Dropping PT voltages with an AC line power resistor

[Baguette]

Hello,

My 6G15 type reverb is done and sounds good, but all the voltages down there are pretty high:
- 6.8VAC instead of 6.3VAC on the heaters.
- 350VDC instead of 300VDC on the B+.

I'd feel more comfortable with lower voltages, therefore I'd like to drop all of them 7% (which would bring the heaters to 6.3VAC and the B+ to 325VDC).

I'm wondering if I could use a power resistor between the live wire from the wall and the PT???

Here are my calculations, please tell me if I'm ok or off:

POWER CONSUMPTION:
- the B+ draws approx. 30mA at idle (calculated from power tube cathode resistor) at 350VDC = 10.5W
- the heaters draw approx. 1A at 6.5VAC (1 6K6 + 2 12AX7) = 6.5W
TOTAL = 10.5 + 6.5 = 17W at idle.

CURRENT DRAWN BY PT:
- line voltage = 230V and power consumption = 17W
current drawn = 17 / 230 = 75mA

POWER RESISTOR'S RESISTANCE
I want a 7% voltage drop in the line voltage, from 230V to 214V. That's a 16V drop.
The power resistor will have to drop 16V @ 75mA.
R = 16 / .075 = 220 ohm.

POWER DISSIPATED BY THE RESISTOR
The 220ohm resistor will pass 75mA of current:
P = 220*.075² = 1.25W

CONCLUSION
If I throw in a 10W (or even 25W, they're so small and cheap) 220 ohm resistor, I'll have my voltages right, and no issues with the power resistor getting too hot.
Is that right?

Now, where should I put this resistor? Before or after the fuse?

Thanks a LOT!!!  

[John]

Baguette, please do NOT take this as gospel,wait for the more knowledgeable to chime in. But I'm pretty sure you do not want to put a resistor before the PT. From everything I've read here they always go after. I think there's some diagrams around here showing resistors in parallel with caps to drop the B+ down to where you want it. Since you only have to drop around 25 or 30 volts, it shouldn't take much. And really, from what I've read here 350 isn't all that high to start with. Hope this helps.

[Baguette]

Thanks for the answer John,

The thing is I want to drop the B+ AND the heater voltage... The drop I want is not huge really (7% approx), and the power requirement of this circuit is tiny (less than a 5F1 Champ).

So why not mess around with the PT primary instead? Kinda like getting a 240V transformer instead of 230V.

[labb]

Give this a try:

www.geofex.com/Article_Folders/vintvolt/vintvolt.htm

[Baguette]

Give this a try:

www.geofex.com/Article_Folders/vintvolt/vintvolt.htm

Thanks, but I'd change the PT rather than adding an extra one.

I'm wondering if my AC line resistor calculation is right, and if it's safe.

[kagliostro]

I never do that (put a resistor in the AC primary of the PT)

but I have a friend that has do that more than one time for 110v low consumption units

as to adapt it to 220v, he use a big resistor in the primary

sure the "Vintage Voltage Adapter" is a more elegant and reliable solution

that can be used also if the consumption is higher

Kagliostro

[stingray_65]

Hi Baguette,

I've been in a very similar situation a few times. I used to like to build amps with salvaged Iron a lot and ran into the same thing when the tranny was designed to run on 110V and I'm pouring 125V into them.

The voltages you quote are NOT out of spec.

you have a + or - 20% tolerance.

Your 300V target can run as high as 360V or as low as 240V (20% of 300V is 60V) and still be considered spot on.

If I had measured 6.8V under load I wouldn't even blink at it.

Your heater voltages CAN run as high as 7,56V. (I do feel that is excessive tho and have ran simple diodes to bring readings like that in line)

If you still want to knock down that B+ reading, there are LOTS of ways to go about that.

You just have to let us know which direction you want to take with that.



Thinking outside of the box is a great thing in my opinion. I encourage it in my children and applaud it in life where I see it.

I'm sure your Idea would work, but I am sure I have never seen such an approach for safety reasons.

I wish I had enough engineering knowledge to site reason why not, but I'm going to have to leave that to someone else with a more classical training.

Ray

[jhadhar65]

>Now, where should I put this resistor?

Back in the bin.

Like stingray_65 says, your voltages are fine.  As has been stated, these are not precision machines.  +/- 20% is as precise as Leo got and the 6G15 schematic says as much.

[eleventeen]

Myself, I have zero problem with this approach, although I will point out that you're not including the pilot lamp in your 75 mils calculation. Taking your numbers (and thus neither am I including the pilot lamp, if it's a 6.3 volt incandescent) I get 213 ohms = 16/.075   Close enough. I concur on your wattage calculation.

We have all seen glass fuses with resistors inside. No?

Your only challenge is mounting the resistor safely and securely and I will assume you know one or more of the dozen or so ways you might do this.

Anything like this should go after the fuse, because if the fuse blows, you want the absolute minimum number of items inside the chassis to remain "hot". If you place the line-drop resistor before the fuse, then those resistors remain hot even after the fuse blows. To state it another way, the very first thing the hot side of line cord should see coming into the amp is the fuse.

[Baguette]

Thanks all for the comments.

Couldn't help it but I slapped a 135 ohm 15W cement resistor between the fuse and the switch (the big dudes I use for biasing AC30s). It's safely attached and I don't see any reason why it would be any more hazardous than a wire there.

Heaters went down to 6.5V, and B+ down to 330V. The resistor drops 12V, pass 88mA of current, and dissipate 1W... so the 15W rating should be good enough for a long while.

I think I'll just leave it there for a while... those voltages look spot on now. I'll take away only if it gets hot.

Thanks for your answers!