Power Supply Seems to outputting high voltage

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I can get my measurements without killing the tubes. The voltage across the 4.7K cathode resistors is 35.6 (for v3a) and 34.9 (for v3b).
 
bryce Goggin said:
I can get my measurements without killing the tubes. The voltage across the 4.7K cathode resistors is 35.6 (for v3a) and 34.9 (for v3b).
Click to expand...

So 35 v across 4700 ohms is 1/4 w and yet they have 2 w resistors; this is a bit strange.
I'd expect up to a watt [69 v, 15 mA] unless this is a really high reliability amp.

And 35 v across 4700 ohms is 7 mA and that's a bit low based on the tube typical values.

Since the 240 v is OK the grid voltage [51/(475 + 51) = 23 v] is close enough.

If the current went higher with the grid staying at 23 v then there'd be more voltage across the two watt resistor and the tube would be turned on more since the relative grid-cathode voltage wouldn't be quite as negative.

The transconductance is 2200, but the graph says a 5 v change grid to cathode should give us ~10 more mA.
[5 x 2200 = 11000 microamps which then equals 11 mA which is ~10 mA. Maybe that's what the micro symbol on the data sheet means in this case].

Are these good tubes?

Maybe there is some other reason the power supply voltage is so high?

This doesn't make sense, and that means I'm assuming something that isn't true.
But what?
 
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Hi Bryce,

I have studied your measured voltages and found that they did not fit what is expected from a normal 12AU7 valve characteristic curve. Simply put, at the measured grid-cathode voltage level, the cathode current should be several mA higher than ~8mA. I don't know why but as both channels show similar voltages, the chance of they are both at fault are slim.

This leads us back to the power supply section where you have replaced the components trying to located the fault. If you have not throw away the replaced power resistors, it is best to measure their resistance again, especially the three 470Ω resistors to confirm that they are really 470Ω. You should measure the voltage of TP1 which is also the DC tube filament voltage as requested by Eric in previous post.

At this moment in time, I think the best way forward is to email the manufacturer about the high voltage problem and ask for the design plate or cathode current in the final stage. It would be extremely helpful to request the design current loading figure on the +270V line and +240V line to aid troubleshooting.
 

Yes.

Or, if graphs for the other tubes can be found the whole circuit can be reverse-engineered and all the design current draw accounted for.

Another way would be to reduce the power supply voltage by putting a resistor in series with the supply, but this is kind of a band-aid solution.
If you do this, though, whatever problem is causing the high voltage may surface, assuming it was obscured by the tubes being ruined.

When the tubes failed, did they fail by opening or by shorting or by transconductance going out of spec?
 
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The voltage tp1 is 11.3V. I will investigate the condition of the capacitors in this section of the supply. If these prove to be fine, should I then look to tweaking the value of R216?
I will also try contacting the fine people at LYNDCRAFT to see if they can shed some light on the plate current question.

Perhaps a discrepancy of .7 volts be the source of this problem?
 
As far as "assuming something that isn't true", I think you should check the tube voltages with a scope. The circuit may be oscillating.
 

Unlikely.

You should hold off tweaking the +12V supply until the fault on the +270V side is cleared. By then you can change the value of R217 9K09 to 9K6 and raise the voltage to +12V.
 
eblc1388 said:
Unlikely.

You should hold off tweaking the +12V supply until the fault on the +270V side is cleared. By then you can change the value of R217 9K09 to 9K6 and raise the voltage to +12V.
Click to expand...

I can't find R217 or the TP; can you post the schematic section that you are looking at?
Thanks.
 
Willbe said:
I can't find R217 or the TP; can you post the schematic section that you are looking at?
Thanks.
Click to expand...

Hi,
This may help.

BTW the heaters are rated at 12.6V
 

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ericgibbs said:
BTW the heaters are rated at 12.6V
Click to expand...

Hi, Mr. G. . .

