Classic Tone P.T Bias Tap?

revolver1 · Post by **revolver1** » Wed Oct 24, 2018 3:08 pm

This kinda runs on from my last post but I just wanted to confirm I have this right?

So I have wall voltage of 230v and I want to hook the P.T up on the 220v tap. I've taken the hot (live) to the DPDT switch then on to the fuse carrier and conected the Grey P.T primary on the other side of the fuse. The Black on the other side of the P.T connects to the DPDT switch and the neutral is taken from the other side of the switch. I then made a small tag board to connect the Black/White and the Brown together. Does this sound O.K?

I wanted the highest B+ possible so I went with the red secondary's using the Red/Yellow center tap to ground with diode rectification. Does This sound right?

This is where I'm really uncertain, can I use the 50v bias tap Red/Blue for the bias circuit and this works on the same Red/Yellow center tap as above? Or should I take the feed from one of the red secondary's pre switch at the standby switch? I'm getting 525Vdc after the rectifier with no tubes.

I had NOS caps and an NOS trimmer and the voltages were jumping all over the place so I ripped It all out and put in some B.C caps and an new trimmer now its much more stable just not sure I'm giving it the right supply?

I'm trying to bias 2x 6550's at 25ma so as a starting point I have a 150K on the bias input and a 47K on the range resistor. I'm only getting a range of about -7vdc to -9vdc so somethings not right, just don't want to get it wrong and blow anything up... There's alot coming out of the rectifier

revolver1 · Post by **revolver1** » Thu Oct 25, 2018 5:33 am

Thinking about it 525vdc off the rectifier will be too much for the mains filter can

danman · Post by **danman** » Thu Oct 25, 2018 8:01 pm

You can use the bias tap but the standard 50 watt circuit taps bias voltage from one of the secondary leads. The bias tap is giving you a much lower voltage for the bias circuit so you will need to adjust the circuit values to achieve proper voltage. It would be easier to stick with the standard way of feeding the bias circuit.

revolver1 · Post by **revolver1** » Fri Oct 26, 2018 4:40 am

Cheers Denman. I re-wired it yesterday for the 240v tap and re-wired the bias off one of the secondarys and it's much better. The bias is now giving -58.5vdc to -39.9vdc, I now need to play around with the range resistor to get it into class B range.

I was shooting for 450vdc on the plates so with 230v wall voltage and only the option of 220v or 240v tap I thought I would be up against it to achieve this but I think it's going to get surprisingly close.

I'm getting 474vdc off the rectifier un burdened and 6.5vac across the heaters so I still need a little adjustment there.

danman · Post by **danman** » Fri Oct 26, 2018 7:29 pm

Maybe a "brown box" type device to drop about ten volts on the incoming AC would get your voltage down just a touch more and make the tube filaments happy.

If those numbers you posted are the unloaded measurements, they should drop down once all the tubes are installed.

revolver1 · Post by **revolver1** » Sat Oct 27, 2018 4:38 am

Thanks Danman, I'm going to fire it up today and take some fast readings

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Classic Tone P.T Bias Tap?

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