Lower B+ in Champ with Resistor before B+

[TurboGuitarMelton]

Hi guys,


Sorry if this has been covered already. I couldn't find the answer using the search function.


My 5f1 champ is running 400v on the B+. 390v on the plates. I rebiased with a 680 ohm cathode resistor but the amp is VERY loud and clean. And id like to lower voltage to keep my RCA 6v6gt alive.


My question is this: After the first filter cap and 10k resistor the second power supply voltage is 360v. I want my B+ to be around 360v. Could I add another 10k 3w resistor between the rectifier and the B+ to get the B+ down to 360?


Thanks!

[DummyLoad]

you have a 10K R with about 4mA load (6V6 screen current + preamp triodes current), so yes that's about 40V drop. 

you're looking for a 40V B+ drop with about 44mA total current consumption at 360V, so ~1K for R and power ~1.6W - use a 1K 5W


install the resistor in series with power trans HV secondary CT lead to ground. IOW, you lift the HV CT off ground and insert the 1K 5W part - one end of the 1K connects to CT lead and the other end of the 1K connects to ground.


--pete

[TurboGuitarMelton]

Thanks for the reply pete!


I added an 820ohm 5watt resistor between PT secondary CT and ground.


Wow! Lowered my B+ all the way to 325v. Im going to split the difference and try a 470 ohm resistor.


Side note, the 820 5w resistor got pretty hot. Not too hot to touch but pretty toasty. Is this okay? Ive seen a lot of people talk about using zener diodes in this application. Any reason they dont just use a resistor like this?

[sluckey]

Side note, the 820 5w resistor got pretty hot. Not too hot to touch but pretty toasty. Is this okay?

Measure the voltage across the resistor. Now use this formula to calculate how much power the resistor is actually dissipating...

     P = E²/R

Let's say you measure 50V across the resistor. 50² ÷ 820 = 3.05 watts. So, 3 watts is the actual dissipated power. Double that value and choose the next higher wattage resistor for a safety margin. In this case use a 10 watt resistor.

People that use a Zener are looking for a particular voltage drop that is not dependant on the load current.  The voltage drop ***WILL*** be different for different loads when using a resistor.

[jjasilli]

1. Per tube charts @ max signal a 6V6 draws 40 - 50mA.  There's a 75V drop.  75V X .045A = 3.375W.  Double that for safety = about 7W.  You should use the next higher standard wattage, a 10W R.  That's how to spec W for an R.


2.  Usual rule of thumb is to drop up to 50V with a dropping R, so as not to overly limit current draw for the power tubes.  But for SE you're probably OK, because plate draw is pretty much constant.


If you prefer the low voltage, another alternative is a bucking tranny.  I've used a bucking tranny + dropping R to drop over 100 volts.

[vampwizzard]

 jj

2.  Usual rule of thumb is to drop up to 50V with a dropping R, so as not to overly limit current draw for the power tubes.  But for SE you're probably OK, because plate draw is pretty much constant.

Thats actually an important consideration. It doesn't take much to drop the available current either. When I was designing  600V PTs you'd be surprised how many calls Id get for folks running induction coils/heaters and couldn't figure out why they kept blowing their breaker boxes. You get a mix of higher inrush current at startup and lower current ratings in steady state. If you want a radical voltage change the buck/boost (auto)transformer is a good way to go.

[TurboGuitarMelton]

Okay, messed around with various resistor values. Got 355 B+ with a 360 ohm resistor. Thats where i wanted it to be.


Here are some measurements I took:


Voltage across resistor is -15.05 DC or 18.68 AC.
Not sure which to use... I know Ohms law is for DC. Im a bit confused because the secondary is AC. Dont think that calculation applies here.


Actual resistor value measured is 359.3


According to my measurements:
((15.05)^2) ÷ 359.3 = 0.63W


Its weird but my multimeter also has temperature measurements i got 58°C on the resistor. (136°F) According to Google i should only start to get worried around 200°C (well within spec at this point) Someome said if you can touch it still it isn't too hot.


Makes me think about the cathode resistors... Seems overkill to use 5w resistors there.

[jjasilli]

It's not just worry / parts failure, but also performance.  When an R gets hot its resistance rises - out of spec.  To keep it cool, or at least warm (not hot) a higher wattage rating is needed.

[PRR]

> cathode resistors... Seems overkill to use 5w resistors there.

Pdiss of 6V6 is 12W-14W.

Mu of 6V6 is 10.

G2 current is small.

Simplifying: RK need only throw 1.2W-1.4W, 1/Mu of Pdiss. However the rating on a resistor is pretty hot. Generally a wise assembler will at least double the expected dissipation to pick a resistor. So 2.4W-2.8W.

More than a 2W. "3W" resistors exist, mostly as over-rated 2W parts. If you count every fractional-penny, that is where you go. In general DIY the 5W part is a fine choice.

> Ohms law is for DC. Im a bit confused because the secondary is AC

Ohm is valid for any current.

If you put a resistor between 1st and 2nd cap, the 1st cap is "DC enough" that the math is simple.

If you put a resistor between Rectifier and 1st cap, the waveform here is both AC and DC in narrow spikes. The numbers on your meter are not correct for these spikes. Ohm is valid but working it out for the very ugly wave is too much brain-pain. Even commercial stuff often slow-toasted a resistor in this position.

[shooter]

install the resistor in series with power trans HV secondary CT lead to ground

I think this is where the ac/dc confusion started.
DL threw me there, but I think  , it's the "return" leg for what happens after the diodes so, DC math it is, with a few R's on hand for fudging 

[TurboGuitarMelton]

Yes, I was confused about the calculations involved with putting the resistor on the PT center tap. (That what Ive devided to go with) Since the voltage there is AC I was confused about using ohms law there to find my resistor wattage.


Ive upped my resistor to 10w just to be safe. Still gets warm (cooler than the 5w did) but so does my cathode resistor on the 6v6. (They are both warm to touch after the amp has been running a while)  I think its good to go!

[jjasilli]

The voltage at the PT secondary CT is rectified DC.  It is the return leg part of the circuit for the 2 outer legs which are joined after the diodes.


Anyway the difference between the AC Ohm's Law vs. DC Ohm's Law is insignificant for amp purposes.  Everyone just uses the DC law.

[TurboGuitarMelton]

So I've noticed using my amp that I am getting a fizzy ghost note below certain notes on the guitar. (Especially on C's) Sounds like electronic static/ringmod like. I first thought it was my speaker cone. But i changed speakers and its still there. Tried other guitars/tubes and still didnt go away. I grounded the one end of the resistor i put on the center tap and it went away. Anyone else experienced this? I just went ahead and ordered the correct PT for the amp.

[jjasilli]

I have been plagued by this problem in more than one amp, including my VibroChamp.  I finally fixed it with lots of NFB.  Note the AB764 VibroChamp has both local NFB @ the power tube: 330pF cap from G1 to K; and global NFB.  I used the cap + higher NFB voltage.

[TurboGuitarMelton]

Jjasilli, were you using a resistor on the CT when that happened to you?


I bought and installed a mojotone 759 and everything is exactly as it should be. Voltages are good, the ghost notes are gone, and the amp is lighter with this tiny PT! Champs are pretty loud!!


Long story short, I should have gotten the correct PT to begin with. I experienced all kinds of weird issues using the resistor on the PT CT/ trying to make the wrong transformer work.


Thank you guys for your help!

[jjasilli]

No R in CT.