 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
68.3.102.14
....filament?What I would like to do with these Heathkit AA-151's I'm modding is have a B+ delay, and in looking at the schematic it seems to me the easiest (lowest voltage) place to accomplish this is to simply delay switching on the filament of the 5V4...
since other secondaries power up all the other filaments, this would not prevent all the other amp tubes from heating up, it would just delay B+ getting to them,
but - I don't know if it would hurt the rectifier tube to have high voltage on it's plates from the HT secondary with a cold filament for about thirty seconds.
Anybody know?
best, charles
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
Follow Ups:
I'm modding the heck out of a Heath AA-32 and faced a similar issue. I have solid state rectification. I'm using an outboard 12V supply for the heaters. I'm going to switch on the heater power supply, which will energize the heaters and start a simple rc timer on the B+ relay. You could do something similar, energizing all the heaters, and then letting a time delay switch on the B+.
Just add an Amperite delay relay for high voltage. There relays come in standard octal or 9 pin envelopes and come in different delay times, from 15 to 45 sec.
.......in this particular case I have a secondary that is dedicated to just the rectifier tube, so I have an opportunity here to delay B+ by just delaying the low voltage to the rectifier filament instead of switching high voltage.yeah, those delay relays are way cool, thanks for the tip, but I have to wonder about what their contacts will look like ten years from now; 400 volts is hard on contacts. Anyone have really long term experience with an amperite delay relay when used with 400 volts?
I think my question is kind of a dumb one, though; the rectifier tubes have high voltage on their plates while their filaments warm up, and it doesn't appear to hurt them, so maybe there's enough space between the plate and cathode in a rectifier tube that cathode stripping doesn't happen.
Also, it's a DHT, but I think it's still coated, so that part probably isn't relevant.
best, charles
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
I use two such relays and they work very well switching on a choke input filter, 0.45 A at 360 VDC. Because contacts are in vacuum, and because they practically work only on turn-on, they do not go bad due to burning/oxidation. The same type of relay is found in electric irons, where they work flawlwssly for many on-off cycles even without vacuum.
thanks, that's very useful information.
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
The 5V4, being an indirectly heated tube, will take about 15 secs to heat up and start conducting. that should be enough for the other tubes, without bothering to delay it.However, if you really do want to delay B+ even further, then your method of switching the heater of the rectifier would be a good one, because it would cause the B+ to rise slowly and cause less stress to capacitors in your smoothing filter. Just don't forget that the cathode is strapped to the heater (at pin 8), so your relay contacts are going to be at the B+ voltage once the tube is warmed up.
....heats up more slowly than the others...or maybe it's that even though it heats up at the same rate, the B+ comes on softly, so cathode stripping doesn't occur anyway...
in either case, I see now - yeah, you're right, I don't need the delay.
Well, how about that - Heathkit knew what they were doing after all!!
Another great reason to use tube diodes, sure saves a lot of trouble. And gives better sonics. Well worth the voltage drop and filament current, I think.
Thanks for the caution about the cathode ending up at B+, good reminder.
best, charles
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
...As you surely must know, the 5V4 isn't the stock AA151 rectifier tube. It's the 5AR4. Unless of course you're using one of those large envelope Phillips ECG 5V4s that are really 5AR4s. The point here is that although both are indirectly heated, the 5V4 warms up in about 11 sec, same as most other IDH tubes thus limited "protection". OTOH, the 5AR4 (even Russian and Chinese) takes 15-20 sec to reach full operation so B+ is truly delayed...not that there is any real benefit to other tubes for this effect.Which causes me to wonder why there is continuing concern about cathode stripping? If one looks to past literature, the primary benefit of IDH rectifiers touted back then was the cost savings in PS caps, not cathode stripping. The delayed B+ allowed electolytic caps and other voltage /cost sensitive parts to be rated at lower voltages, thus lower costs. I've never seen any evidence of tube mfgrs having developed the IDH rec for consumer use for anything other than this...but I haven't seen everything either. Transmitting tubes are another story.
Another thought: If we have concerns about cathode stripping on our beloved signal tubes, why aren't we concerned about cathode stripping of the rectifier?
If it is cathode stripping you are concerned with then you have nothing to worry about.I don't know how this notion got started that putting full voltage on a stone cold tube causes cathode stripping but it is wrong. Many vintage amps did this and it never caused any problems at all, even with solid state diodes. Now don't get me wrong I wouldn't keep full plate voltage on for longgggg periods of time with a cold tube but for start up it isn't a problem.
No doubt many will argue.
Russ
Some people worry about it, others don't; some say it's only a problem above 1,000v; some say it only happens with certain types of cathode material; some say there is a danger of bombarding the cathode with positive ions from the plate. Without deliberate destruction testing by some one, we'll probably never know the whole truth but I prefer to err on the side of caution.An SS bridge rectifier, with its zero voltage drop, followed by a filter that includes a TV damper diode in series (giving 30 second warm-up, soft start and low voltage drop ~10v), seems like good medicine. It delays B+ until the tubes in the amp are good and ready - about 30 seconds or so. In addition, because the voltage through a therminoic diode ramps up slowly, it avoids the severe inrush that you can get with SS rectifiers due to sudden charging of filter caps. Got to be good for the caps, right?
Putting a damper in series with an SS bridge is one way of using it. A better way, which incurs a bit more of a voltage drop, is to use two dampers together with two SS diodes in a hybrid bridge. This has the added advantage of suppressing any switching noise the SS diodes might produce.
Goddamittohell, yeah, 5AR4, not 5V4, didn't know there was a difference in warm-up time.I agree, the only way to settle this confusing mess is some destructive testing; I know damn well I read that in one of the older reference books, but....
perhaps it was referring only to ham transmitter tubes with a kilovolt on the plates....
that would explain a lot.
The problem here is it's all just opinion against opinion.
I guess I'll have to sacrifice an EL84 to get to the bottom of this, but don't anyone hold their breath, I still gotta make a living first.
I do love a mystery, though....(laugh)
best, charles
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
As I mentioned there are several vintage amps that applied full B+ to stone cold tubes. Not a single one of them had shortened tube life.But perhaps a 40 year track record isn't enough testing?
Russ
P.S. Now a power blink of such a duration that allowed the tubes to stay warm enough to conduct but allowed the bias supply voltage to go to zero could cause a problem.
thanks, one less thing to worry about
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]