| Author | Message | ||
     Mael Bellec (mircasi) Username: mircasi Registered: 03-2009 |
Hi there, I'm new on this forum but I'll try to be as clear as possible. A friend of mine gave me an 2275-65 directly comming from hall of rust... Abiously nothing was ok and this amp really needed complete lifting. And that's I've done. I've changed all caps, cleaned everything, changed blown fuse, resistors etc etc. Finally I've changed driver trans (je1692) by new 2N6488 as one was out. I've set the bias correctly, the amp is now alive but...only makes heavy distortion at any levels, any input. So i took my scope, and start troobleshooting by following a sine wave injected. All is ok in preamp, LM1458 too ( if no tubes set). Signal is strange on base of drivers (like round), and completely distored at collector going to tube. (both drivers) I really don't know where problem is coming. Any idea ? Thanks a lot ! | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I've posted a re-print of somebody using the sub drivers and talking about imbalance when not changing both. Have a look. ########################### I've now tried tried both the 2N6292 and 2N6488. They both sound fine and work fine. However, the 2N6292 do not seem to be as consistant from one to another as the 2N6488 from one to another in terms of hFE (and probably other things, but that's all I can measure), which will cause a bias im-balance. I discovered this after installing a pair of Central Semi 2N6292 from Mouser and finding that my bias differed by 15mV! Whereas the 2N6488 from RCA and ST all measured very closely to each other. I eventually "matched" a pair of the Central 2N6292, and that's what I have in now. Although I probably should have kept in the RCA 2N6488, but I was too curious to let it be. ################################ This might be a problem. MIke. | ||
| Mael Bellec (mircasi) Username: mircasi Registered: 03-2009 |
Thanks for your prompt response. Unfortunately, I've ever changed both drivers, and even tried other pair. it has changed nothing. I can make a copy of my scope tomorrow, maybe it can help. | ||
| Mael Bellec (mircasi) Username: mircasi Registered: 03-2009 |
Here are some news. I've changed everything on driver board so that all is new now (except pot). I always have the same problem : sound is completely distored at driver's out. With scope I've followed the the signal, and the feed from LM1458 is ok clean. Things are getting differents when arriving on driver's base. I've made a shot of scope (sorry for quality...) Down is signal coming in base (look at his strange face for a sin !) Up is collector both 1v/div, finject = 1khz, no saturation and no distortion out of LM1458. http://www.taxila.eu/up/Vrac/IMAG0220.jp g Any idea ?....where's the deal ? | ||
| Mael Bellec (mircasi) Username: mircasi Registered: 03-2009 |
Nobody can help me ?... | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I ASSUME that you have tried a different set of tubes? Are these the real 6ca7s or are they EL34's? | ||
| Mael Bellec (mircasi) Username: mircasi Registered: 03-2009 |
In fact I made the tests without any tubes. When I put, the feedback distroy signal more and more...and nothing good... | ||
| Thomas Leong (tyguitar) Username: tyguitar Registered: 03-2009 |
Sirs, When I restored my MM One Thirty amp, it was distorting like crazy. Not sure if it it is the same type you are facing. Finally detected it as partly mechanical distortion, it comes on as I hit the strings hard, and more distorted at certain notes! The fault I painfully discovered, the MM solder joins are notorious when old, and re- melting them all over the circuit solves mine...now amp is rocking fine. My amp's distortion was, especially coming from the 12AX7 PI area, I also changed the socket with the exercise. | ||
| Mael Bellec (mircasi) Username: mircasi Registered: 03-2009 |
Thanks for advice. I will try, but my problem seems to be little bit different as yours. Anyway I will do it, maybe it will help ! |