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Soft start power supply?


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5/23/2000 9:23 PM
Chris Harden
Soft start power supply?
I've just been thinking about the idea of having an automatic soft-start or delay on the HT, rather than having to switch on the Heaters then the HT. Once the amp is switched on, it would either bring up the HT slowly, or wait a fixed time for the heaters to reach opperating temp.  
 
The advantages of this for the design I intend to use is that since I will be using a large filter bank, it will remove the sudden surge at switch-on when the capacitors are completely discharged.  
 
I haven't yet thought about what I would need to switch it back into standby mode during a break, but I'm sure it could be done.  
 
I've seen low-voltage supplies where there is normally a resistor in series with the output of the rectifiers or even the mains, and when the voltage on the capacitors reaches a set level, the resistor is automaticly shorted by a relay, and the supply is ready for use.  
 
Any comments/ideas, it's just something I'm pondering while I'm waiting for bits such as PA valves and OT's to build the amp.  
 
Chris
 
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5/24/2000 11:14 AM
Stephen Conner

That surge can be a killer, KG knows all about it due to his Big Ass Guitar Amplifier (an ear bleeding 600 watt unit using 12 KT90s IIRC) I also had the same problem when building a solid-state hi-fi amp. KG got round it by installing NTC thermistor current limiters. These have a high resistance when cold and go to practically short circuit when hot. They are widely used in switching power supplies for computers. You just put them in series with the mains supply.  
 
I got round it with a more intricate circuit using a resistor in the mains line, which got shorted out by a relay once the filter caps were charged up. By using on and off buttons instead of a switch I was able to have the amp turn itself off automatically in the event of a fault.  
 
The circuit for that is at http://homepages.strath.ac.uk/~cnbp111/amp1.html  
under 'Power Supply'  
 
You need to use quite a substantial resistor though. My hi-fi amp had two 160VA toroids each feeding a pair of 10,000uF filter caps. The 47 ohm 5 watt wirewound resistor I tried exploded quite spectacularly. I finally settled on a 33 ohm 50 watt thick-film resistor with a huge surge rating.  
 
Steve C.
 
5/24/2000 3:53 PM
Chris Harden

Thanks for your input. KG's amp must be the equiv of about 6 Marshall heads in one unit! One is loud enough! I wonder what sorr of speakers he uses, or does he just run most of the power into a dummy load and use a few speakers. You'd need either 8x12in each with 100W handling, which would make for a stack that would need fixing to the ground, or a lot more lower power speakers. It must be on a site somewhere, so I'll have a look.  
 
Your circuit involving the resistor is the sort of thing I am looking at. During standby, the HT would be reduced rather than removed, so that the amp is still kept ready, but the tubes aren't having such a hard time. Would a resistor in the HT work the same, as I wouldn't want the reduced supply to affect the heaters or the bias. I quite like the idea of automated power up, just switch on main power and it'll power up the heaters, then cut in the HT afterwards, then just have a snooze button. It's probably a bit more than is actually necessary, but it'd be a nice touch.  
 
Chris
 
5/24/2000 8:14 PM
Doc

You may be able to accomplish your goal of soft-start B+ by adding a large resistor in series with the rectifier & filter stage, something on the order of 50k to 100k. Across this resistor will be relay contacts to short across after the predescribed timing period. (I've seen a small high voltage disc capacitor placed across the current limiting resistor to absorb the switching transient when the relay contacts closed or opened.) Now all you need is a low voltage timing circuit to energize the relay coil and close the contacts. Timing circuits can be built using a 555 timer IC, or even a single transistor and a slow charging capacitor.  
 
There are thermal time delay relay tubes manufactured by Amperite (numbers like 6NO30), which used to be commonly used in well designed classic equipment. I was going to try a tube circuit using a small rectifier tube, a 6AL5, which would allow relay coil current to ramp up as its cathode heated up. It's 6v heater would be energized with the rest of the amp tubes' heaters, and could have its heater current interrupted by a manual standby switch.  
 
My simple circuit ideas don't provide any built-in protection circuitry, but that probably isn't necessary. Just limiting the peak current through tube rectifiers, limiting the initial surge current to the first filter cap, and applying B+ to tubes after they've warmed up will add much useful life to tube amps.
 
