troubleshoot after recap

[martin manning]

Ouch! Also, be sure that your meter's battery is good.

[hebaton]

Carefully remeasure DC resistance from the CT to each plate lead, and then the two voltage drops, and post them. Should be straightforward.

I am confused. O.K. I get 439 Plate voltage and 440 Ot ct voltage. Resistance is 39ohms on one side 38,4 on the other.
Does this make sense ?

[martin manning]

V/R = I, so (440-439)/39 = 25.6 mA. Measure the voltage across each side of the OT primary (CT to plate lead) to get a more accurate reading.

[hebaton]

How do I get this to 44ma

is OT ct voltage a constant in the circuit ? Or is plate voltage ?

should I be looking at this as 44 = ( 440-x ) /39 ?
And then, does changing the bias pot position change plate voltage, ot voltage ?
I don't see how I get to where I want to be. sorry for being a dummy

[martin manning]

If you want 44 mA and you have 39 ohms across one side of the OT, CT to plate, then you need to adjust the bias pot until you have 0.044 x 39 = 1.72 V CT to plate. The plate voltage will change as you adjust the plate current, so you have to check it, recalculate the target plate current, and readjust.

[hebaton]

O.K but My meter does not give me decimal readings on those. So I am in guess land. I either read 1 or two v drops...
I guess when my meter in moveing back and forth between a difference of 1 and two volt difference between plate and CT, Ishould be in the ballpark...

[martin manning]

What if you put one probe on the CT and one on the plate?

[hebaton]

Got it, finally. Thank you so much for thehelp. You have been patient and tolerant. And i have learned a lot !
I think all will be fine, including my finger ( was afraid for a bot .-)

[martin manning]

How does it sound now?

[martin manning]

Got it, finally...

There is something missing here, a factor of two.

You have two tubes on each side of the OT, so the estimated current per side should be (2 x 0.65 x 30W)/440 = 88.9 mA, and on the 39 ohm side of the OT primary that would mean 0.0889 x 39 = 3.47 V.

This would be near the maximum recommended 70% of max dissipation, but you could operate it as low as 50%. That would be 50/65 x 0.0889 = 0.0684, which is 2.67 V across 39 ohms. Over that range the plate voltage will change noticeably (it will be lower at higher current), so you will have to recalculate and reset it at least once.

[hebaton]

Got it, finally...

There is something missing here, a factor of two.

You have two tubes on each side of the OT, so the estimated current per side should be (2 x 0.65 x 30W)/440 = 88.9 mA, and on the 39 ohm side of the OT primary that would mean 0.0889 x 39 = 3.47 V.

This would be near the maximum recommended 70% of max dissipation, but you could operate it as low as 50%. That would be 50/65 x 0.0889 = 0.0684, which is 2.67 V across 39 ohms. Over that range the plate voltage will change noticeably (it will be lower at higher current), so you will have to recalculate and reset it at least once.

i'll go back to it, but i don't think the bias ajustment pot will allow that much range, i think I was already wel past the half on the pot...

[martin manning]

...i don't think the bias ajustment pot will allow that much range, i think I was already wel past the half on the pot...

In that case you would have to adjust the value of a resistor in the bias circuit. As already noted, someone has already modified it to convert from bias balance to bias level, and added a resistor in parallel to an existing one to adjust the range.

[hebaton]

The maximum I can get now with the pot completely turned counter clock is -2,55v
The two resistors read a total of 11K
But the amp was working before the recap, it had an issue, a ghost note after the played note, pointed towards a leaky cap so I swapped all the electrolitycs for new ones. Why would that change the bias range ?

[johnnyreece]

Just to clarify: Have you replaced the electrolytic caps in the bias section yet?

[hebaton]

No, those are coming in today.
And I had a tube redplating when I first turned it on ( after the recap ). But that could have ben the case before, I didn't really look back there then...