Bias Circuit Comparison

[mwelch55]

I was looking at the Soldano Hot Rod 50 schematic and I noticed that the bias circuit was different than the standard bias circuit.  The Hot Rod 50 bias circuit has the bias voltage coming from the wiper of the pot.  Isn't that risky?  If the wiper looses contact with the pot internally, the bias voltage would go to near zero, which would cause the output tubes to roast. 

In the standard circuit, you would still get negative voltage if the pot failed.

Am I missing something or is there a design flaw in the HR50 bias circuit?

[2deaf]

The HR50 has a design flaw.  You can fix it by putting a 1M resistor from the wiper to the junction of the pot. and the 22uF cap.

[sluckey]

I don't like that Soldano bias circuit either. But all the Fender Blackface AB763 amps were just like that. A lot of those are still kicking today with no bias mods. When I built my Deluxe AB763 about 10 years ago I used that same flawed circuit. I also used a quality pot like Fender did. If I was gonna use one of those 25 cent trimmer pots like you see in Marshalls, I'd build a foolproof bias circuit.

[mwelch55]

Thank you for the response. I was pretty sure that was a flawed design, but wanted to run it by you guys first.  I am surprised that Mike Soldano let this one out the door.  I know you can avoid most failures by using good quality parts, but then there's Murphy's Law.

Mike

[tubeswell]

The HR50 has a design flaw.  You can fix it by putting a 1M resistor from the wiper to the junction of the pot. and the 22uF cap.

Or easier still, put a jumper wire between the pot wiper and the pot lug that goes to the 22uF cap (making the pot a variable resistor). This will default to maximum -ve bias voltage if the wiper contact fails.

[2deaf]

Or easier still, put a jumper wire between the pot wiper and the pot lug that goes to the 22uF cap (making the pot a variable resistor).

That will change the bias range rather dramatically.

[tubeswell]

Or easier still, put a jumper wire between the pot wiper and the pot lug that goes to the 22uF cap (making the pot a variable resistor).

That will change the bias range rather dramatically.

It won't make any difference. The circuit will still see the same resistance between the pot wiper and the ground, and it will be 'better' in terms of decoupling because the pot wiper will be shorted directly to the filter cap.

[2deaf]

Make the pot in the HRD50 10K.  Now with the HRD setup, the range is from 0.33 to 0.67 times the diode voltage.  With your setup, the range is from 0.5 to 0.67 times the diode voltage.

[alerich]

It won't make any difference. The circuit will still see the same resistance between the pot wiper and the ground, and it will be 'better' in terms of decoupling because the pot wiper will be shorted directly to the filter cap.

I do not believe that is entirely true. Yes, the circuit will see the same resistance between the pot wiper and ground but you will be bypassing part of the resistance of the voltage divider as you turn the pot. Marshall does that in their JCM800 bias circuit (bypasses part of the voltage divider)  but it's a slightly different circuit that is designed to behave that way. In practice, it might not matter in this example depending on the output tubes you are running and negative voltage they require at the grids but the circuit will behave differently with the jumper in place since you'd be progressively losing 47K of divider resistance with the jumper. You might just end up sacrificing a little bit of range or shifting the range one way or another or both. If it works for the tubes you are using it's probably the cleanest fail safe method to counteract wiper failure.

This is the bias circuit for the Soldano Hot Rod 50 with component values:

[2deaf]

So there is a 100K resistor that Mike didn't show between the HT winding and the diode.  That makes the bias voltage very sensitive to the total load on the bias side of the diode.  Soldano's version has the same load at any setting and installing a 1M resistor from wiper to upper lug has essentially the same load at any setting.  But tubeswell's version has a radical change in load at the setting extremes.  When tubeswell's is at the 67K extreme, the load is the same as Soldano's and the bias is the same.  When tubeswell's is at the other extreme, the total load is 20K which will bring the absolute value of the voltage at the diode way down.  Then the bias is taken at the halfway point of the load, so its absolute value will be lower still.  The net result will be that tubeswell's range will be similar to Soldano's range.   

[alerich]

Honestly, I have never seen an official Soldano Hot Rod 50 schematic. That circuit image is from a schematic floating around that purports to be for the Hot Rod 50 and/or the SL60, its short lived Asian made predecessor. The hand drawn HR50+/HR100+ schematic that is available online has a slightly different configuration that is not wiper dependent but who knows if it is even correct. I have a 1992 HR50. The next time I open it up for something I'll take a look at the bias circuit and stare and compare it just out of curiosity. The bias pot is pretty sturdy so I doubt I'll mod it if I find that it resembles the circuit I posted.