5/24/2000 11:34 PM
Ken Gilbert

hey chris,  
 
quote:
"KG's amp must be the equiv of about 6 Marshall heads in one unit!"
 
 
or you can think of it like two SVT's...  
 
quote:
"One is loud enough! I wonder what sorr of speakers he uses, or does he just run most of the power into a dummy load and use a few speakers."
 
 
oh no, no dummy loads here, i went through that phase about 5 years ago. speakers are now unavailable--they were the eminence 12df from new sensor. 150w pd, 57Hz fs, 3 mm xmax, heavy cones, packed 4 to a marshall jcm900 1960 cab for a total capacity of 600W each cab. still, i don't think the speakers like me. i abuse them a bit!  
 
quote:
"You'd need either 8x12in each with 100W handling, which would make for a stack that would need fixing to the ground, or a lot more lower power speakers."
 
 
the modded cabs, which are also stuffed with about 10 cu ft of fiberglass, weigh considerably more than stock cabs.. 125 pounds each. bitch on the back, lemme tell ya. biceps by ken's rig!  
 
quote:
"It must be on a site somewhere, so I'll have a look."
 
 
let me point it out for you:  
 
http://www.geocities.com/SunsetStrip/Garage/5701/tubestuff.html  
 
go to the sections on the BAGA.  
 
i'm still coaxing more power out of it ;-) next is true pentode CF's to drive the output grids, and paralleling the grid stoppers on the outputs with 100uH inductors... MORE GRID CURRENT!!!  
 
kg
 
5/25/2000 12:57 PM
Chris Harden
BAGA? more like HFFSA
HFFSA = Huge, F**K-off, floor-shaking, stomach-turning, amp.  
 
I've now looked at your amp, and it is some piece of engineering. The only real question which remains is why? Was it just an exercise in 'how loud can I go without the ceiling giving in'? Or do you, or have you done some huge stadium gigs without a PA? With suitable ear protection, what does it _feel_ like to stand in front of a 600W amp? My current experiment has a single 6V6 running into 2x6in speakers, just what I had to hand, and it is loud! Have you ever measured the SPL?  
 
An alternative speaker setup could involve say 2x15in running off 1 side and 4x12 off the other, that would probably give you some scope for low end riffs, esp. if you get hold of a 7 string with the low B. Ever tried it as a bass amp?  
 
I'm sticking to 50+50W with 2xEL34 each side, which should be more than enough. 2x12 (closed back) and 2x10 (open back) should be a good combination I think.  
 
Keep up the good work.  
 
Chris
 
5/25/2000 1:32 PM
kg
hye chris,  
 
quote:
"The only real question which remains is why?"
 
 
well, i USED to use a crest vs900 solid state power amp for my rig. into 2 ohms, that amp cranked out 1100 watts total, and it sounded pretty good--at least i thought so at the time.  
 
eventually i had a lot of problems with the amp and had to send it back to crest. on the way BACK to me it was damaged by RPS, and i had to send it back once again. during this time i had no power amp for my guitar rig, and it was killing me not to be able to play. so i began feverishly working on a mother of all tube power amps.  
 
i based the design on the ampeg svt, for obvious reasons. when i decided on output tubes, there was no problem getting them--that's a different story now: kt90's are like unobtanium. oh well, it's a bummer, but there ain't too much i can do about it.  
 
once i started using this baga, there was NO going back to the SS amp. though it was far less output power on the bench, this tube amp totally obliterated the solid state one in terms of output projection and raw viscous power.  
 
quote:
"Was it just an exercise in 'how loud can I go without the ceiling giving in'?"
 
 
well part of it was that i got a good deal on a matched pair of hammond 1650W's, which are good for at least 300W each. that meant that to take advatage of the OPTs i had to design a circuit of comparable power. and yeah, i like power. ;) my drummer is a seat-of-the-pants kind of guy who plays like an animal, and we've got to be able to contend with one another. (he plays softly too, of course, but somtimes you just gotta let it all hang out.)  
 
quote:
"Or do you, or have you done some huge stadium gigs without a PA?"
 
 
we have done a couple of outdoor gigs, and without any reflective surfaces around, the SPLs disperse quite a bit. you really DO need a loud as hell rig to play outside, especially w/o a pa, which is usually the case. the pa has enough trouble projecting the vocals over the din from the rest of us, nevermind loading it down with instruments!  
 
quote:
"With suitable ear protection, what does it _feel_ like to stand in front of a 600W amp?"
 
 
feel is the operative word. it's not so earbleedingly loud, really... i just like solid bass, and as you know that can suck up a lot of power. i like extended frequency response, and to overcome the natural HF rolloff of the speaks you've got to really pump some voltage down the line... therefore, to avoid HF compression in the amplifier you've got to have adequate headroom. it's certainly not a CONTINUOUS 600W output while passing guitar signal. in fact, since i've decided to leave the voltage-doubled supply in there the initial transients are quite a bit louder than the sustained notes, due to the sag of the 620V rail down to about 580V. believe it or not, the amp is compressing itself. still cranks like a sunufabitch though... everyone who hears it wants one.  
 
next project is a 1kw bass amp for my bass player, probably using svetlana el509's (a very rugged sweep tube with top anode cap). i've got a suitable power tranny already on hand for it, and i will probably go with the same dual output trannies as the BAGA (hammond 1650w's).  
 
spl can get up to 127-130db on the peaks. i don't PLAY that loud, but just for shits and giggles i ran it wide open and had a brave soul stand there with the rat shack SPL meter. do not attempt this at home.  
 
peace,  
kg
 

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