DX-60 audio breaks into oscillation

[KX5JT]

It happens from time to time.  I used to think it was just feedback from audio escaping my headphones as I would monitor myself in the phones, but I recently wired up the mute on the reciever so I can fill my room up with received audio.  Well, it still occurs.  It's just a tiny amplitude riding on the carrier on the scope and a fairly high pitched tone, probably above 1Khz.  Cutting back the audio gain pot on the DX-60 chassis makes it dissapear but I then my modulation level is cut back from nearly 100% to more like 70%.  I figure it's the 12ax7 going into oscillation.  Is there something I can do easily to resolve it?

John

[KX5JT]

I think I found it.  I had put in a 12ax7lps from Sovtec when I was trying to solve AC Hum.  It has a spiral filament but it also has a larger plate area and is more prone to microphonics.... I see if changing it back to the old tube solves it.


From the tubestore.com:

Sovtek 12AX7-LPS - This is an entirely new design from Sovtek and a great step up in sound quality. They have very large ribbed plates and great sound reproduction. I found them very smooth and well balanced in terms of bass, mids and treble response. The large plates make them more prone to microphonics and in combo amps, so they can be a problem if you like to run things wide open. It is still the best thing Sovtek has produced in a 12AX7, with very good gain and low noise. I would advise against using them in compact high-powered combo amps where they will be subjected to lots of vibration. One other note about the construction of these tubes is they have filaments that are almost completely encased in the plate structure. They often don't "light up" when working properly. This is not a problem, it's normal for the LPS.

[ke7trp]

Let us know if that fixes it.  If not, Then I have some other solutions for you.   I had to fight this same issue with the GK500C.  You can change the resistors to alter the gain of the tube and most importantly, You can change the Grid resistor leading to the mic. Once matched, Your audio will really sound nice and you dont have that instability.

Look in your old ARRL handbook.. Every AMer should have one. There will be designs for that tube in the modulator section. You can compare those values to your DX 60.  You can also look up the tube in the tube handbooks and use those values like you where building a HiFi preamp.

Its so bad on the GK, that its best to run a powered D140. This allows you to Lower the gain on the radio and put more audio in.  This was an easy short term fix that got me 100 % with no Squeal or oscilation.  Later, I just fixed the problem. Even so. I dont really care for most Mic preamps in these radios. They really tend to limit your audio.  For example. The GK and the T3 WILL NOT hit 100% if I go into the first stage. If I hook the audio into the second stage, It will run and easy over 100%.



C