Hiwatt dr103 Bias voltage to little

[Ampzone]

So I have a stock 70's DR103 with the bias voltage AC appx 356v. Its very anemic. I can get the new el34's to about 36ma at 503v DC with a bias pot added. But, I would like to see it around 30-32ma, not as hot.

[g1]

Not sure what you mean about the bias circuit AC being 356V? Does it not have the bias winding?
Here is the bias circuit with pot added from Mark Huss's site

[drewl]

Yeah for Hiwatt and Sound City amps this is common.

Google search for "Hiwatt bias doubler" to find a schematic to double the bias voltage.

[Ampzone]

Not sure what you mean about the bias circuit AC being 356V? Does it not have the bias winding?
Here is the bias circuit with pot added from Mark Huss's site


[ATTACH=CONFIG]51947[/ATTACH]

Im sorry 36v ac.

[g1]

Here's a couple: (single and dual control)
https://hiwatt.org/Schematics/BiasCkts2.gif

[The Dude]

Sure, you could build a doubler- no arguement. I have to wonder if you couldn't hit the goal with a simple resistor change or two, though.

[Ampzone]

Sure, you could build a doubler- no arguement. I have to wonder if you couldn't hit the goal with a simple resistor change or two, though.

I know, IT sucks to have to add extra parts. I just need to cool it down a bit more from avg 36ma per tube to 32ma. I get 36v ac on the other side of the diode. Strange how little there is on tap!

Thanks for the link

[g1]

What is the VDC at the anode end of the diode?

[Ampzone]

What is the VDC at the anode end of the diode?

Appx 36vdc. plates are appx 503v, I hope these new 34's can handle it.

[g1]

Have you tried replacing the cap there? I think you should get more than 36VDC with 36VAC going to the diode.

[The Dude]

^^^^^^ I agree. I might also try upping the value of the 47k across the cap. I don't think you need a large increase in bias voltage to get to 30-32mA bias.

[Ampzone]

^^^^^^ I agree. I might also try upping the value of the 47k across the cap. I don't think you need a large increase in bias voltage to get to 30-32mA bias.

It was stock 100uf cap 47k giving about 48ma cooking per tube at 510v. I put a small 25k trim pot and changed the 47k to a 100k and it got me to appx 36ma per tube 503 plate but its still too hot. I have left the 100uf stock cap in there. So you suggest changing it?

[drewl]

If the cap is bad or starting to go bad it can drag the bias voltage down.

Plus if it shorts or does go bad it can do alot of damage.

Bias supply filters are very important.

[The Dude]

I'd try either changing it, or temporarily clipping another one across it to see if anything changes.

[Randall]

"I'd try either changing it, or temporarily clipping another one across it to see if anything changes."

Yeah, but if the cap is partially shorting and pulling the voltage, what will clipping in another across it tell us?