# Dropping B+ - advice wanted

Discussion in 'Amp Central Station' started by alexwilds, Aug 24, 2019.

1. ### alexwildsTele-Meister

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I have B+ out the wazoo, 407 volts, where about 300 volts would be better for the Harvard build. I also have a stack of various 5 watt ceramic sand block resistors, most between 200 and 500 ohms. Is there any reason why I should not put a big resistor between the rectifier and node 1? And if that is a reasonable move, what is the calculation for dropping DC from 400 to 300 volts?

2. ### FluddmanTele-Meister

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Could try a zenor diode or two.

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What are you using for a 5Y3 rectifier? Is it of modern or vintage manufacture? Vintage 5Y3 output voltages vary a lot and you could try a different tube.

R=E/I

To drop 100 volts, you need to know the current draw. If it is 20ma, then 100 divided by .020 would be a 5000 ohm resistor. Also calculate power. IxE=Watts. So disssipating 2 watts, you should go with a 5 Watt resistor.

Good luck!

4. ### Axis29Poster ExtraordinaireAd Free Member

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I used copper caps in two of my rebuilds (a 5f8-a and 5f6-a) to drop ~67 volts on the B+ of both.

I contemplated zener diodes... But, the copper caps ended up being cheap and easy with no alteration fo the circuit itself. Not that big a deal, but the CC really is easy, just replace the rectifier. But, if you're already measuring 407 with a 5y3, my solution won't work. I am also still slightly concerned about the transformer still seeing elevated voltage (I am not smart enough about electricity to know if it makes a difference).

5. ### keithb7Friend of Leo's

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I had good success with Zener Diodes in a 5F2A build. If I recall I dropped it down to pretty much bang-on the printed vintage schematic spec.

6. ### dan40Tele-AfflictedSilver Supporter

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Are you measuring your B+ with all of the tubes installed?

7. ### alexwildsTele-Meister

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Yes, up and running I have 407 volts on pin 8 of the 5Y3. I would use a different rectifier tube, but everything I have is even hotter.

I could order a pile of zener diodes, but in the meantime I guess I'll go alligator clip in various resistors and see what I get with what I got on hand - the old seat of the pants, trial and error approach I always end up using. Thanks guys.

8. ### IntubatorTele-Meister

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9. ### Mexitele BluesTele-Holic

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I think you are on the right track with the resistors. My gibsonette has a big 500ohm resistor between the first filter cap and the OT power node that cuts the 350v from the 5y3 to almost 320v. Clip in 2-3 of your 500Rs in series and see where it gets you.

10. ### dougstrumTele-Holic

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Those sand resistors will surely drop your voltage. They will also get very hot, so watch where you mount them.

11. ### DrPepperTele-Afflicted

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Power transformer with correct specs would be the optimum route...

12. ### DrPepperTele-Afflicted

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Changing rectifier type is another...

13. ### gusfinleyTele-Holic

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What voltage are you feeding your power transformer (Mains Voltage)? What voltage does is want ( input voltage spec)?

Shoving 123VAC into a transformer spec's for 110V has the potentional to raise your DC voltage by quite a lot!

A good indicator of appropriate plate voltage is your heater voltage. It should be right around 6.3VAC. If this goes over by 10% or more, than you are damaging you tubes.

The best way to mitigate this problem is by giving your power transformer the voltage that it was designed for.

If this is the root of your problem, then there are a few ways to drop your AC/MAINS/WALL voltage to an acceptable level that can be discussed.

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14. ### gusfinleyTele-Holic

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More on this. It appears that the Harvard Power Transformer has 275-0-275 secondary windings. For a "Modern" 120VAC primary, that would give a turns ratio of 2.29. If a vintage transformer were to be designed for a 110VAC primary then it would have a turns ration of 2.5.

If we then connect the 2.5 turns ration transformer to the modern 125VAC Mains then we will have a secondary that runs at 312-0-312.

Judging from this Classictone spec sheet:
http://www.classictone.net/40-18027.pdf

The 5Y3 DC ration is about 1.15, so 125VAC into a transformer that wants 110 will produce a B+ of 360VDC. Whereas 110VAC into the same transformer will produce a B+ of 316VDC. That is an increase of over 40 VDC in the B+ voltage!

So, just bringing your MAINS voltage into spec for the transformer could get you in the right B+ range.

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