lowering B+ with Zener diode

[paulj]

Question to all,

I have lowered the B+ voltage in my amp using a Zener diode reverse biased between CT of the HV winding and ground. I know this works well.

What if I plug the Zener diode after the rectifier tube, on the DC voltage going to the first filter stage and the output transformer. I know that I would have to isolate it from chassis since it will be very hot in terms of voltage, then give it a good isolated heatsink for cooling, but would it work the same? Are there any no-noes for doing this? For example, more noise introduced to DC, as opposed to using the diode before the rectifier tube?

[kg]

it will work exactly the same. just watch that voltage!

[paulj]

it will work exactly the same. just watch that voltage!

Thanks,

I thought maybe noise from the diode or something but I also don't see what the difference would be.

But yeah!, everyone reading this, you must isolate that diode from chassis. There will be high voltage on it. An absolutely don't try touching it to check temperature or something like that!

[olddawg]

One of the usual methods is to mount several zeners on a zigzag patern on a piece of perf board suspended.

[paulj]

Great idea!
Do you have a photo?

[olddawg]

Great idea!
Do you have a photo?

Unfortunately no, but you just mount them in series on the perf board in a zigzag pattern. You can use lower wattage zeners and just add the drops together. It will also distribute the heat better. When I did it I used 3 or 4 five watt zeners.

[paulj]

And did you also install it after the rectifier on the high voltage line, without any negative effects?

[olddawg]

And did you also install it after the rectifier on the high voltage line, without any negative effects?

Yes... It is a very common practice when doing a conversion project or using used salvaged transformers to tweak the B+ this way.

[paulj]

Yes... It is a very common practice when doing a conversion project or using used salvaged transformers to tweak the B+ this way.

Cool, it looks like I will be employing this to adjust the output stage and leave the preamp section alone.

[Bruce / Mission Amps]

If you put the zener diodes in the center tap, the circuit is working with 1/8th to 1/10th of the voltage.
That seems safer to me.

Also, if this is an amp with a fixed bias supply, and you put the zener diodes in the center tap, ...your negative bias voltage will be shifted to the same dropped volts as the positive side but deeper negative now..... which means redesigning the bias supply's dropping resistors.

[paulj]

If you put the zener diodes in the center tap, the circuit is working with 1/8th to 1/10th of the voltage.
That seems safer to me.

Also, if this is an amp with a fixed bias supply, and you put the zener diodes in the center tap, ...your negative bias voltage will be shifted to the same dropped volts as the positive side but deeper negative now..... which means redesigning the bias supply's dropping resistors.

Understood. So those wires/diodes would have to be isolated/protected well from shorting to anything.

The advantage that I see in this is lowering just the power section voltage and leaving the preamp running on the full "unlowered" voltage. Just speculating. Does it make sense?

[paulj]

Thanks,

I thought maybe noise from the diode or something but I also don't see what the difference would be.

But yeah!, everyone reading this, you must isolate that diode from chassis. There will be high voltage on it. An absolutely don't try touching it to check temperature or something like that!

Responding to myself but someone could have misunderstood the earlier statements:

It is not like any of these methods is any safer (may be but that is too complicated to discuss here, unless someone volunteers). Either way, it will NEVER be safe to touch anything on a live or charged circuit.
Just to clarify, because discussing safety on the internet forum is somewhat controversial. It could encourage someone without knowledge in electricity to attempt things they NEVER should even approach, without having a professional preparation.

[Bruce / Mission Amps]

Understood. So those wires/diodes would have to be isolated/protected well from shorting to anything.

The advantage that I see in this is lowering just the power section voltage and leaving the preamp running on the full "unlowered" voltage. Just speculating. Does it make sense?

Yes, you could even put the zener diode(s) in the B+ lead to the OT or the center tap lead of the output transformer if you want. You might run into an issue where the screen(s) node is higher voltage then the plates of the power tube(s). I guess a separate node for plate supply and screen supply would work then but you'd need to break out the screen voltage after the zener and have a new dropping scheme with Screen B+ filtering.
So, like I said earlier, maybe the zener(s) should go in the grounded center tap of the PT and then you just adjust your preamp B+ supply dropping resistors to compensate!!

[paulj]

Yes, you could even put the zener diode(s) in the B+ lead to the OT or the center tap lead of the output transformer if you want. You might run into an issue where the screen(s) node is higher voltage then the plates of the power tube(s). I guess a separate node for plate supply and screen supply would work then but you'd need to break out the screen voltage after the zener and have a new dropping scheme with Screen B+ filtering.
So, like I said earlier, maybe the zener(s) should go in the grounded center tap of the PT and then you just adjust your preamp B+ supply dropping resistors to compensate!!

Bruce,
Thanks, I understand completely and have been thinking about these things for a while. I have come to the same conclusion by weighing the benefit/added complication ratio.
I too prefer simple solutions whenever possible.