• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Strange experience with Dynaco ST70 input tubes

This is my first thread here.Just wanted to share some strange experience with Dynaco ST 70 that happened this week.
I have coupled it with Tannoy Glenair 15 and initial run did not give satisfactory sound at all. It was blurr and muddy sound to our surprise.
with little investigation it was found that at "0" volumes levels also left channel was passing some kind of signal to speaker and speaker surround was oscillating in and out at @ 100-110 times a minute.
I attempted changing volume control,I/O cables,shuffled L/R speakers, but problem was still there at left output on amp.
Checked all circuit, NFB,PFB. Did find nothing really.
Finally interchanged 7199 input tubes and to my surprise that oscillations disappeared. After interchanging tubes problem did not go to right channel,which was expected.
Now I am getting clean o/p and no oscillations or DC current at speaker terminals. Earlier it was giving average of 60-70 mA of DC current at speaker terminal.
I have shared this problem here expecting some feedback on rectifying and fixing this problem for ever.
Please give your valuable suggestions.
Thanks in advance.