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6l6 mystery amp

what do you mean the voltage stays. Up or down?

I'm still seeing a lot of really old caps in here. Leaky coupling caps drive up the current load, so will a shorted cathode bypass cap on the 6L6. Likely pin 8 on both tubes will tie together, then go to a resistor to ground with a cap across it. If that cap is bad it screws things up.

Have to know where the screens (pin 4) are meant to go. They could go straight to the same B+ feed as the output transformer center tap. They could also be meant to feed from a lower voltage source. If they are supposed to get less voltage, the cathode resistor will not be the correct value to feed them from full B+.
 
This uses a balancing pot between the 2 output tubes then a shared 100ohm resistor to ground to ground for bias. It has individual 22uf cathode bypass caps.
 
pull the caps going to pin 5.

see what that pot value is too. Probably ought to be roughly in the center, and verifed to not be open.
 
I have the pots centered. I removed the caps from pin 5 and it still has low voltage. With full b+ to the screens and lower voltage on the plates isn’t that going to make the tube flow like crazy?
 
Not triode mode, 3 and 4 would be tied together if so. You can certainly run the screen at full B+, it just flows a lot of current without compensating with a larger cathode resistor or more negative grid voltage
 
gadget73 said:
Not triode mode, 3 and 4 would be tied together if so. You can certainly run the screen at full B+, it just flows a lot of current without compensating with a larger cathode resistor or more negative grid voltage
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They kind of are but the plate is getting voltage through the output transformer and the screen is getting it straight from b+.
 
Screen direct to plate is triode mode
Screen to a transformer tap is ultra linear
Screen to a DC source is pentode

The relative voltages dont really matter. You can have the screen at plate voltage, but sometimes its much lower.

Power supply is standard cap input, nothing odd. What sort of voltage is across the 100 ohm resistor?
 
gadget73 said:
Screen direct to plate is triode mode
Screen to a transformer tap is ultra linear
Screen to a DC source is pentode

The relative voltages dont really matter. You can have the screen at plate voltage, but sometimes its much lower.

Power supply is standard cap input, nothing odd. What sort of voltage is across the 100 ohm resistor?
Click to expand...
Ok I thought the screen voltage had to be lower than the plate voltage for current flow. I’m getting 16v on one channel and 21v on the other channel at the 100ohm resistor
 
It can be equal but if you look at typical 6l6 sheet conditions its like 360 plate and 270 screen.
 
But since the plate voltage flies through the output transformer it’s actually lower than the screen voltage. I’m just trying to understand how all this works and why the outputs are dragging the voltage down so much.
 
Yes, but not significantly so. Its still pentode mode. In theory the screen could be much higher voltage than the plate. The screen would melt from that sort of thing since the dissipation would be way too high, but current will flow until that happens. You can even leave the plate unhooked. Did that once by accident, it flowed current for a second or so and I got a flash like a camera going off as the screen vaporized. So much for that one.

either its drawing too much current or there is a problem with the power supply that significantly limits current. A voltage read across the 100 ohm resistor will tell us how much current is flowing and from there we can figure out if its a bad rectifier or something wrong in the output stage.
 
that 125ma, if thats per-tube, its entirely too much. Need either a larger cathode resistor or the screen needs to be fed from a lower voltage source.

if its per-pair its less nuts but if its 250ma total for both channels its pushing the rectifier to it's limits and if the rectifier is weak the voltage will tank pretty badly.

alternate thought, do the grid resistors go to ground, or are they supposed to be getting negative voltage from that rectifier stack on the front? If the negative supply is part of the bias circuit and its not working correctly, it'll do this.
 
The measurement is per pair of 6l6. The grid resistor goes straight to ground there’s no negative voltage. I replaced the 5u4 with a 5y3gt just to test and it still drops. I’m leaning towards cathode resistor change but I may try a screen resistor first.
 
Move all 4 screens down a node in the supply where they can connect to a filter cap. Don't just put a big resistor in there, it will have poor decoupling and regulation. Not that its going to be awesome anyway but if there is some current draw from the other tubes also in the mix, it will be less of a swing from idle to full output screen current, so the voltage change won't be as significant.

if you're aiming for something like 6l6 datasheet conditions, which honestly isn't a bad idea, you want 360 volts plate to cathode and 270 screen to cathode. Thats -22.5v of bias, so 385 and 295-ish from supply to ground. The cathode will be around 22.5 volts give or take for tube condition and exact voltages applied. Should be 9-10 volts across the 100 ohm resistor if all is happy. The rest will be in the pot. Per sheet conditions if the voltages are right, total resistance from a common cathode to ground is 242 ohms, so I'm hoping that 100 ohm resistor goes to a 250 ohm balance pot. If not, its just not the correct values for any of this to work.
 
The balance pots are 150ohm. I put 310 ohm resistors in place of the 100 ohm just because I had them laying around and I’d try them. It brought the b+ up to 350v but with 54v going through the cathode resistor. With the 150ohm pots I’m guessing I’m going to need 200ohm resistor?
 
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