Our GM handed me a 1980 2203 and said "I'm borrowing this, check it out". Tubes tested OK and I set the bias.
I noticed a tiny bit of fuzz on the output, so I flipped the scope to FFT mode. When cranked the amp was oscillating at about 30kHz and IIRC 40kHz, and sometimes other frequencies.
In the past, someone installed a loop right before the MV. So I ran into the return and there was no oscillation.
I got the scope probe out and found that V1B was clean on the plate, V2A was clean on the grid, and there was 30kHz on V2A's plate. (This apparently was perturbing the amp and thus causing the additional frequencies.) I swapped V2, which made no difference.
I tried putting different caps across V2A's plate resistor, and when I got to 1000pf/.001uf, the oscillation went away.
Not to worry about loosing any top end, it can peel paint if you want it to, and sounds quite good when adjusted "properly".
2203 Oscillation (and solution).
Moderators: pompeiisneaks, Colossal
-
TimmyP1955
- Posts: 21
- Joined: Wed Dec 10, 2014 12:50 am
- Location: Indianapolis
-
Reeltarded
- Posts: 10189
- Joined: Sat Feb 14, 2009 4:38 am
- Location: GA USA
Re: 2203 Oscillation (and solution).
No swampers or something...?
That would do it on a cascade. You can use way smaller stoppers than plate bypass to kill the ultrasonic. Just wondering..
That would do it on a cascade. You can use way smaller stoppers than plate bypass to kill the ultrasonic. Just wondering..
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
-
TimmyP1955
- Posts: 21
- Joined: Wed Dec 10, 2014 12:50 am
- Location: Indianapolis
Re: 2203 Oscillation (and solution).
The cap was the first thing I thought of, and it worked. A grid stopper might also have done the job. But the cap was easy, and the amp sounded good, with the tone controls able to take it from too bright to too dark. The boss is happy, so I'm happy 
It was suggested on another forum that in iffy supply bypass cap could have been contributory, and if it's the original cap, it certainly might be getting iffy (high ESR or ESI).
It was suggested on another forum that in iffy supply bypass cap could have been contributory, and if it's the original cap, it certainly might be getting iffy (high ESR or ESI).
Re: 2203 Oscillation (and solution).
Did you shield the input and also the wire from the gain pot to the input of the second stage? Thats the fix for this problem.TimmyP1955 wrote:The cap was the first thing I thought of, and it worked. A grid stopper might also have done the job. But the cap was easy, and the amp sounded good, with the tone controls able to take it from too bright to too dark. The boss is happy, so I'm happy
It was suggested on another forum that in iffy supply bypass cap could have been contributory, and if it's the original cap, it certainly might be getting iffy (high ESR or ESI).