Adding 5V Supply

[chipaudette]

Hi All,

I've got a 65 Deluxe Reverb Re-Issue that I've been modding (for fun) to give me a high-gain sound by cascading the "Normal" and "Vibrato" channels. It's working out pretty well, but when switching between clean and overdrive sounds, I've got to flip 3 different switches on my amp. I'd like to have it operate on just one switch. So I'm thinking about replacing those switches with relays.

Not knowing much about relays, I found and purchased a couple of Omron signal relays. They're DC relays and draw 40 mA when active. Having 125 Ohm resistance, they need about 5V each. My problem is, my amp has no low voltage power supply rail. What is the best method for getting the supply that I need to operate the relays?

The schematic to my amp is below...

http://www.fender.com/support/amp_schematics/pdfs/65_Deluxe_Reverb_Schematic.pdf


Notice that I've got a bunch of high voltage DC rails (B+, X, Y, and Z) and one lower voltage DC rail (C-, I think it's DC). I need two relays, so 80 mA total. That can be a lot of current if the output resistance of the power supply rail is high.

Any suggestions?

Chip

[Enzo]

I think I might have chosen higher voltage relays like 24v or even 12v.

WHy not take that red/blue wire that serves the bias supply and use it? Add a diode at CP10, I would say pointing the other way, but really, the relay doesn't care about polarity. On the other hand making it positive means any current peaks would be on the phase not used by the bias rail. FIlter it and drop it to what you need. A tenth of an amp ought not to challenge the winding.

Are both these relays operating together? if so, run them in series, use the same current for both. That drops you to 40ma, plus you wouldn't need to drop the supply to 5v, you could leave it at 10v.

[chipaudette]

Is it advisable to pull off my current instead at the point labeled "C-"? It's at -50VDC and looks to be filtered a bit already. I'd need fewer parts if I tap in here.

If it's solid at -50VDC (if!) and if I put my two relays in series, then I need 10V across my relays. The two relays in series have a total resistance of 250 Ohms. Therefore, I guess I'd need to tap off the C- point with a (50V / 10V * 250 Ohms) = 1250 Ohm resistor.

If I understand things correctly, when I close my one user switch to connect the output of this new resistor to my new relays, the voltage should drop through the resistor until I get my 10V and 40 mA through my two relays.

It all hinges on the supply capabilities (and ripple?) of the C- point. My ability to analyze power supply circuits is poor, so I can't judge the ability of C- to deliver. What do you see?

Thanks,

Chip

[Enzo]

Yes, it is already rectified and filtered, but saving parts is minimal, one 5 cent rectifier diode and another 60 cent cap.

The reason I would make a separate supply for it is the isolation. When the relays are on, even that small current will make ripple, and if any of it gets into the bias rail, it can be amplified by the power tubes,

Remember to add a reverse biased diode across the relay coils. It can span the both in series. This will prevent transients when the relay de-energizes and the coil field collapses. And you don't want them on your bias either.

[chipaudette]

The problem with parts is not cost but the problem is availability. Resistors I can get locally, but caps I can't. I have a bunch of small-valued caps on-hand, but filter-size caps are a different story. Therefore, I'll have to order some. That'll take a week. I hate that. But, your arguments are convincing so i suppose that I can wait. I'm just such an impatient person.

How big a cap would you recommend for filtering?

Chip

[Enzo]

I wouldn't think it was critical. WHat is the larger size you have on hand? Use it and see if it falls short. You can replace it later with something larger.

Where do they sell resistors but not caps?

You don't need to make a filtered 50v - or whatever - supply to start with, you can drop voltage after the rectifier with a resistor and then filter a lower voltage. SO if you have lower voltage caps on hand, that might help?

[jrfrond]

I agree with Enzo. 24V relays are a better choice. In this case, you could take that 50VDC (or so) supply an create a 2:1 voltage divider across the output of that supply using a couple of 270?/3W resistors, and tapping 25VDC off the center of the two. That's close enough, but you could trim one of the resistors higher or lower in you are anal about it. All you really need is two 1N4007 diodes, a filter cap (100uF/50V) and two resistors. Don't forget a couple of terminal strips to hold the whole she-bang.