NYDave One Bottle Preamp power supply issues--bizarre voltage drop

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C12VR

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Hello,
I recently built the NYD one bottle as my first point-to-point (at least if perfboard counts) project, but ran into problems with the B+. The power supply I am using is a generic Apex 460 with the regulating diodes d5 and d6 cut out to give 286v B+ with nothing plugged in. When I run the power supply into the preamp this voltage drops like a brick to 102v. I used the attached schematic to assemble the pre. What might be causing this and what can I do to fix it? I am sure this is the result of some stupid simple elementary property. Please excuse my ignorance. Thank you!
 

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Gus

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Use ohms law
(235VDC -55VDC)/220k = ? mA
(235VDC-106VDC)/22k =? mA

Add up the mA
 

C12VR

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Sorry, I'm not sure what you mean--what is the relationship between the current and the drop in voltage? Is the added current draw somehow decreasing the voltage?
 

Gus

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There is a chance the transformer is not rated for the current the circuit needs.

Plate current in tube microphone is often under 1mA
If you look at the NYD print there are voltages marked on it you can get a good idea of the current needed using ohms law.
Things that are good to know
The transformer
winding(s) current rating
VA rating
 

ruffrecords

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Sorry, I'm not sure what you mean--what is the relationship between the current and the drop in voltage? Is the added current draw somehow decreasing the voltage?
What he is giving you is a way to work out the current draw from the schematic. The answer is about 7mA

If the supply you are using is unregulated then its output will drop as you take current out of it. It would help if you posted a schematic of the power supply.

Cheers

Ian
 

C12VR

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Thank you Mr. Thompson-Bell and Gus,
here is the schematic--
Thinking it over the power transformer only needs to provide current to one heater in the original mic. I assumed that it had enough current to supply two triodes without voltage drop, but this may not be the case. The schematic says at 120v it has 500ma-- this should be enough though, no?
 

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grid_stopper

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cant recall if the one bottle uses a 12av7 like the MILA, but the apex psu youre using is barely adequate for a single 12ax7, which is definitely a lower current draw than the 12av7 if memory serves correct. Thats definitely where i would look.
 

ruffrecords

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Those 20K series resistors in the HT supply smoothing chain are the culprit - R1, R2 and R3. Change all of them to 2K 2W and try again. If that does not work it may well be that the transformer is just not man enough for the job.

Cheers

Ian
 

rock soderstrom

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Edit: Ian was quicker

Just to be clear. Your microphone power supply will not be able to deliver 500mA on the B+ rail at any time. Not even close.

Your power supply will sag because you draw many times more current than originally planned. This current then leads to a multiple voltage drop at the very large 20k resistors. The result is your B+ voltage collapses.

As already mentioned before, nothing bizzare just Ohms Law.
 
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C12VR

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Is there any drawback to lowering the value of the resistors in question as suggested by Mr. Thompson-Bell? I will have to buy a new transformer if I plan to build more channels but just to get an idea of the sound, I wish to at least scrape by for this one.
Thank you all very much for your help.
 

rock soderstrom

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Is there any drawback to lowering the value of the resistors in question as suggested by Mr. Thompson-Bell?
Not really, as long as the transformer doesn't get too hot from overloading.

The 20k resistors form filter stages with the corresponding 22uf capacitors to filter the ripple of the rectified B+ voltage.

With the 2k resistors Ian mentioned, there is still plenty of filtering. If you made these resistors even smaller, you would eventually hear the ripple in the form of an increasing 120Hz hum.

As said, the transformer is not suitable to permanently supply 1 or more One Bottles. A short test of one amplifier might work, keep an eye on the temperature of the transformer, it will get warmer with time.

B+ will still be too low, but might be enough for a short test.
 
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hodad

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Just for reference: I built a 2 channel one bottle many years ago, & this is the power transformer I used: XFO

Works fine for two channels. Could be a good choice if you're in the US or Canada.
 

ruffrecords

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Just for reference: I built a 2 channel one bottle many years ago, & this is the power transformer I used: XFO

Works fine for two channels. Could be a good choice if you're in the US or Canada.
That one is ideal for one or two channels. It can supply 25mA on the HT secondary. SInce the power supply has a capacitor at the input. the raw output dc will be approximately 1.4 times the ac voltage. The current you can take out the supply consequently has to be at least 1.4 times less. To allow for losses in the rectifiers a factor of 1.6 is usually uses. So 25/1.6 = 15mA ad since we already established one channel needs about 7mA, it can definitely supply two of them.

Cheers

Ian
 
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