ggjgjhkihkjhiuyiy8788 7/26/2018 5:53 PM
Hiwatt Bias mod - an alternative idea
I have a Hiwatt DR103 on my workbench with the usual anaemic bias supply issues; squeezing every last drop out of the circuit gives me -38 to -40VDC, which gives borderline bias levels of 70-80% max dissipation, depending on EL34 age and brand.

I know the common procedure seems to be modding the circuit for a voltage doubler, but it seemed like excessive modification when a few more volts is all that's really needed. My simple solution was to lift the grounded leg of the bias tap and attach it to one side of the heater/filament supply. That boosts the bias supply by 3VAC, which equates to about 5VDC more on the grids, which actually shifts things into cold-ish bias (~60%). Then all I needed to do was swap out the ground resistor with a resistor+trimmer arrangement so I could dial the bias back to the usual 70% of max.

I've done some searching and haven't been able to find an example of anyone doing this before, which makes me wonder if I'm overlooking something that makes it a bad idea? It certainly seems to work and sound fine, but I'd appreciate any feedback on whether my idea is clever in its simplicity or stupid in its ignorance.

Thanks,
J
 
Leo_Gnardo 7/26/2018 6:05 PM
Points for originality! I have used the voltage doubler solution a couple of times, Hiwatts and Sound City. Looking forward to comments from other techs on your unique technique.
 
The Dude 7/26/2018 6:57 PM
Agreed on points for originality. I've never considered this. That said, I can't think of any pitfalls or reasons "why not" off the top of my head. The bias supply is nearly "currentless" so there wouldn't be any additional stress on the filament winding.
 
cerrem 7/26/2018 7:01 PM
You might need a HUM pot to re-balance the "Center-Point" of the heaters to minimize hum...
For a Half-Wave Voltage Doubler....you just need another cap and diode...
 
The Dude 7/26/2018 7:16 PM
Here's a schematic for reference.

http://music-electronics-forum.com/a...7&d=1444568031

It doesn't show the heater supply, but, if memory serves, I believe it's a center tapped winding. I don't think you'd have to do anything with the center point. since the filament supply stays effectively the same.
 
Leo_Gnardo 7/26/2018 7:54 PM
Quote Originally Posted by cerrem View Post
You might need a HUM pot to re-balance the "Center-Point" of the heaters to minimize hum...
For a Half-Wave Voltage Doubler....you just need another cap and diode...
I hardly think ggjg's circuit would steal enough current from the filament supply to unbalance it enough to cause more hum than already exists. BUT, if at some point he wanted to install a humdinger pot or float the filaments on DC then his scheme would have to be abandoned. If it's working fine as is, I say go with it. Kiwi technology - it's different but by golly it works!
 
g1 7/26/2018 8:30 PM
I was actually wondering if the stock unit already had a hum balance pot, and if so, would tweaking it now mess with the bias.
 
66 Kicks 7/26/2018 8:57 PM
I'm pretty sure that Paul over on EL34 brings up that bias/heater idea every once in awhile.

If you are only looking for a few volts, you could probably change the 1K current limiter resistor to a 470 Ohm. Then replace the 47K resistor with a 20K in series with a 50K potentiometer that has its wiper tied to ground.
 
ggjgjhkihkjhiuyiy8788 7/28/2018 2:09 AM
Quote Originally Posted by 66 Kicks View Post
If you are only looking for a few volts, you could probably change the 1K current limiter resistor to a 470 Ohm. Then replace the 47K resistor with a 20K in series with a 50K potentiometer that has its wiper tied to ground.
^ This is exactly what I did - I had tried it beforehand to see if I could get enough bias voltage, then moved the bias ground to the heater when it was coming up short.

Thanks for the feedback! Yeah, I'm sure someone's thought of this before, but google searches tend to pull up results relevant to using the -DC bias supply to (?de-)'elevate' the heater supply, not the other way around. I realise the whole idea goes out the window if you elevate the heater supply off the centre tap.

I'll report back if something untoward arises - hopefully not!

Cheers