ST120 - Noise in one channel after valve red plated

Hi Everyone,

I have one of Bob's ST-120 amps (been running for around 2 years) and just the other day had a KT88 red plate on me which took out the rectifier tube as well.

I have had tubes red plate a couple of times before and on one of those occasions it took the fuse out, his time the fuse was ok.
On those other occasions I simply replaced the faulty tube, rechecked the bias and everything is good again.

This time however looks like some damage has been done in the left channel where the tube red plated.

After replacing the faulty KT88 and rectifier tube I now have a hash type of noise comming through the speaker and this is accompanyed by the woofer cone moving back and forth very slowly ... probably 5 to 8 mm of movement at random times.

Also the bias voltage is now unstable and swings between 0.470 and 0.560 ... trying to adjust the bias will just see the swing continue at either a higher or lower value.

The right chanel is silent ... no woofer movement and the bias is rock solid with no fluctuation at all.

Swapping the pair of KT88's from one channel to the other makes no difference ... the left channnel still has the problem.

And it still plays music fine from both channels.

Any ideas what might have gone wrong?
Has it fried a bias resistor or a cap?

I haven't taken the bottom cover off yet ... thought I would ask the question here first.

Thanks for any help.

Cheers,

Alan.

my first reaction is that your multicap or perhaps even the smaller SCM module took a hit, check your B+ is see whats happening there, and yes, measure your BIAS resistor on what was the fried tube as well...with power off of course!

Alan-14 wrote:Hi Everyone,

I have one of Bob's ST-120 amps (been running for around 2 years) and just the other day had a KT88 red plate on me which took out the rectifier tube as well.

I have had tubes red plate a couple of times before and on one of those occasions it took the fuse out, his time the fuse was ok.
On those other occasions I simply replaced the faulty tube, rechecked the bias and everything is good again.

This time however looks like some damage has been done in the left channel where the tube red plated.

After replacing the faulty KT88 and rectifier tube I now have a hash type of noise comming through the speaker and this is accompanyed by the woofer cone moving back and forth very slowly ... probably 5 to 8 mm of movement at random times.

Also the bias voltage is now unstable and swings between 0.470 and 0.560 ... trying to adjust the bias will just see the swing continue at either a higher or lower value.

The right chanel is silent ... no woofer movement and the bias is rock solid with no fluctuation at all.

Swapping the pair of KT88's from one channel to the other makes no difference ... the left channnel still has the problem.

And it still plays music fine from both channels.

Any ideas what might have gone wrong?
Has it fried a bias resistor or a cap?

I haven't taken the bottom cover off yet ... thought I would ask the question here first.

Thanks for any help.

Cheers,

Alan.

replace the 10ohm cathode resistors. It's small money and they might not show on the outside
if they are fried.

Bob Latino solved a similar root problem for me by suggesting re-tightening the tube sockets. This, after a red-plating, loss of a very expensive GZ-33, etc., but fortunately no "under the hood" issues. If Pin 5 is even a bit loose you can get a lot of grief.

Bob Latino wrote:Hi Alan,

I do not think that the quad cap or the SCM is at fault. If they were, it would affect BOTH channels. It has to be some issue on the left side of the driver board or something on the two left channel output tube sockets.

1. Try swapping the left and right driver tubes and see if the woofer movement moves from the left channel over to the right channel.

2. Check the bias system > set all bias pots to their center position. Pull out all the tubes. Turn the amp on and measure the negative DC voltage on pins 5 and 6 of each output tube on both channels. You should get between -45 to -65 VDC on pins 5 and 6 of all four output tubes.

3. Check the resistance to chassis ground (with the amp OFF) at each of the four bias measuring points and see if you get about 10 ohms.

Let us know what you find ?

Bob

yes, that is a very good point, if it were the cap/s it would affect both channels for sure!

deepee99 wrote:Bob Latino solved a similar root problem for me by suggesting re-tightening the tube sockets.

Like I always say, if your root problem lasts more than four hours, seek professional help!

~~~

Systematic approach is best. Start with the easy stuff, then work your way through. Nothing really all that complicated here, so step by step.

- As mentioned, check the sockets, and I'd add, check all mechanical connections on that side of the amp. Cold solder is also a killer.
- Bias resistors are inexpensive and an easy potential fix. You really need to lift them to get a good read, so might as well replace them while you're at it, starting of course with the problem socket.

I'd add that the only red plate I ever had was due to a bad bias pot, so that'd be my next stop.

And most important ... post back with what all ya did, and especially what worked!

Thank you everyone ... I'll get back to you tomorrow after I try things suggested.

Bob Latino wrote:
Hi Alan,

I do not think that the quad cap or the SCM is at fault. If they were, it would affect BOTH channels. It has to be some issue on the left side of the driver board or something on the two left channel output tube sockets.

1. Try swapping the left and right driver tubes and see if the woofer movement moves from the left channel over to the right channel.

2. Check the bias system > set all bias pots to their center position. Pull out all the tubes. Turn the amp on and measure the negative DC voltage on pins 5 and 6 of each output tube on both channels. You should get between -45 to -65 VDC on pins 5 and 6 of all four output tubes.

3. Check the resistance to chassis ground (with the amp OFF) at each of the four bias measuring points and see if you get about 10 ohms.

Let us know what you find ?

Bob

Thanks Bob ... your diagnosis steps worked and the problem solved!


1) The first thing I tried was to check the bias resistor to ground ... all four points measured 10 ohm as normal.

2) Next I swapped the left driver tube into the right channel ... and all the hash/woofer movement and unstable bias voltage moved over to the right channel with the left now going back to normal.

So I replaced both driver tubes with some Bugle Boy 12AT7's that I had bought a while back and all was fixed.
No more noise or unstable bias!
But I thought I would continue on and check the bias system anyway.

3) Removed power tubes, Set the bias pots to centre position and measured the -ve DCV of pins 5 and 6 ...
LF= -59.2 VDC,  LR= -60.5 VDC, RF= -59.4 VDC, RR= -60 VDC
So all points measure well within spec.

And I've learned something new ... the symptoms that faulty driver tubes can produce

sKiZo wrote:

Like I always say, if your root problem lasts more than four hours, seek professional help!
~~~

Systematic approach is best. Start with the easy stuff, then work your way through. Nothing really all that complicated here, so step by step.

- As mentioned, check the sockets, and I'd add, check all mechanical connections on that side of the amp. Cold solder is also a killer.
- Bias resistors are inexpensive and an easy potential fix. You really need to lift them to get a good read, so might as well replace them while you're at it, starting of course with the problem socket.

I'd add that the only red plate I ever had was due to a bad bias pot, so that'd be my next stop.

And most important ... post back with what all ya did, and especially what worked!

Thanks sKiZo ... yes you are right ... a methodical process of elimination starting from the easy stuff is the best way  

And thank you everyone for your input