| Author | Message | ||
     Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
http://members.aol.com/axeist/axeist.htm l There is a Mod removing the feedback from the PI tube. Any thoughts ??? What would it do soundwise ?? | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
interesting question, andrew. from what I've read though, it would require extensive modification to reduce enough of the resulting hum caused by the loop's removal to make the amp playable. (i don't KNOW that for a fact, it is just what i've concluded from my reading on the subject.) an interesting article discusses the NFBL and its use by manufacturers to quiet-down amps that would otherwise be already quiet if they had been built to tighter specs with better quality parts (larger filters, chokes, etc). mr grimwood's contention is that the NFBL came into general use as a cost-saving measure....can you imagine a guitar amp built to specifications much like a macintosh hi fi amp? (well, yes there are a couple actually: some very spendy boutique amps are built with this as their design philosophy.) you might want to check out "screen grids in audio and rf modulator power tubes" by dennis grimwood. the article is located at: oops, the link is dead. try google using the author/title of the article (unless of course you are into those gambling sites....then the original site is a good one  ) it is a really nice (and not too technical/math-orented) article dealing mostly with screen grids but it does have a couple of discussions about negative feedback loops. the material on screen grids is really excellent. are your anti-distortion measures still holding up? good luck with your project. (my 210 sixty-five has become a "project amp" lately. I'm gonna be one happy camper when I'm finally able to use it for gigs again!) ed s | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
Thanks for the info and yep, I think it might be fine now, I am getting some other clipping kind of early in the PI section. Funny thing is that it clips the positive going and not the negative going signal. I tried replacing the cathode res which measured high but it still scopes the same. I am begining to think I have a cap leaking DC. But overall, it sounds fine so maybe its nothing to really worry about. | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
andrew, I don't remember whether you said you had (or not) recapped your amp. even if you have renewed your filters, occasionally QC slips up and a bad cap hits the retailer's shelves. (after purchasing such a cap we begin kicking ourselves for buying the cheaper DMM without capacitance tester.... I know I have regretted that momentary lapse into the "penny wise, pound foolish" behavioral set.) if you haven't recapped (all electrolytics--th), you should. if you already have done so it is easy enough to test an electrolytic cap if you have an analog VOM Simpson 260 has saved me a lot of headaches) | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
disregard that scrambled mess of a post, andrew. I accidentally hit the post control whilst in the middle of my proof-reading/editing process. sorry. I'll try again later (after I've stopped cussing my laptop for having "dainty" controls--will they ever make a laptop that a man's hands can use accurately?). e |