zener on ct...again

[Bruce / Mission Amps]

Those 5 watt zeners will get hot, real hot. Hot enough to desolder themselves. The math may say they'll only see 3 watts, so they'll be fine, in reality they'll get very hot. You can help keep them cool with some heat sinking, I used copper pennies (not the cheap zinc ones, use real copper pennies) mounted between them. Just drill the penny and slip it over the lead between the zeners and solder. It's a trick I learned here on Ampage years ago. Probably from Bruce, but I don't recall exactly who it was.

Yes, that is my "tweak" and as you described, exactly what I use to do when facing this problem!
Especially with those old VOX amps, like the AC30s from the 60s.
The stock AC30 PT is made to be run with a 220v to 110v mains voltage and there is no good rotary switch setting for modern line voltages here in the USA.
So, when used here at 120v-126v, the PT and rectifiers made too much B+, greatly under biasing the four EL84 power tubes. I've seen them idling at over 17 watts each in some amps!
In that application, three 9v-12v, 5 watt zeners, with the pre-drilled, solid copper pennies soldered between the leads as a heat sink, made all the difference in the world and still allowed the amp to sound like a fat idling, cathode biased AC30.
The idea of course is to drill two small 1/16" (or less) holes in each penny to be mounted between the diodes and shove the pennies up close to the zener diode's bodies so they can dissipate the heat like cooling fins.
Leaving some of the zener's lead length can actually help cool them too so don't make then leads too short.
Yeah, it looks a little weird but when mounted to a couple small terminal strips near the PT, it is a pretty good remedy and really extends the life of the power tubes without changing the tone of the amp at all.
This works well with all smaller powered amps.
If you need to do this with a high powered amp, I'd use a 50 watt zener diode and maybe or a 2N3055 circuit set up in "pass" as a voltage regulator with a low wattage zener in the base, but still in the grounded center tap lead of the rectifier/PT circuit.

[TD_Madden]

Since I have a number of early 60's Gibsons and a mid-60's AC50, I decided just to use them with a less elegant solution: a Weber Browner (line of zeners, rotary-switch and a volt-meter) It wasn't cheap, but it fis my needs ok.

One of the things I like about it is being able to set the AC50's voltage selector to 115, the Browner to match, and seeing 6.3v on the heaters.....

[Merlinb]

Good post on the mosfet booster. It actually gets me thinking about trying a power scaling mod with the regulator at the CT.

Theoretically that's a good idea, but practically you run into problems. The reason being that the regulator is now before the reservoir cap. At switch on, the inrush current can peak at 5A or even more, so you need some hefty transistors, or a more complex limiting circuit, to withstand it. A high-voltage, lowish current regulator, placed after the reservoir, end up being cheaper usually.

Similarly, the problem with a zener is that you need it to cope with the ripple current, which is usually at least 5x the average current. The power factor that results means the zener ends up dissipating MORE power than would be expected simply by multiplying zener voltage x average DC load current.

Of course, this problem can be easily avoided by placing the zener between the reservoir cap and the CT of the OT, instead, since it's usually the power valves we want to drop the voltage for. The preamp voltages are easily controlled using the PSU dropping resistors.
Taking this further, we need only place the zener between the cathode resistors and ground, moving the grid-leaks to the top of the zener, so that we effectively 'jack up' all the power amp voltages by the zener voltage. Now the zener only sees the cathode current, and no nasty voltage spikes at switch on! What's more, because the preamp voltages remain unchanged, you can hit the PA with more signal, which equals more gain to the people!

[vince76]

about amplified zener

excuses for replying in a old thread.
since I have a too high B+ (30 v) in my 5f6-A ,I was thinking abut lowering the voltage and searching around the forum found this interesting thread.
I take a look at geofex too in the section mosfet follies.
I don't have an electronics background so please pardon my mistakes.
Was wondering if I can do something like that in the picture attached (hope I understood the schem on geofex) with the amplified zener R.G. was explaining.
The idea was to make the lowering B+ circuit mod excludible using a spdt switch or better a on off switch.
if the voltage I want to drop is 30V what values for R1, R2, D1. which mosfet to use?

thanks a lot, Vincenzo

[tubeswell]

Hi Vincenzo

A simpler way is just to put 2 x 12V 5W zeners in series off the centre tap. This will be 24V @ 10W which should be more than enough for dropping the B+ by about 30V. (Or use 3 x 9V 5W zeners if you need even less). Wire the zeners to a decent terminal strip to help dissipate the heat better.