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In Reply to: RE: Thanks posted by Awe-d-o-file on July 30, 2015 at 17:27:23
But, the 1570A were lighter metallic green, like the 340/350, with wider bandwidth opt iron. The pairs I have seen in print or online were "tabletop" looking without their rack faces. Apparently, they may have sat on shelves inside some Altec racks of yesteryears.
The 1570B or 1570BT can easily be modded for better than stock bandwidth; even with the B iron. They run cool but do have a bit of mechanical buzz when nearby. As they run 400 watts each, turning them on can dim your lights. When it comes to "huge" sounding amps, airy and open sounding with fantastic soundstage, the 1570s are sweet contenders...
Follow Ups:
Oh yeah????!!!
Somewhere, I have four chassis stashed away. What scared me off was the B+ of 900 volts. In fact, I remember the schematic stating the primary power winding should be set at 115 volts unless the B+ was too low. Where the hell did I put those behemoths now?
In fact I remember one theater tech telling me he got shocked and the amp wasn't on and his finger was about a 1/4 inch away from the cap. Of course he may have been pulling my leg, but I was impressed by the very high voltages. Still have a bunch of oil caps to rebuild them.
Got them from a local drive in theater when they closed down. BTW.
Mine had the traditional Altec colors not the hammertones of the 340/350, which I love appearance wise and would love to find a duplicate. Even the automotive paint suppliers could not match the color, BTW, at least not in my area.
very small output iron for such large power. I guess high voltage conquers the need for the large iron.
They used smaller than usual opts for two reasons, IMHO. First, they did not expect us hifi guys to use them for the lowest bass. Then, consider that smaller iron makes it easier to get the highs right. Smaller iron means smaller coils; thus easier to fabricate windings with less reactances.
You are still researching iron, right ? Think of the finest large opt iron that has extended yet not overly bright highs. Immediately, I think of the Chicago 32007 on the HF-50, Heath W6 opt by Peerless, Scott LK-150 by Stancor or Magnetic Windings, HK Citation II by Freed or Magnetic Windings...there really are only a few choice large classics...
Another lovely aspect of the 1570 is low overall feedback, allowing it to yield a HUGE soundstage. I am not usually a fan of cathode follower drivers; but this amp can be forgiven of using CFs and missing the lowest low bass...
Concerning HV, SAFETY FIRST ! Use only one hand while probing with a test lead; preferably with leather, pigskin or deerskin gloves on your hands(or genuine HV gauntlets). Yes, keep one hand in your pocket. Before even considering working underneath, you MUST discharge the B+ HV rails. Your first HV shock can be your last and a small (low current but high voltage)jolt can kill you easily. Research safe work practices with HV devices...Remember not to lean the chassis on any body parts while testing. Shirts are not good insulation. Sweaty body parts mean your body is less resistive. When I guide apprentices, I remind them not to get caught up in the theory of how voltage/current flows; but, to remember not to allow any body part to become the conductor of any electrical current. With AC, there is a "flinch" response possible sometimes. With DC, your body part gets "pulled in." Safe practices are essential. BE AWARE ! BE CAREFUL...but do restore those 1570s when you find them...
One zap at a 1000 volts makes any further warning unnecessary. The day I did that accidentally, my meter ( the probe slipped) was flung 25 feet away and I lay flat on my back, with my left arm trembling for 15 minutes....I now fully understand why those military oil filled caps have the leads all shorted out.
As a more humorous (luckily) aside a friend, a Raytheon electronics tech who loves tubes, built a couple of DIY projects including a preamp loaded with oil filled caps. He moved cross country to New Hampshire and took a month of side trips while doing so, so he felt the caps would discharge by then, especially as it was built up and in circuit.
At any rate after opening up the unit to double check the rigors of moving he brushed against the caps and received a big jolt. Pissed he took a screw driver and jammed it across the terminals figuring he had already discharged most of the voltage. Imagine his surprise when the screw driver welded itself to the terminals. His meters had yet to be unpacked, in his defense.
Still high voltage and oil filled caps some big no-nos... A big discharge resistor which remains in place as those damned oil filled caps rebuild their charge is a necessity....
As for your trannies, I do agree, although I do not have any Scott gear. I have a W-6 and am so impressed with them particularly with its bass control. Kills a Mac 275 and a Marantz 9 IMHO.
Thanks for your memories and warning, wish someone gave me those warnings early on....
Edits: 08/02/15
In looking at the schematic for the 1570, I am ( only now) struck by the fact they use a power tube to drive the 811's , which I know need a lot of current to achieve power
Still in thinking about older designs, I am struck by the fact that many used small pentodes as drivers and sounded very good. This modern trend of all triode design, believe, bears a closer looking into. Certainly even the Yamamoto amp ues the WE 717 pentode a=to drive the 300B.
I am of the believe the extra gain is beneficial for the low Mu ouptus
The 811s are positively biased; between +1 and +2.2 on the grids in 1570s !
Class A amps are negatively biased and most A1 amps simply require voltage gain. That is why we have seen the rebirth of WE 91A type front ends with pentodes driving triodes. Since the late '80s, French and Asian designers have spoken/written how the distortion products of pentodes are complimentary to the triode opts' distortion. Be that as it may, I favor all triodes....
Power drive is always a good thing; like having "reserve power" for loud passages. Power drive requires low grid impedance; hence cathode followers or interstage trannies with 1:1 or Step-Down Ratios. Have you seen the Japanese MJ or other experimenter books/magazines ? Every issue seems to have had at least one amp, from over a dozen schematics per issue, with a SE triode amp driven by a power tube or a cathode follower. Some have feedback; even around an IT ! Why this topology ? Because these tricks help an opt tube to give a bit more output. When we are dealing with a flea power amp, it can sound better if an extra watt or two can be pushed on signal peaks.
Having tried many topologies for SE or PP, including over a decade with Class A PP triodes using a step-up intertsage tranny, I now have almost two decades using Power Drive with 2A3s or 45s. Just as an aside, the absolute world reference triode amps through the years was/is Brook ! Their supreme 30 Watter uses power drive. The 1950 Peerless A-100 amp that pushes 18W out of back-biased PP 6B4s, also uses power drive; with a pentode as the amp input tube...thumb through those magazines my friend...online research is inadequate...ENJOY !
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