Negative bias circuit

C Moore · Post by **C Moore** » Sat Jun 04, 2011 3:37 am

http://www.mojotone.com/amp-parts/trans ... -120V-Only

I have the above transformer. I plan on using it in a kind of solid state rectified, 18 watt Marshall. It has Two Brown wires that supply 21 volts for the Blues Juniors relays, switching, op-amp, type circuitry. My amp will nor have that "stuff".
Can I use the wires to build a bias circuit.? Can I ground One wire and use the other wire as a 21 Volt tap, and then construct a negative bias circuit from there.?
Thank you

Post by **Colossal** » Sat Jun 04, 2011 4:02 am

Hi Hand,

Yes, you could use your 21VAC tap. You might set it up using a bridge rectifier to produce about -27VDC or a negative full wave voltage doubler circuit if you need about twice that voltage. How much bias voltage are you needing for your project?

diagrammatiks · Post by **diagrammatiks** » Sat Jun 04, 2011 4:21 am

That tap is used on the blues jr for it's fixed bias circuit. So it should be fine.

C Moore · Post by **C Moore** » Sat Jun 04, 2011 4:44 am

Colossal wrote:Hi Hand,

Yes, you could use your 21VAC tap. You might set it up using a bridge rectifier to produce about -27VDC or a negative full wave voltage doubler circuit if you need about twice that voltage. How much bias voltage are you needing for your project?

I have a great capacity for thinking well beyond my abilities.....
I do not know what I would need, or if it is even a good idea. The amp will pretty much be a 18 Watt Marshall, which is a cat biased circuit. I was thinking, since I have the 21 Volts handy, I could make a fixed bias 18 Watt Marshall. So, I am not even sure if that will get me there. The Fender Blues Junior does it with this PT and with 2x EL84, so I thought I could make it work for a Marshall 18. My amp will be solid state rectified.
Thanks

tubeswell · Post by **tubeswell** » Sat Jun 04, 2011 6:33 am

That secondary should easily provide you with enough bias voltage for EL84s

C Moore · Post by **C Moore** » Sat Jun 04, 2011 7:29 am

tubeswell wrote:That secondary should easily provide you with enough bias voltage for EL84s

I guess my biggest question is how to implement it.
Would I ground one lead and grab 21 Volts off of the other lead.? Or would I use both leads into a full wave rectifier of some sort.?
Thank You

diagrammatiks · Post by **diagrammatiks** » Sat Jun 04, 2011 7:31 am

you can full wave it.

tubeswell · Post by **tubeswell** » Sat Jun 04, 2011 8:22 am

You could use a bridge rectifier. But even just a 1/2 wave rectifier would probably be good enough for biasing EL84 off that winding, because you probably only need -12V to -16V or so (at a fairly crude guess. Anyway a range from -10V to -20V ought to get you wherever you need to be)

Post by **Colossal** » Sat Jun 04, 2011 1:04 pm

Hired Hand,

Here is a bias supply that might work for your needs. I used a bridge rectifier because its performance is much better than the old school half wave but you certainly could use a half wave if you wanted (less diodes). The R and C values are estimated but I ran this in SPICE to get the voltages shown and they are reasonable. Tubeswell's guess of -10 to -20V seems right on the money. I looked up an EL84 tube data sheet and found the grid voltages for AB1 push-pull operation. They suggest -10.4V at 300V on the plates. You will of course need to adjust this for your plate voltages. You don't have to add R2 and C3 but they are considered modern enhancements for safety and less ripple. C3 could be a film cap. TRIM1 could be a little trimmer pot or an external bias pot if you want to adjust your bias from the rear panel with test points.

C Moore · Post by **C Moore** » Sat Jun 04, 2011 1:24 pm

Colossal.....Everybody -
Thank You So Much.
Looks like I am all set. The info is perfect. Kind of funny.....I just gave away a bunch of 100 Volt diodes because I thought I would never have a use for them.
At any rate....this was the last (for now) piece of info I needed to turn my Crate V-18 into some sort of Marshall 18. This will definitely be my first 2x EL84 amp that is fixed bias. I am looking forward to that.
I still do not quite see any big advantage with having external bias points. But maybe I am just old fashioned. Anyway, I will have a trim pot on the inside of the chassis.
Thanks Again

tubeswell · Post by **tubeswell** » Sat Jun 04, 2011 6:34 pm

hired hand wrote:I still do not quite see any big advantage with having external bias points. But maybe I am just old fashioned. Anyway, I will have a trim pot on the inside of the chassis.
Thanks Again

Yes - with cathode bias, I still just clip the meter straight onto the cathode resistor. Altho' I've been putting external bias points in a few fixed bias amps now and they are handy (but it can be a bit fiddly finding the real estate in a smaller chassis). The main reason I do it is to minimise the amount of 'gator clips that I need to have inside the chassis at any one time.

Post by **Colossal** » Sat Jun 04, 2011 7:18 pm

IMHO, I really like external bias test points and bias trimmers for fixed bias and even adjustable cathode biased amps. They are very convenient and especially as an added safety measure if the amp will be for someone else (keeping them <i>out</i> of your chassis when replacing tubes). I usually note the plate voltage and bias mV range on the chassis or in the user notes if the amp is for someone.

C Moore · Post by **C Moore** » Sat Jun 04, 2011 7:52 pm

With externals.....
Do you find that plate voltage remains pretty much constant (static). Even after, lets say, a years time and maybe 4-5 power tube changes that required a bias adjust each time.?
Thanks

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