Power Supply Seems to outputting high voltage

[bryce Goggin]

My Limiter began eating 12au7's at an alarming rate. Probing through the supply at the test points revealed the 270volt test point to be 330volts. The 240v test point reads spot on. Loaded or unloaded I see the same results. I have replaced all the silicon and caps in that portion of the supply. Why does this voltage get so high? What is the next step I should take in trouble shooting this supply? The transformer itself reads 270 ac.

 

[on1aag]

Hi Bryce,

Check R206.

on1aag.

 

[Diver300]

I don't think it matters that TP6 is at 330 V. In fact, I would expect it to be higher with no load.

The 270 V ac transformer is rectified to produce the voltage at TP6. The peak voltage of a 270 V ac waveform is 382 V, so you should expect that sort of voltage with no load. You might even get a bit more voltage, as the transformer may be rated at 270 V with no load and will give out a bit more when unloaded.

As you load the supply, the transformer voltage will drop due to the winding resistance (and other effects), and there will be a significant drop across R201, R202 and R203. The result is that the voltage at TP6 could vary a lot.

That is why the 240 V regulator has been fitted. If it is working at 240 V then I don't think that you should worry what the voltage at TP6 is at, as long as it is between about 260 V (The point where the regulator won't be able to produce 240 V) and 420 V (A voltage that is too high to have come from a working 270 V transformer)

Is the 270 V supply on TP6 connected to anything else?

 

[Diver300]

Hi Bryce,

Check R206.

R206 is needed to get the regulated 240 V at the correct voltage. As that seems to be OK, I don't think you need to worry about R206.

If the voltage had been higher or lower than 240 V, R206 would certainly be worth checking.

 

[on1aag]

R206 is needed to get the regulated 240 V at the correct voltage. As that seems to be OK, I don't think you need to worry about R206.

If the voltage had been higher or lower than 240 V, R206 would certainly be worth checking.

That's right, I got the two voltages mixed up.
Check the filament voltage.

on1aag.

 

[ericgibbs]

Hi Bryce,

If you are still concerned about the "270v" I would suggest you measure the voltages along the filter resistor chain, see what the voltage drops drops along the chain are, its possible that one or more have failed to a low resistance value.

That would give a high +270V.

 

[eblc1388]

Hi Bryce,

Please tell us how many physical 12Au7 tubes are there in the limiter.

 

[bryce Goggin]

ecc82 count

There are 4 ecc82's (12u7's)

 

[Nigel Goodwin]

Which valves are dying?, and have you checked C1 and C3, grid coupling capacitors leaking is a common cause of valve failure.

 

[eblc1388]

Hi Bryce,

Can you measure the five voltages marked A~E shown in the image using a DC meter, with meter black(negative) lead connected as shown. You should have the ECC82 in place and wait until the filament is fully orange before making the measurement and make sure the measured voltage is stable. If you see the tube plate growing red, switch off to let it cool before making another measurement.

If you measured the voltage in your previous post "without" the tube, you will get high voltage like 330V. The 270V is the nominal circuit working voltage when the tube is operating.

 

[bryce Goggin]

Can I use the ecc82's that tested bad for this analysis or do I need to put in fresh tubes? I suspect the later.

 

[eblc1388]

I understand your concern.

You can test it first using the bad one in this case.

 

[Willbe]

. . .the 270volt test point to be 330volts.
The 240v test point reads spot on.

I'm not totally following this thread, but I think. . .

the supply is putting out considerably less current and higher voltage than it's suppposed to because there is an open circuit somewhere in the downstream circuitry.
The 240v is OK because it's regulated.

I guess you could measure the Vdrop across the cathode resistors to gauge current draw in each stage. I'd start with those two 2 W resistors R16 & R17.

Also, using the graph from a tube manual for a 12AU7 and the circuit values shown you could figure out the design current flow through the tubes, and then substitute resistors, plate to cathode, for the bad tubes, until you find the open circuit.

I think it's less likely that one or more of those three resistors in the power supply have shorted and so your PS filtering is compromised.
A scope should show increasingly smoothed waveforms on the filter caps as you proceed to the right (assuming some current flow out of the supply).

 

[Willbe]

. . .the 270volt test point to be 330volts.
The 240v test point reads spot on.

I'm not totally following this thread, but I think the supply is putting out considerably less current and higher voltage than it's suppposed to because there is an open circuit somewhere in the downstream circuitry.
The 240v is OK because it's regulated.

I guess you could measure the Vdrop across the cathode resistors to gauge current draw in each stage. I'd start with those two 2 W resistors R16 & R17.

Also, using the graph from a tube manual for a 12AU7 and the circuit resistor values shown, you could figure out the design current flow through the tubes and then substitute hi-wattage resistors [plate to cathode] for the bad tubes until you find the open circuit.

