1976 Vox AC50 - How works the bias circuit ?

[kagliostro]

I'm not able to understand how acts the bias circuit of the Vox AC50

it is a bit different from the usual schematics that obtain bias voltage from HV winding

thanks for any explanation about

Kagliostro

[sluckey]

The main difference between this bias circuit and one of the old Ampegs or Marshalls that use the bridge for the B+ is the orientation of the rectifier diode.

This circuit picks up the AC input voltage from one side of the PT HT winding. This AC voltage is applied to the rectifier diode thru 2 series connected caps (just a 2M nonpolarized e-lytic). This diode is not the familiar series connected diode that blocks the positive half cycles of the AC and passes the negative half cycles on to the filter caps. Instead, this diode is connected so that it shunts the positive half cycle to ground, leaving only the negative half cycles to be passed to the filter caps. Both circuits only allow the negative half cycles to reach the filter caps. But the Vox way just looks ugly to me.

Does that make sense?

[kagliostro]

Many Thanks Sluckey

after reading your explanation everything is clear about how it works

But the Vox way just looks ugly to me.

to me odd ....

(just a 2M nonpolarized e-lytic)

I think if I want to obtain a non polarized e-lytic I must connect the two caps +--+ or -++-

am I wrong ?

MANY THANKS for the given explanation

Kagliostro

[sluckey]

My mistake. You're right about the non polarized e-lytic.

[PRR]

It is and can be a polar electro.

It is two caps because the main PT winding voltage is too high for one electro (see the main filter caps top-right: stacked). If the main PT HV winding were under 300VAC you could use single 450V caps in both places. 33uFd for main and 2uFd for bias.

I too say "ugly". Was there a law against 50VAC bias windings?

[SoundmasterG]

They also have cathode bias and fixed bias in combination on this amps....weird....but they sound great when set up right.

Greg

[PRR]

> also have cathode bias and fixed bias in combination

Not really. 47 ohms and say 50mA idle is 2.5V "cathode bias". OTOH there is 34V of fix-bias. The fix-bias is WAY more; it is much more fixed- than self-bias.

Thoughts:

We didn't all have milliVolt DVMs in 1976. Standard meter might have a lowest range of 3V. A 2.5V reading is easy to see on a 3V scale.

Part of the distortion in a power amp is the internal cathode resistance falling at high current.

200 ohms at 50mA to 80 ohms at 200mA. In this stage, at 50mA both tubes are working but when one is pushed to 200mA the other is near off. So the gain is effectively 200/2 or 100 for teeny signals and under 80 ohms for large signals. Adding 47 ohms of fixed resistance makes gain more constant and lowers distortion a hair.

[Tiny_Daddy]

I replaced that with a Marshall-style bias.

[kagliostro]

May be that adding a cathode resistor to a fixed bias amp can give a form of protection against bias supply failure ?

Thanks

Kagliostro

[Tiny_Daddy]

I add 20 ohms cathode bias to fixed-bias Crate Vintage Club to prevent blowing the HV fuse when played hard.

[SoundmasterG]

> also have cathode bias and fixed bias in combination

Not really. 47 ohms and say 50mA idle is 2.5V "cathode bias". OTOH there is 34V of fix-bias. The fix-bias is WAY more; it is much more fixed- than self-bias.

Thoughts:

We didn't all have milliVolt DVMs in 1976. Standard meter might have a lowest range of 3V. A 2.5V reading is easy to see on a 3V scale.

Part of the distortion in a power amp is the internal cathode resistance falling at high current.

200 ohms at 50mA to 80 ohms at 200mA. In this stage, at 50mA both tubes are working but when one is pushed to 200mA the other is near off. So the gain is effectively 200/2 or 100 for teeny signals and under 80 ohms for large signals. Adding 47 ohms of fixed resistance makes gain more constant and lowers distortion a hair.

Thanks for elaborating PRR. On a Conn amp that I modified, the original circuit was using a single 18 ohm resistor in the cathodes, and then a single cathode bias resistor for the pair that was a 250 ohm 10 watt. I would guess from your description that may have accomplished the same thing?

Greg