Damm relay question

[LETHAL]

Help... help  Can some one explain doug's hot switch relay power supply...trying to adapt to jmp marshall. amp has grounded centre tap standard...I done the mod and presto rectifier smoke.Showed it to a mate who is pretty cluey and he said circuit can't work...have to get separate small transformer with no centre tap to power relays.I wan't my input plug for foot switch to be ground on the sleeve(to chassis) because my floor effects looper is ground referanced.

[sluckey]

Doug's circuit with the bridge works fine. But leaving the center tap grounded will smoke the bridge for sure and maybe even the PT. You cannot use the center tap. You must build the circuit exactly as in the schematic, including the artificial center tap (two 100Ω resistors).

[LETHAL]

Thanks...finaly got it through my thick head, so just to clarify...I can't use the heaters filament voltage because jmp has centre tapped tranny....have to use separate power source eg.small tranny, create artificial tap with resistors to supress cycle hum ?

[John]

You can't have the heaters CT grounded. The CT wire should just be taped off, install the 100 ohm resistors as shown, and then you're good to go. Lots of guys use the "elevated ground" to help reduce hum, whether or not they are using the heater wires to power a DC supply. Hope this helps!

[LETHAL]

Thanks John,
Excuse my uneducated electrical knowlege...but if I lift the centre tap off my marshall jmp tranny, I'll get hum... cause that's the ground for the tranny (filaments etc) isn't it. I've got a small non ct tranny I can mount under chassis to supply ac before retification...wouldn't it be a better way to go..or can I achieve
power supply without.  If you can't use 6.3 tap with a ct, what is the differance of using artificial type..it goes to ground aswell..it will smoke the bridge as well ?

[sluckey]

You don't need an extra transformer. Your PT will work just fine. Just remove the center tap lead from chassis and tape it. Connect the two 100Ω resistors as shown. This is the artificial center tap and works just fine to reduce heater hum (maybe even better than the actual center tap). The artificial center tap will not smoke the bridge. You will need to use a new bridge because you done let the smoke out of the one you hooked up wrong.

EDIT...

When we talk about center tap we're talking specifically about the center tap for the heater winding. This CT is totally separate from any CT that's associated with the high voltage winding. The heater CT is the only CT you must disconnect from chassis and tape off.

[John]

Lethal, look halfway down that page and see if the wiring scheme looks familiar. If you have the 2 green wires for heaters with a gr/yel wire going to ground, that gr/yel is the one you want to tape off. Then,  you'll want to cut the green wires between the tranny and the first power tube, and install the 100 ohm on each wire, and ground those resistors. They form the fake center tap. Then of course your green wires continue on to the tubes as normal. Hope this helps.

Also attached little schem for elevating ground on heaters. If Sluckey would proofread to be sure I'm not leading you astray that would also be great! I'm still scratching the surface of what I need to know too.

http://www.el34world.com/Transformers/marshall.htm

[LETHAL]

Thanks Sluckey,John
Understando......
I've got another plexi 50 ceriatone which I wan't to do the same...It has a spare non ct 5v tap (for rectifier) which is not used...Would I remove the Ct on heaters and do same thing or is it a differant set up ?????

[sluckey]

Would I remove the Ct on heaters and do same thing or is it a differant set up ?????

You could but since you have an unused 5VAC winding, why not use that? It would be kinda like using a separate transformer.

If it looks like the PT in the pic below just use the 5V for the relay power supply and dont even mess with the 6.3 heater circuit. And you don't need the artificial center tap in this case. Just Connect the 5VAC leads to the AC input on the bridge.

[LETHAL]

Thanks Sluckey,
For being patient & informitave..
Just one more Question to ask....
My 74 Jmp 100 watt which i"m doing the mod to...in relation to current draw,i've installed a tube fx loop,works great ...no problems.4 x p/tube..4 ax7's  =7.2 amps..by adding relay which draws 30 to 40 ma gives total 7.240 ma....do you think I will have problems considered p/tranny spec(dagnell 2668) is 7 amps????????

Cheers

[sluckey]

Probably not.

Did you get the relay working?

[LETHAL]

Hi Sluckey,
Just got time to work on amp this weekend..

No relay not working...smoked out bridge again..Doing my head in................

1.taped off small ground wire from tranny..(ct Heaters)
junctioned 100 ohm resistors to ac heater lines ..to earth.

Drawn up little diagram of my board..

[John]

Lethal, attached is what I *think* you should be doing with your power supply. The biggest thing I saw was your ground post on the rectifier was hooked to your footswitch. That has to be grounded always.

I drew up the wiring from memory, too lazy to take the bottom off mine and be sure it's how I have it wired,but pretty sure it's drawn right. If not, Sluckey will slap me straight! Also, be sure your diodes across the relay poles are right. The banded ended faces the + of the relay.

