zener diode to decrease high voltage....

[acorkos]

i want to drop the high voltage in this amp by a couple dozen volts, using a reverse polarity 10w zener. not sure where to connect it....between center tap and the first capacitor, or between first capacitor and ground...? thanks for your help

[Chuck H]

My rectifier know how is limited, but I did read that you can change the funky "full wave bridge voltage doubler" to a standard "bridge" rectifier for the same output voltage. You would leave the CTR floating and add a 220k 1W balancing/bleeder resistor parallel to each main filter. At that point you would put the zener on the bridge 0V reference.

Not sure what would happen if you placed the zener there with the "full wave bridge voltage doubler".

[Prairie Dawg]

My memory of this is a little rusty but IIRC-need to check me on this-the zener can be installed between the HV center tap and ground. I did one on a Rose Morris AC30 with diode rectification and it had no center tap, whereupon it got installed as you say on the zero reference. You need a bit of real estate there to properly dissipate the heat that the zener is going to generate, some heat sink compound, and a cap across the zener to quiet any noise.

[Chuck H]

WTF??? I didn't add any link to my post about the balancing resistors. Why then am I all of a sudden endorsing some product I've never seen?

[g1]

Not really a voltage doubler but another way of balancing totem pole caps without the resistors. Like Chuck mentioned you can change over to the resistor balancing and put the zener between the bridge and ground.
Or you can leave it as is and use 2 zeners as mentioned at the bottom of page 1 here: MetroAmp.com Forum • View topic - Methods For Dropping Plate Voltage -

[g1]

Not seeing any link there Chuck, is it an ad? If so I won't see it (adblock plus which I will happily endorse ).

[hasserl]

It's been discussed around here many times, a reversed biased zener diode between the HV center tap and ground will drop the voltage about the same amount as the value of the zener. A 5 volt zener will drop about 5 volts. But they do develop a lot of heat. You need to calculate the wattage dissipated and at least double that value for the rating of the zener, more is better.

[Chuck H]

I think the OP gets tht part. He was wondering where to place the zener in the above posted rectifier circuit.

[acorkos]

ok...the reason i am thinking about dropping voltage with a zener is because this amp has been burning through power tubes and power tube screen resistors. i had a long look at the rectifier diodes, and i think they have been removed and replaced incorrectly. instead of the configuration as shown in the schematic above, they are arranged like this:



would this be the cause of my too high voltage readings? i am going to reconfigure them as in the schematic...

[Chuck H]

Hmmm... Your drawing looks correct. The schematic looks wrong.

[hasserl]

I think the OP gets tht part. He was wondering where to place the zener in the above posted rectifier circuit.

Sorry, not taking enough time to look at things fully, discussion about putting the zener before a filter capacitor threw me off. I thought he was talking about putting the zener in the rectified DC B+ prior to first filter cap. That's an unusual arrangement. Why is the center tap grounded thru a capacitor?

[JoeM]

Fender did something similar in the UL amps.

[g1]

Like Chuck said, schematic error.
Diode polarity should be like this:

[acorkos]

From the MetroAmp forum...
----------------------------
"If you're using a full wave bridge rectifier (typical operation on a 100W) the easiest way to deal with this is to tape off the centre-tap and use a pair of balancing resistors on the first bank of caps. The centre-tap is used only as a 1/2 B+ voltage reference to ensure the caps are balanced, but the resistors will take care of that.

It'll be a lot easier to drop a Zener in once you've simplified the power supply like that."

"Yes I'm aware of the voltage balancing...and that's what I figured. Just install balancing resistors, omit the center tap, then place the zener between the bridge negative and ground."
-----------------------------

I think this is what Chuck H was referring to in post #2

[Chuck H]

Right. Just make sure to secure the CT so it is not connected and insulated. And be sure to heat sink the zener on the chassis (if it's aluminum) or a heatsink. Here's a modified schem (with corrected diode polarity):