Non CT Bias Supply Questions

[Littlewyan]

I'm building myself an amp that uses a non CT power transformer and so as a result I've had to use a Capacitive Coupled Bias Supply. Now I've read about people having issues with this bias circuit not having voltage when the amp is on standby and I've unfortunately wired up my standby switch into the AC side of the bridge rectifier.....

Anyway, I see that Marshall have also done this on the JCM900 but their's obviously works fine? Is the trick to only switch off one side of the PT Winding with the standby switch?

[ToneMerc]

I'm building myself an amp that uses a non CT power transformer and so as a result I've had to use a Capacitive Coupled Bias Supply. Now I've read about people having issues with this bias circuit not having voltage when the amp is on standby and I've unfortunately wired up my standby switch into the AC side of the bridge rectifier.....

Anyway, I see that Marshall have also done this on the JCM900 but their's obviously works fine? Is the trick to only switch off one side of the PT Winding with the standby switch?

Yes, you should be able to move the input of your bias supply to the hot side of the standby switch, right?

TM

[sluckey]

That bias circuit gets it's ground through the bridge circuit. So moving the bias circuit to the hot side of the switch wont work. The bridge still needs to be connected for the bias circuit to work. If you want bias voltage all the time you need to move the STBY switch to the B+ side of the bridge.

But why are you concerned about not having any bias voltage when the amp also has no B+?

[Littlewyan]

Because the valves will be under strain for a few seconds whilst the bias supply is coming on. I might just put a 47K resistor across the standby switch. It shouldn't give enough voltage for the power valves to conduct but the bias circuit will be on

[sluckey]

JCM900 has a couple 10µF filter caps for the bias supply and the B+ reservoir cap is 100µF (50+50). I'd think the bias voltage would come up faster than the B+. What size caps are you using?

[Littlewyan]

Thats a good point, i'm using the same size caps. Overkill really as its a 6V6 amp running on 340VDC. I thought there was a way of making the bias circuit work off standby though? Putting a 0.1uf cap to ground on the other winding I heard works.

[Littlewyan]

Do I really need to use a class x capacitor? As they tend to fail open which obv would be very bad.

[sluckey]

You may be over-concerned about this. Class X caps are typically used as part of line filters placed on the AC mains. Class X are placed across the hot and neutral. Class Y are placed across hot to ground and/or neutral to ground. The caps are designed to fail open as a safety feature. And they are generally a lot tougher, more reliable than regular coupling caps.

Look at page 6 of this pdf for an explanation of this bias circuit...

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

Regardless of whether the cap fails open or shorted, you will lose bias voltage and put the output tubes at risk of meltdown. If you decide to use a regular cap then get at least a 1KV voltage rating. If I were building this circuit for my own use I'd probably use a regular coupling cap just because I don't have any spare Class X caps laying around. But if I were building an amp to sell I would spend the extra and put a Class X cap in that circuit. Surely if my name was Marshall I'd use the Class X.

[Littlewyan]

I think you're right. This all started because of a Pignose G40V (long story) and that amp has been using a standard poly cap for the bias circuit no problem. I just want to make this amp bullet proof.

I was struggling to find a Class X Cap that isn't massive but now I think I've found one. It has the right voltage rating and the lead spacing is just right for the turrets.

http://www.ebay.co.uk/itm/Capacitors-Fi ... Swux5YSX6X

Also, what is the cap that goes from the positive side of the rectifier to ground for?

[pdf64]

JCM900 has a couple 10µF filter caps for the bias supply and the B+ reservoir cap is 100µF (50+50). I'd think the bias voltage would come up faster than the B+. What size caps are you using?

Bear in mind that the time constants governing the initial voltage rise of the 2 supplies will also be determined by their supply impedances.
In the case of the HT, that will the low, mainly the ESR of the HT winding + rectifier ESR, so maybe 50 ohms max.
Whereas for the bias supply, its ESR will be much higher, the capacitive reactance of the 47nF cap (~56k@60Hz) will dominate.
So the bias reservoir is likely to take longer to charge up than the HT reservoir.
And the bias reservoir has to then charge up the 2nd stage of filtering before the control grids get bias voltage.

But, why implement the clodge of HT standby in the first place? It has all the finesse (and benefit) of the switch Igor throws in Dr Frankenstein's lab.

[rp]

I wouldn't buy a safety part off ebay, many bootlegs supposedly. Why a box cap, aren't these good enough? Unless I'm wrong and got the wrong X/Y cap here, please correct.

http://www.mouser.com/ds/2/427/440lseries-239711.pdf

I need one of these to put on an amp power switch that pops on shut down, right now I just have a 600V 716. TOUGH buggers to find, at least at e-shops, you'd think they'd be common, needed in computers, flat screens, radios - all that modern electronic stuff waiting to become e-waste in land fills. Maybe I answered my own question.

[Littlewyan]

I only installed a standby switch because the faceplate and chassis had cutouts for one and it kept the amp looking standard (Its a 10W JMP Master Volume built into a proper JMP Chassis). However I wanted to make sure I wired it up in the best way possible, hence why I put it between the PT and the Bridge Rect, NOT thinking about the bias circuit, doh. I just thought that would be best for the switch. I wanted to make the amp fool proof in case anyone borrowed it .

I think the best way forward is to either short out one side of the standby or put a 47K resistor across it. Its either that or rewire it so it comes after the first filter cap but this option involves a lot more work.

With regards to the capacitor I now have one that I ordered from RS. I did see some at Mouser but they wanted £12 delivery which seemed ridiculous when the part was only £1! I can post a link for the one I bought if you like? It was a box cap that I used in the end but it wasn't ridiculously huge.

[Phil_S]

Maybe I'm missing your point. Just fill the hole with an unwired switch. If someone borrows the amp, they will turn it on and it will be on. So what if the standby is a fake? They'll figure it out. This is considerably less of a problem than trying to deal with where to put it in the circuit, especially when it isn't needed and probably is not desirable.

[sluckey]

If you want to really get obsessive about protecting this 10 watt homebrew amp, consider using an old thermal time delay relay. Looks like an octal tube. Use the STBY switch to control 120V to relay heater. Wire the relay switch contacts in series with the B+. Using the relay in the ebay link below would allow the bias and tube heaters to start powering up immediately but there will be a 60 second delay before B+ is applied to the tubes. Take a look...

http://www.ebay.com/itm/Amperite-115N06 ... SwcUBYNPqo

[rp]

http://www.ebay.com/itm/Amperite-115N06 ... SwcUBYNPqo

OOO! An other tube - does it glow? Nixies, Magic Eye, 83 Rectifier, and now this thing - be the making of a damn cool Steam Punk Galactic Star Cruiser Amp.