Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: alerich on February 10, 2013, 10:57:03 am
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Relay switching newbie here. I want to add a solo boost to my amp and make it relay switchable via footswitch. I understand all of the other basics but I have one question. If my amp already has DC heaters for V1 and V2 (see schematic) can I just use that to power one 5VDC relay? I don't have the amp open at the moment and I have not measured this DC voltage in the real world yet. Just kicking around ideas. What is the operating range of the 5VDC relay that Doug sells and how close to 5VDC (high or low) does it have to be to operate properly and not damage the coil?
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R47 appears to connect one side of the heater system to C20 which appears to be a +200V(?) DC point.
This circuit snippet is confusing, or drawn incorrectly.
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I have the amp open on the bench and have confirmed that I have 6.2VDC at the heaters for V1. My guess is that the print is not accurate. I say "My guess" because this amp is an odd mix of point to point and small PCB construction. Much of the interconnecting power wiring is laced together in bundles with waxed string. I really don't wish to unbundle it since the amp is otherwise working properly. That point in the circuit is actually about 350VDC (screen supply).
I'm assuming that an extra 40ma from the relay is not going to cook the filament tap on my Mercury PT. Ignoring the print for a moment is 6.2VDC in the safe operating window for a 5VDC relay like the one Doug sells?
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Ignoring the print for a moment is 6.2VDC in the safe operating window for a 5VDC relay like the one Doug sells?
Probably. But just to be safe I'd put a resistor in series with the relay coil.
You need to drop 1.2v @ 40mA, so a 30Ω resistor would be perfect. 33Ω@.5W would be close enough.
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If you are going with OMRON G5A-234P, remember that this little bastards have polarity - check datasheet!! I have struggled all morning and then bum! reversed supply wires and it started working...
Hope this helps
Best Regards
R.
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Thank for the limiting resistor info, Steve. I'll tack it all in and test/verify everything before I hard solder it all.
Good call on the polarity issue, rzenc. With the whole top view/bottom view thing happening I am sure to get things reversed if I am not careful. I am using the OMRON G5A-234P 5VDC relay that Doug stocks.
Thanks for the assist, guys. I'll have it wired up and running by the end of the week - parts enroute - and report back.
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It works! I ended up tapping off of the DC heater string right at the last filter cap for that circuit. It was 6.9VDC so I adjusted the dropping resistor accordingly. It only drops my heater voltage at V1 about 0.4VDC when the relay is engaged. Yeah, I had to futz around with the relay to get it to work. First time learning curve. I recited the Doug Hoffman mantra "If you had wired it correctly it would work" until I got it straight.
I'm using it to implement a solo boost function (see pic below). It really works well. An added benefit was that I eliminated the low input and the dedicated gain control it had on this amp. It's a Sovtek Mig60 that had been modded by Splawn amps in 1998. He added another gain so that the low and high inputs would have independent gain controls. However, he located the high gain control way over by the pilot light - all the way across the amp. Long runs with low level signals traversing power connections = bad. I relocated the high gain control to the spot next to the input jacks where it should be. This shortened the leads to only a few inches. In addition, I cleaned up a lot of dodgy soldering (he must have been in a hurry that day) and replaced a few dodgy original Russkie components. The amp is noticeably quieter now and seems to have a tad more gain. I probably inadvertently corrected a few cold joints. I may even retire my ubiquitous Marshall Guv'nor overdrive pedal.
The amp rocks and the mod turned out great. Thanks for the help, fellas.