Yeah, the schematic pin numbers should have told me that they're applying 12v to a 12v heater, just like they should.
For some reason I can't comprehend that this tube is a dual triode, even with the data sheet. I think I have lost some of my original 10^13 brain cells since 1963.



Has all of the revelant schematic been posted? I get lost without a road map [and sometimes, with].
What is this circuit chunk fed by? What is it feeding? 4700 uF at a delta V of 10 v at 8 mS is ~6 A, a hefty bit of current for a vacuum tube circuit.

I assume 9K09 means 9.09 K?

You guys still have that "Red Lion" chain of restaurants [I was at Matra Marconi in the early 90's]?
 
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hi,
Bryce has posted two dwgs , one of the psu and the other ECC83/82 section.

Do you want a ECC82/83 datasheet.?
 
ericgibbs said:
hi,
Bryce has posted two dwgs , one of the psu and the other ECC83/82 section.
Do you want a ECC82/83 datasheet.?
Click to expand...

I count two power supply schematics and the one tube wiring schematic in this thread. There also seems to be a lot of circuitry not shown (e.g., "PL 1") that the tube schematic shows as gozintas and gozoutas.

Probably I only need ECC82 and 81 datasheets if they are the ones that are dying, but the What the Heck, post it! Then we can reverse engineer the entire tube schematic.
From their resistor wattage values the 82s and 81s don't seem to draw much current and this does seem to be a current problem.

If we can figure out the Thevenin equiv. power supply resistance looking back into the supply then from the full load/no load voltage difference we can figure out exactly how much current is/is not being drawn.

I think now it was the Royal Lion Pub, near Portsmouth.

Fixing this or not, we are definitely having fun now!
 
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hi,
Ref the valve or should it be tube data.

I think now it was the Royal Lion Pub, near Portsmouth.
Click to expand...

UK or USA.?
 

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For the power supply,
Voc = 370 V
Rthev = 3 x 470 = 1400
So we need to draw 70 mA in order to get down to 270 V.
We got 330 V which means we were somehow drawing 370-330/1400 = only ~30 mA.

V3A & B probably are supposed to draw ~10 mA min. each, 20 mA min. total, from the graphical data and typical values I have from the Internet.

I should not have thrown out my RCA tube manual.

V2A & B cannot draw more than 240 V/200 K/2 = ~ 3 mA total no matter what both sections of the tube is doing. I don't have data for this tube.

For V1A & B I don't know what value R3 & R4 have but if you divide 110 by half of R3 + R4 you can figure how much max current this stage must draw.
It looks like 3 mA per section is the typical value with this tube from what data I could find on the Internet.

So we're up to ~30 mA for this amplifier.

Where does the +110 V that feeds V1 come from?
 
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Are you aware this is a stereo unit? This should double the current draw.
As far as the 110 V goes. This is on the supply drawing.
 

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bryce Goggin said:
Are you aware this is a stereo unit? This should double the current draw.
As far as the 110 V goes. This is on the supply drawing.
Click to expand...

I guess I need new glasses; I still can't find it but my drawing is cut off on the left.

So all the current is accounted for and yet the power supply puts out more voltage, and not less voltage, than it is designed to?
And those 470 Ω resistors are good, and filter cap values are not way high?
Is your AC input excessive by a 330/270 ratio?

This is very strange. This unit may need an Exorcist.

I guess you could load down the supply with a large resistor until the plate voltages are within spec, and if no more tubes burn out you're home free.
But I hate to leave a mystery like this unsolved, especially if you use the resistor and then the power supply bridge diodes start burning out from excessive current draw.

I'm baffled. Ohm's law has never let me down before.
I'd confirm these readings with a scope. They were taken with no signal in, right?
 
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"I'm baffled. Ohm's law has never let me down before.
I'd confirm these readings with a scope. They were taken with no signal in, right?"

No signal running at the time.


Readings taken with a DMM
 
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