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

[wildbill]

I used a 56v 50w zener diode between B+ centertap and ground to lower voltage to the plates of a '74 vibrolux reverb. This exact mod is mentioned in the q/a section of a desktop reference of hip vintage amps. The 74' VR has been "blackfaced" so it has a bias adjust circuit. When the centertap is wired to the zener the bias supply voltage reads -50v (which is normal) until the standby switch is turned on and then it goes up to -99v!? I have reconnected the centertap to its original ground and checked bias supply voltages which remain at -50v wether the standby switch is on or off. I can't find any explanation for why moving the b+ centertap would effect the bias voltage and what is more confusing is that changes when the standby switch is switched on or off?

[John]

Please don't take this as gospel, but I think that your bias supply, since it's negative voltage, sees the elevated ground, thereby making it *more* negative. Since you're using a 50volt zener, the math is about right.

I am still working my way through Merlin's Power Supply book, so I could be totally wrong. 

[kagliostro]

can you post a link to the schematic of the amp

Kagliostro

[wildbill]

This is a link for the aa964 blackface circuit, it is what I followed when converting my 74' vr to blackface specs, I dont have a centertap for the filament heater winding (no green yellow wire) and the voltages coming of my power transformer are higher (which is why I am using the zener diode to lower them) 
first link is layout
second is schematic
thanks
http://www.kbapps.com/audio/schematics/tubeamps/fender/vibroluxreverbaa964layout.html
http://www.kbapps.com/audio/schematics/tubeamps/fender/vibroluxreverbaa964.html

[sluckey]

Put the zener in the B+ line rather than the transformer center tap. Then only the B+ will be affected.

[kagliostro]

There is something that escapes me, with the zener you should not have the same bias voltage for the bias and you should make an adjustment to have the right voltage

the schematic show exactly your bias circuit and intake ?

Kagliostro

[wildbill]

Put the zener in the B+ line rather than the transformer center tap. Then only the B+ will be affected.

But it IS inline with the B+ centertap it should only affect the B+ winding not the bias supply winding or the filament heater winding right?

[sluckey]

But it IS inline with the B+ centertap it should only affect the B+ winding not the bias supply winding or the filament heater winding right?

The bias tap shares the transformer center tap with the HT winding. That's why the bias is affected. Put the zener in the B+ line, ie, where the standby switch is located. The center tap is called B-, not B+.

[wildbill]

But it IS inline with the B+ centertap it should only affect the B+ winding not the bias supply winding or the filament heater winding right?

The bias tap shares the transformer center tap with the HT winding. That's why the bias is affected. Put the zener in the B+ line, ie, where the standby switch is located. The center tap is called B-, not B+.

Ok so you are suggesting that I put the zener in line with the B+ after the rectifier tube?

[sluckey]

yes

[wildbill]

I really appreciate the help Thank You
Im not really sure how this would be done... in series with the b+ line off the rectifier tube? Before the standby switch? which direction anode side/cathode side?

[sluckey]

Disconnect the red wire from pin 8 of the rectifier tube socket. Connect that wire to one side of the zener. Connect another wire to the other side of the zener and connect the other end of this wire to pin 8 of the rectifier tube socket. If the B+ is still the same just reverse the connections to the zener. You won't damage anything by connecting the zener backwards. It'll just act like a normal diode.

You'll likely need a heatsink for that zener. Don't forget to use an insulator.

[HotBluePlates]

Please don't take this as gospel, but I think that your bias supply, since it's negative voltage, sees the elevated ground, thereby making it *more* negative. Since you're using a 50volt zener, the math is about right.

I am still working my way through Merlin's Power Supply book, so I could be totally wrong. 

Look at it this way:

1. Due to the number of turns in the winding of the PT, the voltage relationship between the CT and either end of the PT secondary is fixed.
2. Normally, you connect the CT to ground (0v) as a reference for the secondary. To lower the effective B+, you can place the zener diode in between the CT and ground. This moves the reference voltage of the CT.
3. Typically, the zener is oriented so that the CT is connected to an end which places the CT at an effective negative voltage; that is, for a 50v zener, the CT thinks it is attached to -50v instead of 0v. The secondary still outputs all the voltage it always did, but since the CT is referenced to a negative voltage, the rectifier output is lowered by the amount of the shifted reference.
4. The bias, if derived from a bias tap, now has the zener voltage added to the original voltage. The bias supply is probably still referenced to 0v; if it were referenced to the zener, it might not develop enough bias.
5. Even if you reference the bias to the zener, you still have shifting voltage problems. Until the amp is taken off standby, there might not be enough current flow through the zener to define its zener voltage, and therefore drop the B+ (or add to the bias). So with this arrangement, even if you meticulously reference to the zener, you might still have bias shift.

