Dropping Resistors

[MattB930]

Is it bad practice to add a resistor before the Output Transformer CT node on a power supply section? Example.. PT secondary>5ar4 rectifier>capacitor>resistor>OT capacitor node>resistor>screen capacitor node>resistor>PI capacitor node.....etc.

I have a very beefy heyboer PT that puts out 360-0-360 and I am building a 4 6v6 amp. After rectification I read near 500v. I believe it'll drop a bit under load and with the tubes in but I don't want to chance putting anymore than 450MAX on the plates of these JJ 6v6's.

Is it really just a waste of using such a big PT to drop enough through that first resistor?

[MattB930]

running a sim in psu designer doesn't seem to be getting me real life readings. I'm not real familiar with the program either.

[MWJB]

Use a reverse polarity zener diode rather than a resistor before the OT CT node, if you want. The JJs will take 500vdc on the plates. With 360-0-360VAC, SS rectifier & a reasonable current draw, I reckon you'll see 490vdc (+/- 10v) at the plates. Do you have a 5VAC rectifier winding? You could use a 5U4 to drop some voltage?

[MattB930]

yes sir, I was planning on using a 5ar4 as I had one laying around.

[MattB930]

If I remember correctly I was getting almost 500v with the 5ar4

[MWJB]

What kind of plate current were they at? You should be good for 16-20mA each? Were you seeing 360VAC at each B+ secondary feed the rectifier?

Theoretically, you should see 1.41x the AC at each leg of the B+ winding with an SS rectifier...that's assuming the current load on the B+ is enough to bring the voltage into check, I usually reckon on a little less than that, more like 1.36x, less a handfull of volts for a GZ34. If you have a vastly overrrated B+ or heater current capacity then B+ may be a litle high?

[MattB930]

I am seeing just that. 360 * 1.36ish and get 489.6. Oh but you were saying for a SS rectifier. This with without tubes installed by the way. Just some initial voltage checks in the preamp.

I will check when I get home later. I think that is what I was getting.

I do read 360 on 4 and 6 of the rectifier.

[MWJB]

OK, when you fit the tubes there will be greater current draw on the PT secondaries & B+ voltage will drop some. Unloaded voltages are unlikely to give you any meaningful figures.

[MattB930]

what would one expect the B+ to drop adding the tubes? 20 volts or so? 4 6v6's in this case. I'll reply back when I install the tubes and check.

Also, would I be correct in stating that by adding a zener to ground, reverse polarity, would drop x amount of voltage and more importantly be more constant where a resistor would fluctuate under heavy load?

[MWJB]

Can't say what the voltag drop would be for sure, without specific experience of that PT, with the tubes biased to taste, your wall AC voltage...if the PT is accurately specced, my guess would be about 485vdc...but if you'r seeing 490 with no tubes installed than it may well be less than that. Any PT specs to hand?

Yeah, a 50v 50W diode will drop about the 50v, maybe a shade less. It's more transparent sounding than resistors in series with the B+, which can sound wheezy. Don't forget your GZ34 has some internal resistance, so this will fluctuate slightly too, adding a warmth to the dynamics...which is why people like them.

[Ronsonic]

How about a less brute force approach. Use a choke input filter, where the B+ hits the choke before the first cap. This will drop the voltage a LOT but you can then bring it back up with a smaller cap, like <1uF before the choke. Simply adjust that first cap to get what you want.

Makes for a very low ripple, stable supply and is easy on the rectifier.

[MattB930]

that's a very good suggestion also! I may try to see what it drops down to without a zener or a choke for the moment. If installing the tubes drop it from 490 to closer to 450 I may leave it for the time being since JJ 6v6's can take up to 500v on the plates.

Can you recommend a choke? I've seen 3h and 5h's. Not sure what I would use. How are Hammond's?

[Ronsonic]

I don't have a specific recommendation. There is a formula for L size for a choke input filter based on current draw. Since the plan is to adjust for minimum smoke it won't be critical - 3H, 5H sure. I do have experience with Hammond, good stuff.

Just be sure you spec enough current rating.

[guitarmike2107]

You could also use a power mosfet to drop the voltage to a fixed level.

The 50v 50w zener method will get got, so make sure its in a position that wont burn up. or use more than one.

I dont know about the choke option, sounds interesting, but I suspect you will need a biggish choke for 4 x 6v6.

[J M Fahey]

You could also use a power mosfet to drop the voltage to a fixed level.

Excellent idea !! Consider it a sort of "fixed power scaling".
To boot , you will have no ripple or ripple induced hum.
The main advantage, however, is that a TO218 or TO247 MOS bolted to a heat sink will safely dissipate *lots* more than any Zener you can get.