If it makes you feel better at all, I bet I used a foot of solder by the time I got mine working right. I've never had anything that is really so simple make me feel so , well, simple. 

Remember, it's not until the - side of the relay is touching ground that current flows through it, activating it.

[LETHAL]

Thanks John...
I'm not electrical minded...just a carpenter.............
I have the relays working now with no smoking bridge.
I lifted the bias ground wire instead of ct...both were green....taped of and done.....
I have the ground post connected to foot switch jack sleeve,which is always grounded on chassis.
I have the twisted ac wires coming from my installed fx tube to save running all the way across amp, Idid as you suggested and installed artificial tap between p/t & first power tube...
Probally something I would like to know...should I have posistioned power supply over in the spare space under p/t.???..where I have put it all on one board other end of chassis as my diagram shows...do you think it might induce noise into audio signal????

[John]

I have the ground post connected to foot switch jack sleeve,which is always grounded on chassis

Ah, got it. Sorry!

I have the twisted ac wires coming from my installed fx tube to save running all the way across amp

I was thinking that after I posted, makes perfect sense 

Glad you got it working! As far as noise goes, you'll just have to try it. I know mine does put in some hum, but I think that's because I have the wires going right over (unshieded) signal wire from the pots, so now I have to change that to shielded. Before I actually finally got the relays to work  the amp was dead quiet. I'm a real newbie at this too.

[sluckey]

Probally something I would like to know...should I have posistioned power supply over in the spare space under p/t.???..where I have put it all on one board other end of chassis as my diagram shows...do you think it might induce noise into audio signal????

Your location sounds logical to me. The PT area is very noisy and you would likely need to run long signal wires to the relays if mounted in the PT area. Keeping the relays as close to the circuit as possible will minimize signal wire lengths and probably minimize noise/hum pickup. And since you already had 6.3VAC in the vicinity it's just logical to use it to power the board rather than run separate wires back to the PT area.

For absolute minimum hum/noise, you could split the power supply (bridge and filter cap) from the relay board. Now mount the power supply near the PT and mount the relay board near the circuit to be switched. Run 2 DC wires between the boards. The benefits of doing this would probably be so slight as to make it not worth the effort. If I was building the relay board, this is the way I would have done it. I'd use a style bridge that can be bolted to the chassis, then solder the filter cap directly to the bridge +/- terminals. Conveniently mount the bridge near the PT and run DC wires to the relay board, which can now be smaller and probably fit closer to the circuit to be switched.

[LETHAL]

Thanks Sluckey,John
You both have been so helpful & I really appreciate it.
Leigh

[LETHAL]

Thanks Sluckey,John
You both have been so helpful & I really appreciate it.
Leigh

Maybe being fussy...
When switching from clean to boost, no pop.. Beautiful.. When switching back to clean I get pop.. Is there a way to swamp pop... If so... Something close to the relay preferably as I use a moem looper to foot switch jack.. Would appreciate any answers ...

[John]

The only thing I can think of is a cap across the switch inside the footswitch itself. The diode across the relay is supposed to eliminate the pop, as I understand it.

I bet Sluckey has a better for you though!

[LETHAL]

Thanks John..
I think it has got something to to with dc potiential.............
tried putting cap across switching poles,tried few meg resistors  to ground  negitive side to  output couplers to switching poles on relay as described on a blog..
seems no matter what i try get pop from going to clean from added gain stage unless quickly mute strings ...no pop ..but that doesn't help in live stage playing as you need to go straight to first count of what ever you are playing...mmmmm stuck with maybe ???

[John]

Did a bit of googling, nothing definite. But if it were me, I'd try 2 caps across the relay poles in parallel. I am guessing 2 500p would be plenty (maybe even too much?), but it will really be trial and error. To make it easier, use alligator clip leads to try 'em so you're not soldering so much. Those relays are a PITA. (for me) Good luck!

[stingray_65]

Do you have a diode across the coil?

You may be hearing the flyback from the coil when it is denergized.

[LETHAL]

Thanks John... I'll give it a go...by the way PITA means ???                                                                                                                                                                                                                                        Put 2.2m  ground referance resistors to output of coupling caps to drain caps on switch side.Might have helped a tiny bit...!

Yes stingray I have a diode across the coil....
switching fantastic appart from pop spike going from gain stage to clean.....
There's go to be a solution in my mind or it's a big effort for a 90% outcome...

[John]

Just to be sure, you have the banded end of the diode pointing towards the positive side of the relay, right? Here is the thread where I was so confused -
http://www.el34world.com/Forum/index.php?topic=11919.msg110396#msg110396

And PITA = Pain in The Ass.