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Old 9th November 2007, 05:35 PM   #1
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Default calculating b+ voltage drop, decoupling?

My first post, and I'm asking if someone can point me in the right direction. For some reason I just don't get it. I'm new to DIY, and I've been reading anything I can get my eyes on (including your great sticky). I have an amp I'm modifying and I just need to wrap my mind around this because things aren't adding up.

I have an RCA shf-7 phono amp from 1957. My issue is figuring out voltage decoupling on the B+. My schematic has basic values present BUT when I try use Ohms law ( V = I/R ) I get lost.

According to the schematic, after the 5y3 I have a 302V 72.5ma supply first to the center tap of the OT with a 40mf 450v cap, then going through a 2700ohm resistor and regulated by a 40mf 450v capacitor, it drives 292v to 2-6v6gt plates. Further it goes through a 4700 R, a 20mf 450V cap, and puts a voltage I can't define on the B+ for the pre-amp tubes. I'm really more concerned with figuring out the math first. I think where I'm stuck is the current? Change in voltage should be 10v right? Anyhow, I don't have a meter yet. I suppose I need to know the current draw for this to make sense. Is there any way to figure this out mathmatically, or in the tube data sheets?

Sorry for the complicated and involved question on my first post, but I don't expect anyone to do the math for me.. I just want to know where I'm making my error. Thanks for any help - jimy
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Old 9th November 2007, 06:04 PM   #2
Klimon is offline Klimon  Belgium
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Welcome to the forum.

What do you mean with "B+ voltage decoupling" and why are you trying to figure it out?

Simon
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Old 9th November 2007, 07:53 PM   #3
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I hope I don't have my terms confused, but the B+ is the voltage that powers the anode/plate? If this isn't B+, I appologize. I'm trying to figure this out for two basic reasons.

1. I think I understand simple examples, but when I apply this logic to my schematic.. I come up with odd results. I must not completely understand how to apply it. Is there a good link for decoupling voltages to power tubes, and pre tubes. I think once I see it clearly, I'll understand how to apply it.

2. The whole reason I want to mess with the voltages, or atleast understand them, is because my pre-tubes are 6AV6, and 6CG7. Since the filiment amps are greater than the sum of 2-12ax7 at 6.3v, and those tubes are quite similar in most respects.. I would like to modify my pre stage to work well with 12ax7 tubes. And I don't want to unknowingly overload the rectifier or 12ax7 tubes, thus causing a meltdown. I figure if I start with the voltages, then I can work out the reason for the resistor values, and eventually understand how to properly bias the tubes and not blow anything up.
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Old 9th November 2007, 08:27 PM   #4
pchw is offline pchw  United States
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I assume you are using the specs from the original schematic.
Those voltages and current are likely to be different from your measurements due to the modern electricity supply is typically higher than the 50's/60's and the components are aged. I suggest not to make any mods, just restore it to according to the schematic to make sure all components are functioning.

B+ is usually referred to the +ve power supply. At you can see, each stage needs a different supply voltage. The way it is done in your amp is having multiple filtering sections and each section feeds to a particular stage. Typically, the current coming out from the 5y3 is the total current various stages consume. If the schematic provides enough measurements such as voltages at different points, you can calculate much current each tube draws based on their data sheet.

Notice that filament supply is from a different transformer. Replace the 6V6 and 6CG7 with lower filament demand will not affect the B+. However, the 12AX7 is not going to provide enough current as the driver tube (6CG7??). Also, the bias point of the 12AX7 is very different, this means it is going to be more than just replace the tubes.
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Old 9th November 2007, 08:29 PM   #5
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Just to make this a little clearer.. Maybe decoupling isn't the right term either, but it seems to show up a lot.

Basically I take it to mean; One main voltage is decreased by means of a resistor, and then decreased again by a second resistor in series. In my case, the voltage tap after R1(2700ohm) powers the 6v6gt at 292volts, and the tap after R2(4700ohm) powers the 6av6 and 6cg7 at a lower voltage (I'm guessing ~250volts). This is all gathered from my schematic
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Old 9th November 2007, 09:25 PM   #6
pchw is offline pchw  United States
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This is essentially the power supply design for a specific circuitry.
Your RCA probably has 3 stages - the voltage amplifier, driver, and the output. Let's use this as a simplified example.
The designer first decided what is used in the output and their respectively plate voltage and current drawn.
Then do the same for the driver and voltage amplifier.
Once that is done, there are 3 sets of numbers, each is a voltage (B+) and current pair. Let say the values are 350V/100ma for the output, 250V/20ma for the driver, and 200V/6ma for the voltage amplifier.
Further assume the voltage after the rectifier is 400V.

In order to obtain 350V we need to drop 50V. Since the total current drawn by the circuit is 100+20+6=126ma, you need a resistor of 50/0.126 ohm after the rectifier to achieve that (ohm's law). Going down the chain, the next target voltage is 250V, and the current drawn is 26ma, so a (100/0.026) ohm resistor is needed. The last section will need (50/0.006)ohms. This is an oversimplified example, but it should give some ideas why there were chosen. If you are really interested in more detail, get Morgan Jones Valve Amplifiers book. I believe someone posted a link to google book of this book recently. Search it. I personally will recommend to buy the book. Nothing beats a hardcopy, call me old fashion :-)
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Old 10th November 2007, 12:02 AM   #7
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Thank you for that explanation! I was missing the fact that the milliamps load carries through. It makes complete sense. Now I just need to determine the load, which I guess is a function of bias and such? I'll look at it again tonight, but surely I'm figuring this out, and I thank you. I'm going to give a link to the schematic at http://techpreservation.dyndns.org - RCA shf-7http://techpreservation.dyndns.org/s...SHF-7_151.djvu
This is just for those curious. You'll need their free plugin to view it. It's also an enourmous archive of beitman archives and more. Check it out here Tech Preservation

This is what I'm doing roughly. I have a schematic to work from for the Pre section. I just need my voltage to be correct and hope the draw isn't too much.

current amp

In - vol - 6av6(pre) - tone/tone - 6cg7(1/2 pre) - 6cg7(2/2 PI) _ 6V6GT - | output
|_6V6GT -|^

what I want

In - 12ax7(1/2 pre) - vol/tone - 12ax7(2/2 pre) - 12ax7(1/2 pre) - 12ax7(2/2 PI) _ 6V6GT - |
|_6V6GT -|^
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Old 10th November 2007, 12:02 AM   #8
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oops.. the 2-6v6gt is Push/pull, but it probably won't look right here in the forum text. Trust me .. it isn't in the pre-amp
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