B+ dropping zener - Champ got diode rectification now

[Guitarist]

I have a Champ here, just need to drop the remaining 6V6 screen voltage just 15 V or so for the robust JJ. Even with a 2.7K screen resistor it is seeing 485V. JJ specs allow 450V.
The now rectified voltage is 502V on the B+. So via Aiken's article (http://www.aikenamps.com/index.php/what-is-back-biasing) I'd like to insert a 1n5352a zener on the transformer secondary center tap. But eBay shipping is insane for a couple diodes (I live in Canada currently). Is there a difference with the 1n5352a vs 1n5352b zener diodes? I only need one really but they look super handy

This output tube is cathode biased. And I've humdinged that cathode junction onto a Y'd pair of 100Ω resistors to tame the heater hum. But the zener should not affect this.

thanks, Guitarist

[Chuck H]

I might be inclined to reduce voltages a bit more. The JJ tubes are indeed tough, but those high voltages they're capable of handling are probably more applicable to push pull circuits that don't idle the tube at or over 100% dissipation like a single ended amp will. I've never seen a JJ spec sheet that included operational examples but I'll bet if they did that the class A example wouldn't be at the max rated voltages.

[Leo_Gnardo]

Mouser has an outlet in Kitchener. www.mouser.ca If they do like Mouser in the USA, shipping is blessedly cheap - well comparable to almost any other supplier. (US$7.95 for some of my orders. What would that be, about CD$10? Let's hope so.) And if you can get your parts there, no duty to pay. I'm sure you can dream up some other items you can use or add to your stockpile or toolbox so the shipping would be proportionally less compared to the cost of items you buy.

A quick glance at US Mouser shows the A version of your diode as out of stock, whilst the B version - thousands ready to ship. If the Kitchener facility has the B zeners... well I'd stop there and not worry what you might be missing in the A version besides a long long wait.

[Bloomfield]

Ordering from mouser in Canada (.ca website) is great. Often it comes from the US but it is fast and I don't think I've ever had extra fees due on delivery. Shipping is like Leo says, and might even be free over a certain amount.

[garytoosweet]

I believe the Mouser Kitchener location is just an office.

Mouser always comes from USA. But free next day shipping on orders over $100CAD. No import duties, but you do pay provincial sales tax.

[Dave H]

You could just use a resistor and capacitor to make a pi filter (like the 5D2 Princeton) to reduce both B+ voltage and ripple.

[Chuck H]

You could just use a resistor and capacitor to make a pi filter (like the 5D2 Princeton) to reduce both B+ voltage and ripple.

This is what I've done. The amp is class A so there's no real sag until the amp is cranked and the bias voltage on the cathode rises out of class A range. Then the added resistance helps to emulate the now missing rectifier tube, except there's no rectifier tube to add heat, get in the way of using a larger speaker or worry about replacing. Add the benefits of reduced noise and it's a win/win

[g1]

If you use the Digikey .ca site (CDN dollars), shipping is $8 and always next day. Mouser is the same shipping price, but not overnight. (has taken as long as 2 weeks for me)

[Guitarist]

Hey, I was going to add a question and see a few more replies! I so really do appreciate this forum and have learnt so much here over the decade/s. Now I'm fixing everyones amps around this neck of the woods—friends mainly
I was kinda thinking of adding that resistor but I was worried about too much sagin' and bagin'!
When I redid this poor amp, all I had was a F&T 2x100uF at 500V cap for that spot. I knew that a tube rectifier would soon be history with that. But after the rebuild the voltage ended up rising that high.
I'm sorry to say, that Ohm's Law isn't my string suit. To drop 50 V going into the OT and 6v6 plate and its 45mA draw (+plus 5 mA for the screen) , what resistor value may you suggest, 1.8kΩ? I have a 2.7kΩ on the screen now but that barely drops volts at this point. IIRC, there is a 680Ω bias resistor plus the cap.

I've stalled on that order as, between CEDist and Digikey, the parts totals crept up to like $600 US! No wonder I can't afford a pass to the Whitewater ski hill;(

Also, down the road when I get some zeners, for other amps etc, do they need heat cooling with Aiken's placement on the PT center tap?

thanks again...

[dcoyle]

I find it useful to use several zeners of lower voltage to get the desired drop to spread the heat. Also, the longer the leads, the less chance of melting the solder.

[Guitarist]

Ok, so I see with zeners they have the assumedly, drop rating in Volts. The also are rated in Watts, go with 5 Watts or adding up to that? Perhaps roughly the same Watts value as with a dropping resistor. These are kinda new to me. Another types I might look at ordering?
Will they also smooth out the B+, for lower noise (I read someplace they have regulating characteristics)?
I'll throw in the resistor to get it going for now.

thanks.

[Chuck H]

So are you planning to use the 100u/100u cap? I think making a pi filter with that and the 1.8k resistor (or any relatively higher value for this application) might make for some long startup charge time. I don't know that it's a problem.?.

The reason you didn't get a lot of voltage drop with the screen node resistor is that the screen doesn't draw all that much current. The plate circuit draws more current so that makes for more voltage drop. A 1k resistor used in a pi filter arrangement should drop about 50-55V. I didn't even do any math for this. I just plugged the values into Duncan PSUDII.

[Helmholtz]

A zener diode regulates (better term: stabilizes) its own voltage drop. So in a series wiring as intended it will neither improve nor worsen total regulation.

With a series dropper resistor, the voltage drop depends on current. While in a class A powerstage the plate current doesn't change much, the screen current may change by 5mA or more between idle and full power.

So with a resistor voltage variation will increase somewhat. But a little more sag at full power may be a good thing.

[SoulFetish]

I'm sorry to say, that Ohm's Law isn't my string suit.

Neither is spelling.
(just kidding, I'd be doomed without spellcheck)

Now I'm fixing everyones amps around this neck of the woods—friends mainly

Don't be afraid to start charging them for that. Ass, Gas, or Grass. Nobody rides for free. as the old bumper sticker goes...

[Dave H]

I'm sorry to say, that Ohm's Law isn't my string suit. To drop 50 V going into the OT and 6v6 plate and its 45mA draw (+plus 5 mA for the screen) , what resistor value may you suggest,

Put the resistor in the B+ line where the 470 ohm resistor is on the Princeton. The B+ current is 50mA so a 1k resistor will drop 50V. The resistor will have to dissipate 2.5W. I'd use a 5W or higher rated resistor.