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Old 29th September 2014, 02:36 AM   #1
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Default Help me modify Peavey Blazer Amp

I want to make the channel and tone switches footswitchable. I want them to switch in tandem because the "vintage" tone position sounds best w/ the gain channel and the "modern" position with the clean.

They are both controlled with dpdt switches. I am thinking I want to use a 12vdc 4pdt relay (or possible 2 DPDT relays). There is room inside the unit for the relays and I believe there is appropriate voltage to trigger them available (which is where I need help).

I'm not really good with schematics, but the Blazer/ Rage schematic shows several points where 13v is indicated.

Here are my questions:
1) Can I just tap into one of these points to power the relay(s) without it effecting the amp's sound or making noise when switched on/off, or would I need to install a dedicated 12vdc transformer for them.

2) There are dirt cheap 12v relays on ebay that I plan to use. Will 13v be okay with them?

3) If this sounds like it will work without a dedicated transformer, I will also need help identifying the point to tap power from the board.

Thanks for helping a newb...
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Old 29th September 2014, 04:00 AM   #2
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Don't use the 13V, you could end up drawing more power than the circuit is designed to provide. A 4-pole relay is likely to be fairly big and draw a lot of current (comparatively).

If you make a mistake you could kill the amp; if you use a transformer, you're just diverting some wiring from some switches. The worst thing you will blow up if you get a short in the foot switch is the transformer. Wire in a fuse to prevent even that.

If you pick the right relays you could probably get away with it, the 13V is no problem, but it'll be safer to go with the transformer.
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Old 29th September 2014, 04:16 PM   #3
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It will likely pop when you switch.
I'd use led-LDR switchers, that change slow enough to not pop. You need to have a SPDT foot switch at least to energize two leds on one side and two leds on the other side. The CDS cell you use like a contact. You need some DC source to energize the LED, 10-20 ma each, which is low enough you might not need a new transformer. Or you can run a wall transformer in your foot switch with the right power socket. You need to calculate series resistor values for the LED's, say
((DC PS voltage)-1.5v)/.02amp=R.
One source of these is the Eagle Power International LCR-0202 which goes < 1kohm on (20 ma in) I think somebody said alibaba sells these. Customs duties overseas freight and long delivery from China.
Or the Excelitas vactrol VTL5C4 which goes 1.2 kohm at 20 ma input current. I think Newark.com stocks these. The VTL5C3 has lower resistance.
You sure don't want to switch relays using an analog power supply for op amps or high gain transistors, the PS inside the guitar amp. The turn off spike might blow up the high gain inputs and for sure will make a huge pop. the "answer" back emf diode across the coil limits the pop to 1 microsecond, which is way audible and can still blow up stuff.
Don't hang component connections out in the air, they will short against something or break. I use Cinch solder terminal strips for this sort of project, you'll need a 6 terminal one in the amp at least and another in the pedal. These are rare, sold by tubesandmore.com(AZ), apexelectronic.com(CA), triodelectronics.com(IL), electronicsurplus.com(NY),mcmelectronics.com some of which charge $10 freight minimum, apex and mcm $15. The first four sell little coils of tin plated wire which you can't get from newark or mcm, they are not ROHS compatible (for europe). Get your screws and elastic stop nuts from one of your major suppliers to save freight, #6 machine US or 3 mm metric. Newark charges me $6 freight, if you get a residence charge specify USPS which doesn't charge residence fees.
Good luck.
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Last edited by indianajo; 29th September 2014 at 04:28 PM.
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Old 30th September 2014, 02:46 AM   #4
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I don't think there will be a popping problem from the relays switching. They will simply be replacing the toggle switches on the front of the amp, which do not pop. I was more thinking that tapping into the circuit board for power might cause a pop when the relays are energized or de-energized. If I install a dedicated transformer for the relays, that will take care of the problem, unless someone here comes along and assures me that they can walk me through getting power from the board and it will work and not make noise.

My footswitch will just be a spst switch that completes the circuit to allow the 12vdc to energize the relays.
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