Hum After Zener Diode Voltage Reduction

[Daver]

So I decided to drop the B+ on this '66 Ampeg Gemini VI I'm working on to something the 7591's will live with. I got a 50 Watt chassis mount 47 Volt zener to put in the center tap. After installation, the voltage is down about 45 volts as expected. The problem is now I get a 120 Hz hum. I added a cap across the zener starting with .1uf going up to 22uf. That didn't do much at all. I tried another zener I had rated at 26 volts and that made no difference. I tried grounding the zener to places other than the chassis like the can cap ground, power tube cathodes and even the input jack. That didn't change anything. The pilot light is connected to the center tap, so I disconnected both wires thinking that might have something to do with it. Now for the odd part. I can get rid of the hum if I remove the phase inverter (7199) or disconnect the pentode from the triode. I did try another 7199 and a 6U8 (after rewiring the socket). If I bypass the zener, the hum goes away. Any ideas or other things I can test? Thanks!

[mozz]

Put a big-ass resistor right after the diodes. Are your filament voltages also higher than they should be? Maybe a bucking transformer is in order. How are you figuring the voltages are too high? There is nothing shown on the schematic. Found another schematic but can't read it too well, electrotanya website. You may have to register to get it, 450 b+?

[Daver]

The filaments are 6.6 vac. Not terrible... The plates are 503. The original schematic calls for 435. The 7591A screens are only rated for 440. It's already killed a pair in short order. The big resistor will create extra sag and a fair amount of heat. I've used the zeners before but never had any noise problems. Thanks.

[Chuck H]

You posted a stock schematic. Does the amp have a safety ground and the death cap removed? I don't know that it would make a difference (I actually don't) but I know I've never had any trouble doing it with a grounded chassis.

[g1]

Any chance it's related to the bias circuit coming off the HV winding?

[Daver]

The death cap is still there, but it has a grounded three prong cord. I don’t “think” it’s bias related because when I remove the phase inverter the noise goes away. Thanks for the ideas.

[kanks]

Try lifting the heater center tap coupling cap or just bridging it out. You could also try a humdinger.

[Daver]

Try lifting the heater center tap coupling cap or just bridging it out. You could also try a humdinger.

I have removed the cap and also tried making an artificial tap with two 100 ohm resistors. What do you mean by "bridging it out"? Thanks.

[kanks]

I have removed the cap and also tried making an artificial tap with two 100 ohm resistors. What do you mean by "bridging it out"? Thanks.

Just grounding the filament center tap.

Have you got an oscciloscope?

[mozz]

As was stated here and el34 world, "Beware of gremlins if you put zeners on the CT of a fixed bias amp whose negative bias voltage has an AC source that also depends on that CT."
"Sorry I forgot to mention that potential problem. Glad sluckey did. It gets complicated fast. I think that you can still take your negative bias voltage off the PT secondaries, but if the zener on the center tap lowers the unregulated/unfiltered secondary voltage then you have to make changes to the bias circuit. You also have to pay attention to which "ground" you use for the bias circuit. That simple & cheap MOSFET circuit avoids those pitfalls."

Seems maybe putting the zener in line with the b+ might solve it.


Zener diode on B+ centertap has doubled bias supply voltage!

[Daver]

I do have an oscilloscope. I'll try grounding the center tap, but I'm not sure that will make a difference.

I guess the bias circuit could be the problem, but as I stated the hum goes away when I remove the phase inverter. So, the hum is being created or injected there, apparently. Thanks!

[dstrat]

with out anything to drive the output tubes you will hear very little of anything, likely why it stops humming with PI tube pulled.

[Chuck H]

with out anything to drive the output tubes you will hear very little of anything, likely why it stops humming with PI tube pulled.

I don't get this. The power tube grids still have a load and block DC in the upstream direction because of the coupling caps. If the noise was from the bias circuit I would expect to hear it even with the PI tube pulled.

A wild guess would be a bad decoupling cap in the HV rail.?. Maybe the elevation is making some noise that isn't being shunted. It may even be that more filters should be replaced but the push/pull power amp is mitigating excess hum so you can't exactly hear some symptoms.

[dstrat]

right not the bias I dont think per say.
Question is how much does the bias change with the zener?

Is the lamp and stand by switch still in the center tap?

[dstrat]

Problem with that arrangement (standby) the bias does not come up until the standby is closed, not great for your tubes.
by adding the 47v zener you have lowered that bias by 47 volts, even worse for your tubes.