zener diode

[studioit]

Can anybody verify my sucspicion- I ordered this from Mouser to lower the B+ voltage on an amp. It's supposed to have the suffix AK which would mean the cathode is the stud end, which bolts to the chassis. The NTE bag says AK but the zener says A and the drawing on the zener also shows it biased the other way. This is the first time I've used 1 of these so I'm not familiar with them. They don't test like normal diodes either. When tested with an ohmeter it tested as follows:
neg ld to stud= 11 meg, + ld to stud 300K. This would lead me to believe that
the stud is the anode? Anyone Know for sure? Don't want to use it if it's backwards.
thanks
Ian

[Bruce / Mission Amps]

The stud is the anode if it is as printed on the actual device I see in your picture.
My first assumption would be they sent the wrong one.

[Enzo]

DOn't trust ohm meter tests. Does your meter have a diode/continuity test function? That will tell you clearly which way she flows.

[studioit]

The stud is the anode if it is as printed on the actual device I see in your picture.
My first assumption would be they sent the wrong one.

Yeah, That's what I thought too, just from a common sense standpoint, but having never seen one of these before I thought it wise to investigate.
Thanks

[solderstain]

I'm doing the Zener trick for the first time myself. My project is that I'm using a 30v Zener to lower the B+ voltage on an early-'70s Fender Pro Reverb (I'm converting it into a 2x12 BF Deluxe Reverb).

What I'm experiencing is:

1) The voltage drop at the B+ level is only half of the rated voltage - it's a 30v Zener and I got about a 15 volt drop.

2) The bias voltage got driven even further negative - I wasn't expecting that - the bias voltage is 20 volts MORE negative. As a result I can't bias the amp with the current resistor/pot network in the bias supply.

Okay - is the 50% effectiveness of the Zener rating a function of it being in an AC circuit?

Okay - is there a solution to the bias voltage change - I think I probably have to readjust the structure of the bias circuit, right?

I was trying to avoid just installing a replacement DR PT, but maybe that's the most sanitary solution...

[studioit]

I'd like to know the answer to the voltage drop as well as I bought a 56 volt and was expecting a drop of the same. If I'll only get half that I'll need to rethink - I may have to use a different PT as well. Anyone know before I order another zener?

As far as the bias voltage change I think thats to be expected as the bias tap is a tap of the same winding that is affected by the zener.

Ian

[solderstain]

As far as the bias voltage change I think thats to be expected as the bias tap is a tap of the same winding that is affected by the zener.

Ian

Thanks, Ian - I kinda knew that, and am somewhat embarrassed about being so surprised about it. I've been so myopically focused on my B+ reduction effort that the bias end didn't show up on my radar until it happened.

I hope someone has an elegant solution about the bias part of this trick - I'd need to add another 30v Zener to get what I'm looking for at the B+ end, and that will, of course, drive the bias even further south. It looks like the cleanest solution will be a replacement PT. RATS!

[Steve Conner]

The problem you're getting is because you put the zener on the centre tap of the PT. If you spliced it into the B+ line after the first filter cap, it would have no effect on the bias tap, and would hopefully lower the B+ by an amount equal to the zener voltage, instead of just half.

However, the case of the zener would now be live and you could no longer bolt it straight to the chassis. Sorting out an insulated heat sink should still be less hassle than replacing the PT, though.

Then again, maybe you got the bias tap and the centre tap mixed up when installing the zener?

[solderstain]

However, the case of the zener would now be live and you could no longer bolt it straight to the chassis. Sorting out an insulated heat sink should still be less hassle than replacing the PT, though.

Then again, maybe you got the bias tap and the centre tap mixed up when installing the zener?

Thanks for the response -

In reverse order, no, I don't have the center taps mixed up.

And as far as the case of the Zener being live, it is anyway. No matter what, the case of this D04 case is the cathode, so it's live no matter where it's installed.

It's a done deal now. I've already got a new PT on the way.

The question that is still unanswered is why the Zener only produces half the rated voltage in an AC circuit - I can only think that it's doing half-wave rectification, but that's still just a guess.

[hasserl]

Thanks for the response -

In reverse order, no, I don't have the center taps mixed up.

And as far as the case of the Zener being live, it is anyway. No matter what, the case of this D04 case is the cathode, so it's live no matter where it's installed.

