Hilgen Troubador T-1506 grounded plug/biasing

[shortcircuit]

With the cap disconnected measure the resistor value. If it's OK (125ohms) power up the amp and measure the voltage across the resistor and the plate voltage. It will run OK without the cap to check the bias. If the EL84s are running too hot the resistor could be changed to 150ohms

now have 3.87vdc across the resistor at idle and 175vdc is where the voltage settles on both el84 at pin 7

[shortcircuit]

No one making cathode biased amps are expecting anyone to "rebias" it. That requires circuit modification. The selected the value for likely good operation with most any proper tubes installed.

heat is a big cap killer. Mount your replacement cap spaced away from the resistor to keep the resistor heat off the cap.

Unless you have specific performance goals requiring change, I'd stick with the stock cap value. You can use higher voltage if you wish, no problem, but the voltage on the cap is not usually very high.

Aha: Duded just beat me to it.

The Dude , Enzo , Dave H thanks wish I knew one tenth of what you've forgot

[Dave H]

now have 3.87vdc across the resistor at idle and 175vdc is where the voltage settles on both el84 at pin 7

Those voltages are too low. What resistance is there between pin 3 of the EL84s and ground. It should be 125ohms (the common cathode resistor value)

[The Dude]

^^^^^^ Agreed. We still have a problem somewhere. Did you get the cap in with the correct polarity?

[shortcircuit]

^^^^^^ Agreed. We still have a problem somewhere. Did you get the cap in with the correct polarity?

have not replaced cap

[shortcircuit]

Those voltages are too low. What resistance is there between pin 3 of the EL84s and ground. It should be 125ohms (the common cathode resistor value)

119 ohms both el84's , voltages start out 300+ then drop and settle

[The Dude]

Ok, then do as Dave suggested and measure the cathode resistor in circuit. If it measures correctly, pull the output tubes, power up, and see if your plate voltage comes up closer to the correct voltage. You could have a bad output tube and that may be what started the whole mess.

[The Dude]

Oops, you snuck in while I was typing. Ok, the resistor seems fine. Try the second part of my post. Pull the tubes and check B+. If it comes up with tubes removed, install tubes one at time monitoring B+. See if you can find one that pulls the voltage down.

[shortcircuit]

Oops, you snuck in while I was typing. Ok, the resistor seems fine. Try the second part of my post. Pull the tubes and check B+. If it comes up with tubes removed, install tubes one at time monitoring B+. See if you can find one that pulls the voltage down.

OK with no tubes I get good b+ , put in tubes ,both start good and and as soon as they warm they drop, both tubes , both sockets . Tubes passed Knight tube test.

[The Dude]

Do you have a rectifier tube to sub in? Maybe it's giving up under load.
FWIW: If a tube tester says a tube is bad, it's bad. If it says the tube is good, it might still be bad. Most testers don't use real world voltages for accurate testing.

Also, you're not on a light bulb limiter or other limiting device, are you?

[shortcircuit]

Do you have a rectifier tube to sub in? Maybe it's giving up under load.
FWIW: If a tube tester says a tube is bad, it's bad. If it says the tube is good, it might still be bad. Most testers don't use real world voltages for accurate testing.

Also, you're not on a light bulb limiter or other limiting device, are you?

Just tested rectifier, 70/80% looking for replacement ,YES running off light bulb limiter

[The Dude]

Forget the rectifer. Remove the light bulb limiter. That is likely the problem. When the amp starts to draw current, the voltage will go down when using the limiter. That's its job. After the amp is fixed, you no longer need the limiter.

[shortcircuit]

Forget the rectifer. Remove the light bulb limiter. That is likely the problem. When the amp starts to draw current, the voltage will go down when using the limiter. That's its job. After the amp is fixed, you no longer need the limiter.

Yes that was it , I keep getting tunnel vision , instead of the big picture, didn't considered fixed until I get the cap installed. One more opinion, change filter caps on vintage amps because of age or not ??

[Dave H]

I forgot about the light bulb limiter (I don't have one). What voltages do you measure now for the EL84 cathodes and plates?

[Enzo]

Even if just for testing, most any cap would work for those cathodes. Just tack solder a part in there. Got a 22uf 500v filter cap handy? That would work. We only want to see what difference it makes, not a permanent install. You can get a more pleasing part later.