Removing the 6L6 and leaving the other tubes, I get 536V on V4 (1st 6L6) and 533V on V5.
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Help Ampeg B-15NC B+ voltages low when pluging 6L6 Power Tubes
- Thread starter Dom65
- Start date
Since the problem arises only when I put the power tubes in the equation and that they connect directly at the first pin of the can (essentially at pin 8 of rectifier), it was looking to me as the filter can was not the problem. Does my assumption make sense?
Or I'm missing something?
4AR4 was a typo. The traces I provided are from a 5AR4 datasheetRef: https://frank.pocnet.net/sheets/093/5/5AR4.pdf
It's possible both rectifiers are bad, but I think it's more likely there is another problems such as a leaky filter cap.
- Feb 12, 2006
- 61,041
- 5
- 220,978
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
No, the capacitor factors into the effectiveness of the power supply.
If the cap is incorrect (wrong value or fake) or is failing, the ripple will be much higher and the rms (DC in this case) voltage will be low.
About 540V DC unloaded is what I expected. This seems to point back to the rectifier. As I mentioned the original filter caps were only rated for 500V. If you have a variac, dial the line AC down enough to avoid pushing the filter caps over their voltage rating.
I believe we have concluded: With the tubes in, the AC from the PT is where it should be but the DC is too low. So the PT appears to be okay.
This may be harebrained: Are you sure the rectifier is wired correctly. AFAIK the output on 5AR4 is supposed to be take off pin 8 because the cathode is indirectly heated. If the wiring is reversed and the output is taken off pin 2 the voltage is being pulled through the heater windings.
Not sure exactly what the result would, but perhaps it would produce the reported symptoms
. Different tubes like 5U4GB have directly heated cathodes, so you can take the output off pin 2 or 8.
5U4GB has a lot more sag.
But, I get same result (low DC on plate) even with only the rectifier and one 6l6 plugged.
At this point, the filter can is not even involved right?
I don't have a Variac, but I can get one. What test would I actually do with it? I would lower it so that 6V6 plates are below 500VDC and then?About 540V DC unloaded is what I expected. This seems to point back to the rectifier. As I mentioned the original filter caps were only rated for 500V. If you have a variac, dial the line AC down enough to avoid pushing the filter caps over their voltage rating.
I believe we have concluded: With the tubes in, the AC from the PT is where it should be but the DC is too low. So the PT appears to be okay.
This may be harebrained: Are you sure the rectifier is wired correctly. AFAIK the output on 5AR4 is supposed to be take off pin 8 because the cathode is indirectly heated. If the wiring is reversed and the output is taken off pin 2 the voltage is being pulled through the heater windings.
View attachment 7089159
Not sure exactly what the result would, but perhaps it would produce the reported symptoms
Different tubes like 5U4GB have directly heated cathodes, so you can take the output off pin 2 or 8.
View attachment 7089161
5U4GB has a lot more sag.
I can test with a third rectifier...
Yeah, it seems the PT is ok.
DC is definitely taken from pin 8 of rectifier.
- Feb 12, 2006
- 61,041
- 5
- 220,978
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Isn’t the cap part of the B+ filtering?
What happens at that node loaded versus unloaded?
I'm not sure that there's any filtering happening at the first nod of filter can. It is like connecting yourself at pin 8 of rectifier. Isn't it?
If I don't plug 6L6, the voltage at the first node of filter cap is way higher than loaded with the power tubes. It reflects up to the pin 8 of rectifier.
CE Mfg makes 525V multi-cans in both sets of values:
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Make sure to get the correct insulating base plate.
Which value should it be?
The 40/40/20/20MFD of the schematics or the 40/40/40/40MFD that is on now?
The voltage goes over spec when the tubes are pulled. The variac allows you to prevent this.
Perhaps it would be a good idea to devise a way to figure out how much current the rectifier is delivering.
Another potentially harebrained idea. You know the voltage is getting to the plate pins on the tube socket, but are you sure both plates are actually being fed. Perhaps the tube socket is damaged, dirty, or loose, so only one plate is active.
Measure the voltage across the resistor in the Pi filter.
Edit: I mis-spoke. I see the OT ct is connected to the first filter section.
How much AC ripple is on the first filter section?
Edit: I mis-spoke. I see the OT ct is connected to the first filter section.
How much AC ripple is on the first filter section?
Last edited:
- Feb 12, 2006
- 61,041
- 5
- 220,978
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Of course there it, that’s the primary “capacitor input filter” node. If that’s bad, the DC voltage will fall faster under load.
Ok, then, although I know I want to change the filter caps can, I would like to know how I can test it to find its fault (s). Cause, I'm really suspecting it now. I made the mistake of ruling it out as the problem...
