|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
66.167.213.95
In Reply to: Certainly,.. Try 'Em, You Might Like the Dukes... posted by Interstage Tranny on March 13, 2007 at 07:19:12:
...This type of info is more encouraging. Thanks.A point of clarification: I don't have anything against corrective feedback as such if properly applied and not used to compensate for marginal open loop performance. I do become concerned when it is applied globally across more than say three stages. In the 475, technically, five stages are involved although the fourth stage is a split load phase inverter with gain <1. The amp also employes the traditional phase compensation network at the output of the second stage so combined with the output snubbers and direct coupling for two of the stages, maybe stability is OK. Hope to find out soon enough.
Regarding age of the 475, the date in title block of the schematic I have is blurry but looks like 2-25-61 or 64. I tend to think the 61 date is correct because I recall seeing a rack of four of these amps in the auditorium of the Jr. High I attended which was built in 61 IIRC.
Follow Ups:
Steve O, regarding "BTW, as a result of this discussion, I'm going to fire up and verify operation of one of the 475s. If all is OK I'm going to do some objective measurements and then AB it with a SS amp of decent quality on a pair of difficult-to-drive moving coil monitors. I don't have great expectations but who knows?"...Did you ever get to doing this? I'd love to hear your thoughts. I personally feel this amp will do better as a bass amp actually then a midrange amp in my personal case, in my migration towards tri-amping my horns.
Hello there, I'm glad that info was helpful. Since this amp can sometimes sound hot up top, with certain systems, I would not remove the snubber or compensation networks. Try the amps stock, first, at least. With a light Variac warm-up, they usually work, even if stored for long time periods. They were overbuilt.
Some of the versions I have seen use some questionable grade coupling caps. Those can certainly be upgraded. I would beef up the first power supply cap, which is after the input filter choke. That will give the bass some more speed and punch ability.
Speaking of smaller output transformers, sometimes the smaller ones have better high frequency extension, than their larger brethren. This might be because the smaller coils tend to yield less interwinding capacitances and leakage inductances, thus less high filtering. Whatever the reasons, sometimes the smaller ones are good ones. In this amp, the PPParallel tubes have a plate to plate load that is low, which is another justification for a smallish output, not needing high primary inductance. Let us know what you hear and Good Luck bringing these heavyweight brutes back to life...
Regarding mention of "with a light Variac warm-up, they usually work"... I'm hoping to learn under what conditions should that be done? Only if the amp hasn't been used for a long time, or are there other situations warranting this? (I asked this in my Bogen thread so please ignore here if it was answered there... thanx much everyone for all this info)
This post is made possible by the generous support of people like you and our sponsors: