Hi all, I'm new here, and this is my first post - hope I don't break any of the forum rules!
I've put an amp together before - essentially a rebuild of a stock 1973 Marshall Superlead, but want to do another, based on the 50w Plexi circuit but with an additional saturation channel and a few other extra bits and pieces...
The first of which is going to be a soft start - or at least a softer start than usual. I was also going to put a 'sag' resistor in the main HT line, and hit on the idea of combining the two, and making the sag switchable via a relay which will engage the resistor automatically when the amp fires up, but is switchable after a second or so
Does that sound feasible, or is the relay concept a bit of a non-starter, given the voltages involved... Hopefully the video will explain things a bit better... It seems to make sense, but am wondering if I'm missing something... Or maybe a better way to do it (and why, so that I can learn a bit too)
Thanks!
http://smg.photobucket.com/user/walkdev/media/Amp Build/PSU_zps1nexjcpp.mp4.html
I've put an amp together before - essentially a rebuild of a stock 1973 Marshall Superlead, but want to do another, based on the 50w Plexi circuit but with an additional saturation channel and a few other extra bits and pieces...
The first of which is going to be a soft start - or at least a softer start than usual. I was also going to put a 'sag' resistor in the main HT line, and hit on the idea of combining the two, and making the sag switchable via a relay which will engage the resistor automatically when the amp fires up, but is switchable after a second or so
Does that sound feasible, or is the relay concept a bit of a non-starter, given the voltages involved... Hopefully the video will explain things a bit better... It seems to make sense, but am wondering if I'm missing something... Or maybe a better way to do it (and why, so that I can learn a bit too)
Thanks!
http://smg.photobucket.com/user/walkdev/media/Amp Build/PSU_zps1nexjcpp.mp4.html
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Chris, it sounds feasible to me, but I have a couple of suggestions that will make the relay more likely to survive.
Wire the "sag resistor" permanently in the B+ line (not switched in, as shown in your schematic), so that if the relay never operated, you'd always have a soft start, and a "saggy" power supply.
Then wire the relay N/O contacts in parallel with the "sag resistor", so that when the relay is operated, it shorts out the sag resistor. (Or, if you choose, shorts out part of it, to leave some "sag").
In this way, there will never be a huge DC voltage across the relay contacts. The relay will only ever "see" the voltage drop across the sag resistor. This makes it much less likely that the resistor contacts will arc and burn.
The other issue I see has to do with the amount of delay. I'm skeptical about using an RC combination (as shown in your schematic) to delay the relay enough for your purposes. I think you need tens of seconds of delay, to allow for heater warm up, right? At the same time, the resistor in series with the relay coil has to be fairly small, to allow enough current to operate the relay. That in turn means you'll need an enormous capacitor to get enough delay - probably too enormous to be practical.
This is a problem that is fairly easily solved, though. Since you'll already have a regulated DC voltage, you can add a simple delay circuit using a 555 timer IC or something similar, to operate the relay after the delay you want.
-Gnobuddy
Wire the "sag resistor" permanently in the B+ line (not switched in, as shown in your schematic), so that if the relay never operated, you'd always have a soft start, and a "saggy" power supply.
Then wire the relay N/O contacts in parallel with the "sag resistor", so that when the relay is operated, it shorts out the sag resistor. (Or, if you choose, shorts out part of it, to leave some "sag").
In this way, there will never be a huge DC voltage across the relay contacts. The relay will only ever "see" the voltage drop across the sag resistor. This makes it much less likely that the resistor contacts will arc and burn.
The other issue I see has to do with the amount of delay. I'm skeptical about using an RC combination (as shown in your schematic) to delay the relay enough for your purposes. I think you need tens of seconds of delay, to allow for heater warm up, right? At the same time, the resistor in series with the relay coil has to be fairly small, to allow enough current to operate the relay. That in turn means you'll need an enormous capacitor to get enough delay - probably too enormous to be practical.
