Bias voltage source - HV or 50vac tap

[pjd3]

Happy Sunday all,

Would you think that there is a benefit (or not) to either tapping off one of the HV coil ends OR, to have a PT that provides a 50vac tap?

I was just now getting ready to solder in the resistors, diodes, electrolytic caps that make up the bias circuit in Sluckeys Plexi and realized that I would or could be ordering soon a PT for a Fender Deluxe Reverb (since the output section of this amp is pretty much just that).

I'm leaning towards just going forward just as is with Sluckeys 6V6 schematic since the circuit, board layout and anything else is already set of for tapping the bias off a 330vac HV. And I could see about ordering a PT, same specs but without a dedicated 50vac bias tap. The worse it could be is that a 50vac tap gets unused which aint the end of the world I guess.

What would you do?
thanks,
PJD3

[wpaulvogel]

The only advantage I can see with a separate tap is that the bias supply can be easily put somewhere else in the chassis. Someone else may have a good reason.

[pjd3]

Thanks wpaul,

I'm tending to think that what ever draw (if much at all) is coming from the HV coil probably isn't critical since its coming from the same or similar place.

But sure, like you said, lets see if anyone else has an angle on that.

PJD3

[wpaulvogel]

If my transformers have a bias tap, I use it. Most of the time I spec my transformers and I don’t include the tap. Sometimes I use shelf stock and then I get what I get.

[pjd3]

Thanks wpaul,

And that makes alot of sense. My rumination with this is as long as there are no detriments to tapping off an HV (red - as in the Sluckey plexi design) as compared to using a dedicated 50vac bias tap then I'm fine in keeping with whats already designed into the board, the resistors, caps and layout that's already there to accept the 330vac - just to keep things clean and not have to re-design things. Not that that would be a very difficult thing, it would probably be as simple as bypassing the resistor that is bringing down the 330vac to and acceptable voltage level for the rest of the bias circuit to be in range with. I think its just a 220K metal oxide.

IN trying to keep cost down, I found a classic tone PT for Deluxe reverb that's 66 bucks. That will be an easier battle to win with the wife than some of the other ones that look good for this amp. It seems that once you get into the multiple primary taps the cost starts to go much higher which would be expected.


Thanks!
PJD3

[wpaulvogel]

Just a note if you’re getting the Classictone power transformer. I’ve used their Trainwreck output transformer 6.6k, it’s much better than the Deluxe Reverb output transformer. I used the TW transformer with 6V6 and 6L6GC. The power transformer has enough gas to support 30+ watts.

[M Fowler]

Those TW transformer the PT are rated for 300mA enough to power a 60w amp easy. Same for the TW OT's.

It doesn't matter if you shrink tube the bias tap and then create the circuit Slucky shows off one side of the HT.
It's your choice no problems.

Mark

[sluckey]

Guess why I used one side of the HT secondary for the bias circuit.

[pjd3]

Thanks everyone, you guys rock.

I'm so confident now, that I'm going to wire all of the HV wires together and tap off all of them!!!!.............Kiddin.

Sluckeys 330vac tap it is.

Thank you
PJD3

[frankdrebin]

There's a reason for a dedicated low impedance winding

[sluckey]

Please tell.

[martin manning]

It can be made to work either way but, as frankdrebin hints, the large dropping resistance required for HT-derived bias makes it a high impedance source. That means it needs a couple more parts (another RC filter) to make a bias supply that has both low ripple and reaches the required negative voltage quickly. If the main rectifier is a FWB, then yet another capacitor is required. Some people like to add a couple more parts, and draw from both legs of the HT.

[pjd3]

Hey thanks all for some good insight responses there.

I guess since its all set up for a single HV tap I'll go with that. If in doing that the desirable qualities of the HV and bias remain intact then I'm good letting it be.

I'll try not to lose sleep when the amps all done and I know there may be a wasted 50vac tap. Things could be worse - like not having a $600.00 AC power cord.

Thanks all, good information.
PJD3

[sluckey]

Jim Marshall made a fortune with that simple, reliable bias circuit.

[pjd3]

I bet he did.

Its an interesting thing. That when someone is involved with a very high volume yield and lots of employees salary to pay, the driving principles must be very different than being say, just a guy like me in local cover bands that tinkers on "one-offs".
The good thing about being a guy like me is that I can buy all kinds of ridiculous snake oil caps and voodoo majic cloth wire, and all I have to sacrifice is a pizza or two. Good deal.

PJD3