I was listening to music last night when the sound got very distorted and low. I went around the corner to see what was going on. One output tube and the rectifier were glowing bright red, almost yellow. I quickly pulled the plug and let things cool down. I pulled the outputs and checked voltages, things seemed OK. I dug out my ancient Jackson 648S tube tester (my favorite ever, BTW) and one output tube is shorted K-G1. I have never seen that happen before. Ordered a quad of KT120, should be here Monday and I will report back. Tube gear is so durable. If it was solid state the magic smoke would have been released.
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ST-120 Meltdown ?- NOT !
bobgroger- Posts : 18
Join date : 2021-02-12
peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
bobgroger wrote:I was listening to music last night when the sound got very distorted and low. I went around the corner to see what was going on. One output tube and the rectifier were glowing bright red, almost yellow. I quickly pulled the plug and let things cool down. I pulled the outputs and checked voltages, things seemed OK. I dug out my ancient Jackson 648S tube tester (my favorite ever, BTW) and one output tube is shorted K-G1. I have never seen that happen before. Ordered a quad of KT120, should be here Monday and I will report back. Tube gear is so durable. If it was solid state the magic smoke would have been released.
KT120 needs the grid resistors ( grid to bias DC ) to be at most 51k
VTA120 has i think 100k resistors . Too large resistors might have the kt120 tubes self-destruct.
bobgroger- Posts : 18
Join date : 2021-02-12
Would you bias them at the same .5V?
peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
In the interval 0.45 - 0.5V.bobgroger wrote:Would you bias them at the same .5V?
LeGrace- Posts : 389
Join date : 2016-08-07
Location : Ontario, Canada
0.5 for KT88 or 6550, 0.6 for KT120.
Bob Latino- Admin
- Posts : 3276
Join date : 2008-11-26
Location : Massachusetts
peterh wrote:KT120 needs the grid resistors ( grid to bias DC ) to be at most 51k
VTA120 has i think 100k resistors . Too large resistors might have the kt120 tubes self-destruct.
Although Tung-Sol may spec that those resistors should be 51K resistors, in practice a 100K resistor here works fine. I have had 100K resistors here in my own VTA ST-120 for about 8 years now. Never had a problem with TWO different quads of KT120 output tubes in this amp.
Bob
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Bob Latino- Admin
- Posts : 3276
Join date : 2008-11-26
Location : Massachusetts
LeGrace wrote:0.5 for KT88 or 6550, 0.6 for KT120.
My advice > You should use .500 VDC bias (per each tube) for either 6550, KT88 or KT120 output tubes in the VTA ST-120.
Bob
peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
Bob Latino wrote:peterh wrote:KT120 needs the grid resistors ( grid to bias DC ) to be at most 51k
VTA120 has i think 100k resistors . Too large resistors might have the kt120 tubes self-destruct.
Although Tung-Sol may spec that those resistors should be 51K resistors, in practice a 100K resistor here works fine. I have had 100K resistors here in my own VTA ST-120 for about 8 years now. Never had a problem with TWO different quads of KT120 output tubes in this amp.
Bob
bobgroger had a problem with kt120 and 100k, that was his problem.
It's no guarantee that > 51k will get the tube into trouble, but there is an enhanced risk for trouble.
bobgroger- Posts : 18
Join date : 2021-02-12
peterh wrote:Bob Latino wrote:peterh wrote:KT120 needs the grid resistors ( grid to bias DC ) to be at most 51k
VTA120 has i think 100k resistors . Too large resistors might have the kt120 tubes self-destruct.
Although Tung-Sol may spec that those resistors should be 51K resistors, in practice a 100K resistor here works fine. I have had 100K resistors here in my own VTA ST-120 for about 8 years now. Never had a problem with TWO different quads of KT120 output tubes in this amp.
Bob
bobgroger had a problem with kt120 and 100k, that was his problem.
It's no guarantee that > 51k will get the tube into trouble, but there is an enhanced risk for trouble.
To clarify when I had the tube failure I had EH KT88s installed. Looks like the only damage was one KT88 and 1 cathode resistor.
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