This is a problem that is fairly easily solved, though. Since you'll already have a regulated DC voltage, you can add a simple delay circuit using a 555 timer IC or something similar, to operate the relay after the delay you want.
-Gnobuddy
My personal view is this is a solution looking for a problem, but taking it at face value, I see two issues.
WHy use your sag feature as a soft start? Soft starts have not been in guitar amps for most of the last 60 years or more. they show up more in the last decade than in previous. But where they do exist, a simple thermistor does the trick neatly and with no relay or support circuitry. A series thermistor in the mains lead does it. Say 100 ohms cold which drops to an ohm or less hot.
First and second generation Bugera heads have a handfull of them in series on a small board with the IEC connector. I find that lacking in elegance, but it work just fine. recent era Fender amps like the Hot Rod DeVille use a C60-11 thermistor. never have problems with them.
Now you can go about implementing your sag feature as an element of amp dynamics without worrying about it also doing double duty. For instance a simple panel switch can turn it off and on, unless you want it to be a remote feature controlled by a pedal. I have added sag resistors to amps, but the effect never rose to the level of "effect" like I'd want it on a footswitch. Like the rectifier selector on the rear of some Fender amps and some others. It usually is turned one way and left.
WHy use your sag feature as a soft start? Soft starts have not been in guitar amps for most of the last 60 years or more. they show up more in the last decade than in previous. But where they do exist, a simple thermistor does the trick neatly and with no relay or support circuitry. A series thermistor in the mains lead does it. Say 100 ohms cold which drops to an ohm or less hot.
First and second generation Bugera heads have a handfull of them in series on a small board with the IEC connector. I find that lacking in elegance, but it work just fine. recent era Fender amps like the Hot Rod DeVille use a C60-11 thermistor. never have problems with them.
Now you can go about implementing your sag feature as an element of amp dynamics without worrying about it also doing double duty. For instance a simple panel switch can turn it off and on, unless you want it to be a remote feature controlled by a pedal. I have added sag resistors to amps, but the effect never rose to the level of "effect" like I'd want it on a footswitch. Like the rectifier selector on the rear of some Fender amps and some others. It usually is turned one way and left.
I think that describes a good 50% of the projects we discuss here on diyAudio, no? 😀
-Gnobuddy
Thanks very much, Gnobuddy! The delay that you mention is more of a standby thing. Probably loads of threads on here about that (I know I've seen one or two already!), but my understanding is that it doesn't really do much to save the tubes as cathode stripping doesn't happen at the plate voltages in guitar amps; cathode poisoning can happen if the tubes are left on standby for too long. That's just going on what I've read (KOC, Aiken, etc) I know some manufacturers shift their standby after B+ (Ceriatone for one), so I might do something along those lines...
Thanks very much, Enzo! I don't really know anything about thermistors, and have certainly never used one. I'm guessing that the time they take to heat up is in the region of seconds, rather than milliseconds. In your experience, what would be the best way of switching them back out of the circuit once they're hot, so that they get a chance to cool down while the amp is still working?
You don't switch them out. They sit there conducting. They heat themselves just as tubes do. Tubes take a few seconds to heat up, and the thermistor will be up and warm before they come on.
Look at the schematic for a Fender Hot Rod DeVille, the mains circuit is utterly basic including the thermistor.
Look at the schematic for a Fender Hot Rod DeVille, the mains circuit is utterly basic including the thermistor.
I thought you wanted to delay the B+ voltage until the filaments were up to temperature, more or less. IIRC those series-heater TV tubes were specified to take about 10 seconds to warm up.
IMO, Enzo's suggestion is an absolutely excellent one. These NTC thermistors start out cold and high-resistance, providing the slow start you wanted. As the milliseconds tick by, the thermistors get hotter, and their resistance falls. In a short time, they're at full temperature, and their resistance is so low that they have essentially no further effect on your amp.
Simple, straightforward, already proven to work. What more could you possibly ask for? 😀
-Gnobuddy
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