Zener diode B+ dropping question - allowable distance from voltage source?

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yello
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Zener diode B+ dropping question - allowable distance from voltage source?

Post by yello »

I've used the zener diode method outlined here successfully in the past to drop B+.

Image

I'm currently working on an amp that would benefit from the same treatment, but there is limited room to place a turret and a couple zeners in the capacitor area. It would work better to have a wire run leading to the turret and zener, then straight to OT from there.

That has led to a general question about the zeners and their implementation - Does the zener have the same effect when placed further away, closer to its intended destination, the OT? Any concern about the wire run? My instance only needs the zeners to affect the OT point to get plate voltage down, nothing downstream needs voltage reduction, but for completed reference, would a wire run back then to feed the next stage be problematic?

I would think it is all fine, but doublechecking with those that have more experience implementing the zeners.
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by JMPGuitars »

Your description is a little ambiguous.

The Zeners can work in different positions, but they're more efficient at or after the 1st filter cap. This is covered in the Zener thread.

To make things clear, draw what you're thinking of.
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by yello »

JMPGuitars wrote:
Wed 03/27/24 9:32 pm
Your description is a little ambiguous.

The Zeners can work in different positions, but they're more efficient at or after the 1st filter cap. This is covered in the Zener thread.

To make things clear, draw what you're thinking of.
Image

*1st drawing is the standard way you have it in your Zener thread.

*2nd drawing, the gist of my question, has the zener far away after a wire run. And only affecting one stage, the OT. Does zener placement distance from filter cap A matter?

*3rd drawing was the related question I posed, difference from 2nd drawing being the zener affects the whole B+ string as opposed to just the OT, which is the current design of the amp I am working on. Also has two long wire runs.

I can get into specifics of the amp I am working on if it is interesting, but wanted some general guidance on zener distances first.
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by JMPGuitars »

I'm only a pseudo-engineer, so I can't give you a proper long-winded answer on this one. I will say that drawing 3 is electrically about the same as drawing 1. So that would be the way to go. I say about the same because physical placement can have an effect on things (heat issues, long wire run issues, etc.).
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by yello »

JMPGuitars wrote:
Thu 03/28/24 4:38 pm
I'm only a pseudo-engineer, so I can't give you a proper long-winded answer on this one. I will say that drawing 3 is electrically about the same as drawing 1. So that would be the way to go. I say about the same because physical placement can have an effect on things (heat issues, long wire run issues, etc.).
That is what I figured, electrically equivalent but distance and proximity to other parts, and heat can induce other issues. But the zener would function the same.

I am working on a 2x6V6 vintage amp that has a factory schematic and PT that puts 450VDC on the 6V6 plates with 117VAC primary. With 122VAC at my wall, in this actual amp, its knocking on 480VDC.

Also unusual in that it splits the B+ rail after the first filter cap which feeds the OT, with one resistor going to the power tubes, then another resistor and subsequent string feeding the preamp.

I'm thinking through how to have the power tubes and the first filter cap be able to handle such high voltages, and come up with:

1) Keep PT, reduce B+ (zeners, resistors, etc., whatever method) - and also likely need to series connect two filter caps at first node to increase voltage handling, currently a 475VDC 16uf cap there.
2) Use 6L6 tubes which would require a new OT, or rewiring current OT (I think I can shift the outputs one way or another, like use 8 ohm out for 16 ohm speaker?)
3) Use a different more appropriate PT, and adjust B+ network for appropriate voltages throughout.

Its a Bogen CHA-20A -Here is the schematic:

Image

Image

My amp was slightly different than schematic as it was missing the 68l 2w resistor that bridges the 6V6 screens and cathodes. And my filament CT was just grounded, not wired to 6V6 cathodes.

Lastly, there is another factory mod that others CHA-20a amps have, that mine didn't, which is a 22r 2w resistor between the HV and Filament CT's.

The amp runs fine and sounds pretty good (with variac managing the voltage during testing, currently about 100-110vac) - and only voltage reducing (and noise filtering) step I've taken is a new B+ node A in front of the stock one with a 16uf 475v cap then a 1k 5w resistor then on to the rail per schematic.
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by JMPGuitars »

Keep in mind that you don't need to put the Zeners in the most efficient position. If you place them before the filter caps, you'll just need more of them. For me it's 50% of stated value, others have had different results.

I would use a string of three 5W 10V Zeners after the rectifier tube if there's more room there. Or you could do two 10V and one 15V if you want to drop a little extra.

Are you using the 6AX5GT rectifier in your amp?

Thanks,
Josh
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by yello »

JMPGuitars wrote:
Fri 03/29/24 2:24 pm
Keep in mind that you don't need to put the Zeners in the most efficient position. If you place them before the filter caps, you'll just need more of them. For me it's 50% of stated value, others have had different results.

I would use a string of three 5W 10V Zeners after the rectifier tube if there's more room there. Or you could do two 10V and one 15V if you want to drop a little extra.

Are you using the 6AX5GT rectifier in your amp?

Thanks,
Josh
I do remember reading them were less efficient between rectifier and filter cap, thus was avoiding that, but worth a try per your suggestion as there is more room over there and that simplifies it. I am using the 6ax5gt rectifier. PT doesn't have a 5vac tap. Tempted to put in a different PT and run a 5 volt rectifier, but easy to try zener first.

Only zeners I have are 16v or 30v 5-watters.

I also need some non-vintage 6v6 tubes that can handle the higher voltage - I have a pile of brand new Ruby branded tubes - I think those are JJ's? Anyone know?
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by JMPGuitars »

Use four of the 16V Zeners near the rectifier where you have space. See what your voltage looks like then.
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by yello »

JMPGuitars wrote:
Sat 03/30/24 2:50 pm
Use four of the 16V Zeners near the rectifier where you have space. See what your voltage looks like then.
I'll try that.

Looks like I need to order zeners though, only have one 16v 5w left, the rest are 30v or 33v. I assume you can't parallel zeners or something to make them into "15v" or "16.5v" like you would a resistor?

Also, I know you don't personally like JJ's, but what high voltage handling 6V6's do you use? Electro Harmonix?
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Re: Zener diode B+ dropping question - allowable distance from voltage source?

Post by JMPGuitars »

I have a pair of EH 6V6s. They only really get used on my tube tester; I'm not a 6V6 guy. The only reason I don't like JJ anymore is because their quality and reliability went in the toilet. I would look at Sovtek, EH, TungSol, Mullard, and maybe TAD.
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