60 Hz Quest

[trem]

This kind of bias supply doesn't do well with big filter capacitors. They can take over a minute to charge up to final voltage. See if you can find the schematic for Vox AC50/4 Amplifier Mk III drawing OS/072. It uses two 4uF electrolytics in series in place of the .25uF cap.

Oh Boy.....keeping in mind I have a limited knowledge of electronics.
This raises Two questions from me.

1. Are you suggesting that I Do Not use a (Much Larger) two stage filter for the bias supply.?
I was thinking of using two 50-150 caps. That is kind of a hard value to find....I THINK they are about 5-8 bux each from Mouser + shipping. My friend has Two 100-160 caps I can have.
I kind of thought that would solve my hum troubles (increasing capacitance in the bias circuit).

2. I have No Idea what that 0.25uF cap does. I guess it is limiting current somehow, at some frequency.? But I have No Idea why it is needed here....never seen it before. (I really do not know how it works).
Thank You

[loudthud]

Refering to the schematic drawing number OS/036 the high voltage and bias are derived from a floating 360V winding connected to a bridge rectifier. This means that either side of the transformer winding swings from -0.7V to about +500V because of the way the output of the bridge is connected. C15 is connected to one side of the winding. When that side of the winding swings to +500V, the diode in the bias supply turns on holding the bottom side of C15 to +0.7V and C15 charges up to 500V. On the next half cycle, the top side of C15 swings to -0.7V and tries to dump that 500V into R38 but C15 is too low in capacitance so it must discharge down to about 65V. After several hundred cycles, C14 hopefully charges up to (minus) 65V (It takes longer when C14 is bigger.). Back to the cycle where C15 charges up, notice that R38 provides a path for C14 to discharge because the right side of R38 is being pulled towards ground by C15 as it charges up.
　
So C15 does limit current and voltage delivered to the bias supply. When the mains frequency changes, so does the bias voltage. The zener diodes regulate the bias voltage but this will cause the output tube bias to be cold when the mains voltage is low or too hot when the mains voltage is high. Not a very good circuit.

[trem]

You are not the first person I have heard say this bias circuit is "not very good".
So let me ask you guys this.....keeping my dual bias pots, is there some reason I cannot build a bias circuit similar to what Marshall used on the 50 watt "plexi".?
It grabbed bias voltage from one side of the PT secondary (like the AC100) and knocked it down to a usable voltage for EL34 power tube grids.
Can I implement the bias circuit that is shown in the Marshall Schem below.?
Thank You

Marshall - jmp bass 50w 1986 - ampix

[g1]

No, you do not have a center-tapped PT secondary.

[loudthud]

The Plexi bias circuit won't work on the AC100 because the high Voltage winding on the AC100 does not have a grounded center tap.

[trem]

That's right.....full wave vs, bridge.
I will just proceed as planned.
Thanks

[52 Bill]

The zener diodes regulate the bias voltage but this will cause the output tube bias to be cold when the mains voltage is low or too hot when the mains voltage is high.

This is a really good point.

[nickb]

Refering to the schematic drawing number OS/036 the high voltage and bias are derived from a floating 360V winding connected to a bridge rectifier. This means that either side of the transformer winding swings from -0.7V to about +500V because of the way the output of the bridge is connected. C15 is connected to one side of the winding. When that side of the winding swings to +500V, the diode in the bias supply turns on holding the bottom side of C15 to +0.7V and C15 charges up to 500V. On the next half cycle, the top side of C15 swings to -0.7V and tries to dump that 500V into R38 but C15 is too low in capacitance so it must discharge down to about 65V. After several hundred cycles, C14 hopefully charges up to (minus) 65V (It takes longer when C14 is bigger.). Back to the cycle where C15 charges up, notice that R38 provides a path for C14 to discharge because the right side of R38 is being pulled towards ground by C15 as it charges up.
　
So C15 does limit current and voltage delivered to the bias supply. When the mains frequency changes, so does the bias voltage. The zener diodes regulate the bias voltage but this will cause the output tube bias to be cold when the mains voltage is low or too hot when the mains voltage is high. Not a very good circuit.

All true....and crucially, the zener diodes have a low ac impedance which greatly attenuates the ripple on the 8uf. Put the zeners back and adjust the bias and the hum will be gone. Of course you still have the mains voltage / frequency issues but I personally think that for most circumstances it's a minor problem.

[trem]

All true....and crucially, the zener diodes have a low ac impedance which greatly attenuates the ripple on the 8uf. Put the zeners back and adjust the bias and the hum will be gone. Of course you still have the mains voltage / frequency issues but I personally think that for most circumstances it's a minor problem.

I wish I could remember. I started this many months ago, and then had hand surgery half way through, and could not work on the amp for 2 months.
With 50 volts of Zeners, I was having trouble getting enough bias voltage on the grids...so I yanked the Zeners. But like I say, that was 3 months back.
At this point, the board is open, so I can jump in the Zeners, and the 2 stage bias caps (separately) and see which way works best.
Thanks For All The Advice

[trem]

So.......I jumped in the two 100-160 caps, and it was almost right...just a slight buzz.
I had forgotten about the U7's.
I yanked the X7 from V1 and V3...installed the U7 and the amp is just about dead quite. With the volume knob on 10 there is very little white noise, or any other kind of noise/hum..
The 100-160 are so big, I cn hardly fit them on the board. I have ordered some 50-150, and have no doubt those will work just as well.
Thank You Everybody...!!

[Chuck H]

You might try a value smaller than 100uf. And possibly reducing other cap values in the bias supply. Sans troubling with any math it's pretty easy to put in a smaller value and see if the amp remains quiet. My concern is the charge time for the bias supply. It may "work" but there could be some later issue due to a longer charge time. Glad you worked it out. Good thread.

[trem]

Hey Chuck -
I get what you are saying.....appreciate it.
I ordered some 50-150 (47-160 actually).....and I have some 10-150 I can try.
I think I lost my head a little bit when you guys helped me to Finally get the hum killed.
2x 2200uF-150V