Sweet. Thanks guys. Just for clarity's sake, what we're talking about here is a nice 10w resistor (say, 150 ohms...) wired in between the recto and the first filter cap, effectively dropping the voltage there and everywhere else downstream, correct? If so, then this is perfect as I have a variety of 10w power resistors lying about.
Also, what about this: Hard-wiring in the resistor of the appropriate value (once that is determined by me testing various ones...) and then, running a switch in between the recto and the first cap, so that I can bypass the resistor if I want to bump up the voltages for some kicks? Is this feasible? I mean, wouldn't the resistor be bypassed, even if it is hard wired between the recto and the cap, as the resistence would be less through the switch, right?
dehughes wrote:
Also, what about this: Hard-wiring in the resistor of the appropriate value (once that is determined by me testing various ones...) and then, running a switch in between the recto and the first cap, so that I can bypass the resistor if I want to bump up the voltages for some kicks? Is this feasible? I mean, wouldn't the resistor be bypassed, even if it is hard wired between the recto and the cap, as the resistence would be less through the switch, right?
Yup. That will work fine. Use a good switch and turn things off before switching.
Dan
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dehughes wrote:
Also, what about this: Hard-wiring in the resistor of the appropriate value (once that is determined by me testing various ones...) and then, running a switch in between the recto and the first cap, so that I can bypass the resistor if I want to bump up the voltages for some kicks? Is this feasible? I mean, wouldn't the resistor be bypassed, even if it is hard wired between the recto and the cap, as the resistence would be less through the switch, right?
Yup. That will work fine. Use a good switch and turn things off before switching.
Dan
Right on. And by "turn things off", I assume you mean power the amp completely off, right?
dehughes wrote:Right on. And by "turn things off", I assume you mean power the amp completely off, right?
Thanks!
Yes, I think so. But it may not be written in stone. All you are doing is switching 150 ohms in and out of the circuit so it's probably easier on the switch than a standby application. First couple of times try in with the supply off. Then give it a go without. The more it think about it the less I'm concerned.
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
This is kind of interesting. If you are saying that you have a switch installed in parallel with the sag resistor then you will only be switching the voltage difference between the sag and no sag voltage. In other words if you sag is 30v and your b+ is say 400v then effectivly your only switching from 370 to 400 or 30 exta volts. That would be the extreme because you probably aren't going to see the 30v sag unless you got the amp cranked and wailing on it. If it is just setting there with no input there will be very little sag present. So I guess this is a long winded explanation to say that as long as the switch can handle the same power as your standby switch then there is very little need to shut the amp down or put in in standby before operating your sag switch. I doubt seriously that you will even hear a pop and you surely shouldn't hurt anything.
Dana:
Yea, that's what I began to realize mid sentence in my reply. You are right on the money.
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
I bet it will pop but it depends on layout & lead length and that kind of thing.
Standby just allows you to not hear the pop.
Anyone ever tried using something like a varistor or thermistor in this application? You could put a something like this in parallel with the "sag" resistor to get it to sag more under load ...?
I haven't investigated the availability of suitable components with the correct curves...
Just a cautionary note here: lowering B+ with a resistor in line is fine for lower powered amps that run Class A or heavy AB, like the AC 30 does. This is because the current draw doesn't vary much from idle to full output in these classes of operation. Even so, the lower the B+ voltage drop the better. The resistor will interfere with normal current draw variations.
I would NOT do this on an amp operating in more usual AB1/AB2, especially with fixed bias, because here the current draw normally varies greatly from idle to full power and the resistor magnifies this and can really stress out the PT.
In this case I would connect a large zener diode rated for the voltage drop you need at max current required, from the PT CT to ground. It should be connected forward, not backwards. If you need 50V drop, get a 60 V zener, as they lose about 7 or 8% voltage capability typically in this application.
David:
I like this very much. The hi fi crowd would cringe at this but with a solid state rectifier to begin with, ain't no way you'ld hear it. It will take a heafty zener though. P=IXE. Series 3each 5watt 20V zeners.
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
These usually come in a 220, 247 or similar case size so as long as you have the space to bolt them to the chassis they will work fine. Don't forget the thermal grease.
I am also a "NO SS in signal chain guy" but this doesn't violate that principle!
You're only cutting off half the cycle though. I wonder about the ripple picture. When I finally get my lab set up (in a storage unit, live on a boat) I want to look at this on the scope. Have you done that?
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
To drop the overall voltage in my AC30-inspired amp I will be fine with a hefty power resistor in between the recto and the first filter cap, BUT, if this were a larger amp (say, 50w Marshall, etc...), I'd be better off with a zener diode connected between the PT center tap and ground. Correct?
David Root wrote:Just a cautionary note here: lowering B+ with a resistor in line is fine for lower powered amps that run Class A or heavy AB, like the AC 30 does. This is because the current draw doesn't vary much from idle to full output in these classes of operation. Even so, the lower the B+ voltage drop the better. The resistor will interfere with normal current draw variations.
I would NOT do this on an amp operating in more usual AB1/AB2, especially with fixed bias, because here the current draw normally varies greatly from idle to full power and the resistor magnifies this and can really stress out the PT.
In this case I would connect a large zener diode rated for the voltage drop you need at max current required, from the PT CT to ground. It should be connected forward, not backwards. If you need 50V drop, get a 60 V zener, as they lose about 7 or 8% voltage capability typically in this application.
David
I don't understand this. An amp circuit is full of voltage dropping resistances. A tube rectifier alone can drop 50V - that's what sag is all about. How is this any different?
