Fender bias change for more negative bias

[imo1]

I loved my "deluxe" scratch build so much that I wanted to make my mid 70's more like it.
I used @martinmanning bias set up. I liked the idea of the more stable bias with the additional reference. In order to get enough negative bias I had to go back down to the 470R off the bias tap and lower the R-C resistor from 10k to 1.5k. I kept the 15k, going to ground. I was able to get my bias settings with the 5U4b good with that mod on his bias design

I liked the stridency of the other amp with the GZ34 in it. I tried subbing that rectifier in. I knew that it would raise the voltage quite a bit and I might have to change some of the bias components to fit
My understanding was that if you didn't have enough negative voltage, you would change that first resistor to a lower value, so less AC was dropping before the diode. I did q quick power up with the 470 and indeed, my current in the tubes was way too high. I tried paralleling another 470, but still wasn't low enough. I tried a 5W 130R and it started smoking! Am I lowering the resistor too much? Does it see the diode as almost a ground ref, since it is converting it to DC at that point? In that case would i be looking to lower the resistor after the diode(in my case a 1.5k?) to get more negative voltage?

I guess I am missing something. For now, I'm going to put the 470R and 5U4G back in. This is stable and works. I thought i understood how this bias circuit worked.

[martin manning]

What is the DC plate voltage on this amp, and what is the bias tap AC voltage? 6V6's, right?

[imo1]

Well, the smoking was the diode fail. I think I may have killed it with all the resistor subs. I may have solved it..

[imo1]

You are using the 5U4G. Makes sense.
Using the 1.2k resistor, and then going to DC, changing the 10k to 6.8, I'm able to bias the tubes at about 20mA @ 435-440, though that is with the pot maxed out. According to JJ that only has me at about 60%, so i could go a little hotter.
I'm seeing 48v AC at the bias tap. I'm gonna try one more resistor(prob 5k). Id like to have it where i could change rectifier tubes without having to change the bias circuit

[imo1]

The 4.7k puts me right in the middle of the pot sweep. I’m sitting at 435 and 21mA. Quick listen and I think I do prefer the gz34. I think I will have enough range to go back and forth between the 2 rectifier types.

So just for summary, 1.2k coming off of bias tap. 4.7k between the 2 caps, 10k bias pot, and 15k to ground. That gets me to the best balance for my deluxe bias circuit

[martin manning]

That sounds good. 435 is pretty high for 6V6, but not scary. PETA (People for the Ethical Treatment of Anodes) will probably leave you alone. The 15k to ground along with the 10k pot drives the adjustment range (so don't increase the 15k to get more negative voltage). Once you set the voltage on the first cap with the resistor coming off the bias tap, the resistor between the caps is where the variable voltage drop (rise, in this case) happens. It's the fixed leg of a divider, where the adjustable leg is the pot plus the 15k. You don't want the resistor between the caps to be too small, otherwise you lose the ripple voltage attenuation. There is more going on in that little circuit than one might think ;^)

[imo1]

The JJ’s are rated to 450 and 14w.
I really like them for production tubes. I’ve had good luck for years with them on the road, and way better results than any other modern brand.

Yeah! Thanks. I understand the 15k to ground 10k relationship. Since I don’t have any other value of bias pot, the 10k has to stay, which means for me, I have to reduce that 10k between the caps to get to where I need to be. That 4.7k is perfect!