to the rest of the filter caps, I get the steady -200 back!

OK. Do you have the standby switch "after" the bridge such that the bridge is always connected to the transformer winding weather the SB switch in ON or OFF?

[Satamax]

i Chipbotle.

Have you put the 75KΩ resistor back in the circuit? I think for a diode to work you need a difference in potential, if both sides of your diode stand at the same voltage at all times, this can't work, like a tap between two barrels with the same level on each side, pressions equate, so there's no transfer of fluids. Same if you don't have anything on the other side of the diode, as if it was connected to nothing, it's like a closed tap, nothing can go through. But i don't know if the 75KΩ would work, or, as i think need to move the standby to the ground leg of the bridge, as there's nothing to close the circuit in your case. Think, the electrons "flow, oposite to the polarity, so when one side of the AC on your bridge rectifier is at positive potential, the other side is at negative potential, the electrons flow from the negative, to the positive, difference of potential, here in your circuit, positive wave is clamped by the diode, it can't go through, since the "travel" of the electrons is blocked by the diode, then when you get to the negative wave, which normaly should let the electrons through, where's your positive side? On the oposite AC leg of your bridge rectifier which is connected to what? In standby mode? The neg side diode of that side of the bridge blocks the electron "flow", and the positive side is open circuit, connected to nothing because the switch is open, so there's no difference of potential between the side you've connected the bias circuit to and the positive side (realy the other side, but the bias is working only "half wave", so the oposite side to the bias is positive, when the negative half wave is hiting the bias circuit) While in the ampeg circuit, the negative side of the diode in the bias circuit is always conected to ground via the 75KΩ and there's difference of potential between the AC leg to which the bias is connected and ground, so this works. But i can't seem to work out how the circuit is closed between the two AC legs! Anyway, chuck that 75KΩ in and see what happens, should work. Someone more knowledgeable will chime in and finalize my explanation i bet!

Bye.

Max.

[Steve Conner]

Well, now you know why Ampeg put the standby switch in the negative side If you rewire it according to the Ampeg schematic, it should work.

[Satamax]

Well, now you know why Ampeg put the standby switch in the negative side If you rewire it according to the Ampeg schematic, it should work.

Steve, you don't think it should work with only the 75K back in circuit? Myself, i can't see the way the circuit is closed in the ampeg! I can't work out the electron path of that bias supply.

[Steve Conner]

The bias supply circuit closes through the bridge rectifier diodes back to the B+ and ground rails. So some ways of wiring the standby switch will break it. In the light of Max's comment, I'm not too sure that it needs to be in the negative return like Ampeg did it, but I'm still pretty sure that putting it between the bridge rectifier and the first filter cap (in either wire) will break the circuit and kill the bias voltage.