It's a done deal now. I've already got a new PT on the way.

The question that is still unanswered is why the Zener only produces half the rated voltage in an AC circuit - I can only think that it's doing half-wave rectification, but that's still just a guess.

Why not just adjust the resistor in the bias circuit? It's a whole lot easier and cheaper than replacing the PT.

[solderstain]

Why not just adjust the resistor in the bias circuit? It's a whole lot easier and cheaper than replacing the PT.

Maybe... but for me - and this is no reflection on anyone else - this is becoming too much of a kluge. Too much doctoring and so on. I personally don't find changing a PT to be that much extra work. For my own work, I prefer the more tidy solution of having a PT that produced the B+ voltage I'm looking for without any special treatments or doctoring of the bias circuit. This is also the kick I needed to just go ahead and replace the OT as well - get something with a primary more suited to 6V6 tubes. I'm changing the choke as well. Might as well get rid of all the Silverface iron. Once I slide the chassis back into the cabinet, I want to be done with this amp, so I like the idea of getting it all done at once.

My own sense of this Zener trick is that it's best suited for minor alterations in B+ voltage. For large changes, it doesn't seem to be the best overall solution, especially when correct PTs are available. At least for me.

[bob p]

My own sense of this Zener trick is that it's best suited for minor alterations in B+ voltage. For large changes, it doesn't seem to be the best overall solution, especially when correct PTs are available. At least for me.

For your application, a PT swap makes sense. The SF Pro Reverb has really HIGH B+ voltages -- something on the order of 440 or 450 VDC. That would definitely be high for 6V6. Considering where you want to go, an iron swap makes sense.

[solderstain]

For your application, a PT swap makes sense. The SF Pro Reverb has really HIGH B+ voltages -- something on the order of 440 or 450 VDC. That would definitely be high for 6V6. Considering where you want to go, an iron swap makes sense.

With 6V6s in the sockets, the Pro gives me right around 457 volts with the input voltage set at 120vac with a variac. JJ 6V6s are supposed to handle that, but I don't want those kind of voltages, even if the tubes handle them. The right PT, OT and choke are in the mail, which is where I should have started anyway. But I learned a lot by doing this.

[Rob Mercure]

Perhaps I'm a tad late on this one - have been on the road working - but bringing up bias source voltage is "old hat" if you've ever had to work on 1960-70s vintage Sound City and some other British amps. The bias source was originally marginal and modern tubes sometimes need much more so you simply add a 1/2 wave voltage doubler - or tripler if needed. As these amps used 1/2 wave bias sources originally the ripple is the same but you might have to replace the bias filter with a higher voltage unit (this mod usually came while I was doing a recap anyway so the bias filter replacement was moot).

Rob

[bob p]

bringing up bias source voltage is "old hat" if you've ever had to work on 1960-70s vintage Sound City and some other British amps. The bias source was originally marginal and modern tubes sometimes need much more so you simply add a 1/2 wave voltage doubler - or tripler if needed. As these amps used 1/2 wave bias sources originally the ripple is the same but you might have to replace the bias filter with a higher voltage unit.

You know, one of the things that we should take advantage of now that we have this new site, is getting a few How-To threads going about useful tricks of the trade. Unfortunately, many pearls of wisdom were lost when threads scrolled off of the old Ampage site. As we return to address those same topics on this new site, hopefully the threads will stick around to serve as reference material.

Rob, I was about to start a related thread about where to get fixed bias when you don't have a suitable fixed bias tap. It would be really handy to have experienced users address things like half-wave taps off of the HV secondary, using resistive dividers, capacitive dividers, voltage doublers/triplers and whatnot.

The only reason I'm thinking about this right now is because I need about -100v of bias for my KT88 project. All of the stock iron that I've been able to find that comes remotely close to fitting the bill is either missing a bias tap, or if it has one its only 50v winding, which just won't cut it.

Since you nominated Dai as the official archivist, could I return the favor and nominate you to be the Bias King? If you have any sketches or schematic snippets handy, they'd sure help to make an interesting thread.

[bob p]

I've started a new thread about where to get fixed bias if you don't have a suitable bias tap. If anyone has pearls of wisdom that they'd like to contribute, please feel free to chime in -- especially if you've fixed a lot of old british amps that needed their bias reworked. Hint, Hint!

Fixed Bias Thread