Is this a dumb idea......

[TIMBO]

Hi guy's, Is this a dumb idea  :think1:I'm going to need a relay to do some channel switching and I can get 12 and 24v relays at the local electronics store.

But this rules out both the 6.3v and 5v taps I already have.

I was thinking that I have a FIXED bias tapped off one leg of the HT and at the dropping resistor (82k/3w) it drops the HT from 305v down to 192v. If I was to use this AC source and add another resistor and drop it down to a suitable voltage then it could be regulated for either 12 or 24v relays.

Whatya think. Thanks

[6G6]

It sounds doable.

[sluckey]

What is the current requirement for the relay you will use? A bias supply is a very low current circuit and the PT may not provide enough current for the relay.

Although the bias supply is a simple circuit, it is a critical circuit and if it fails it will likely take something else with it. That's reason enough for me to not tack anything else onto a bias supply.

[kagliostro]

As Steve say bias windings usually have low current disposable

if you have a 5v winding for tube rectifier, unused, there you can find enough current to use a voltage multiplier

also with 6.3v winding you can use a voltage multiplier, if there is enough current disposable satisfied the request of current of the tubes

K

[birt]

5V and 6.3V in series should be enough for 12V relays

[alerich]

What is the current requirement for the relay you will use? A bias supply is a very low current circuit and the PT may not provide enough current for the relay.

Although the bias supply is a simple circuit, it is a critical circuit and if it fails it will likely take something else with it. That's reason enough for me to not tack anything else onto a bias supply.

I doesn't sound like he's planning to incorporate this into the bias supply itself but rather to tap off of one side of the PT secondary in the same manner that a Marshall amp taps off the PT to create a bias supply. It would be essentially parallel to the bias supply. It shouldn't impact the bias supply directly. Making sure to calculate and be wary of current is good advice.

[HotBluePlates]

5V and 6.3V in series should be enough for 12V relays

If he has a tube rectifier, that 5v winding is floating at B+ voltage. That = ZAPPED Relay!!!

[birt]

5V and 6.3V in series should be enough for 12V relays

If he has a tube rectifier, that 5v winding is floating at B+ voltage. That = ZAPPED Relay!!!

but he didn't say there is one..

[HotBluePlates]

Fair enough.

My point though is that we might forget that the 5v winding for a directly-heated rectifier (like a 5Y3 or 5U4) or a rectifier with an internal connection from heater to cathode (like a GZ34/5AR4) has the 5vac riding on top of the B+ voltage.

[spacelabstudio]

It wouldn't hurt to draw a schematic so everyone can agree on what you're talking about.   

[TIMBO]

Thanks for all the replays, My computer was down so I have not been able to add any info

I think the relays draw 44mAs
The idea I had was to use a AC supply that is on hand and that the 6.3v (3A) and 5v (2A) were not enough to power a 12v relay and the HT supply could be reduced similar to that of the Bias supply possibly taped off the 82k resistor of the bias supply but if this is a bad idea (For safety reasons not to upset the bias supply ) then if using the other HT leg and by a dropping resistor or two, to reduce it to useable voltage

OR, if the 6.3 and 5v taps ( on tube rect so 5v tap is spare) could be used in a better way, I'm am all EARS 

To use a 12v transformer is not a problem other than adding extra weight, albeit small

But to be able to utilize an on hand supply (safely) would be great. Thanks

 

[alerich]

The only catch I see to this plan is that a bias supply draws negligible current. It is designed to develop a largely static negative voltage. A similar circuit off of the HT leg in that fashion that is going to draw a some current will require higher power ratings on the dropping resistors. Make sure you do all the math beforehand.

[jojokeo]

If it were me I'd likely use a small seperate tranny or design it in a way not tied into the main workings of your amp because all you need is Murphy's Law to come along and it usually does. Especially before or in the middle of a gig. Always think reliability (after safety which is always first). What if the little relays' coil shorts or something else? Goodbye fuse or worse, goodbye gig, and/or embarassingly plug into your mate's other channel (if you can?). All because you wanted to be clever or save a couple bucks. Can you spell "stupid"? Please know that I'm not calling anyone stupid - unless - by definition, you've been told something but do it anyway. Then the definition fits and is warranted.

[TIMBO]

Thanks guys', I'll stick to what works best and wack in a tranny.

[DummyLoad]

Thanks guys', I'll stick to what works best and wack in a tranny.