Reeltarded wrote:(screams like a little girl) AAACK!
Wires!
Yeah, I know. I'm still learning.
I'm gonna fire it up first and see what's what. But, I'm thinking about replacing the screen leads with something shielded. Maybe add terminal strips at pre tubes to ground the shields.
I got the AC, bias adjust and OT hooked up last night. I powered up to check AC voltages. As I was checking the recto socket, it dawned on me that the SS filter caps were charging even though the recto switch was in standby. So, to keep my thinking straight and avoid making any stupid mistakes, I powered down and removed the 355V from the SS recto.
I'll power up again this evening and see where I am with voltages at the heater pins and bias. Then add the recto tube and check DC.
I powered up with the rectifier tube on the dim bulb tester and and started checking voltages. Something strange going on.
AC at pins 4 and 6 is 315VAC.
F1 is 335V, F2 is 320V and F3 is 317V.
The plates on all 3 pre tubes are in the 305V range and the plates on the power tubes are at 335V. I checked all the R values and jumpers and everything is spec except for 2 mix Rs that should 470K are 470R. Oops!
Just as I was about to power down, I saw smoke coming from the doghouse. I unplugged right away and then checked as usual for DC to drop...it held for nearly 15 mins...I think I smoked the 33K 5 watt bleeder resistor.
With no tubes other than the rectifier in and 315VAC, I would expect something like 400V on all the plate pins. Seems like there is some high current draw somewhere, hence the smoke. A 33k bleeder is too small. I'd use something like 220k 3W. A 33k will draw 12mA at 400V, and dissipate 4.8W. I think there is something else wrong, though.
martin manning wrote:With no tubes other than the rectifier in and 315VAC, I would expect something like 400V on all the plate pins. Seems like there is some high current draw somewhere, hence the smoke. A 33k bleeder is too small. I'd use something like 220k 3W. A 33k will draw 12mA at 400V, and dissipate 4.8W. I think there is something else wrong, though.
Thanks Martin.
Mystery solved on the smoke. The bleeder was 33K 3 watt not 5. So, I'll pick up some 220K 3 watt today and see where I am.
I looked for 220K 3 watt resistors but the local supply didn't have them so I bought two 500K 2W and wired them in parallel for the bleeder. Powered up and no smoke this time. It idled without the limiter for about 10 mins then I checked voltages.
I'm a little puzzled about F3 being down 130V from the other stages. The end of the 10K 2W is the first place in the circuit I can meter it. Would my filter cap size have anything to do with this? It's using a 22uF/450V.
No, it's not the filter size. You're pulling all of the preamp current, maybe 6-7 mA, through 20k (the two 10k 2W resistors in series), so that seems to add up. The rest of the voltages look reasonable, except the -63V bias is pretty cold.
martin manning wrote:No, it's not the filter size. You're pulling all of the preamp current, maybe 6-7 mA, through 20k (the two 10k 2W resistors in series), so that seems to add up. The rest of the voltages look reasonable, except the -63V bias is pretty cold.
That's good news!
I realized this morning that I wired the bias adjust incorrectly. I have the 47K going to ground and it should be in line with the lead coming from the 15K and filter caps. And wiper straight to ground.
Well, I made the change and got mixed results. The attached pic shows how the circuit is wired and I noted voltages at 2 points. The power tubes are seeing -71v and so is the MV.
60vac tap, through one 1N4007. 47K R to 47K pot. The bias voltage is -84v after the diode and -64v after the 15K. How the heck is it getting back to -71v?
I'm stumped...which doesn't take much.
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I don't kmow about the voltage drop from the 15k to the MV pots, but the 15k should be more like 42k to get a usable range for biasing 6L6's. You need to be about -40 to -55V.
martin manning wrote:I don't kmow about the voltage drop from the 15k to the MV pots, but the 15k should be more like 42k to get a usable range for biasing 6L6's. You need to be about -40 to -55V.
Yeah, I should have questioned that R value since I didn't have a similar circuit in the BC30 to reference...it was cathode biased.
I think I'll drop the MV for now and replace the 220K resistors until I get things working properly.
I moved the bias circuit to fit the eyelets on the Bassman board. This is what it looks like on the board. I can't figure out how the voltage is going from -64 to -71 just over a jumper.
Maybe I transposed the circuit wrong?
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