I did the unthinkable: VTA ST120 powered amp on with NO speakers connected!

Keverdawg wrote:I did it, my fault. I powered on my VTA ST120 with KT120s in it. switched it right off. It was maybe on for 10 seconds at the most. I was leaning over the amp to turn it on and I was looking for tubes to light up. I had installed the warmup timer when I built the amp. Tubes began to light up and I noticed some arcing in the right rear power tube. I immediately turned the power off. I looked for what I had done and saw that the speaker wires weren't connected. I hooked them up and turned the amp back on. I didn't sound good and had much less power. The amp was stored for about a year and I had just bought a tube preamp to put with the amp. l played one album side and I also checked out a FM tuner to it and tried it out. The sound was getting worse so I thought it was time to reset the bias on the Tung Sol KT120's. I wrote Bob Latino to asked about the biasing procedure as it had been a year or more since I fired up this amp. This is what I got:
Front left biased fine at .500 VDC
Front right biased at .500 VDC
rear Left biased could not be set the meter read 18.5 VDC turnd all the way down CCW on pot.
Rear right same as rear left.

What did I break?  Rectifier tube? Power Tubes? or all.
Bob help me please...
Thanks

Bob Latino wrote:

Keverdawg wrote:I did it, my fault. I powered on my VTA ST120 with KT120s in it. switched it right off. It was maybe on for 10 seconds at the most. I was leaning over the amp to turn it on and I was looking for tubes to light up. I had installed the warmup timer when I built the amp. Tubes began to light up and I noticed some arcing in the right rear power tube. I immediately turned the power off. I looked for what I had done and saw that the speaker wires weren't connected. I hooked them up and turned the amp back on. I didn't sound good and had much less power. The amp was stored for about a year and I had just bought a tube preamp to put with the amp. l played one album side and I also checked out a FM tuner to it and tried it out. The sound was getting worse so I thought it was time to reset the bias on the Tung Sol KT120's. I wrote Bob Latino to asked about the biasing procedure as it had been a year or more since I fired up this amp. This is what I got:
Front left biased fine at .500 VDC
Front right biased at .500 VDC
rear Left biased could not be set the meter read 18.5 VDC turnd all the way down CCW on pot.
Rear right same as rear left.

What did I break?  Rectifier tube? Power Tubes? or all.
Bob help me please...
Thanks

Hi,

Yes - You should never turn the amp on without speakers attached. The amp is expecting a "load" on the speaker posts and sometimes "things can happen" when you don't have speakers connected. You didn't take out the rectifier. If you took out the rectifier, you would get no bias voltage on any of the 4 bias measuring points ..

1. With the amp OFF, measure the RESISTANCE to chassis ground at the 4 bias measuring points. Yes - you normally have the amp ON and you are normally measuring DC volts but this time measure the RESISTANCE to chassis ground with the amp OFF. You should get about 10 ohms at all 4 bias measuring points. If one or more of the 10 ohm resistors are blown, you will get no reading or a very high reading on one or more points and the bad resistor(s) must be replaced.

2. Center the 4 bias pots. Take off the bottom cover. With all tubes out of the amp turn the amp ON and measure the DC voltage on pins 5 and 6 of all output tubes. You should get -50 to -60 VDC at all points. Pins 5 and 6 have a 1000 ohm 1 watt resistor across the two pins.

Let us know what you find ?

Bob

Bob Latino wrote:If your readings are correct, then two of those 10 ohm 2 watt resistors should be replaced. The FL and the RR. If one of those resistors reads as open and the other 3 megohms, then those two resistors have to be replaced.

Bob

Bob Latino wrote:1. With the amp OFF, measure the RESISTANCE to chassis ground at the 4 bias measuring points. Yes - you normally have the amp ON and you are normally measuring DC volts but this time measure the RESISTANCE to chassis ground with the amp OFF. You should get about 10 ohms at all 4 bias measuring points. If one or more of the 10 ohm resistors are blown, you will get no reading or a very high reading on one or more points and the bad resistor(s) must be replaced.

tubes4hifi wrote:one of the reasons I don't agree with using 1 or 2 or 5w resistors for the 10 ohm cathodes.   At normal bias, those resistors could be 1/20 watt.  So a 1/8 or 1/4 watt is more than plenty.
I'd rather replace a 10c resistor than a $50 tube anyday!