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OK, been going thru this measuring the various voltages, and it appears that I have a problem with the bias/heater string portion. At the input to the 15 ohm resistor, I have around 25 v, that is split between the bias/DC balance circuit and the heater string, which comes after the 15 ohm resistor (which measures right on 15 ohm). At the bias/balance circuit, I have 25 vdc that is sent to the pots, the output from the pots is 15 vdc which is further reduced by the balance pots to around 13 vdc, the biasing on the schematic shown to be 16 vdc, so that may be one reason I am not getting enough voltage to bias the output tubes, brand new tubes will almost get there, but none of the used but tested good tubes will. The output from the 15 ohm resistor to the heaters is 19.8 vdc, and across the heaters of the tubes is 9.6 vdc, which would appear to be way low for those 12AX7 tubes and may be one reason the top end sounds anemic. Any suggestions on where to go from here? thanks
Follow Ups:
As Jim said, ignore the grid voltage - it's idle current you need to set. If you're using a low-cost DVM, it may read low with the 270K grid resistor in series - but should measure the voltage at the pots accurately.
Have you changed the caps in the negative supply? The first one carries about a half-amp of ripple current, and if it has a high ESR, heater voltage will be low. I've seen caps in a bias supply that measure nearly open without creating hum, due to the cancellation in the push-pull output stage.
Also, any leakage in the coupling caps will be a POSITIVE voltage, canceling some of the negative bias voltage. If you haven't changed them, do that first.
Thanks, Jim and Tom, I have rebuilt the bias supply with new caps, 100 ufd @50 vdc, and heater supply was also rebuilt using 1000 ufd caps, all new. One thing I also did was to install two CL-80s, one in each primary leg, to slow down the current inrush as I upgraded the caps in the B+ to 560 ufd and 100 ufd (thanks, Jim), I read that should be done to prevent transformer clanging. It may be that two CL-80 is too much and is dropping the primary voltage too low. It may also be that one of the 12AX7 tubes is shorted and causing the severe drop in the heater voltage, that will be my first area of attack, thanks!
See if this doesn't help:"OK, been going thru this measuring the various voltages, and it appears that I have a problem with the bias/heater string portion. At the input to the 15 ohm resistor, I have around 25 v, that is split between the bias/DC balance circuit and the heater string, which comes after the 15 ohm resistor (which measures right on 15 ohm)."
Not true, the voltage is not split. You should have -25 volts at the input to the bias pots AND the heater string. And remember - all the voltages in that circuit are negative.
"At the bias/balance circuit, I have 25 vdc that is sent to the pots,"
Which is what you'd expect to have, except it'd be -25 volts."the output from the pots is 15 vdc which is further reduced by the balance pots to around 13 vdc, the biasing on the schematic shown to be 16 vdc,"
First off, the voltage out of the pots can be changed by adjusting the pot. Are you saying 15 volts is the highest you can get?
Second, as I posted many times before and in the tube FAQ:
"Keep one last thing in mind - often you see a voltage "spec" on a schematic at the control grid of a tube. DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO THAT SPEC! The negative voltage at the tube grid should be whatever value is required to set the tube's cathode current to spec. If the tube is passing the correct amount of current and the voltage at the grid is -20 volts then the hell with the "spec" of -22.5 volts or whatever it is. Making the grid voltage more negative with respect to the tube's cathode is the equivalent of stepping down harder on a garden hose to restrict the flow of water. And if you let up the pressure on the hose and more water flows - that's the same thing that happens if the tube grid is less negative!!"
"so that may be one reason I am not getting enough voltage to bias the output tubes, brand new tubes will almost get there, but none of the used but tested good tubes will."
If the voltage at the tube grids was LESS NEGATIVE then the current through the tube will INCREASE. So when you say "brand new tubes will almost get there" do you mean they will almost get to a low enough current through the tube or a high enough current through the tube?
"The output from the 15 ohm resistor to the heaters is 19.8 vdc, and across the heaters of the tubes is 9.6 vdc, which would appear to be way low for those 12AX7 tubes"
That's what you'd expect, the tubes divide the voltage supply between each of the two tube series strings.
"and may be one reason the top end sounds anemic."
Could be, I don't know for sure. But lower heater voltages drops the transconductance of a tube - which IMHO is the LAST thing you need from a 12AX7!
> Any suggestions on where to go from here?
Yes, here's what you need to do. The voltage out of the rectifier for that winding MUST be increased. To get a reasonable heater voltage you need at the very least -23 volts so each tube will see -11.5 volts across the heater. To make the voltage more negative you can replace the diodes with a couple Schottky diodes. This will make the negative DC volts more negative because Schottkys have less voltage drop than conventional diodes. You can also increase the value of the C4A&B dual cap. I'd double the value. Then if the voltage is still not high enough you can reduce the value of the 15 Ohm resistor to get less drop across it. With the 3 series tube strings you'll have about 350 to 400 ma of current passing through the resistor - that current would be higher but the heaters draw less current at the lower voltages you have now. Add a bit for the bias circuit to the heater draw and you'll have maybe 425 ma. or so. Using Ohm's Law that means that for every Ohm of resistance change in the resistor you'll get approximately .4 volts difference coming out of it. Tweak away until you get about -24 volts out of the resistor - you can get by with less, but -12 volts would be nice! It will also give you more voltage to feed the bias pots.
That may be enough to get the bias voltages where they need to be. In any case I'd replace the C26 50 uf cap in the bias supply with a 220 uf or so. You can use better filtering with the bias supply sharing a common supply with the tube heaters!!
If you still can't get the bias correct then you may have to rejigger some of the resistor values in the bias circuit.
Finally keep in mind the sequence of bias/balance setting - be sure the amp is fully warmed:
1. Center the balance pots
2. Adjust the bias pot until you read the correct current through the tube. If the tubes' have different bias currents then adjust the balance until it is equal.
3. Repeat steps 1 and 2 until the current is the correct value and both tubes are passing the same current.
4. Repeat for the other channel, then recheck the first channel. Repeat all the steps until everything is set correctly.
All written above assumes healthy tubes and a basically healthy circuit. If there is a bad part, corroded wire, bad pot(s), bad transformer, etc. then that needs to be fixed first before moving on.
I hope this was some help anyway.
Edits: 02/15/15 02/24/15
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