High B+ using SS rectifier

[expat]

Also, I found a 100uf / 450v cap in my stash... should I replace C14 (100uf / 350v) or am I cool?

 

[expat]

Yes, either.
Also, use double pole switches for AC mains.

Uh ohhhh, I have this rad 50's (edit: single) pole in there. What's the reason for this?

 

[ccaudle]

getting 3.5v on either heater winding

That does not seem like enough for the tubes to operate properly. How much current is flowing in the plates?

I found a 100uf / 450v cap in my stash... should I replace C14 (100uf / 350v)

Are you asking whether you should use a 450V or 350V cap on a supply which is currently running at 472V?! Is there a "none of the above" selection? Generally the voltage rating of the cap should have a generous margin above the highest power supply voltage (i.e. taking into account line and load variation).

 

[expat]

hey ccaudle, thanks for chiming in. so much to try and understand!

so, am I wrong that the heaters are AC and at 3.5v per side accounts for 7v swing, which would be over the amount needed for the heaters?

not sure about current. i don't have tubes plugged in yet.

regarding the cap, i must admit i thought it was based on the 'under load' requirements. So these should all be 500v caps you're saying?

Anyways, I'm thinking I might be looking into the 20v (or more?) zener from the CT to reign this in. From what I can tell, that should put me in the safe range once tubes are in.

 

[abbey road d enfer]

hey ccaudle, thanks for chiming in. so much to try and understand!

so, am I wrong that the heaters are AC and at 3.5v per side accounts for 7v swing, which would be over the amount needed for the heaters?

It is correct. Heaters tolerate significant variation from nominal.

not sure about current. i don't have tubes plugged in yet.

It shows you haven't grabbed the basic notions of voltage drop. If you draw no current, there is no voltage drop.
Put the tubes in and check voltages; that will be a useful info.

regarding the cap, i must admit i thought it was based on the 'under load' requirements. So these should all be 500v caps you're saying?

When you turn on the amp, the tubes are cold and draw no current, so for about 20-30 seconds, there is no voltage drop. That's why you want the capacitors to be able to withstand this elevated voltage.
Yes you need 500V or more caps.
Electrolytics are limited at 500V. A better solution consists in using two caps in series. They can be rated at about 300-350V, but they must be accompanied with balancing resistors.

 

[ccaudle]

the heaters are AC and at 3.5v per side accounts for 7v swing

The RMS voltage is

 

[ccaudle]

Goofy phone behavior.
The RMS AC voltage is also referred to as the heating equivalent. The RMS AC value produces the same heat output as a constant DC voltage with the same magnitude as the RMS measure of the AC voltage. So 3.5V RMS AC produces the same heat as 3.5V DC.
As Abbey points out maybe that is ok, heaters work over a wide range. Over 44% below nominal is lower than I usually see referenced in guitar amplifiers, but I am not familiar enough with the limit conditions for tubes to know for sure what is definitely too low.

 

[Cqwet Dbdfte]

Uh ohhhh, I have this rad 50's (edit: single) pole in there. What's the reason for this?

Never trust that the AC wires are correctly polarized. This is a safety thing. Single pole AC mains switches is a legacy US thingy,

 

[Cqwet Dbdfte]

The RMS voltage is

Depending on the voltmeter it may not respond correctly to P-P, but an "open circuit" voltage, or "no load voltage" is one number, with a load it changes, how much depends on circuit resistance, (impedance for AC), which includes all losses between the power source and the load, according to Ohm's Law.
The power companys is low enough to be ignored in this context.
Disconnect the +B from the circuit, then with tubes installed measure the heater voltage across a tube socket.
For typical 6.3V tubes, it should be within +/-0.1V.

 

[ccaudle]

For typical 6.3V tubes, it should be within +/-0.1V.

Traditional designs have unregulated power supplies, so the heater voltage will vary by at least the line variation.

 

[Cqwet Dbdfte]

I never seen 120V AC go up 10%, but well down to 105V with a three phase transformer using the wrong tap for 120V.

 

[expat]

Allrighty, just ordered up a couple of 6at6's since I can't find that one I had. (years in storage and i'm still sifting through 300 boxes!).
Also grabbed a few 20v 1W zeners.
They should be here Wednesday - if I find the old one sooner I will of course pop it in and report back.
Thanks for everyone's input!!!

 

[moamps]

...
Yes you need 500V or more caps.
Electrolytics are limited at 500V. A better solution consists in using two caps in series. They can be rated at about 300-350V, but they must be accompanied with balancing resistors.
.

My great respect for your patient explanation. BTW, you can find electrolytic capacitors with an operating voltage of up to 700V.

 

[Cqwet Dbdfte]

Yummy, 808 joules of energy storage.
$150 bucks a pop, whats not to like?
Sure beats a spare Tesla battery.

 

[Brian Roth]

Out of curiosity, I checked Mouser for 525 V and higher. Surprised at so many choices in stock.

https://www.mouser.com/c/passive-co...ectrolytic-capacitors|~Voltage Rating DC&pg=5

Bri

 

[abbey road d enfer]

My great respect for your patient explanation. BTW, you can find electrolytic capacitors with an operating voltage of up to 700V.
View attachment 123715

My answer was based on what I was tought in school in the late 60's in chemistry.
It seems like things have changed in the meantime...
Then I wonder why Marshall (among others) continue to use 350V caps in series. Tradition?

 

[abbey road d enfer]

As Abbey points out maybe that is ok, heaters work over a wide range. Over 44% below nominal is lower than I usually see referenced in guitar amplifiers, but I am not familiar enough with the limit conditions for tubes to know for sure what is definitely too low.

The OP is actually measuring 3.5Vrms on ach side of the center-tapped heater winding; that makes for a full 7Vrms heater voltage, which is well within the tolerance.

 

[moamps]

.... Tradition?

Maybe. And maybe they still have a lot of stock in the warehouse and they don't want to change the mechanical design or in general a design that is fully functional.

 

[expat]

I put a 47ohm 3 amp thermistor in series with the black AC line to the fuse connection to help with in-rush current.

Changed out C14 (100uf) from 350v to 450v.

I know its been advised to replace all the PS caps with 500v, but I'm going to try to avoid that for now if i can (!!!!).

I'd rather do the zeners and bring this thing down into a decent voltage range.

So, I'm seeing 472v at pin 3 of the 6v6.

I can add a 20v zener between the CT and ground giving me 452v (correct?).

I can add two of them in series and get 40v (at the same 1W dissipation), correct?

From what I can tell, I can expect 20% or more voltage drop once the tubes are in (correct?)

If I do a 40v drop using the zeners, I think that puts me around 367v, which sounds like it might make the 6v6 happy.

I count on you guys to decimate my limited knowledge and total hack guesses.

Gonna hopefully have a Variac soon, my buddy said he's got one for me... oooooo nice!
He invented the first color SEM (RGB color Spectron Microscopy photography setup)! You know, those color photos of bug eyes!? Also builds music gear and is a great player, too. I built his site for him and we're about to update it, check it out if you like: Dave Scharf Photography

 

[abbey road d enfer]

I can add two of them in series and get 40v (at the same 1W dissipation), correct?

You probably want to use 2W zeners for 1W permanent dissipation.
You could use a single 5W 39V zener.

From what I can tell, I can expect 20% or more voltage drop once the tubes are in (correct?)

From what? Do you want to have both zener and voltage dropping resistors?

If I do a 40v drop using the zeners, I think that puts me around 367v, which sounds like it might make the 6v6 happy.

My math say 472 minus 40 is 432...