Blues Junior power transformer smoking - can it be rewound ?

[chazpope]

well I did not measure it but if even you account for the 5% drop in B+ voltage it still is 11.2W - which is 95% (not 70%) or 12W!

not sure what to make of that, I don't want my valves to wear out otoh I don't want to change the sound too much. I will install a bias pot at some point - do you think it's worth trying to also adjust for valve differences or would one be enough

[cagamp1]

Wow, that's kind of scary. I don't know how you shunted what but you were frying something. That could have led to something else burning up...which does seem serious to me. Be careful.

[ThermionicScott]

well I did not measure it but if even you account for the 5% drop in B+ voltage it still is 11.2W - which is 95% (not 70%) or 12W!

Now that the plate (and screen) voltage is lower, the current draw will be a little lower as well. I'd re-check the idle current before making any circuit changes. If it's still a little too hot for comfort, I'd lower the value of the 33k resistor before making the 22k resistor bigger. That'll improve the RC time constant of the bias supply a little.

I wouldn't fret the difference between the two tubes, as long as the amp isn't humming badly.

- Scott

[chazpope]

yes but when I check it (I assume by measuring voltages across OT) what current should I aim for ?

[ThermionicScott]

At 330V, 36mA max down to... whatever sounds best to you. Fixed-bias push-pull operation from the manuals is a lot cooler than you'd expect -- the plates idled at 4.5 W each, for the exact same power output and THD as cathode-bias operation.

http://tdsl.duncanamps.com/show.php?des=EL84

- Scott

[Jazz P Bass]

Output Tube Idle Current

You cannot measure the current draw of the output tubes by measuring "across" the output transformer.
You can measure voltage.
To measure idle current you can:
-Buy a bias reading adapter.
-Insert a 1 ohm, 1% tolerance resistor between each output tubes cathode & ground.
A voltage reading across the resistor can be converted to current (using Ohms Law :I=V/R)
Disconnect the blue (& then the brown, one at a time) primary wire from the OPT at the tube socket. The meter gets set to measure DC Current. The meter leads then get connected to the Blue wire & the Pin that it was connected to. The meter (set to read Current DC) now completes the circuit that was broken when you disconnected the wire from the tube socket.
The reading that you get will be the DC current in ma's.
This is the hairy way to do it. You have high voltage flopping around on the test leads.
Stay AWAY from ground.

[ThermionicScott]

You cannot measure the current draw of the output tubes by measuring "across" the output transformer.

Sure you can. Measure the resistance between the OT CT and a primary tap, and then measure the voltage drop across that when the amp is on. It's called the "transformer shunt method."

Although I do prefer the 1-ohm (or 10-ohm) cathode resistor option for ease and safety.

Disconnect the blue (& then the brown, one at a time) primary wire from the OPT at the tube socket. The meter gets set to measure DC Current. The meter leads then get connected to the Blue wire & the Pin that it was connected to. The meter (set to read Current DC) now completes the circuit that was broken when you disconnected the wire from the tube socket.
The reading that you get will be the DC current in ma's.
This is the hairy way to do it. You have high voltage flopping around on the test leads.
Stay AWAY from ground.

This and the transformer shunt method have the advantage of reading only the plate current, but I personally don't like any method where you have to temporarily hack the amp to do it. You could permanently add a 10-ohm resistor in series with each plate lead, and not only would you be able to easily read the plate current, but it would add a little protection from oscillation.

- Scott

[Jazz P Bass]

Sure you can. Measure the resistance between the OT CT and a primary tap, and then measure the voltage drop across that when the amp is on. It's called the "transformer shunt method."

I stand corrected, Thanks TS.
My reason for the statement was I did not want the amp checked, in this way, with the meter set on Current DC.

[ThermionicScott]

I stand corrected, Thanks TS.
My reason for the statement was I did not want the amp checked, in this way, with the meter set on Current DC.

I went and looked it up again after posting, and I guess there are two ways -- one of them is to measure the voltage drop across the primary, and the one that is actually called the "shunt method" is to use an ammeter across it! Apparently the high impedance of the ammeter keeps things from blowing up and is still reasonably accurate. I definitely prefer to measure voltage drop across a known resistance for this stuff, though.

- Scott

[cagamp1]

Hey guys, the resistor method of cureent measurement is easier and more consistent.

The transformer shunt method is good in a pinch if nothing else is available. You have to take into account that the transformer primary will heat when voltage is applied therefore changing its resistance/impedance slightly.

[chazpope]

so isn't it BEST to measure voltage across the OT in order to gauge current - as opposed to invasive and dodgy methods like inserting resistors or OMG shorting the OT with an ammeter that has (almost) 0 resistance ? At idle there is only DC going through the OT - so it should be just like a big resistor. And about heat - at idle .35 ma the OT is dissipating 0.12W (say as much as an LED) - hardly enough to heat the big lump to the point of changing its wire resistance.

[melvin]

The OT is like a very small resistor to the DC. I don't know of anyone measuring the resistance of each leg of the OT and then measuring the voltage across it to find the idle current.

The transformer shunt method is adequate. I don't find bias to demand much precision to warrant adding bias sensing resistors. However, the shunt method doesn't account for screen current. But there are usually other ways to determine that.

[Jazz P Bass]

OMG shorting the OT with an ammeter that has (almost) 0 resistance ? .

You are Not shorting the OPT with your ammeter.
You are completing a broken path.
As long as you keep away from ground potential, you have milliamps going through the meter.

[cagamp1]

When electrical power is applied to anything heat will be created and increase resistance. In this amp with that voltage and those tubes, it will only vary about 1mA on average. So, sorry I brought it up.

I do like to check both sides of the OT to make sure the two tubes are idling at close to the same current.

[Chuck H]

The resistance of the meter is so great (some 20M) that it can be ignored. But the meter is designed to sense the current at this resistance via it's sensiivity. That's why it works. More important is the fact that specs given for tubes are typically plate specific. Either method of bias measurement fails to isolate the actual plate current and is actually measuring total tube current. Not seemingly as much an issue with the big bottles but with EL84's it tends to confuse people.