Hilgen Troubador T-1506 grounded plug/biasing

[shortcircuit]

Hello
I would like to put a grounded plug on this, run primary to fuse, then to the switch and out to T1 ,safety ground to chassis , remove C18 ?? Just double checking . Would really appreciate biasing info on this amp

[The Dude]

Grounded plug and wiring- yes
Remove C18- yes
Bias- measure the voltage drop across the shared cathode resistor, use ohms law to find current, divide by two to approximate each tube (assuming they are somewhat matched). If you want to change bias, change the resistor value.

[shortcircuit]

Grounded plug and wiring- yes
Remove C18- yes
Bias- measure the voltage drop across the shared cathode resistor, use ohms law to find current, divide by two to approximate each tube (assuming they are somewhat matched). If you want to change bias, change the resistor value.

Thank you , I don't see the shared resistor, I see resistors off pin 2 of 470k , and 220 to 330k , what am I missing ?? sorry I'm 101

[The Dude]

The shared resistor is from both output tube cathodes to ground. It's the 125 ohm 10W.

[shortcircuit]

The shared resistor is from both output tube cathodes to ground. It's the 125 ohm 10W.

The Dude you are great , open thy eyes and see !! Thank You

[shortcircuit]

The Dude you are great , open thy eyes and see !! Thank You

OK grounded plug success, Biasing has run into a snag , my 125ohm shared resistor measures .8 and the 25uf cap 0 , current limiter is lit. Nothing shows any burn , or signs of leakage , it's not blowing the fuse. I have to order all my parts so it will be a few days . Any suggestions otherwise ?? Vintage amps like old pipes , fix this and it just might spring a leak downstream. Thanks all !!

[The Dude]

........my 125ohm shared resistor measures .8 and the 25uf cap 0 , current limiter is lit......

.8 what? Ohms, voltage drop, etc. 0 what? Ohms, voltage drop, etc. The two are paralleled, so the readings don't make sense? Please explain what you measured and how- in circuit out of circuit, voltage, resistance, other.

[shortcircuit]

.8 what? Ohms, voltage drop, etc. 0 what? Ohms, voltage drop, etc. The two are paralleled, so the readings don't make sense? Please explain what you measured and how- in circuit out of circuit, voltage, resistance, other.

I'm sorry, actual resistor reading .8 ohms in circuit connected as close to the body of the resistor as possible, cap also measured in circuit on 200u scale climbs then reads 1 , voltage reading across the resistor at idle was .157vdc . does this make even less sense??

[shortcircuit]

I'm sorry, actual resistor reading .8 ohms in circuit connected as close to the body of the resistor as possible, cap also measured in circuit on 200u scale climbs then reads 1 , voltage reading across the resistor at idle was .157vdc . does this make even less sense??

I also did a plate voltage reading off pin 7 to chassis ground of el84 started off 350vdc then started dropping I shut off the amp when it was in the low 200's

[The Dude]

Resistors almost never short. I suspect your cathode cap (that is paralleled with that resistor) is shorted. I'd pull it out of circuit and check it's resistance to be sure.

[Enzo]

That^^^

In 60 plus years of soldering, I have never encountered a shorted resistor.

If not the cap shorted, look for a solder bridge from the cathode socket pins to ground or maybe heater.

[Dave H]

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

[shortcircuit]

Resistors almost never short. I suspect your cathode cap (that is paralleled with that resistor) is shorted. I'd pull it out of circuit and check it's resistance to be sure.

Once again I've been educated , out of circuit resistor is within tolerance , cap is junk. Since the cap is in the shared bias circuit should I stay as close to the cap rating as possible ?? 50v 25uf. And in your opinion did they use such a heavy resistor so you could switch tubes without much chance of having to re bias ?? As always can't thank you enough

[The Dude]

Yes, I would stay close to that value- you can use a higher voltage cap if that's what you have. It won't hurt anything. Also, if the cap is mounted near the resistor, be sure to keep the new one away as much as possible. Heat from that resistor will age the cap more quickly. The large resistor was chosen simply for it's ability to dissipate the power created in that circuit.

[Enzo]

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.