Could someone review my bias pot diagram for a Hiwatt dr103?

[dvlp]

I'm building a DR103 using Mark Huss's diagrams and Heyboer transformers and want to add 2 bias trim pots. On the attached diagram I used Photoshop to move one 100k resistor over one terminal post and to remove/reroute a couple jumper wires on Mark's original diagram. Will this work? Thanks

[loudthud]

No. It looks like all you are doing is adding resistance in series with the grid leak resistors. That won't adjust the bias Voltage.

[Tom Phillips]

The circuit schematic comes first. The layout is an implementation of the schematic. In order to clearly understand the effect of your changes it would be best to work with a schematic view rather than a layout view.

[dvlp]

Thanks guys. I appreciate you looking

[dvlp]

will this work? I don't know much about electronics but it dawned on me that the DR201 has dual bias pots and the circuits are similar to the DR103 so I used the DR201 schematic to come up with this.

[loudthud]

That looks better. On each pot add a 100K resistor between the blue and orange wires. That will make the pots "fail safe". If the wiper (center lug) gets intermittent, your tubes won't burn up because of lack of bias. The resistors could be mounted on the turret strip if there is enough room or an extra couple of turrets.

[dvlp]

That looks better...

Awesome. Thank you

[Chuck H]

Almost! And not bad for a guy that's never done this. It will probably "work", but...

The problems with you're proposed circuit are threefold. It doesn't allow adjustment below the stock bias voltage. That means you can only bias the tubes cooler than stock. It also elevates the AC ground above the interaction point of the grids resulting in local negative feedback. That could result in noise and loss of gain. Ideally you want an AC ground (filter or decoupling capacitor) right at the bias resistor junction. With an independent bias circuit you will certainly need to add another capacitor (at the very least) to do this. But most important...

You are not working with a stock transformer. Which, incidentally, is somewhat known for not providing adequate bias voltage. Voltage doubler circuits have been devised for the stock amps to overcome this problem. But you will not be working with the stock transformers. Any transformer you use will almost certainly have this "flaw" worked out. That renders the values in any proposed circuit arbitrary for a new build. It would be tough to know the actual voltage under load with a non stock transformer prior to the build and some initial testing. That doesn't mean we can't have a proposed starting point circuit. But understand that it may require adjustment to values other than the pots. Finally...

New tubes in matched quads are pretty damn well matched. That wasn't always the case when HiWatt devised the DR201 bias circuit. Using a single bias for all four tubes should be eminently sufficient for a new build if you plan to use matched tubes. I honestly don't think you need to pair balance the bias circuit. I mention this because an individual bias for either "pair" will probably require the addition of another cap (at the very least) and will certainly skew the notoriously "neat" appearance of the HiWatt layout. Also...

It would take some doing to work it out well. And some more doing once real life voltages are known. It's a lot to step up for. Probably a couple of hours before it's done. That is, I'm not stepping up this moment. But here's a schematic version of your proposed circuit in case anyone else wants to chime in. I'm providing this because interpreting the layout is frigging awful for any one that isn't intimate with the circuit as you are. The schematic format offers faster understanding and diagnosis.