If you go the resistor route, you might be able to use incandescent bulbs for these resistors; the problem is that the R of such a bulb varies with V^2 [the cold resistance can be 10x-15x less than the hot resistance] so you might have to cut-and-try.

I think it's less likely that one or more of those three resistors [R201 - R203] in the power supply have shorted and so your PS filtering is compromised.
A scope should show increasingly smoothed waveforms on the filter caps as you proceed to the right (assuming some current flow out of the supply).

 

[bryce Goggin]

Hi Bryce,

Can you measure the five voltages marked A~E shown in the image using a DC meter, with meter black(negative) lead connected as shown. You should have the ECC82 in place and wait until the filament is fully orange before making the measurement and make sure the measured voltage is stable. If you see the tube plate growing red, switch off to let it cool before making another measurement.

If you measured the voltage in your previous post "without" the tube, you will get high voltage like 330V. The 270V is the nominal circuit working voltage when the tube is operating.

Here are the measurements for both channels:
A-320 vdc A-321 vdc I tested both tubes and they are good
B-33.4 vdc B-35.6 vdc
C-320 vdc C-320 vdc
D-32.4 vdc D-34.9 vdc
E-23.6 vdc E 23.6 vdc

 

[Willbe]

Here are the measurements for both channels:
A-320 vdc A-321 vdc I tested both tubes and they are good
B-33.4 vdc B-35.6 vdc
C-320 vdc C-320 vdc
D-32.4 vdc D-34.9 vdc
E-23.6 vdc E 23.6 vdc

So plate to cathode is ~300 vdc and if R17 is 4 kΩ then your plate current is ~8 mA. From
**broken link removed**
according to the graph it looks like your tube is "off" [you're nowhere near the middle of the operating region so your output signals would be clipped] but see typical characteristics for 250 v(?).

You're over max values for plate voltage so the tube is being overstressed.

I should have seen this earlier, but from your power supply voltages [(337-304) v dropped across 470 Ω] I'd say your supply should be sourcing about 70 mA.

BTW, I'm not so sure I agree with putting tube filaments in series, because of voltage [power] sharing issues. Maybe they can get away with just two in series.

 

[ericgibbs]

So plate to cathode is ~300 vdc and if R17 is 4 kΩ then your plate current is ~8 mA. From
**broken link removed**
according to the graph it looks like your tube is "off" [you're nowhere near the middle of the operating region so your output signals would be clipped] but see typical characteristics for 250 v(?).

You're over max values for plate voltage so the tube is being overstressed.

I should have seen this earlier, but from your power supply voltages [(337-304) v dropped across 470 Ω] I'd say your supply should be sourcing about 70 mA.

BTW, I'm not so sure I agree with putting tube filaments in series, because of voltage [power] sharing issues. Maybe they can get away with just two in series.

hi Willbe,
This is in line with the dwg I posted a few days ago, I asked for the 470R/C psu filter chain to be checked out, added some voltages for guidance.

 

[bryce Goggin]

hi Willbe,
This is in line with the dwg I posted a few days ago, I asked for the 470R/C psu filter chain to be checked out, added some voltages for guidance.

Hi eric,
I checked the voltages along the psu filter chain and the voltage didn't drop as much as in your chart. I got 365 v after the rectifier, 350v after r201, 337v after r 202 and 320 v after r3. I put in new 470 ohm resistors and the results were the same.

 

[ericgibbs]

Hi eric,
I checked the voltages along the psu filter chain and the voltage didn't drop as much as in your chart. I got 365 v after the rectifier, 350v after r201, 337v after r 202 and 320 v after r3. I put in new 470 ohm resistors and the results were the same.

hi Bryce,
The fact the resistors are OK, indicates that one/more of the valve/tube circuits are not drawing the design current.

I'll relook at the circuit.

EDIT: have you checked the heater/filament voltage pins 4 and 5.?

 

[Willbe]

hi Bryce,
The fact the resistors are OK, indicates that one/more of the valve/tube circuits are not drawing the design current.

Yes. My bad. I read the tube graph wrongly [I haven't done this since 1963].
It seems the tube is operating right where it is supposed to. What fooled me was that they show + grid voltages.

The plate voltage is high, though, and so you can substitute 4 w resistors [250/11mA = ~23 kΩ] for the tubes that are burning out or you can hurry with your measurements.

If you want to take unpowered measurements, look for an open resistor in the plate-cathode circuits.

I recommend reading voltages across the cathode resistors to see where the current is going, or not going.

Don't go changing resistors until you get the whole picture.

Compare your measurements with the specs for the other tube types that you are using.