Which is why you do it Sluckey's way. Or don't use zeners to drop B+, but that's a different can of worms and maybe more $$.

[John]

HBP, great explanation of what I was thinking, but got all wrong explaining. Of course, the center tap is not elevated but "sunken".  I was getting my heater wires involved in there! 

Thanks!

[wildbill]

Hey thank you guys, I appreciate getting feedback on both the "how" and the "why" of it.
Like I said on page 221 of desktop reference of hip vintage amps there is this question "my 75 VR is changed to blackface but still doesnt sound like my friends 64" and Gerald answers by saying "the PT voltages are higher, place a 56v 50w zener between centertap and ground and rebias the amp you will notice a remarkable difference in tone." Sounds simple right... not to mention there is a zener diode mounted to a small heatsink bracket being sold as the V Dump which is also wired to the PT ct.
Regardless this setup did not have the desired effect of earlier "blackface" like breakup in my '74 vibrolux, it lowered plate voltages but raised my negative bias voltage so much I couldn't properly bias my tubes. I don't think I will place the rather large zener inline with my B+ it would be difficult to heatsink it and insulate it from ground... but thanks again.

[HotBluePlates]

You could alter the bias supply to get rid of the extra unneeded voltage; it would be nice to not have to do that.

[sluckey]

Gerald answers by saying "the PT voltages are higher, place a 56v 50w zener between centertap and ground and rebias the amp you will notice a remarkable difference in tone." Sounds simple right... not to mention there is a zener diode mounted to a small heatsink bracket being sold as the V Dump which is also wired to the PT ct.

Good salesman! That's a good simple solution for some amps. But not for yours.

Zeners on the HT centertap work fine on cathode biased amps and even fixed biased amps that have a separate source of AC for the bias circuit. But if the AC for the bias circuit shares that centertap there will be problems. And now you know.

I don't think I will place the rather large zener inline with my B+ it would be difficult to heatsink it and insulate it from ground...

Insulating power SS devices is a simple matter of using the proper mounting kit. If you have a stud mount all you need is two proper sized mica washers and a nylon bushing/shoulder washer. A mounting kit would contain everything you need including nuts, flat washers, ring lug, mica washers, and bushing. And they are cheap. There was a time when the mounting/insulating hardware was included with power SS devices.

[Geezer]

May I suggest this as an option>  http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

I built one into a 4x outlet box, can use it on any amp, & it works perfectly.

G

[jjasilli]

Another possible solution?  Double pole standby SW:  one side of the SW operates the standby function.  The other side switches in/out a shunt resistor to the bias supply, which cuts the bias voltage in half when standby is switched ON.

[HotBluePlates]

Cool idea JJ!

I humbly suggest we'd like to fix the problem rather than the symptom, but that still is great thinking!

[tubeswell]

Or keep the zener on the CT and don't use the bias supply tap for the bias PS. (Instead, you can take the bias power from one end of the HT winding, like in a BF princeton or the like)

[kagliostro]

What about to move the standby switch between zener and ground

or better a double pole switch one in the B+ rail, the other between zener and ground ?

Kagliostro

[sluckey]

What about to move the standby switch between zener and ground

That will almost certainly pop the bias caps when the STBY switch is open. Try it and see.

It's a bad idea to do anything to the HT centertap (except connect to ground) when the bias supply depends on that same centertap.

[kagliostro]

Try it and see.

No, thanks, I believe you

thanks for answering

Kagliostro

[wildbill]

Or keep the zener on the CT and don't use the bias supply tap for the bias PS. (Instead, you can take the bias power from one end of the HT winding, like in a BF princeton or the like)

That is an interesting idea... I might look into it

[MZANNETOS]

I almost have the same issue.
I built a cathode 4 x EL84 cathode biased amp and used a PT that I had available
Issue is that the PT is giving me 350-0-350 where I only need 290-0-290
So I am getting some high voltages
I tried the zener diodes in series with the cathode going to ground from the CT, but it seems for this amp it does not work
I am using 5W 15V zener diodes
And have them in series on a terminal board
What I noticed is that if I measure from the first diode I do see the drops and if I keep going will see subsequent drops of voltage until I get to the last one connected to ground. And then I get the same original reading of 350-0-350
Would anybody have had the same issue?