3 Tech Q's

[UltraHookedOnPhonix]

Three tech questions:

1. Am I OK using FR-4 with 1oz/SqFt of copper for the “Precision” PS and Bias/Rectification board? I’m going to use a layout very similar to the one attached.

2. Should I insulate the relay’s voltage regulator from the chassis since the TO-220’s case tab is connected to the ground pin? In looking at pictures gathered from this forum, it doesn’t seem that other builders are doing this. Does it matter?

3. Can I get away with using 1/2W, 470K balancing resistors in the PS?

Any help is greatly appreciated!

[paulster]

2. Should I insulate the relay’s voltage regulator from the chassis since the TO-220’s case tab is connected to the ground pin? In looking at pictures gathered from this forum, it doesn’t seem that other builders are doing this. Does it matter?

If your relay power supply is isolated from other parts of the circuit (i.e. it's on its own transformer or independent secondary winding) then you're fine using the tab directly to the chassis, since it's at 0Vdc potential and it'll provide the ground reference for this part of the circuit.

If you've derived your relay power from a winding that is used for anything else then you've introduced a potential ground loop path as it will be referenced to the chassis ground elsewhere.

[martin manning]

Some good stuff on 1) here:

http://circuitcalculator.com/wordpress/ ... alculator/

Looks like 1-oz x 3mm (0.125 in) width traces should be good for close to 4A on an external surface. Maintain that as a minimum and I think it should be fine, no?

MPM

[ChrisM]

I isolated my regulator with a little piece of mica, nice little insulator.

The middle lug is grounded anyways with a wire. So not insulating the tab you could potentially have a ground loop. Doesn't look from the photos I have seen that the regulator gets isolated though.

[butwhatif]

just a thought--- using the bias winding for the relay supply doesn't have to have regulation-- a relay will work with a pretty wide v range, and are avail in several coil v values . i gen use a pair of resistors -a drop and pull down . never have used a regulator for this, can't see the need

[UltraHookedOnPhonix]

Paulster,

I'm using a auxilliary PT to power the relays and as it is right now I have the Vreg un-insulated, meaning the Vreg tab is bolted to the chassis and the ground pin is tied to a lug on a terminal strip where the PAB & OD Relay coils ground.

ChrisM,

I don't think it's enough to use a piece of Mica since the tab is still in contact with chassis ground through the nut & bolt (unless you've insulated it with a nylon spacer). Aavid Thermalloy have these nifty little kits, Mouser part #'s: 532-4880M, 532-4880 and 532-4880G. I know for a fact that the Ceriatone OTS amps do not insulate the Vreg.

Martin,

Thanks for the URL. I'm doing a 100W 6L6 BM so current draw will be in the neighborhood of about 4.5A. Using the calculator and reading off the, "External Layers" column, I get an required trace width of 2.39mm. Should be enough. Also, if I were to fill in the copper traces with solder, I'd think current handling capacity would increase no?

[ChrisM]

Opps ya sorry, forgot to mention that. I do have a nylon spacer, the regulator is totally isolated from the chassis.

[Structo]

On the supply I built for my relays the regulator is standing vertical with a small heat sink that is made for it.

That way I didn't have to worry about grounding it or having any issues with it touching the chassis.

Remember to use heat sink compound between the tab and what ever you mount it to for good heat transfer.

[glasman]

If you are using a 78XX regulator in a TO220 packege, the mounting tab is ground. I mounted mine (SOD II) straight to the chassis with some heat sink compound. I have seen many commercial pieces where they actually cut off pin 2 as it is redundant with the tab grounded.

Of course don't try this with a 79xx series .

I would recommend the 470K resistors, Allowing the caps to self balance is not desirable. The 470K also act as safety bleeds for the various stages.

Yes they are normally 1/2W. This is fine.

Gary

[glasman]

just a thought--- using the bias winding for the relay supply doesn't have to have regulation-- a relay will work with a pretty wide v range, and are avail in several coil v values . i gen use a pair of resistors -a drop and pull down . never have used a regulator for this, can't see the need

This will probably work, but I would not want to have the relay switching noise on my B+ lines. Also most Bias taps have a pretty low current rated and this might imbalance your PT secondary a bit with all of the relays enabled. Even the low current relays draw about 15mA when enabled.

Just my 2cents.

Gary

[martin manning]

...I'm doing a 100W 6L6 BM so current draw will be in the neighborhood of about 4.5A. Using the calculator and reading off the, "External Layers" column, I get an required trace width of 2.39mm. Should be enough. Also, if I were to fill in the copper traces with solder, I'd think current handling capacity would increase no?

The PC boards are only handling the current from the HT, so you have less than half an amp to deal with normally. Still, I'd think the traces should be sized to blow the line fuse (which is a momentary load), so if they are capable of handling that current continuously there should be lots of margin.

MPM

[UltraHookedOnPhonix]

Guys! Thank you very much. I feel I'm ready to tackle the next stage of my build now. Much appreciated!

I'll certainly post pics n' stuff when I'm further along.

[talbany]

I've been running my 5v relays off a tube rectifier tap..Works fine.. If your building it for yourself I wouldn't worry to much about regulating it..Have done this to several of my personal amps going on a couple years w/ no problems.. If your building to sell I would throw on a regulator for some extra insurance.. I agree w/ Gary don't want it near the B+ or on the filaments though I know Henry does it with much success.. Personal preference..


Tony

[wjdunham]

just a thought--- using the bias winding for the relay supply doesn't have to have regulation-- a relay will work with a pretty wide v range, and are avail in several coil v values . i gen use a pair of resistors -a drop and pull down . never have used a regulator for this, can't see the need

Can you provide a little more info on the exact configuration you're using here - I understand the down sides of using the bias supply for the relays, but I haven't noticed any noise on the builds I've done, and it saves an extra transformer since my PT doesn't have the aux supply taps. I used 24V relays in previous builds as the current draw is low, series resistance of the coil is almost 2.9K. I don't have any more of those and wanted to use the 5V ones I have, but the coil resistance is only 120 Ohms, which would draw 41mA - I'm not sure this is a good idea to have two of these on the bias supply, am I missing something? I'm using a 6V Zener diode to "regulate" the voltage, and things are getting a little warm...
Bill

[heisthl]

I've been running my 5v relays off a tube rectifier tap..Works fine.. If your building it for yourself I wouldn't worry to much about regulating it..Have done this to several of my personal amps going on a couple years w/ no problems.. If your building to sell I would throw on a regulator for some extra insurance.. I agree w/ Gary don't want it near the B+ or on the filaments though I know Henry does it with much success.. Personal preference..


Tony

Yeah an extra 5 or 6.2 VAC winding, connected to a FWB with a larger filter cap (1000uF or my favorite of 2200uF@10volts) for the relay supply referenced to the same ground lug as the transformer C.T. has only 2 parts, no noise, takes up very little space and is extremely reliable for running 5 volt relays. A branch off to a 500 ohm resistor/LED pair can run the pilot LED too. It could be argued that the low parts count makes this method more reliable than a regulator setup. The highest parts count is the regulated voltage doubler method. Besides the extra Radio Shack transformer is a PITA to mount and connect.

If you want to run relays off the existing heaters look at the attached Yamaha/Soldano T50 schematic excerpt - this method works pretty good and you only need 2 diodes, 1 resistor(works as a fuse) and 1 capacitor.