Latino ST120 Problem - bad bias resistor

Good Day All,

So I'm listening to music and I hear a quick pop. I didn't immediately hear any change in the sound coming out of the speakers - I was listening at a low level.

I powered off my CD player and preamp to check my bias on the ST120. The left rear bias was high - about 23 VDC I think. The other three bias points were OK at about 0.550 VDC.

Then I saw a quick light show in the SOVTEK GZ34 rectifier tube. I powered off the amp to investigate further.

I found that the 10 ohm resistor on pin 1 of the rear left KT88 tube was physically chipped and the resistance was open so that obviously needs replaced.

What might have caused this and might it have damaged my left rear KT88???

I'm going to test the GZ34 and have a spare if needed. I'll have to look around to find a tube tester to test the KT88.

Assuming I have all good tubes - should I just replace the resistor, plug everything back in and fire it up and see if all is well???

Any suggested methodolgy I might consider so I don't damage anything further?

Thanks,

John
.

Since you are in my fair city I'll post a reply and get the ball rolling. Did you see the light show in the GZ34 after you noticed that you couldn't set the BIAS or was that after the light show? If the KT88 test out OK then I would just replace that 10 Ohm 2 watter and fire it up. Someone else can explain why that resistor opened up in the first place.  I'm assuming nothing else looked wrong under the chassis?

Do you run your amp through line power, or reduce the incoming voltage with an autotransformer? Just curious, I have a ST-70 that's been fully refurbished including a beefier PS transformer and I still run it through a variac at 115 V just to add a measure of safety since these amps are pushed pretty hard.

The rectifier tube tested bad - no suprise. Fortunately, the KT88 tested good. Now I have to find a 10 ohm 2 watt resistor.

I first noticed the high bias - probably due to the bad resistor - then the rectifier flashed.

guentherj2 wrote:


Good Day All,

So I'm listening to music and I hear a quick pop. I didn't immediately hear any change in the sound coming out of the speakers - I was listening at a low level.

I powered off my CD player and preamp to check my bias on the ST120. The left rear bias was high - about 23 VDC I think. The other three bias points were OK at about 0.550 VDC.

Then I saw a quick light show in the SOVTEK GZ34 rectifier tube. I powered off the amp to investigate further.

I found that the 10 ohm resistor on pin 1 of the rear left KT88 tube was physically chipped and the resistance was open so that obviously needs replaced.

What might have caused this and might it have damaged my left rear KT88???

I'm going to test the GZ34 and have a spare if needed. I'll have to look around to find a tube tester to test the KT88.

Assuming I have all good tubes  - should I just replace the resistor, plug everything back in and fire it up and see if all is well???

Any suggested methodolgy I might consider so I don't damage anything further?

Thanks,

John
.

i'd consider the kt88 bad and have it replaced before i continue.
The schenario is typical for a flashing powertube. Your gz34 might have survived.
The resistor is to be replaced with one that is matched to the remaining one, they need to be matched.

Thanks Bob. Yes, those 2 watt resistors are not readily available. I look forward to getting and replacing the new resistor and hope that takes care of it. Lets hope the KT88 tube is OK - it checked out on a tube tester but the old tube tester I used is not 100% reliable so we'll see.

John

Bob quickly sent me a couple of 10 ohm 2 watt resistors. I replaced the bad bias resistor and GZ34 rectifier and am happily back up and running.